Understanding Fixed Limit Gages | PDF | Engineering Tolerance

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Fixed limit gauging uses GO and NOGO gauges to accept or reject parts based on whether they fall within the minimum and maximum acceptable tolerances. GO gauges check the minimum size and NOGO gauges check the maximum size. Ideally, the gauges' tolerances are 10% of the part tolerance to avoid rejecting good parts, but standard gauge grades are often used that have tolerances close to but not exactly 10%. Choosing a gauge with a looser tolerance detects out of tolerance parts sooner but risks accepting more out of spec parts, while a tighter tolerance gauge risks rejecting more good parts.

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• 1. TRAINER GUIDE - II JIGS, FIXTURES & GAUGES (1ST SEMESTER) PGTD / PDTD VERSION - 0 MSME TOOL ROOM INDO GERMAN TOOL ROOM AHMEDABAD
• 2. TRAINER GUIDE - II FOR ADVANCE DIPLOMA IN TOOL & DIE MAKING Subject Area: Tool Design Theory – “Jigs, Fixtures & Gauges” ( Part – I & II)
• 3. CONTENTS Chapter No. DESCRIPTION Page No. A1. Introduction To Production Toolings A1.1 Introduction Of Tools Used In Mass Production…………... 1 A2. Introduction To Jigs & Fixtures………………………………… 4 A3 Elements Of Jigs & Fixtures A3.1 Locators, Locating Methods & Devices……………………. 7 A3.2 Clamps, Clamping Methods & Devices……………………. 35 A3.3 Guiding Elements (Jig Bushings)…………………………... 60 A3.4 Tool Bodies (Jig & Fixture)………………………………….. 81 A3.5 Fasteners (Jig & Fixture)……………………………………. 84 A4 Limit, Fit & Tolerance A4.1 Introduction…………………………………………………… 96 A4.2 Advantages Of Limits & Fits………………………………… 97 A4.3 Tolerances …………………………………………………… 98 A4.4 Limits………………………………………………………….. 100 A4.5 Fits…………………………………………………………….. 101 A4.6 Types Of Assembly………………………………………….. 108 A4.7 Allowances …………………………………………………… 111 A4.8 Deviation ……………………………………………………... 112 A4.9 Maximum & Minimum Material Condition…………………. 115
• 4. CONTENTS Chapter No. DESCRIPTION Page No. A5 Design A5.1 Design Of Jigs & Fixtures…………………………………… 118 A6 Jigs A6.1 Introduction…………………………………………………… 137 A6.2 Function Of Jigs & Fixtures…………………………………. 138 A6.3 Factor Characteristics In Jig Design……………………….. 138 A6.4 Jig Support……………………………………………………. 140 A6.5 Jig Bodies And Rigidity……………………………………… 140 A6.6 Classification Of Jigs………………………………………… 140 A6.7 Types Of Jigs & Their Description…………………………. 151 A6.8 Maintenance, Storage & Safety Of Jigs…………………… 152 A7 Fixtures A7.1 Introduction…………………………………………………… 155 A7.2 Basic Design Consideration………………………………… 155 A7.3 Factors In Fixture Design…………………………………… 156 A7.4 Classification Of Fixture…………………………………….. 158 A7.5 Maintenance, Safety & Storage Of Fixtures………………. 181 A8 Estimation A8.1 Introduction…………………………………………………… 183 A8.2 Purpose Of Cost Estimating………………………………… 183 A8.3 Elements Of Cost…………………………………………….. 184 A8.4 Cost Structure………………………………………………… 187 A8.5 Estimation Of Cost Elements……………………………….. 188 A8.6 Estimating Tool Cost………………………………………… 192
• 5. Chapter No. DESCRIPTION Page No. A8.7 Steps In Making A Cost Estimation………………………… 194 A8.8 Chief Factures In Cost Estimation…………………………. 194 A8.9 Numerical Examples………………………………………… 195 A9 Gauges A9.1 Introduction…………………………………………………… 199 A9.2 Classification Of Gauges……………………………………. 201 A9.3 Design Of Gauges…………………………………………… 219 A9.4 Sub Zero Treatment…………………………………………. 229 A9.5 Maintenance, Safety & Storage Of Gauges………………. 229 A9.6 Numerical Examples………………………………………… 230
• 6. INTRODUCTION A1.1 Introduction of Tools used in Mass Production Production of quality goods in large quantities at high speeds is the requirement of the day. To meet this, there have been considerable changes and developments in the manufacturing industries, with an empha sis on increased efficiency and productivity. As a sequel to these changes the tool technology has also undergone changes, leading to the designing and development of special tools, methods and techniques for the benefit of industry, to ensure quality products at economical rates. Jigs and fixtures are the special production tools which make the standard machine tool, more versatile to work as specialised machine tools. They are normally used in large scale production by semi -skilled operators, however t hey are also used in small scale production, when interchangeability is important. Manufacturing industries in India, on par with their counterpart elsewhere, have brought lot of revolution in manufacturing technology, during the (last 20 years, as a cons equence of which several developments like CNC Lathes, CNC Machine Centers, Flexible Manufacturing Systems, Fabrications Centre, Transfer Machines, Robotics, etc. took place). Our Engineers and Technologists are deeply involved in devising innovative 7 techniques. Lot of modernisation has taken place in Indian Industry. Even with these advancements in the manufacturing indsutries, there is a continued use of jigs and fixtures in some form or the other either independently or in combination with other systems. CHAPTER OUTLINE A1.1 – Introduction of tools used in Mass Production TOPIC OUTLINE A1.1a Jigs A1.1b Fixtures A1.1c Gauges A1.1d Press Tools A1.1e Moulds
• 7. The work tooling refers to the hardware necessary to produce a particular product. The most common classification of types of tooling is as follows : 1. Sheet metal press working tools. 2. Moulds and dies for plastic moulding and die casting. 3. Forging dies for hot and cold forging. 4. Jigs and fixtures for guiding the tool and holding the workpiece. 5. Gauges and measuring instruments. 6. Cutting tools such as drills, reamers, milling cutters, broaches, taps etc. The tool maker manufactu res the above item from the design supplied to him. On gaining experience the tool maker will be able to design and manufacture simple tools. A1.1a Jigs A jig is a device that locates and holds the workpiece. It also guides and controls one or more c utting tools. Jigs are fitted with hardened steel bushings for guiding drills or other tools. Small jigs are not usually clamped to the machine. For holes above 6mm jigs are usually clamped. Drill jigs are used while drilling reaming counter boring, ta pping, chamfering etc. There is hardly a product produced that does not contain one or more holes. The location finish and size of these holes may be critical as in the case of a component for a missile or they may be holes like those punched in a templat e for the purpose of hanging it on the wall when not in use. Holes are produced and finished in a number of ways. They are drilled, reamed, bored, punched, ground, flame cut etc. Drilling is by far the most common method. A1.1b Fixtures A fixture is a device that locates and holds the workpiece. Setting blocks and feeler gauges are used for setting the cutter in relation to the workpiece. Fixtures designated for machining operations always clamped on to the machine. A fixtures is a device for hold ing a workpiece during machining operations. The name is derived from the fact that a fixture is always fastened to a machine or bench in a fixed position. Many machining operations can be performed by clamping the workpiece to the machine table without using a fixture, especially when a few parts are to be machined.
• 8. However when the number of parts is large enough to justify its cost, a fixture is used for holding and locating the work. Further, when the profile of the Component is not regular or when machining has to be done w.r.t. a reference face or bore, application of fixture will be necessary. A1.1c Gauges Modern manufacturing requires extensive uses of gauges for shop work, inspection and reference. A gauge can be defined as a device for investigating the dimensional fitness of a part for a specified function. A1.1d Press Tools Press tools are special tools custom built to produce a particular component mainly out of sheet metal. The principle op erations of sheet stampings include cutting operations (Shearing, blanking, piercing etc.) and forming operations (bending, drawing etc.). Sheet metal items such as automobile parts (roofs fenders, caps etc.) components of aircraft, parts of business mach ines, household appliances, sheet metal parts of electronic equipments, precision parts required for horological industry etc. are manufactured by press tools. A1.1e Moulds for Plastics Plastics did not enter our lives with the fanfare of other revolu tionary inventions, but more by the process of infiltration. Plastics being synthetic materials were at first considered to be cheap substitute for the better known and more expensive materials. Plastic articles are not only replacing wood, metal and oth er materials but because of their particulars qualities they function better than other materials for specific purposes. Through the years plastics have carved the right as materials themselves and not as substitute for other materials. Not only are plastics more useful, adaptable and practical than the materials they have supplemented, but uses for plastics have been found for which no other material can be used.
• 9. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2A5 DESIGN JIGS, FIXTURES & GAUGES 118 Maximum productivity at minimum cost is the demand of modern industry. To meet this requirements designing of efficient and accurate jigs and fixtures is required. Quality, simplicity and economy from the important criteria from the design of jigs and fixtures. To meet this requirement the designer will have to made an economic analysis for using jigs and fixtures and has to device certain principles of design, and finally develop a checklist for the jigs and fixture design. A5.1a Tool Design Objectives The main objective of tool design is to lower manufacturing costs while maintaining quality and increased production. To accomplish this, the tool designer must satisfy the following objectives: Design Economics Maximum productivity at minimal cost is t he demand of the day. Tool designer has therefore an additional consideration of keeping the cost of these special tools as low as possible apart from developing designs for efficient and accurate jigs and fixtures. For this he has to apply the design economy. i.e. to reduce the cost without sacrificing the quality. The following are some of the considerations involved in the economy design. 1. Simplicity 2. Preformed components 3. Standard components 4. Secondary operations 5. Tolerance and allowances 6. Simplified drawings TOPIC OUTLINE A5.1a Tool Design Objectives A5.1b Design Principles A5.1c Major factors in design of jigs & fixtures A5.1d Elements of design (jigs & fixtures) A5.1e Flow chart for development of design solution A5.1f Check list for the design of jigs & fixtures CHAPTER OUTLINE A5.1 Design of Jigs & Fixtures
• 10. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2A5 DESIGN JIGS, FIXTURES & GAUGES 119 1. Simplicity Simplicity is essential in the tool design. Every element in the design of jigs and fixtures should be considered for possible savings in time and materials. 2. Preformed Materials These materials greatly reduce tooling costs by the elimination of many machining operations. Wherever practicable, preformed materials, such as drill rods, structural sections, pre machined bracket materials etc. should be included in the design. 3. Standard Components Commercially available standard components such as clamps, locators, supports, drill bushings, pins, screws, bolts, nuts etc. would contribute greatly in improving the tool quality besides effecting considerable savings in labour cost and time. 5. Secondary Operations Secondary operations such as grinding, heat treating and some machining should be as far as possible be eliminated as they involve additional time and cost. If they cannot be totally eliminated they should be limited to areas necessary for efficient tool operations. 5. Tolerances and Allowances Generally the tolerances of a jig or a fixture should be between 20 percent and 50 percent of the part tolerance, as unnecessarily close tolerances will be add up to the higher cost of the tool. 6. Simplified Drawings Tool drawings will for a sizable part of the total tooling cost, hence it is necessary to keep them low. This is accomplished by simplifying the drawings as follows : a) Wherever practicable words should replace drawn details. b) Elimination of redundant views, projections or details. c) When possible, replace drawn details with symbols. d) Reduce the drawing time by using templates and guides. e) Standard parts should only be drawn for clarity, not detail refer to these by part numbers or named.
• 11. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2A5 DESIGN JIGS, FIXTURES & GAUGES 120 A5.1b Design Principles After the economic and design analysis the tool designer must comply with the following in designing the jigs and fixtures. 1. He has to thoroughly understand the component details, its pre -machined conditions, reference surface dimensions, accuracies and tolerances to be achieved. 2. He has to know on which machine the operations is likely to be performed. A check has to be made for constraints on the design parameters. 3. He has to make provision for easy loading and unloading of the work piece. 4. Facility for quick and accurate positioning of work piece be provided. 5. Fool proof method has to be incorporated to avoid wrong position while loading the work piece. 6. Designer has to take into account the optimum clearance with swarf removal and cleaning facility. 7. Machined surface are be taken as locating surface preferably. 8. Sharp corners in the locating surface must be avoided. 9. Adjustable locations are to be provided for right surfaces. 10. Locating surfaces should be as small as possible. 11. Locating pins should be tapered and easily accessible and visible to the operator. 12. Designer must have the economic approval to the design considerations. 13. As many degrees of freedom of movement should be arrested as necessary to achieve the required accuracies. In general 3-2-1 principle to be adopted. i.e. 3 Points in the first plane 2 Points in the second plane 1 Points in the third plane 14. Make the layout always to a scale, whenever possible. 15. The use of standard items in clamping, locating and fastening elements should be made, whenever possible. 16. Total engineering data in the drawing to be provided. I.e. material, heat treatment of the component, geometrical accuracies, toler ances, surface roughness for manufacturing and inspection purpose etc. 17. Stress to be given to minimise the weight of the jig or fixture for easy handling and to reduce the fatigue on the operator. 18. Care has to be taken for providing suitable support or guidance for preventing work piece bending or movement while operation and clamping.
• 12. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2A5 DESIGN JIGS, FIXTURES & GAUGES 121 19. Attention to be given to tightening up of loose items of jig or fixture. 20. In jig and fixture layout a distinction between work piece and jig or fixture component to be brought by means of chain dotted lines for work piece and full lines for jig and fixture components. 21. Provision to be made for the setting gauges in fixture. 22. Machining table mounting requirements are to be considered while designing. 23. Bill of material to be provided. The good design of jigs / fixtures is that which satisfies the following a) Functional aspect b) Quality c) Cost d) Production schedule e) Safety f) Adaptability to the machine A5.1c Major Factors in the Design of Jigs & Fixtures In planning jigs and fixtures, it is essential to consider three major factors, which have a definite influence upon the design of tools. i) The tool should be designed for efficient operation and for easy manipulation by the operator. ii) The tool should be designed so that it will be produce accurate workpieces on a repetitive basis. iii) The cost of the tool should usually be governed by the number of parts to be produced. 1. Efficient Operation In determining how a jig or fixture can be best designed for its most efficient use by the operator, the following should be considered : i) Type of jig or fixture required for the specific part. ii) Locating and loading of the work a. Clearances necessary for locating the work. b. Methods of foolproofing against improper loading c. Unloading the work iii) Rapid methods of clamping the work.
• 13. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2A5 DESIGN JIGS, FIXTURES & GAUGES 122 iv) Methods of handling the tool especially when it is large and heavy. v) Chip clearances and chip removal. vi) Wearing surfaces and replacement of worn parts. vii) Selection of materials for the special tool. viii) Safety in operation. 2. Accurate Workpieces The design of jigs and fixtures is influenced by the degree of accuracy requiredin the workpiece. The features of design will vary as the requirements for accuracy vary for a workpiece. This is one of the major factors to consider in the design of special tools. When multiple or subsequent operations are necessary, the same locating surface or surfaces on the workpiece should be used in each of the special tools required for the manufacture of that part. The accuracy necessary to obtain the propre relationship between a workpiece and other parts in an assembly is an important consideration in design. Some of the factors to be considered in this respect are : 1. The accurate relationship of operating surfaces on different parts when they are assembled? 2. Adequate rigidity to maintain accuracy in jig or fixture. 3. Economy & Cost The cost of the tool and the number of parts to be produced are other factors in determining the design. If a small quantity of parts is to be produced, a simple low cost tool may be satisfactory. The necessity of keeping the manufacturing cost of a new article as low as possible, or reducing the present cost of an existing article, usually determines the type of jig or fixture that is to be made. In some other instances, the cost of an operation may be reduced by using a more efficien t though more expensive tool. Increased accuracy and interchangeability secured through the use of a more elaborate tool frequently warrants its greater coat. The use of standard accessories requires serious consideration in the economical construction of jigs and fixtures. The designer should familiarize himself with the possible used of all types of standard accessories. Pre-fabricated units such as tool bodies, locating and clamping devices, drill jig bushings, and tool body supports should be used. Standard parts may be purchased or made in quantities and kept in stock.
• 14. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2A5 DESIGN JIGS, FIXTURES & GAUGES 123 A5.1d Elements of Design (Jigs & Fixtures) 1. The work must be Located Properly The ease and rapidity with which the workpiece can be located and removed is an important conside ration in the design of special tools. Therefore, the designer should become thoroughly familiar with the various methods of locating and clamping before a design is definitely decided upon. Parts having rough or irregular surfaces, and parts which varyin size, usually present special problems in locating. To compensate for such irregularities or variations, adjustable locators should be used. The design of adjustable locating stops and supports should provide for positive location and for simple and easily accessible means of adjustment and locking. Locating points on jigs and fixtures should be designed so that incorrect loading of the workpiece is impossible, further more to obtain the proper balance, the locators should be place as far apart as the shape of the workpiece will allow. Consideration must also be given to the position of the locators to allow for the necessary clearance in loading and unloading. The locating points should be made wear resistant in order to maintain accuracy, especially when non-adjustable stops are used.
• 15. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2A5 DESIGN JIGS, FIXTURES & GAUGES 124 Accuracy Location should be done on the most accurate surface of the workpiece. A machined surface is preferable to an unmachined one. When more than one machined surfaces are available, locate from the most accurate surface. For example, the center of the turned part can be located from outside diameters 110 or 80 or form central 50 f bore 80f has the minimum tolerance of 0.05, so the workpiece can be located most accurately from outside diameter 80f. Location form 50 f bore would be less accur ate than location from 80f but more precise than location from outside diameter 110f which has a much wider tolerance of 1mm (±0.5mm). 2. The work must be clamped properly The method of clamping and the design of the clamps depend upon the shape of the workpiece. The designer should consider the following fundamentals : i. The clamps should be positioned to resist the maximum pressure of the cutting tools. ii. The clamps should be located over or as near as possible to some bearing point of the workpiece. This must be done to avoid springing the part. iii. The clamps should be designed so that they can be quickly and easily unlocked and shifted out of the way of the workpiece when it is unloaded. iv. Complicated clamping devices should be avoided if possible. A simple device has fewer wearing surfaces and will stay in working condition for a longer time. v. The kind of material in the workpiece should be considered in choosing a design for the clamps. For example, finished surfaces or soft material require a larger clamping area than surfaces of hard material. The larger clamping face distributes the pressure so that the workpiece is not deformed or spoiled. The work supporting devices opposite the clamps should be large enough to support the pressure of the clamps. 3. Large tools – Weight & Handling Special tools designed for large workpieces are often unduly heavy. It is therefore desirable to make them as light as possible for easy manipulation without impairing their strength. Large cast iron tool bodies can be made lighter by coring out the metal. It is often possible to reduce weight by the use of well designed, fabricated and welded tool bodies, where heavy castings are to be drilled, reamed or bored on several sides, trunnions, thereby
• 16. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2A5 DESIGN JIGS, FIXTURES & GAUGES 125 eliminating the otherwise difficult problem of lifting and turning jig faces up into the operating position. All corners should be well filleted for strength. Heavy tools should be provided with handles or holes for bars or eye bolts to facilitate lifting. In some cases it is often good design to provide smaller jigs with handling devices for convenience in holding them while the enclosed workpiece is machined. Sharp edges and corners which might injure the operator should be avoided. 4. Chip clearance Clearance must be allowed so that the chips will not accumulate and interfere with the cutting operation or workpiece location. The jigs should be designed so that it ca n be easily cleaned. Holes or escapes for draining the coolant or cutting lubricant should be provided. 5. Materials for Jig & Fixture Jigs and fixtures are made from a variety of materials, some of which can be hardened to resist wear. It is sometimes necessary to use nonferrous metals like phospher bronze to reduce wear of the mating parts, or nylons or fibre to prevent damage to the workpiece. Given below are the materials often used in jigs, fixtures, press tools, collects, etc. a. High Speed Steels (HSS) These contain 18% (or 22%) tungsten for toughness and cutting strength, 5.3% chromium for better hardenability and wear resistance and 1% vandadium for retention of hardness at high temperature (red hardness) and impact resistance. HSS can be air or oil hardened to RC 65-65 and are suitable for cutting tools such as drills, reamers and cutters. b. Die Steels These are also called high carbon (1.5 -2.3%) high chromium (12%) (HCHC) cold working steels and are used for cutting press tools and thread forming rolls. Hot die steels with lesser carbon (0.35%) and chromium (5%) but alloyed with molybdenum (1%) and vanadium (0.3 -1%) for retention of hardness at high temperature are used for high temperature work like forging, casting and extrusion.
• 17. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2A5 DESIGN JIGS, FIXTURES & GAUGES 126 c. Carbon Steels These contain 0.85-1.18% carbon and can be oil hardened to RC62-63. These can be used for tools for cutting softer materials like wood work, agriculture, etc. and also for hadn tools such as files, chisels and razors. Theparts of jigs and fixtures like bushes and locators, which are subjected to heavy wear can also be made from carbon steels and hardened. d. Collet Steels (Spring Steels) These contain about 1% carbon and 0.5% Manganese. Spring steels are usually tempered to RC 57 hardness. e. Oil Hardening Non-Shrinking Tool Steels (OHNS) These contain 0.9-1.1% carbon, 0.5-2% tungsten and 0.55-1% carbon. These are used for fine parts such as taps, hand reamers, milling cutters, engraving tools, and intricate press tools which cannot be ground after hardening (RC 62). f. Case Hardening Steels These can be carburised and case hardened to provide 0.6 -1.5 thick, hard (RC 59 -63) exterior. 17Mn1Cr95 steel with 1% manganese and 0.95% chromium is widely used. 15Ni2Cr1Mo15 steel with additional nickel (2%) reduces thermal expansion up to 100 0 C. Case hardening steels are suitable for parts which require only local hardness on small wearing surfaces where costlier, difficult to machine full hardening tool steels are not warranted. g. High Tensile Steels These can be classified into medium carbon steels with 0.55% - 0.65% carbon (En8-9) and alloy steels like 50 Ni2Cr1m028 (En25). The tensile strength can be increased up to 125 kg/mm2 (RC50) by tempering. Medium carbon steels are used widely for fasteners and structural work while alloy steels are used for high stress applications like press rams. h. Mild Steel It is the cheapest and most widely used material in jigs and fixtures. It contains less than 0.3% carbon. It is economical to make parts which are not subjected to much wear and are not highly stressed from mild steel.
• 18. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2A5 DESIGN JIGS, FIXTURES & GAUGES 127 i. Cast Iron It contains 2-2.5% carbon. As it can withstand vibrations well, it is used widely in milling fixtures. Self lubricating properties make cast iron suitable for machine slides and guide ways. The ingenious shaping of a casting and the pattern can save a lot of machining time. Although, the strength of cast iron is only half the strength of mild steel, a wide variety of grades have been developed. Nodular cast iron is as strong as mild steel, while meehanite castings have heat resistant, wear resistant, and corrosion resistant grades. j. Steel Castings These combine the strength of steel and shapabilly of a casting. k. Nylon and Fibre These are usually used as soft lining for clamps to prevent denting or damage to the workpiece under high clamping force. Nylon of fibre padsare screwed of stuck to mild steel clamps. l. Phospher Bronze It is widely used for replaceable nuts in screw operated feeding and clamping systems. Generally screw making process is time consuming and costly. So, their wear is minimised by using softer, shorter phospher bronze mating nuts. These can be replaced periodically. Phospher bronze is also used in applications calling for corrosion resistance, like boiler valves. 6. Construction of Jigs and Fixtures Jigs and fixtures bodies may be made of c ast iron, or they may be built up of steel plates or structural forms held together by screws and dowels or welded joints. Welded constructions are proving desirable because the bodies are strong and light, and addition alterations and additions to the tool can be effectively accomplished. The size of a tool, the quantity of workpieces to be made, and the cost of construction are important considerations in planning a design. 7. Replacement of Worn Parts
• 19. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2A5 DESIGN JIGS, FIXTURES & GAUGES 128 Some parts of tools are subjected to so much wear that the accuracy of the tool may be impaired. For such tool parts, material that can be made wear resistant must be selected and the tool must be constructed so that worn out parts are easily replaceable. 8. Safety in Operation One of the most important considerations affecting the design of tools is the safety of the operator. Any features, which might cause injury, must be eliminated. Adequate operating accessories, such as suitable and efficient levers and locks are essential for safety in operation. The design of jigs and fixtures should provide means of clamping to the machine table if large tools are used or when tooling need not be shifted. Convenient holding devices should be provided as a safety factor whenever necessary. The tool must be designed so that it can be easily set up, adjusted, operated, and cleaned. Features, which safeguard the equipment against misuse, are also very important elements of design. A5.1e Flow chart for development of design solution
• 20. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2A5 DESIGN JIGS, FIXTURES & GAUGES 129 FLOW CHART FOR DEVELOPMENT OF DESIGN SOLUTIONS Initial Design Concept Design procedure 1. Statement of the problem eg. To design a drill jig to hold a support bracket while drilling 3 – 6mm holes To design a lathe fixture for holding a pump housing for drilling and boring of bearing holes. Part Details Operation Classification Equipment Selection Operator Criteria Select Pertinent Items Select Pertinent Items Select Pertinent Items Select Pertinent Items Discarded ideas Preliminary Tool Design Cost Analysis & Evaluation Primary Tool Design Alternate 1 Tool Design Alternate 2 Tool Design Evaluation and Final Decision Completion of Design, Execution of Shop Drawings Phase 1 Phase 2 Phase 3
• 21. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2A5 DESIGN JIGS, FIXTURES & GAUGES 130 2. Need Analysis: (Who, why, how, when, what and where about functional requirements) 3. Ideation (sketches) Information collection. 4. Analysis and synthesis 5. Tentative design solutions 6. Evaluation and testing 7. The finished design A5.1f Checklist for the Design of Jigs & Fixtures The following list of checkpoints should be consi dered before any design of jig or fixture is released for manufacturing. ¨ Check List for Tooling Layout 1. Is the tool layout the latest issued? 2. Is the part drawing the latest issued? 3. Is the part correctly shown on the layout? 4. Are the locating points provided using the thumb rule 3-2-1- -Three points in first plane - Two points in second plane - One point in third plane 5. Are practical considerations made in locating and clamping a part in jig or fixture? 6. Can locators be easily cleaned or replaced? 7. Is the jig or fixture of sound design? Are rigidity and simplicity taken into consideration? 8. Are the locators accessible for cleaning? 9. Are all the clamping requirements properly considered? 10. Are the individual components designed from the point of minimizing machining? 11. Is the layout made to scale? 12. Are the standard items used indicated? 13. Does using separate numbers with leaders and arrows pointing to the details identify the different parts? 14. Is the bill of materials provided in the drawing? 15. Are the details of operations, such as heat treatment and the surface roughness indicated in the drawings?
• 22. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2A5 DESIGN JIGS, FIXTURES & GAUGES 131 ¨ The machine and Set Up 1. Will the jig or fixture fit into the machine for which it is intend? 2. Will the clamping slots or holes in the jig or fixture line up with the T-slots in the table? 3. Will the fixture, when in place, overhand at the end of the table? 4. Will the jig or fixture interfere with any other fixture next to it in the case of multispindle machine? 5. Can a set up operator see whether the cutter/drill are correctly set? 6. Can the cutting tools be adjusted and removed easily for sharpening when the fixture is in place? 7. Can setting blocks, bushings, stops, or collars be used in setting up the cutting tools? 8. Are suitable locating plugs for setting up, provided? 9. Does the set up operator need more than one size of wrench? 10. Are the hold down bolts to make tightening easy provided? ¨ Method of Location 1. Are the locating points widely placed? 2. Are the centralized means required to compensate for variations in the work piece. 3. Is the tolerance on locating points sufficiently close to obtain the specified operational accuracy? 4. Are the locating points as small in area as possible? 5. Are locators safe from damage by cutters? ¨ Method of Clamping 1. Are the loads static or dynamic? 2. Is the work piece supported as closely as possible to the point of load applications? 3. Is the cutting force resisted by a solid support and not by the clamp? 4. Can the cutting force be used conveniently to help securing the work piece? 5. Has the clamp sufficient range to accommodate allowable work piece variations? 6. Is the work piece directly supported under clamping points? 7. Will the clamping force unduly distort the work piece? 8. Will the clamp tend to loosen under cutter chatter or vibration? 9. Can it be planned to have a single standard wrench to tighten all the clamps? 10. Does the work piece size, the required clamping force, or the required speed of action warrant used of pneumatic or hydraulic clamping?
• 23. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2A5 DESIGN JIGS, FIXTURES & GAUGES 132 ¨ Handling 1. Is the part within handling capacities by hand? 2. Is by hoist, are the facilities and necessary sling clearance, loading skills provided to ensure easy handling? 3. If by conveyer, is the correct height maintained? ¨ Loading / Unloading Work Pieces 1. Will cutters such as long drills interfere with work piece while loading or locating? 2. Will clamp interfere while loading or unloading? 3. Is the clearance sufficient to permit the work piece to be easily lifted over or into locating and centering devices? 4. Have any sliding pins or other hand -operated locators been provided with comfortable handles? 5. Are movable locators and adjustments on the side of the fixture nearest the operator? 6. Can the fixture be loaded with one hand while the other hand is used for loadi ng the completed work piece? 7. Are any burrs likely to interfere with unloading? 8. Should an ejector be provided? ¨ Thrust and Torque 1. Can satisfactory blockings be arranged to withstand cutter feed strains and distortion? 2. Are clamps carrying thrust load avoided? ¨ Chips 1. Are channels to allow the collate to wash the chips away provided? 2. Are blind spots and traps avoided? ¨ Capacity 1. Will the proposed design come within the column clearance capacity, table spindle travel of machine? 2. Are the jigs fit large enough to span T-slots in the machine table? 3. Is there sufficient clearance between tools and the work piece for easy gauging?
• 24. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2A5 DESIGN JIGS, FIXTURES & GAUGES 133 ¨ Lubrication 1. Is the machine equipped for coolants? 2. Can suitable coolant reservoir be provided? ¨ Tool 1. Is the design ensures the use of standard tools (drills, reamers, milling cutters)? 2. Can use of existing special tools be incorporated? 3. Is provision made to secure all special tools needed? 4. Can the cutter flutes discharge chips even when covered up? ¨ Standard Parts 1. Does the proposed design incorporated fully the use of all standard parts carried I stock for jigs or fixtures? ¨ Loose Pieces 1. Can the loose parts (if inavoidable) be attached to the fixture with keeper screws or light chains? 2. Is the proper identification or storage facilities provided for the loose parts (Clamps, removable locating plugs and adapters)? ¨ Balance 1. Is the uniform mass distribution in design ensured? ¨ Progressive Experience 1. Does similar type of equipment exist if so what is your experience? 2. Are they suggestions from machine shop or tool room been considered and incorporated? ¨ Manufacturing Considerations a) Built up 1. Can the jig or fixture be built? Does the proposed design includes any impossible (difficult impractical) machining problems? 2. Does it confirm to all known machine capacities? b) Castings 1. Does the design land itself to good pattern making practices and economical castings?
• 25. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2A5 DESIGN JIGS, FIXTURES & GAUGES 134 c) Welded Construction 1. Would welded steel construction offer any advantages? d) Material 1. Have proper steels / materials been selected to make the miscellaneous details? ¨ Production Requirement 1. Is the jig/fixture best suited to meet the requirements? 2. Will available machine burden, require manual semi or full automatic, single or multiple set up? ¨ Safety 1. Does the fixture design protect the operator from coolant spray or flying chips? 2. Is the designed tool safe to operate with? ¨ Sundry Requirements 1. Will the fixture design keep the length of the cutter travel to a minimum? 2. Will the operator, when positioning the jig, clearly able to see all bushings or cutter guides? 3. Are the bushings long enough to provide adequate tool support? 4. Do the tool need guiding for a second operations? 5. Can the use of slip bushings be avoided by used of stepped drills? 6. If the slip bushings are must, are the heads large enough, fluted for easy gr ipping, and provided with locking means? 7. Do all supporting pads and pins stand well clear of chip collecting surfaces? 8. Is the chip fouling the clamp lifting springs? 9. If the work piece is to be measured while still in the fixture can it be easily cleaned? 10. Is there sufficient clearance between tools and the work piece for easy gazing? 11. Can the tools be damaged or made inaccurate through incorrect insertion of the work piece? 12. Have breather holes been provided to allow escape of air from clo se firing plunger holes? 13. Provided the production and economics warrant, have all wearing parts been specified to be hardened? 14. Have means of setting the cutter or cutters in the correct position been provided? 15. In case of revolving features
• 26. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2A5 DESIGN JIGS, FIXTURES & GAUGES 135 a) Has sufficient metal been left on jig to form an integral balance weight or provision for balance weight? b) Have arrangements been made for swarf to be shaken out or be blown out from interior of jig? c) Are projecting screws etc. covered in order to eliminate risk of injury to the operator (counter bored/counter sunk)? d) Have pilot made fool proof, if arranged so that the work cannot be inserted except in the correct way? 16. Is core out of unnecessary metal, making the fixture as light as possible, consistent with rigidity and stiffness made?
• 27. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2A5 DESIGN JIGS, FIXTURES & GAUGES 136 Summary Quality and higher productivity are aimed through the use of jigs and fixtures in any production process. The jigs and fixtures add to the cost of tooling, as such their use in production should be justified economically. The design of jigs and fixtures need to satisfy, functional, qualitative, safety and adaptability aspects to a production techniques. Tool engineer has to adopt certain design principles for the jigs and fixtures. A check list for the design of jigs and fixtures before releasing it for manufacture will greatly help the production process with considerable saving in production time and cost. Questions 1. What are the different elements of design? 2. Explain the design steps for designing of jigs and fixtures? 3. How does the checklist help the tool designer in designing of good jig or fixture? 4. What are the major factors to be considered in design of jigs & fixtures? 5. Explain, why jig has a four feet not three? 6. What are the design principles to be followed while designing of jigs & fixtures?
• 28. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A8 – ESTIMATION JIGS, FIXTURES & GAUGES 183 A8.1 Introduction Cost estimating may be defined as the process of forecasting the expenses that must be incurred to manufacture a product. These expenses take into consideration all expenditures involved in design and manufacturing, with all the related service facilities such as pattern making, tool making, as well as a portion of the general administration and selling costs. Cost estimates are the joint product of the engineer and the cost accountant, and involves two factors : physical data and cost ing data. The engineer as part of his job of planning manufacturing determines the physical data. The cost accountant compiles and applies the costing data. A8.2 Purpose of Cost Estimation Cost is the background of almost every decision the tool engin eer makes in organizing manufacturing operations and in selecting materials, methods, tooling and facilities. An understanding of cost determination is essential to ensure that these decisions are based on sound and dependable estimates of cost. Estimates of cost must be reasonably accurate if a venture is to be successful (realistic cost estimate). If a job is overpriced, it is lost to a competitor. If it is underestimated, it results in financial loss. Detailed cost estimates are prepared to: CHAPTER OUTLINE A8.1 Introduction A8.2 Purpose of cost estimation A8.3 Elements of cost A8.4 Cost structure A8.5 Estimation of cost elements A8.6 Estimating tool cost A8.8 Steps in making of cost estimation A8.8 Chip factors in cost estimation A8.9 Numerical examples CHAPTER OUTLINE A8.3a Material cost A8.3b Labour cost A8.3c Expenses A8.5a Direct Material cost A8.5b Direct labour cost A8.5c Indirect expenses A8.5d Direct expenses
• 29. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A8 – ESTIMATION JIGS, FIXTURES & GAUGES 184 1. Determine the selling price of a product for a quotation or contract, so as to ensure a reasonable profit to the company. 2. Check the quotations supplied by the vendors. 3. Decide whether a part or assembly is economical to be manufactured in the plant or is to be purchased from outside. 4. Determine the most economical process or material to manufacture a product. 5. Initiate means of cost reduction in existing production facilities by using new materials, which result in savings due to lower scrap loss and re vised methods of tooling and processing. 6. To determine standards of production performance that may be used to control costs. A8.3 Elements of Cost The constituents of cost of a product or the “cost elements” are : Material cost, Labour cost and Expenses. We shall discuss each element in turn. a) Material cost Material is divided into two basic categories: (a) material for fabricated parts (b) standard purchased parts. The total cost of thesetwo will give the material cost. Again there are two kinds of materials, which comprise the factory cost of a product. These are : Direct material and Indirect material. i) Direct Material The direct material is the raw material, which is processed in the plant and finally forms the finished product. Any standard part, which also becomes a part of the finished product, will also come under the category of direct material. ii) Indirect Material Indirect materials are those, which hel p in the processing of direct materials into the finished product. These materials don’t form a part of the finished product. Indirect materials include: Shop supplies such as cotton waste, lubricating oil, cutting fluids, coal, oil,
• 30. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A8 – ESTIMATION JIGS, FIXTURES & GAUGES 185 gas, shielding gases used in Arc welding, Emery paper used for polishing, quenching oils for heat treatment etc. Indirect materials form the part of on cost or overheads. b) Labour Cost Labour, which enters into the manufacture of a product, is of two categories : Direct Labour and Indirect Labour. i) Direct Labour The operator or operators, which actually process the raw materials either on machines or manually, form the direct labour. ii) Indirect Labour All the staff excepting administrative and sales office staff, wh ich helps in running the plant, comes under the category of indirect labour. Indirect labour includes: Foremen, supervisors, maintenance staff, stores personnel, time office staff, drawing office staff, etc. Indirect labour forms a part of overheads. c) Expenses Total cost of the product minus the costs of direct material and direct labour constitutes the ‘Expenses’. Expenses may also be either direct or indirect. i) Direct Expenses These expenses like the direct material and direct labour are directly chargeable to the finished product. These are also known as “chargeable expenses”. These include: a) Cost of patterns, jigs, fixtures, dies, drawings or designs specifically prepared for a particular product, which cannot be used for other purposes. b) Cost of any experimental work done specially for a particular product. c) Cost of inward carriage or freight incurred on supply of special material needed for the particular product. d) Hire of special or single purpose tools or equipment for a particular product.
• 31. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A8 – ESTIMATION JIGS, FIXTURES & GAUGES 186 ii) Indirect Expenses These are also called “oncosts” “overheads” or “burden”. These include: cost of indirect material, cost of indirect labour and other expenses that cannot be conveniently charged directly to a particular job. Indirect expenses may be divided into: a) Factory expenses or overheads. b) Office and Administrative expenses or overheads. c) Selling and Distribution expenses or overheads. ¨ Factory expenses These expenses include : indirect materials, indirect labour, expenses, insurance, maintenance and depreciation of machine, power etc. ¨ Office and administrative expenses These expenses consist of all expenses incurred in the direction, control and administration of an undertaking. These expenses include : rent and rates of office premises, salaries of office staff, printing and stationery, postage, salaries of high officers, depreciation of office equipment and insurance on office equipment. ¨ Selling and distribution expenses These expenses include : salaries of sales staff, publicity and advertisement, catalogues, leaflets and price lists, packing and forwarding charges, godown rent, commission to salesmen etc. The overheads may be grouped into two main categories: 1. Fixed overheads or constant overheads These are such items of indirect expenses, which remain constant or fixed irrespective of volume of production. These items include : salaries of higher officers (administrative and management executives), capital taxes, insurance charges, depreciation of building, plant machinery etc., rent of buildings.
• 32. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A8 – ESTIMATION JIGS, FIXTURES & GAUGES 187 2. Variable or floating overheads These are such items of overheads, which vary, with the volume of production. Such items are : internal tran sport charges, power, fuel, stores expense, factory lighting and heating and sales office expenses and repairs of machine tools. Since fixed overheads remain constant irrespective of volume of production, production should be increased to reduce the cost o f the part. There should be some minimum production to meet the fixed expenses and start earning profit. A8.4 Cost Structure The elements of cost can be combined to give following types of cost: 1. Prime cost. Prime cost or direct cost is given as : Prime cost = Direct material + Direct labour + Direct expenses (if any) 2. Factory cost. This cost is given as: Factory cost = Prime cost + Factory expenses. Factory cost is also called as “Works cost”. 3. Manufacturing cost. Manufacturing cost or cost of production is given as: Manufacturing cost = Factory cost + Administrative expenses. 4. Total cost. Total cost is given as: Total cost = Manufacturing cost + Selling and Distribution expenses. 5. Selling price. Selling price is given as: Selling price = Total cost + Profit. The above mentioned cost structure is explained with the help of a block diagram as follows:
• 33. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A8 – ESTIMATION JIGS, FIXTURES & GAUGES 188 Selling price Total cost Profit Manufacturing cost Selling expenses Factory cost Administration expenses Prime cost Factory expenses Directmaterial + Directlabour + Directexpenses (ifany) A8.5 Estimation of Cost Elements Directly material cost, direct labour cost and directs expenses can be found out most accurately by the estimat ing procedure. The indirect expenses items, which are so numerous, are determined by cost accounting section only, which furnishes the figure department wise to the estimator. The various cost elements are estimated in the manner give below. a) Direct Material Cost The cost of standard purchased parts can be obtained from the purchasing section. The raw material chargeable to a product is that in the rough state and includes all scrap removed. Material can be in the form of sheet metal, bar stock, forgings or castings, plastic etc. The weight of the material can be determined from the drawing of the part. An irregular part is divided into simple sections to calculate its volume. Volume is multiplied by density of the material to find its weight. The weight of a part multiplied by the unit cost of the material gives the material cost per piece. If the unit cost covers only the purchase price of the material, the material cost is multiplied by one or more additional factors to account for bulk losses, purchasing and handling costs. b) Direct Labour Cost
• 34. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A8 – ESTIMATION JIGS, FIXTURES & GAUGES 189 For estimating the direct labour cost of a product, the job is divided into operations needed to machine it, and then estimating the operation time for each operation. Total time multiplied by a labour rate gives the direct labour cost. The total time required to perform an operation may be divided into the following parts: i) Set-up time This is the time needed to prepare for the operation and may include : time of study the blueprint or to do paper work, time to get tools from the crib and the time to install the tools also on the machine. ii) Man or handling time This is the time the operator spends loading and unloading the work, manipulating the tools and the machine and making measurements during each cycle of operation. iii) Machining time The elements comprising the machining time are those, which are performed by the machine. This is the time during each cycle of operation that the machine is working or the tools are cutting. iv) Tear down time This is the time required to remove the tools from the machine and to clean the tools and the machine after the last component of the batch has been machined. Tear down time is usually small. It will seldom run over 10 minutes on the aver age machine in the stop. It may require only a few minutes to tear down a set up on a drilling press and 10 to 15 minutes on the average miller or turret lathe. In exceptional cases, it may go upto as high as 30 minutes on very large boring mills and large milling machines. v) Down or lost time This is the unavoidable time lost by the operator due to breakdowns, waiting for the tools and materials etc. vi) Allowance
• 35. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A8 – ESTIMATION JIGS, FIXTURES & GAUGES 190 The total time to perform an operation also includes time for personal needs of the operator, time to change or resharpen the tools etc. The time all these allowances are taken to be about 20 percent of the sum of all other times and then the total time for the operation is obtained. (a) Personal allowances. This is time taken by the operator to attend to his personal needs such as going to lavatory, taking a cup of tea, smoking etc. The time for this is usually taken to be about 5 percent of the total time. (b) Fatigue. The efficiency of the worker decreases due to fatigue or working at a stretch and also due to working conditions such as poor lighting, heating or ventilation. The efficiency is also affected by the psychology of the worker, which may be due to domestic worries, job security etc. For normal work, the allowance for fatigue is about 5 percent of the other times. This allowance can be increase depending upon the type and nature of work and working conditions. (c) Time to change or resharpen tools. Some allowances should also be provided for the time taken by the operator to get the tools changed or to resharpen the tools. This time varies from machine to machine. (d) Inspection or checking allowance. To maintain the uniform quality of the parts, the dimensions of the parts should be checked or inspected at regular intervals depending upon the closeness of tolerances. The checking times for the various instruments are given below, to check one dimension : With rule 0.10 minute Vernier caliper 0.50 minute Inside caliper 0.10 minute Outside caliper 0.05 minute Inside micrometer 0.30 minute Outside micrometer 0.15 minute Depth micrometer 0.20 minute Dial micrometer 0.30 minute Thread micrometer 0.025 minute Plug gauge 0.20 minute Snap gauge 0.10 minute
• 36. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A8 – ESTIMATION JIGS, FIXTURES & GAUGES 191 Set up time and tear down time are perfor med usually once for each lot or batch of parts. Set up time per piece is obtained by dividing the set up time for the machine by the number of pieces produced in lot. Set up time, handling time and tear down time are estimated from previous performanceson similar operations. All work on a particular type of machining tool consists of a limited number of elements. These elements can be standardized, measured and recorded. This is done under Time and Motion study. Standard data is available for set uptime, tear down time and handling time. Machining time is obtained with the help of formulas for each machining operation, which takes into account speeds of cutting, feeds, and depth of cut and tool travel. The actual amount of down or lost time that will occur in a particular operation can scarcely be predicted. Some operations will run smoothly, others may be beset by troubles. The sum of machining time and the handling time is called ‘run time’ or ‘unit of operation time’. The total time to manufa cture a product (from which the direct labour cost will be estimated) may be divided into the following major groups : 1. Set up time 2. Machining time 3. Non machining time 4. Down time The tear down time discussed earlier may be included in the set up t ime itself. The non-machining times will be man or handling time, personal needs, fatigue, cutter or tool sharpening and inspection. The man or handling time, as already discussed, includes : loading and clamping the part, unloading the part, advancing or retracting the cutting tool, tightening a chuck, a trial cut, trial gauging, debarring the machine, cleaning the fixture etc. c) Indirect Expenses Indirect expenses or overheads are those charges which vary in proportion to the production rate, but which are not easily attributable directly to a given operation or part. These expenses are apportioned among the operational units (machines, plants etc.) according to some weighting factor. 1. Percentage of direct labour cost 2. Percentage of direct labour hours 3. Machine hour method 4. Direct material method
• 37. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A8 – ESTIMATION JIGS, FIXTURES & GAUGES 192 5. Unit of production method 6. Space rate method d) Direct Expenses The direct expenses are estimated in the following manner : The engineering (preparation of drawings, blue prints, drafting etc.) and design cost of a product is calculated as a flat hourly rate for each estimated hour of design and engineering time. On the same lines, the cost of any experimental work done specifically for a particular product, can be estimated. The cost of patterns and special tools such as jigs, fixtures, dies and gauges can be estimated as outlined below: ¨ Tool cost Generally tool cost estimating is concerned with tool and other special equipment to be used for production. Much product cost estimating depends upon tooling cost estimating. Therefore, tool costs are often treated separately during a product cost study. Tooling costs are estimated to : 1. Determine how much must be invested i n tools and equipment to manufacture a product. 2. Determine the cost of alternate methods of tooling to help in selecting the more economical method. 3. Find the cost of a proposed machine or tool that promises to produce more economically method. 4. Determine the reasonable cost of a special machine or tool to gauge whether tool room performance is efficient or vendor’s prices are reasonable. The cost of cutting tools, both special and standard, jigs, fixtures, dies and gauges and other special equipment is often separated from the cost of machine tools. This is done to determine what portion of the tooling cost is to be charged directly to the project or proposal and what portion is to be capitalized. A8.6 Estimating tool cost Tooling cost is estimated in the same manner as the cost of manufacturing of a product is estimated. There are four major factors in the cost of any tool. These are: material, labour
• 38. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A8 – ESTIMATION JIGS, FIXTURES & GAUGES 193 and overheads or burden as in manufacturing costs. The fourth factor is the cost to engineer and design the tool, that is, the cost of designing and drafting the tool. i) Engineering and design cost The engineering and design cost represents a large portion of the total cost of the tool, often as high as 20 to 30 percent. This cost can be directlycharged to the individual tool and as a consequence are always considered first in establishing an estimate, the engineering and design cost is applied to the tool cost estimate as a flat hourly rate for each estimated hour of design and engineering time. ii) Tool materials cost The cost of the material for a proposed tool may be calculated and therefore becomes more of an actual cost than an estimated cost. It is the most accurate item in the tool cost estimate and is determined as discussed before. Sta ndard parts such as knobs, hand wheels, bushings, bolts, screws, springs, and similar items that complete the tool bill of material can be accurately priced from catalogues, price lists or invoices. iii) Tool labour cost The labour involved in the machining, assembling, fitting, and tryout of tools is always difficult to estimate accurately, even for the most experienced estimator. The machining time can be calculated. The other times cannot be calculated and must b esti mated. It is difficult to foresee all the problems that may develop in fitting assembly and try out operations, even under most favourable conditions. Therefore, the estimate must include a liberal factor of safety for lost time, which is impossible to an ticipate and will always be present. The sum of all the times will be the labour time for the tool. When multiplied by the toolmakers hourly rate, the estimated labour cost is determined. iv) Burden or overhead The tool room may be considered as a depar tment, and therefore may have an established burden rate, just as production department has. It general, the man -hour or direct labour cost method of burden distribution is applied as discussed under point 3 above. A8.8 Steps in making a cost estimation
• 39. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A8 – ESTIMATION JIGS, FIXTURES & GAUGES 194 The cost of a new product may be estimated by following the basic steps given below : 1. Make a complete and thorough analysis of the cost request to understand it fully. 2. Make an analysis of the part or product and make separate lists of standard partsand the parts to be fabricated within the plant. 3. Make a manufacturing process plan for the parts to be fabricated. 4. Determine the material costs for the standard and the fabricated parts. 5. Estimate the total production time for each operation listed in step 3. 6. Apply the labour and burden rates to each operation. 7. Add the material costs (step 4) and the labour and burden costs (step 6). This will give the total manufacturing cost. 8. Apply the profit factors to arrive at the selling price. A8.8 Chief factors in cost estimation Each cost estimate may not be exactly the same as the actual manufacturing cost. The most significant causes for the cost deviations can be : Fluctuations in ma terial and labour costs, incomplete design information at the time of estimate, unexpected delays resulting in premiums paid for overtimes and materials and the unexpected machining or assembly problems. However, the average of cost estimates over a perio d of time should be reasonably close to the actual manufacturing costs. For this, the following factors should be considered for arriving at an accurate and complete cost estimate : 1. Each estimate should contain complete costs of direct material, direct labour, factory overheads, spoilage, engineering, administration and selling. 2. If the cost of a new product is estimated on the basis of previous estimates of comparable parts, detailed estimating should be used. It is necessary to make substitutes in the past estimates for individual operations, individual parts or individual sub assemblies. 3. The period of time between the cost estimating and the actual production of the part affects the determination of unit prices. During the intervening period, the basic material and labour rates may rise or fall. Therefore, an estimate of what the cost will be at the time of actual production is what is really needed. Thus the estimator should have the ability to project thinking and reasoning into the future.
• 40. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A8 – ESTIMATION JIGS, FIXTURES & GAUGES 195 4. The volume of the pieces to be produced also affects the costing rates since the time and therefore the cost of performing an operation decreases as the number of units produced is increased. 5. The addition of new type of equipment and special buildings require the development of new overhead rates etc. A8.9 Numerical Examples From the following data, calculate the total cost and selling price for a job : Direct material = Rs. Manufacturing wages = Rs. Factory overheads to manufacturing wages = 100% Non manufacturing overheads to factory cost = 15% Profit on total cost = 12% Solution. Direct material = Rs. Manufacturing wages (Direct labour) = Rs. Factory overheads = 100% of Rs. = Rs. Factory cost = Direct material + Direct labour + Factory overheads = Rs. + + = Rs. 11,500. Non-manufacturing overheads, i.e., administrative and selling overheads = 15% of Rs. 11,500 = Rs. Total cost = Factory cost + Rs. = Rs. 11,500 + Rs. = Rs. 13,225 Profit = 12% of total cost = Rs. Selling price = Total cost + Profit = Rs. 14,81
• 41. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A8 – ESTIMATION JIGS, FIXTURES & GAUGES 196 Table 1 - Weight of Rolled Steel (8.843 gm/cc) Size S Round S f kg/metre length Square S Sq. Hexagonal S A/F Sheet S Thick kg/sq. metre 5 5.5 6 8 8 9 10 11 12 14 16 18 18 19 20 22 25 28 28 32 35 36 38 40 41 45 50 55 56 60 63 65 80 81 85 80 0.154 0.19 0.222 0.302 0.395 0.50 0.62 0.85 0.89 1.21 1.58 2.00 2.48 2.98 3.85 4.83 6.31 8.99 9.86 12.49 15.41 18.8 19.34 22.2 24.48 26.0 30.2 31.08 34.8 39.46 0.20 0.24 0.28 0.38 0.502 0.64 0.885 0.95 1.13 1.54 2.01 2.54 2.83 3.14 3.80 4.91 5.82 6.15 8.04 10.18 12.56 15.90 19.62 23.8 24.62 28.3 31.16 33.2 38.5 39.58 44.2 50.24 0.180 0.206 0.245 0.333 0.435 0.551 0.68 0.823 1.33 1.96 2.45 3.29 4.96 6.96 8.81 11.4 18.0 20.6 24.5 28.8 33.3 38.2 43.5 39.2 55 88.5 94.2 109.9 125.6 141.3 182.8 196.2 219.84 251.14 284.68 298.22 313.92 331.36 364.46 398.63 430.88 463.9 498.04 530.18
• 42. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A8 – ESTIMATION JIGS, FIXTURES & GAUGES 197 Table 2 - Thumb Rules for Estimation Group No. Operations involved Total costs 1. Only shaping, turning drilling and fitting. Three times the raw materials cost. 2. Shaping / turning, drilling, fitting and milling. Four times the raw materials costs. 3. Above operations plus heat treatment. Five times the raw materials costs. 4. Above operations plus precision grinding or lapping. Six times the raw materials costs. Example Find the manufacturing cost of 14 f bore collared bush shown in figure. The bush is to be manufactured form 28 f x 35 long alloy steel bar which costs Rs.80 / kg. After rough turning, the bush is to be hardened and finished by grinding. Solution: Referring to Table1, we note that 28 f Steel bar weights 4.83 kg/metre. Wt. Of a 35 long piece = 83.4x 100 35 = 0.169 kg. Cost of material at Rs.80 = 0.169 x 80 kg = Rs. 13.52 Machining involves turning, hardening, and grinding. Referring to table 2, we notice that the bush falls in Group 4, for which the total cost is approximately six times the raw material cost. Total cost = 13.52 x 6 = Rs. 81.12 If the bush is to be sold, profit should be added. Selling cost at 30% profit = 81.12 x 1.3 = Rs. 105.5 (min.) Selling cost at 100% profit = 81.12 x 2 = Rs. 162.24 (max.)
• 43. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A8 – ESTIMATION JIGS, FIXTURES & GAUGES 198 SUMMARY Quality and higher productivity are aimed through the use of jigs and fixtures in any production process. The jigs and fixture s add to the cost of tooling, as such their use in production should be justified economically. The design of jigs and fixtures need to satisfy, functional, qualitative safety and adaptability aspects to a production techniques. Tool engineer has to adopt certain design principles for the jigs and fixtures. A check list for the design of jigs or fixtures before releasing it for manufacture will greatly help the production process with considerable saving in production time and cost. Questions 1) Define cost estimating. 2) What is the purpose of cost estimating? 3) Name the various constituents of cost. 4) Name the indirect material and indirect labour. 5) What are : direct expenses, indirect expenses, factory expenses, office and administrative expenses, selling and distribution expenses. 6) Define : Prime cost, Factory cost, Manufacturing cost. Total cost and Selling price. 7) Define : Set up time, Handling time, Machining time. Tear down time and down or lost time. 8) List the various steps of cost estimating. 9) Discuss the chief factors in cost estimating. 10) Find the manufacturing cost of 14 f bore collared bush shown in figure. The bush is to be manufactured form 28 f x 35 long alloy steel bar which costs Rs.80 / kg. After rough turning, the bush is to be hardened and finished by grinding. 11) From the following data, calculate the total cost and selling price for a job : Direct material = Rs. Manufacturing wages = Rs. Factory overheads to manufacturing wages = 100% Non manufacturing overheads to factory cost = 15% Profit on total cost = 12%
• 44. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 155 A7.1 Introduction Maximum productivity at minimum cost is the demand of modern industry. To meet this requirement designing of efficient and accurate jigs and fixtures is required. Quality, simplicity and economy from the important criteria for the design of jigs and fixtures. To meet this requirement the designer will have to make an economic analysis for using jigs and fixtures and has to device certain principles of design, and finally develop a checklist for the jigs and fixture design. A7.2 Basic Design Considerations In addition to locating and holding the part, the designer must also consider several other factors before a welding jig or fixture can be designed. Heat dissipation is an important consideration with any welding tool. Several methods can be usedto insure that proper heat is maintained in the weld area. The primary factor that determines the amount of heat required is the metal being joined. When metals such as steel and other poor heat conductors are joined, the excess heat should be carried of f to prevent overheating the weld. To do this, backup bars of copper, titanium, or beryllium can be used. For metals that are good conductors or heat, such as copper or aluminum, too rapid cooling becomes the problem. To prevent this, the fixture or jig must be made to contact the part in as small an area as possible. Clamping supports must be provided to prevent distorting the work while it is in a heated condition. Whenever possible, place clamps directly over the supporting elements. Locators should be positioned so that the distortion will cause the part to loosen rather then tighten against the locators. If this is not possible, either power or manual ejectors should be built into the tool. Foolproofing is one feature that is necessary for any t ype of welding jig or fixture. Each tool must be designed so the part will only fit into its proper position. CHAPTER OUTLINE A7.1 Introduction A7.2 Basic Design consideration A7.3 Factors in fixture design A7.4 Classification of fixtures A7.5 Maintenance, Safety and Storage TOPIC OUTLINE A7.4a Types of fixtures based on how the tool is built A7.4b Types of fixtures based on the type of machine on which they are used A7.4c Types of fixtures & their descriptions
• 45. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 156 A7.3 Factors in Fixture Design The major difference between a drill jig and a fixture is that a jig has hardened bushings which guide the dril l, while a fixture for a machining operation is attached to the work table of a machine to hold the workpiece in a fixed position for the action of milling cutters, broaches, or other of cutting tools. Work may be entirely enclosed in a drill jig and is r eached for the drilling or boring through bushings provided for that purpose. However, to enclose the work in a milling fixture would defeat the purpose for which the fixture designed that of securely holding the work in position for the action of cutting tools. The clamps used in fixtures are applied so that the work surface is clear for machining. Milling fixtures are usually fastened to the table of the machine upon which they are to be used. As a rule, they are flat on the bottom so that they will r est securely against the table upon which they are clamped. Clamping lugs or other clamping surfaces are generally provided. The cutter operating with a fixture is usually in a fixed position. The work held in the fixture is “fed” to the revolving cutter by means of the movable table. Broaching fixtures, on the other hand, are usually stationary with the broach travelling through the work. Cutting tool chatter can be greatly reduced by carefully designed work holding devices. These must be designed so that they are properly proportioned and sufficient in number to support and hold the work rigidly in the fixture. One of the major factors in the design of a milling fixture is providing a place in the fixture for the workpiece to resist the thrust of the cutter. The thrust of the cutter should be against the body of the fixture, rather than against the clamps. The direction of rotating of the cutter often governs the placing of the work. An attempt must be made, therefore, to design the tool so that the body of the fixture takes this thrust. The design of fixtures should also permit the use of the clamping collars used on the cutter arbor. Often, though, it is not good economy to use small diameter cutters that allow only a minimum clearance of 3mm or less between the arbor and the work or a projecting part of the fixture, because of the necessity of sharpening the cutter which will reduce the clearance. ¨ Design Points Are all parts well designed to take the loads imposed on them in service? Is the tool study enough to stand considerable abuse? Is the fixture amply proportioned to damp out vibration and chatter? This applies especially to milling fixtures.
• 46. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 157 Is the design of all parts and mechanisms as simple as possible? Have cylindrical plungers and holes been used in preference to square or polymer once? Have holes for headed pressed in parts (such as for accurate location of resigns) been countersunk to allow any excess press lubricant to collect in the countersink (allowing the rest pin to vibrate slightly in service) instead of gradually squeezing out under the head? Have spring pocket holes been countersunk on their open ends? Are the dowel pins in each part as widely spaced as practicable? Where detachable parts need very accurate location, have register keys or pins been used instead of dowels? Is the accuracy of the operation such that the base of the fixture should be scraped to fit the machine table? Have breather holes been drilled to allow air to escape from lose fitting plunger holes? Is it possible to forecast any part design changes and to make allowance for them in the design of the fixture? ¨ The procedure in developing designs for fixtures is similar to the procedure followed in designing jigs. A7.4 Classification of Fixture Fixture Types of Fixtures based on how the tool is built Types of Fixtures based on the type of machine on which they are used
• 47. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 158 The jigs and fixtures used for welding can generally be limited to three basic types; tacking, welding, and holding. Types of Fixtures based on how the tool is built Plate fixture Angle plate fixture Vice jaw fixture Indexing fixture Multistation fixture Profile fixture Types of Fixtures based on the type of machine on which they are used Turning fixture Milling fixture Planning fixture Broaching fixture Grinding fixture Shaping fixture Shaving fixture Forming fixture Stamping fixture Welding, brazing, soldering fixture Assembly fixture Inspection fixture Testing fixture Heat treatment fixture Honing fixture Lapping fixture
• 48. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 159 Tacking jigs and fixtures are used to hold the parts of an assembly in their proper position so they canbe tack welded together. These tools are generally used for assemblies that must be held together in several places to prevent warping or distortion when welding is complete. Parts assembled in a tacking jig or fixture are removed after tacking and eith er finished without special tools or transferred to a holding jig or fixture. Welding jigs or fixtures are used to hold the parts of an assembly in position for welding. The difference between welding and tacking is the amount of welding performed. The tacking tool is used only when the part is to be tack welded. When the part is to be completely welded together, a welding jig or fixture is used. Welding jigs and fixtures are normally built heavier than tacking tools to resist the added forces caused by the heat within the part. Holding jigs and fixtures are used to finish tack welded assemblies. Like welding tools, holding jigs and fixtures must be made rigid enough to prevent distortion and warping. Generally fixtures are classified as - 1) How tool is built a) Plate Fixture b) Angle plate c) Vice jaw d) Indexing e) Multistation f) Profiling 2) Machine on which they used a) Turning b) Milling c) Planning, shaping & slotting d) Broaching e) Grinding f) Holding, Broaching and soldering g) Assembly h) Inspection Types of Fixtures The fixtures are classified based on i. How the tool is built
• 49. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 160 ii. The type of machine on which they are used Though jigs and fixtures are made basically the same way, due the increased tool forces the fixtures are built stronger a heavier than jigs. 7.4a Types of Fixtures based on how the tool is built 1. Plate Fixtures These are the simplest form of fixtures used for most machining operations. This fixtures consists of flat plate with variety of clamps and locators. Its adaptability to several machining operations makes it a popular type of fixtures. 2. Angle Plate Fixtures These fixtures are used to machine at right angles to its locator. If machining is to be carried out at other angles a modified angle plate fixtures can be used.
• 50. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 161 3. Vise jaw Fixtures These are used for machining small parts. With this type of tool, the standard vise jaws are replaced with jaws which are formed to fit the part. These are least expansible and are limited by the size of vises available.
• 51. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 162 4. Indexing Fixtures These are similar to indexing jigs and are used for machining parts which must have machined details evenly placed.
• 52. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 163 5. Multistation Fixtures These are intended for high speed, high vo lume production runs, where the machining cycle is continuous. Duplex fixtures are two station fixtures and are the simplest of the multistation fixtures. This form of fixture allows both the loading and unloading operations while the machining operation is in progress. For example, once the machining operation is complete at station one, the tool is revolved and the cycle repeated at station two. At the same time, the part is unloaded at station one and a fresh part loaded. 6. Profiling Fixtures These are used to guide tools for machining contours which the machine cannot normally follow. These contours can be either internal or external. Since the fixtures continuously contacts the tool an incorrectly cut shape is almost impossible.
• 53. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 164 7.4b Types of Fixtures Based on The Type Of Machine on Which They Are Used Fixtures are generally clas sified based on the machining on which they are used. The following are some of the common production operations in which fixtures are used : 1.Turning 2.Milling 3.Grinding 4.Welding, brazing, soldering fixtures 5.Assembly fixtures / Inspection fixtures 1. Turning Fixtures These fixtures are used for turning, facing and boring operations, and mainly consist of workpiece locating and clamping elements. The standard fixtures that are used in a Lathe machines are, three jaw and four jaw chucks, collectsface plate etc. These jaw chucks are used for holding round hexagonal or other symmetrical works. Collects are used for bar stock. Special jaw chucks, face plates with clamping devices are used for holding irregular shaped turning jobs.
• 54. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 165 CLASSIFICATION OF LATHE FIXTURES Following points are to be noted while designing turning fixtures. i) Grip the rotating work piece to the fixture to resist torsional forces. ii) The fixture should be rigid with minimum possible overhang. iii) Locate the work piece on critical surfaces, which are the areas from which all or major dimensional or angular tolerances are taken. iv) Provides adequate support for frail sections or sections or sections under pressure from turning tools. v) Balance the fixtures to avoid vibrations. vi) Fixtures should not have any projections, as they will cause injury to the operator. vii) A pilot bush for supporting tools should be provided where extreme accuracy is required in boring operations. Figure shows a typical turning fixtures. The fixture body is located on the machine spindle and bolted in position, it carries the work piece location and clamping system. Figure shows yet another special turning fixture in which work piece is located and clamped to a shelf that projects from the fixture body. The fixture incorporates a balance weight (the fixture would other wise be out of balance) and a pilot bush to guide the boring bar. Lathe Fixtures Chuck Type Face Plate Arbor Special Collet Type Pot TypeMandrel
• 55. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 166
• 56. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 167
• 57. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 168 2. Milling Fixtures Milling fixtures are the work holding device which are firmly clamped to the table of milling machine. They hold the work piece in correct position as the table movement carries it past the cutter or cutters. The essential features of a milling fixture are a) Base b) Location elements c) Clamping elements and d) Setting blocks These fixtures are classified based on i) Type of operation performed ii) Method of milling iii) Method of clamping the work piece MILLING FIXTURE Cradle fixture Rotary fixture Drum fixture Indexing fixture Magnetic chucks Vacuum chucks
• 58. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 169 Following design principles be adopted for milling fixtures 1. Pressure of cut should always be against the solid part of the fixtures. 2. Clamps should always operate from the front of the fixture. 3. Work piece should be supported as near the tool thrust as possible. A milling fixture is located accurately on machine tube and then bolted in position. The tube is positioned relative to the cutter with the air of setting blocks. The location an d clamping systems are similar to those used for drill jigs, but as the cutting forces are high, interrupted and tend to lift the workpiece, the clamping forces must be big, hexagonal nuts are usually used to clamp the work piece rather than hand nuts. Fig. show a simple milling fixture and a line or string milling fixtures. The line or string milling fixture shown is used to mill a slot in the end of each of the five cylindrical work pieces arranged in line. This arrangement facilities all the work pieces to be located as required and clamped with one screw. Fig. shows an index milling fixture having a number of surfaces to be milled by successive positioning of a single fixture provided with an indexing arrangement.
• 59. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 170
• 60. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 171
• 61. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 172 3. Grinding Fixtures Fixtures used in grinding depend upon the type of grinding operation and the machine used. GRINDING FIXTURE Angle Plat Fixture Automotive Grinding Fixture Magnetic Chuck
• 62. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 173 Following table gives some commonly used grinding fixtures Sr. No. TYPE OF OPERATION FIXTURES USED 1 External Grinding Mandrel - Taper - Straight - Combined - Straight - And taper 2 Internal Grinding Chucks, special jaw chucks or special fixtures as the lathe fixtures. 3 Surface Grinding Clamped on machine table, held in vise, held in magnetic, vacuum and special features. 4. Welding Brazing and Soldering Fixtures These fixtures comprise of usual locating and clamping elements used in other fixtures. However the effects of heat and prevalence of welding spatter will have to be taken into account while designing them. Some of the consideration are as follows : i) Expansion of heated work pieces and resulting distortion should be taken care of by providing adequate clearances between work piece and locators. ii) Handles subject to heating should be properly insulated. iii) The welding spatter should not be allowed to face on the threaded parts of clamping elements. iv) Parts near the welding area should not be threaded. v) Spatter grooves must be provided below the line of welding of work pieces to the base plate with the weld spatter. vi) Care should be taken to prevent locking of joined work pieces in the welding fixture after welding. vii) Provision for easy tilting or rotation be made to ease welding from various dies. Toggle clamps, without threaded elements are widely used in welding fixtures. Welding and inspection are everyday operations in manufacturing. Like many other areas, these operations can be simplified and improved through the use of appropriate jigs and fixtures. Although welding is specified in the examples, the methods and techniques listed will apply equally well to other assembly operations such as brazing, soldering, riveting, and stapling.
• 63. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 174 5. Broaching Fixtures As broaching is a fast and accurate method of metal cutting involving high cutting forces, broaching fixtures are required to perform one or more of the following functions: a) Hold the workpiece rigidly. b) Locate the workpiece in correct position relative to the tool or the machine table. c) Guide the broach in relation to the workpiece. d) Move the workpiece into and out of the cutting position. e) Index the workpiece between the cuts. Fixtures are used for both internal and external broaching. The fixtures used for internal broaching are the simplest and for many operations consist of a face plate or support place on the broaching machine. The fixtures for external broaching are made quite ri gid so that the workpiece does not move during the broaching action.
• 64. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 175
• 65. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 176
• 66. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 177 6. Assembly Fixtures These are used to hold various components in their correct position, while they are assembled; Assembly operations often involve pressing interference fit pins, bushes and other parts in housings. The assembly fixtures need to be of light construction with adequate rigidity to ensure relative positional relationships of the various components. They may be built up from light castings, steel section or completely from steel. Assembly fixtures generally are of two types : a) Mechanical assembly fixtures used for operations generally performed at ordinary room temperatures with mechanical means. Eg, Reverting Fixtures. b) Fixtures for hot joining methods of assembly work using energy in the form of heat. Eg. Welding, brazing and soldering fixtures.
• 67. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 178 7. Inspection Fixtures Every part made must meet a standard for size and shape if it is to perform its design function. While it is quite possible to measure each dimension separately, this is not the most cost effective means to insure part quality and conformity to dimension. To satisfy the requirements of speed and accuracy, gauging or inspection fixtures are used. The main requirement of an inspection fixture is accuracy. Each inspection fixture should contain only those elements needed to check the specified sizes of forms. Individual gauges that only check one size are preferred over complicated tooling if the dimension being gauged is independent of other part features. An example of this is the size of the threads in a hole. While the location of the hole is important to the part, the size of the thread is independent of the location. There are two general types of inspection fixtures; gauging and measuring.
• 68. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 179 A7.4c Types of fixture & there descriptions Types of fixture Description Vise Fixtures Standard machine vises adopted with special jaws provided on easy way of holding parts for machining Lathe Fixtures Are used on vertical and horizontal turret lathes and high-speed production lathes Chuck fixtures The cheapest type of lathe fixture is the standard lathe chuck with special jaws or inserts machined to fit the part Face Plate Fixtures Is used to machinelarge diameter parts on the vertical lathe Mandrel and arbor type fixtures These fixtures will centre, locate and grip the work from the inside and are normally used for parts that already have machined internal surface Miscellaneous fixtures Lathe operations on parts that are unusual because of their shape or dimensions, at the time fixtures are complicated and expensive yet they are always efficient with respect to the saving of time and the improvement of quality Milling fixtures A mill fixture holds the part in the correct relation to the milling cutter as the table movement carries the part through cutters a) Cradle fixture The work piece is rocked or rotated within a given angle during milling b) Rotary fixture The work piece is rotated under the cutter c) Drum fixtures The work piece is mounted on the periphery of a rotating drum Indexing fixtures Where the work piece is indexed in to the next position during the machining cycle of mill Rise and fall fixtures Which allow raising and lowering of the work piece in conjunction with the mil feed Magnetic chucks Are used to hold ferromagnetic materials in production milling operation Types of fixture Description
• 69. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 180 Vacuum chuck Are being used for holding nonferrous and non magnetic parts for milling operations Boring fixtures Differ from drill jigs in that they are to be used with boring bars Line boring fixture The distance between the holes requires a line boring bar, consequently their type of fixture is called a line boring fixture Stationery fixture Is designed to mount on the table and present the work piece to the boring bar in proper location for matching Universal fixture May be obtained commercially from the manufacturers of boring machines. Indexing fixture Consist of a base with a rotary table or rotating indexing plate mounted on it Automatic loading fixture Is suitable only for long production tuns of a particular work piece Broaching fixture External broaching usually requires a special fixture for each job Grinding fixture Must allow for the unrestricted access of coolant to the work The structural design of grinding fixtures is very similar to that of other fixtures Angle plate fixtures Is used for internal grinding Automotive grinding fixtures Are used in the automotive industries Trunk pin grinding fixtures, cam grinding fixtures, cam shaft grinding fixtures etc. Magnetic chuck Is used to hold work pieces in surface grinding operation Planning fixtures Can be economically applied to smaller parts when they are clamped in a gang fixture Welding fixtures Their purpose is to locate and hold the parts in correct relative position for joining to reduce distortion A7.5 Maintenance, Safety & Storage Provision for Maintenance Has provision been made for lubricating the tool mechanisms? 55
• 70. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 181 Have all wearing parts been hardened? Are these parts easily made and replaced? Have correct materials and heat treatment been specified? Has provision been made for easy removal or pressed in parts? Can vulnerable parts be removed and replaced quickly without disturbing the set up of the fixture on the machine? Safety 1. Does the fixture design protect the operator from coolant spray or flying chips? 2. Is the designed tool safe to operate with? Handling and Storage Lifting Aids Have lifting lugs, eyebolts, or chain slots been provided for slinging heavy tools? Have lifting handles been attached to all awkward or heavy loose parts of the fixture? Loose Parts If loose parts such as spacing pieces, wrenches, or locating pins are unavoidable, can they be attached to the fixture with keeper screws or light chains to prevent loss in storage? Fragile Parts Is there any fragile part of the jig which needs a protective cover in storage? Is the tool so delicate or highly finished as to require a special case, cover, or box to protect it in storage? Identification Has the tool, and all loose items belonging to it, been marked clearly with identification numbers or symbols? Storage Aids Can the tool be stowed safety without danger of tipping over? Is a special storage stand or rack desirable for safe and convenient storage? SUMMARY:
• 71. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A7 – FIXTURES JIGS, FIXTURES & GAUGES 182 The use of fixtures is extending and developing very fast. The quality, type and complexity of fixtures used depend upon the type of job and its method of production. Fixtures are classified into two types depending on how they are builtand based on the type of machine on which they are used. QUESTIONS: 1. What are the two types of classifying fixtures? 2. What is an indexing fixture? 3. What type of fixture is used for machining contours, which the machine cannot normally follow? 4. What is an assembly fixture? 5. What are the rules for selecting clamps of work piece in fixtures? 6. What are the principles to be following in designing of fixtures? 7. Describe the various grinding fixtures. 8. Describe the design principles for a lathe fixture. 9. Name the various work holding devices use on a lathe. 10. How are cutters set in relation to the work in milling fixture? 11. Name the essential features of a milling fixture. 12. Why the proper disposal of swarf or burr is very important in jigs & fixtures design? 13. What provisions can be made to ease the handling of heavy jigs & fixtures? 14. Explain the advantages to be obtained from the use of pneumatic & hydraulic clamping devices. 15. How can a lathe fixture be clamped to the lathe? 16. Write short notes on “Broaching fixtures”, “Assembly fixtures”. 17. What are the checks to be made for fixtures for (a) provision for maintenance, (b) manufacturing & maintenance cost, (c) handling, (d) loading & unloading, (e) storage, (f) human factors?
• 72. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 199 A9.1 Introduction Gauges are inspection tools of rigid design, without a scale, which serve to check the dimensions of manufactured parts. Gauges do not indicate the actual valueof the inspected dimension on the work. They can only be used for determining as to whether the inspected parts are made within the specified limits. A workman checking a component with a gauge does not have to make any calculations or to determine the a ctual dimensions of the part. Gauges are easy to employ. This is one reason for their wide application in engineering. Gauges differ from measuring instruments in the following respects : (a) No adjustment in necessary in their use. (b) They usually are not general-purpose instruments but are specially made for some particular part, which is to be produced in sufficiently large quantities. Gauging is used in preference to measuring when quantities are sufficiently high, because it is faster and easier with resulting lower costs. A9.1a Advantages and Disadvantages Modern manufacturing requires extensive use of gauges for shop work, inspection, and reference. Shop gauges are used by workmen. Inspection gauges are used by inspectors to check manufactured product, and reference gauges are reserved for checking the other two types. A gauge is defined by the Sheffield Corporation as “a device for investigating the dimensional fitness of a part for a specified function”. Gauging is defined bythe ANSI as “a process of measuring manufactured materials to assure the specified uniformity of size and contour required by industries.” CHAPTER OUTLINE A9.1 Introduction A9.2 Classification of Gauges TOPIC OUTLINE A9.1a Advantages & disadvantages of gauges A9.2a Fixed gauges A9.2b Advantages of fixed gauges A9.2c Classification of fixed gauges A9.2d Indicating gauges A9.2e Special gauges A9.2f Classification of Plain Gauges
• 73. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 200 Basically, gauging accomplishes two things: (1) it controls the dimensions of a product within the prescribed limitat ions, and (2) it segregates or rejects products that are outside these limitations. Gauging devices and gauging methods, like other phases of tooling in modern manufacturing, have become standardized. Generally speaking, standardized components that can be obtained commercially are assembled into a unit to gauge a particular product. It is therefore quite important that the tool designer be familiar with gauging equipment and practice. It may be necessary to design special gauges for checking dimensions that do not readily adapt to standard gauges. A gauge of this type may be quite simple, as shown in Fig. Frequently time can be saved by the use of a simple length gauge in place of a machinist’s rule when a quantity of workpieces is involved. It shoul d not be assumed that special gauges are necessarily elaborate or that they are used only to measure close tolerances. The tolerance of the workpiece in Figure may be as large as ±1/64 in., and a machinist’ s scale would be sufficiently accurate for the job; however, it would take a little longer to read. There are many gauging methods used to determine when a product conforms dimensionally with drawings, specifications, or other prescribed requirements. Ho wever, when these methods are analyzed, it will be found that they are designed to check one of the seven basic elements of workpiece geometry : Distance Used to specify the relative location of the various components and elements of the workpiece. Distance is measured by comparison to a known standard. Flatness Used to ensure that every element of a surface is within a specified distance from a nominal surface plane. Determines straightness and alignment of a product. Parallelism Used to ensure that two flat surfaces are parallel to each other. Perpendicularity (squareness) Used to determine that two flat surfaces are normal to each other.
• 74. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 201 Angularity Used to specify the angle between two flat surfaces, other than 900 . Concentricity Used to ensure that points on a cylindrical surface are concentric to a common axis. Surface texture Similar to flatness but concerned with irregularities in a surface rather than straightness and alignment. These elements of workpiece geometry are theoretical and are unattainable in actual machining practice. It is therefore necessary to specify the degree of variation (tolerance) that is acceptable for the product to function properly. Gauges are the means by which the products are checked to determin e whether the elements of geometry fall within the specified variation. A9.2 Classification of Gauges Classification of gauges on basis of accuracy. (With wear allowance) (1/3 wear allowance) (More accurate) For shop purpose for inspection purpose Calibration purpose ¨ Classification on use A9.2a Fixed Gauges Gauges Shop Gauges Inspection Gauges Reference Gauges Gauges Fixed Gauges Indicating Gauges Special Gauges
• 75. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 202 Fixed gauges may be classified as fixed gauges or fixed limit gauges. the master gauge is made to represent the workpiece dimension in its nominal condition and is used as a setting gauge for setting up comparator type measuring instru ments or as a reference standard for calibrating direct measuring tools, which require periodic readjustment. Master gauges are dimensioned to represent the dimension to be gauged. The dimension may be the basic size or the median size of the tolerance zone. Fixed limit gauges are used to determine whether the product is within prescribed limits and are intended for use as inspection gauges. Since there is a high and low limit on the product, two gauges are usually required, which are made to constitute t he design limits of that dimension. A9.2b Advantages of fixed gauges The various advantages of fixed gauges in comparison to comparator type gauges are : (1) Fixed gauges are essentially free from errors due to drift and the original adjustment, non-linear response, effect of power supply variations and other extraneous factors, which necessitate regular calibration and occasional correction on comparator type gauges. (2) These provide positive dimensional information. (3) These are portable and independent of power supply availability. (4) It involves no other auxiliary equipment and set-ups. (5) These can be designed to check combinations of several dimensions comprising lengths, diameters and angles. (6) These can be designed to inspect interrelated features, for size, location, for, alignment etc. so as to check the virtual size of a member (combined effect of all parameters with regard to the functional adequacy of the inspected features). (7) These are particula rly useful in the checking of part members whose meaningful geometric irregularities can’t be readily detected by gauges, which do not provide complete reverse replica of the critical part portions. (8) These provide uniform reference standards. (9) These are not expensive. (10)Comparators have to be set from time to time using master fixed gauges. A9.2c Fixed gauges are further classified as : Fixed Gauges Fixed limit Gauges Fixed Master Gauges
• 76. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 203 A9.2d Indicating Gauges
• 77. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 204 Indicating gauges detect variations in a specific distance and display them on a dial or graduated scale. They have the ability to detect minute errors because the variation shows on the scale in an amplified version. The majority of indicating gauges com pare the actual dimension of the workpiece with the dimension of a master setting gauges. For this reason, indicating gauging methods are often referred to as comparative gauging. The master setting gauge is equal to the nominal dimension to be gauged. The indicating gauge actually measures only the amount and direction of any deviation, which exists in relation to the nominal size. The amplification of indicating gauge movement may be mechanical, pneumatic, optical, or electric. The tool designer will seldom be required to design the amplification system, as this is generally done by a company specializing in the manufacture of indicating gauges. The tool designer’s job will be to design fixtures to adapt the particular amplification system to a practical inspection setup. The following figure shows a series of levers, which is an extension of the simple lever movement making a compact device with greater magnification. However, in this type the errors due to wear, friction, and inertia must be carefully considered. The following figure shows the lever movement applied to a simple depth gauge. The device quickly checks the depth of surface, A, and if the workpiece is resting on a flat surface such as a surface plate, the uniformity of surface A can be checked by rotating the workpiece. The spring ensures that the spindle will remain in contact with the surface, and the pin located in the plunger slot prevents the plunger from falling out when the workpiece is removed.
• 78. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 205 A9.2e Special Gauges Special gauges are specially designed and manufacture for special purpose. A9.2f Classification of Plain Gauges Plain gauges are used in checking plain, that is, unthreaded holes and shafts. Plain gauges are classified as following ways: 1. According to form of tested surface 2. According to purpose 3. According to type 4. According to design 1. According to the form of tested surface According to the form of the tested surface, the gauges are of two types : Gauges for checking the holes and gauges for checking the shafts. Gauges for checking the holes are called “Plug Gauges” and those for checking the shafts arecalled “Snap or Gap gauges and Ring gauges”. a) Plug Gauge Plug gauge are used to check the holes. The ‘go’ plug gauge is the size of the low limit of the hole while the ‘not go’ plug gauge corresponds to the high limit of hole. These are used to check the uniformity of holes. A plug gauge may be straight or tapered and of any cross section. It may have either an integral or replaceable handle.
• 79. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 206
• 80. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 207 b) Pin gauges When the holes are larger than 3 in. (as, for example, automobile cylinder bores), plug gauges are very heavy. In these cases, it is more convenient to use pin gauges. In using a pin gauge, the gauge i s placed lengthwise across the cylinder bore, and the measurement is made in a manner similar to that of an inside micrometer. Pin gauges are also used to measure the width of slots or grooves. In this connection, they are sometimes called width gauges. Figure shows a typical pin gauge. c) Snap, Gap or Ring Gauge These gauges are used for gauging the shafts and male components. The ‘Go’ snap gauge is of a size corresponding to the high (maximum) limit of the shaft, while the ‘Not Go’ gauge corresponds to the low (minimum limit).
• 81. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 208 Double ended type snap gauges can be conveniently used for checking sizes in the range of 3 mm to 100 mm and single ended progressive type snap gauges are suitable for size range of 100 to 250 mm. The gauging surfaces of the snap gauges are hardened upto 720 HV and are suitably stabilis ed, ground and lapped. The other surfaces are finished smooth. d) Ring Gauges These are gauges whose inside measuring surfaces are circular in form. Ring gauges are used to measure cylindrical surfaces, tapers on shafts, and similar work pieces, as well as external threads.
• 82. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 209 Plain Ring Gauges The plain ring gauges are made of suitable wear resisting steel and the gauging surfaces are hardened to a hardness of about 720 H.V. The gauging su rfaces are first suitably stabilised using proper heat treatment process and then ground and lapped and other surfaces are finished smooth. These are protected against climatic conditions by applying a suitable anti-corrosive coating. These are available in two designs, ‘Go’ and ‘No Go’. These are designated by ‘Go’ and ‘No Go’ as may be applicable, the nominal size, the tolerance of the work piece to be gauged, and the number of the standard followed. 2. According to Purpose According to purpose, the gauges may be classified as : (a) Workshop gauge or Working gauge (b) Inspection gauge (c) Purchase inspection gauge (d) Reference or Master gauge
• 83. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 210 (a) Workshop Gauge Workshop gauge or the manufacturing gauge is used by the machine operator to check the dimensions of the parts as they are being produced. These gauges usually have limits within those of the component being inspected. They are designed so as to keep the size of the part near the centre of the limit tolerance. (b) Inspection Gauge Inspection gauges are those used by inspectors in the final acceptance of manufactured parts when finished. These gauges are made to slightly larger tolerances than the workshop gauges so as to accept work slightly nearer the tolerance limit tha n the workshop gauges. This is to ensure that work, which passes the working gauge, will be accepted by the inspection gauge also. (c) Purchase Inspection Gauge The need of such gauges arises when the products of other plants are to be accepted. The purchaser must remember that the parts may have been made and checked by working gauges worn to the maximum permissible degree. Therefore the ‘Go’ side of the purchase inspection gauge must be designed accordingly. Thus, nominal size of ‘Go’ purchase inspection gauge will be equal to the lower limit of the hole. ‘No -Go’ purchase inspection gauge design is similar to ‘No-Go’ working gauge. (d) Reference or Master Gauges Reference or master gauges are used only for checking the size and condition of other gauges. Reference gauges are the reverse or opposite in form to working or inspection gauges. Due to the expenditure involved, reference gauges are seldom used and gaug es are checked by universal measuring instruments optimeters, comparators etc. or gauge blocks (for snap gauges). 3. According to type (a) Standard gauges (b) Limit gauges (a) Standard Gauges Every gauge is almost a copy of the part example, a bushing is to be made which is to mate with a shaft. In this case, shaft is the mating part. The bushing is checked by a gauge,
• 84. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 211 which in so far as the form of its surface and its size is concerned, is a copy of the mating part, that is, the shaft. If a gauge is ma de as an exact copy of the opposed (mating) part, in so far as the dimensions to be checked are concerned, it is called a “standard gauge’. The first gauges to be developed were the standard gauges. The first standard gauges were the opposed (mating) parts themselves. When a component is assembled with its mating part, a (mating) part itself. However, such individual fittings are not convenient or even possible in mass production conditions. Moreover, the two parts to be assembled might be in productio n in two different shops or even at two different plants. Therefore, it is more proper to use, as a checking tool, not the mating part, but its exact copy as far as the tested dimension is concerned. Such a standard gauge has two drawbacks: i) The quality of the manufactured part will depend upon the freedom with which it mates with the standard gauge. The judgment of this freedom is a relative thing and it usually creates misunderstanding between the purchaser and the manufacture. ii) A standard gauge ca nnot be used to check an interference fit. For example, if a bushing of 50 mm diameter is to be made for assembly with a shaft of 50.1 mm diameter, then the standard gauge diameter will be 50.1 mm of opposed part. Such a gauge will not pass into a properly produced bushing of diameter 50 mm. (b) Limit Gauge The system of limit gauges is very widely used in industries. Limit gauges are made to the limits of the dimensions of the part to be tested. As there are two limits of the dimensions of a part, high and low, two gauges are needed to check each dimension of the part. The part is checked by successively assembling each of the gauges with it. Since the dimensions of a properly manufactured part must be within the prescribed limits, one of the gauges called a “Go Gauge” should pass through or over the part, while the other gauge called a “Not Go Gauge” should not pass through or over the part. Gauges should pass through or over a part under their own weight and the part and the gauge must be at the same temperature.
• 85. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 212 4. According to Design According to design, the gauges may be classified as: (i) (a) Single limit (b) Double limit (ii) (a) Single ended (b) Double ended (iii) (a) Fixed (b) Adjustable (iv) (a) Integral end (b) Renewable end (v) (a) Solid end (b) Hollow end 5. Common types of fixed gauges a) Threaded or screw Gauges These gauges are designed along the same general principles as all other types of gauges. These gauges are of Plug and Ring type and are made in the ‘Go’ and ‘Not Go’ models. These are of special quality gauge steel, hardened and seasoned before the threads are ground and finally lapped to dimensions. Nuts or internal threads are che cked with Plug thread gauges and screws or external threads, with ring thread gauges. Thread gauges similar to plain gauges are designed with manufacturing tolerance for new gauges and wear allowance. Both tapered and trilock methods securing the plug gauges into their holders are employed. In case of ‘Go’ and ‘Not Go’ plug gauges, it is
• 86. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 213 common practice to make the wearing side, i.e., the ‘go end’ at least twice as long as the ‘not go end’. The maximum wear in the latter case occurs on the end thread or threads. In screws threads, there are three classes of fit : (a) Close fit (b) Medium fit (c) Free fit. Tolerances for major, minor and effective or pitch diameters for all these three types of fits are given in national standards. Screw or thread gauges take the form of the mating thread and are assembled with the thread to be checked. By suitable designing the gauges, it is possible to provide a limit gauging system, which will control and complex dimensions of the threads within the tolerance limit. There are two types of thread gauges : 1. Plug screw gauges 2. Ring screw gauges b) Receiving gauges These are similar to ring gauges but are used to verify the specified uniformity of size and contour of noncircular holes. They are extensively used to check splined shafts. Three receiving gauges are shown in Fig.
• 87. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 214 c) Flush Pin Gauges These types of gauges are mainly used to check the depths of slots, but can also be used for gauging lengths and tapers. A flush pin is defined as : a gauge for checking the distance between two surfaces, comparising a body having a through hole, and a pin in the hole which projects from a face of the body, a distance equal to the dimension to be gauged when the opposite or indicating end of the pin is flush with the opposite face of the body. The indicating end of the pin, or the adjacent face of the body, has a step of a depth equal to the tolerance on the dimension gauged. d) Form Gauges These are specially designed to check the form or contour of a workpiece. Consequently thi s type of gauge is of particular interest to the tool design draftsman. Gauges used to check a cutting tool. Another form gauge is also used to check the contour of railroad wheel threads. Pratt & Whitney makes a series of similar gauges to the specifications of the American Association of Railroads. These gauges not only check the contour of a tire or flange when new but show the amount of wear and indicate when wheels should be reconditioned.
• 88. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 215 e) Template Gauge These are similar to form gauges but are designed to check the position and dimensions of two adjacent surfaces. Template gauge designed to check two shoulders of a shaft. Template gauge designed to check the ways (or slides) of a machine tool. f) Gauge Blocks Length dimensions used in industry are based on the standard unit of dimensional measurement, such as the international inch or meter. U ntil recent times, the basic reference unit has been the International meter bar or its official duplicates. More recently, the international basic standard of length has been defined in terms of a specified wavelength of light. In theory, all gauges should be checked against the basic international standards, but when millions of gauges are considered, such a procedure would obviously be impractical. Large companies may be able to stand the expense of special equipment needed for achieving reliable length measurements based on wavelength, but the majority of companies cannot. The logical solution is to use the basic standard to calibrate many secondary standards, which are close replicas of the former. The secondary standard would
• 89. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 216 be manufactured in qu antity by mass production methods in order to be relatively inexpensive. The secondary standards that have emerged are known as gauge blocks and have become commonplate in modern plants. On first thought one tends to classify gauge blocks as a master fixed gauge, but in reality they are much more than this. A single gauge block may be classed as a master fixed gauge, but a single gauge block has little use except in special cases. Gauge blocks must be viewed from the concept of several single master gauge blocks being combined into a single gauge bar. The combined single blocks result in a bar whose actual dimension truly represents, within specific limits, the nominal dimension sought for a particular application. A typical set of gauge blocks is shown in figure.
• 90. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 217 The tool designer will not be responsible for the manufacture of gauge blocks, but the needs to understand how they are used. The gauges the designs will be calibrated and checked with gauge blocks before the gauge is placed in use. Gauge blocks are made of steel, chrome plated steel, stainless steel, tungsten carbide, or chrome carbide, depending upon the manufacturer and the price the purchaser is willing to pay. They are stabilized for minimum dimensional change and are hardened to RC63 to 64. Standard blocks may be rectangular (approximately 1/8 x 1¼ in.) or square (approximately 1 by 1 in.) in shape. The gauging surfaces are lapped to a very high finish. Gauge blocks set are classed in grades according to accuracy. When individual gauge blocks are combined, or built up, to provide a specific measurement, they must be wrung together. This is accomplished by a twisting and sliding motion that squeezes out the air between the almost geometrically flat gauging surfaces. When properly wrung together, the blocks adhere to each other to the extent that considerable force must be exerted to pull them directly apart. This phenomenon is usually explained as a combination of molecular attraction and the cementing action of oil or moisture film on the gauging surfaces. Steel age blocks should not be wrung together any longer than nece ssary, as moisture trapped between the blocks may cause corrosion. Gauge block sets are available with various numbers and combinations of blocks. A particular size can be built up by wringing individual slip gauges together. 6. Wringing : Wringing is the act of joining the slip gauges together while building up to sizes. It is due to molecular attraction & cementing action of moisture. Some sets of slip gauges also contain protector slips of some standard thickness made from higher wear resistance steel or tungsten carbide. These are used for protecting the exposed faces of the slip gauge pack from damage.
• 91. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 218 7) Classification of block gauges on basis of grades. ¨ Grades a) Grade ‘00’ accuracy : It is a calibration grade used as a standard for reference to test all the other grades. b) Grade ‘0’ accuracy : It is an inspection grade meant for inspection purposes. c) Grade i accuracy : Workshop grade for precision tool room applications. d) Grade ii accuracy : For general workshop applications. Block Gauges Grade 00 accuracy Grade 0 accuracy Grade i accuracy Grade ii accuracy
• 92. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 219 A9.3 Design of Gauges A9.3a Manufacturing Tolerances We know that is any other manufacturing process, in gauge making also it is economically impractical to attempt to make ‘Go’ and ‘Not Go’ gauges exactly to the two limits of the work tolerance. Thus it is necessary that permissible deviations in accuracie s must be assigned for gauge manufacture. Gauge maker’s tolerance or manufacturing tolerance should be kept as small as possible so that a large proportion of the work tolerance is still available for the manufacturing process. However the small the gauge tolerance, the more the gauge will cost. There is no universally accepted policy for the amount of gauge tolerance. However, the following norms are generally accepted : Limit gauges are made 10 times more accurate than the tolerances they are going tocontrol. That is, the tolerance on each gauge whether ‘Go’ or ‘Not Go’, is 1/10 th of the work tolerance. For example, if the work tolerance is 10 units, then the manufacturing tolerance for ‘Go’ and ‘Not Go’ gauge each will be 1 unit. This makes it poss ible, although the probability is small, for the work tolerance available in the shop to be cut down to 80% of the specified tolerance. The amount of tolerance on CHAPTER OUTLINE A9.3 Design of gauges A9.4 Sub zero treatment A9.5 Maintenance, safety & storage A9.6 Numerical examples TOPIC OUTLINE 9.3a Manufacturing tolerance A9.3b Wear Allowance A9.3c Taylor’s Principle A9.3d Fixing of gauging elements (ends) with handle A9.3e Provision of Pilot A9.3f Correct Centering A9.3g Materials A9.3h Hardness and Surface finish A9.3i Rigidity A9.3j Alignment A9.3k Gauge members A9.3l Gauge marking A9.3m Essential features A9.4a Characteristics of subzero treatment
• 93. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 220 inspection gauges is generally 5% of the work tolerance. Tolerance on reference or master gauges is generally 10% of the gauge tolerance. A9.3b Wear Allowance Mostly the measuring surfaces of ‘Go’ gauges, which constantly rub against the surfaces of the parts in inspection, are subjected to wear and loose their initial size. ‘Not Go’ gauges are not subjected to so much wear as ‘Go’ gauges and there is considerable wear on ‘Go’ gauges only. The size of go plug gauge is reduced while that of go snap gauge increases. It is of course desirable to prolong the service life of the gauges, and therefore a special allowance of metal, known as wear allowance is added to the go gauge in a direction opposite to wear. Wear allowance is usually taken as 5% of work tolerance. Wear allowance is applied to a nominal go gauge diameter before gauge tolerance is applied. A9.3c Taylor Principle This principle is based on the use of a correct system of limit gauges to inspect shafts and holes. According to Taylor it is not adequate to use simple Go gauge on outer dimension only but the shape is an importan t factor, i.e. the Go gauge should be full form gauge and it should be constructed with reference to the geometrical form of the part being checked, in addition to its size. In other words, Go gauge should check all the dimensions of a work piece in the maximum metal condition. As regards Not Go gauges, Taylor was of the view that it was useless for Not Go gauge to be full form, and each feature being dealt should be checked with a specific Not Go gauge. In other words, Not Go gauge shall check only one dimension of the work piece at a time, for the minimum metal condition. Thus according to it, a hole should completely assemble with a ‘Go’ cylindrical plug gauge made to the specified ‘Go’ limit of the hole, having a length at least equal to the length of engagement of the hole and shaft. In addition, the hole is measured or gauged to check that its maximum diameter is not larger than the ‘No Go’ limit. The Taylor principle interprets the limit of size for gauging holes and shafts as follows : 1. For holes If the Taylor principle is followed then the diameter of the largest perfect imaginary cylinder which can be inscribed within the hole so that it just contacts the highest points of the surface, shall not be a diameter smaller than the ‘Go’ limit ofsize. Further, the maximum diameter at any position in the hole should not exceed the ‘No Go’ limit of size.
• 94. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 221 2. For shafts If the Taylor principle is followed than the diameter of the smallest perfect imaginary cylinder which can be circumscribed about the shaft so that it just contacts the highest points of the surface, should not be a diameter larger than the ‘Go’ limit of size. Further the minimum diameter at any position on the shaft should not be less than the ‘No Go’ limit of the size. It may be noted the Taylor principle does not take care of the error of form, circularity or straightness etc., the tolerances for which should be specified separately. Thus according to Taylor principle, we require a plug ring gauge having exactly the ‘Go’ limit diameter and a length equal to the engagement length of the fit to be made for checking the ‘Go’ limit of the work piece and this gauge must perfectly assemble with the work piece to be inspected. The other gauge needed is the ‘No Go’ gauge, which contacts the work piece surface only in two diametrically opposite points and at those two points it should have exactly ‘No Go’ limit diameter. This gauge should not be able to pass over in the work piece in any consecutive position in the various diametric directions on the work piece length. In certain applications, the Taylor principle cannot be strictly and blindly followed. The following deviations are allowed which basically do not deviate f rom the principle as such. 3. For ‘Go’ Limit (i) In case the manufacturing process assures that the error of straightness will not affect the character of fit of the assembled work pieces, it is advisable to go for standard gauge blanks instead of using full form and length of engagement and make the gauge unnecessarily bulky and cumbersome and avoid the special gauge of exactly same working length for one work piece. (ii) If gauge happens to be too heavy, only segmental cylindrical bar could be used provided the manufacturing process ensures that errors of roundness will not have any effect on the character of fit of the assembled work pieces. (iii) For shafts, particularly heavy ones; it is generally not desirable to use full form ring gauges but only gap gauges. But for this purpose the manufacturing process used should take care of the error of roundness (especially lobbing) and the error of straightness.
• 95. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 222 4. For ‘No Go’ Limit The two point checking devices are not feasible and practicable in actual practice because these are subject to rapid wear etc.; these can be safely replaced by small planes/cylindrical surfaces/spherical surfaces. For gauging very small holes and in cases where work pieces may be deformed to an oval by a two point mechanical contact device, the ‘No Go’ gauge of full form may have to be used. 5. Allowable Deviation from the Taylor’s Principle In some applications, difficulties are experienced in conveniently using gauges if they are strictly based on Taylor’s principle. Accordingly some deviations may be permitted. According to Taylor’s principle, a Go gauge should be of full form having length equal to engagement length of fit, but this is not always necessary. For example, if it is known that the manufacturing process ensures that error of straightness of hole or shaft is so small that it would not affect the desired fit of assembly, then length of Go cylindrical plug or ring gauge may be less than the length of engagement. For very big holes, the full form gauge may be too heavy and inconvenient to use. Therefore, segmental cylindrical bar or spherical gauge may be used if it can be assumed that the manufacturing process would not produce the error of roundness outside the permissible limits to affect the character of fit. Similarly ‘Go’ gap gauge can be used in place of Go cylindrical ring gauge (which is often inconvenient for gauging shafts) provided manufacturing process can ensure the errors of roundness (lobbing) and straightness of shaft within permissible limits . The straightness of long shafts of small diameter should be checked separately. Similarly, though Taylor’s principle desires use of two point checking device to check No Go limit, but same is not always necessary in following cases. Since point contacts are subject to rapid wear, these can be replaced by small plane, cylindrical or spherical surfaces whenever appropriate. The two point checking device is also found difficult to design and manufacture for gauging very small holes and for such cases No Go plug gauge of full cylindrical form can be used. However possibility of accepting work pieces having diameters outside the No Go limit should be checked. In certain case the non -rigid workpiece may be deformed by 2 points mechanical contact device and in such cases No Go ring or plug gauges of full cylindrical form have to be used.
• 96. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 223 No Go gauges of full cylindrical form can also be used for thin walled work piece, which may be out of round due to heat treatment, but would become circular when such gauges are applied with force just sufficient to convert the elastic deformation into circularity. A9.3d Fixing of gauging elements (ends) with handle Plug gauges can be of solid type in which the gauging members are integral with the handle or the gauging elements can be separated from the handle and suitable fixed together. There are known as “Renewable” type of gauges. Below 50 mm diameter, solid type gauges are mostly used. For larger diameters, the renewable end types of plug gauges are used. A9.3e Provision of Pilot In case of very closely toleranced parts, sometimes it happens that the plug gauge does not easily enter the hole. This requires skill and practice. Since the gauges are generally used by semiskilled workers in industry, th erefore, to solve this problem, we use
• 97. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 224 what is known as piloting of plug gauge. The diameter of land is the same as that of the plug-gauging portion. A9.3f Correct Centering For the purpose of fabrication of plug gauges, grinding is very extensively used, and sometimes lapping may be used. In grinding, the centre of job and centre of machine plays an important role. Centers for high -grade job should be very perfect. Sometimes before grinding, the centers are even honed after heat treatment. After making the centre in the plug gauges, for heat treatment purposes these centers shoul d be protected so that the same may not get spoiled due to heat and burn. A typical centre in the gauge is shown in figure. The recess and groove in the centre protect the centre from external contacts and burns etc. A9.3g Materials Most gauges are s ubjected to considerable abrasion during their use and must, therefore, be made of wear resistant materials. Hence, the materials for limit gauges should meet of the following requirements : (i) Uniformity of structure and required co-efficient of linear expansion. (ii) Proper workability, especially in grinding and polishing. (iii) Stability of dimensions and forms of parts in the process of operation and possibly lower deformation in heat treatment and during manufacture. (iv) High resistance to mechanical wear and corrosion. (v) Optimal hardness which is a property characterizing a high durability and resistance to damage in use.
• 98. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 225 High Carbon and alloy steel have been used as gauge materials because of their relatively high hardenability and abrasion resi stance. For high volume production runs, gauge wear surfaces are often chrome plated. The durability of steel gauges coated with a layer of chrome of 5 to 8 um is 10 to 12 times that of uncoated ones. For economy in material as well as hardening costs, gauges are designed in such a way that only the parts subjected to wear are made of hardened steel. Handles are made of cheaper MS. For high degree of accuracy, long production runs and excessive wear conditions and particularly in bigger gauges, the ent ire body is made of M.S. and only wear contact surfaces are deposited with welded layer of hard materials such as cemented carbides, Satellite or weartrode. Some gauges are made entirely of cemented carbides or they have cemented carbide inserts at certain wear points. A9.3h Hardness & Surface Finish Recommended hardness for gauges is 60 to 64 Rock well C for plain gauges and 56 to 62 Rock well C for screw gauges. The recommended surface finish is : 0.127 to 0.254 um, Ra for ground gauges and 0.05 to 0.20 um, Ra for lapped gauges. A9.3i Rigidity Rigidity is one of the most important features of gauge design. Gauges such as gap gauges must always be designed with adequate rigidity as well as with robustness suitable for use in shop where they seldom meet with the treatment they deserve. A9.3j Alignment of Gauge Faces In a normal gap gauge, the faces must be parallel and opposite to each other and the points of contact with the work at each face must lie on a line normal to the gauging face. A9.3k Gauge members The gauges and gauges members shall be well finished and free from defects, which may affect their serviceability and shall be consistent with the grade of the product. The gauges and gauge members shall be demagnetized. All sharp corners and edges of measuring surfaces shall be broken.Incomplete starting threads on thread gauges shall be removed and blunt start shall be provided. The gauges shall be free from burrs. Whenever grooves or recesses are provided for colour coding, they shall be of 0.3mm depth and 2mm minimum width and the ir bottom corner shall be suitably rounded. The bottom corners of the grooves on gauges members shall be rounded off.
• 99. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 226 Feature Size Handle Nos. as per standard Double ended handle 1 ~ 3mm 6 ~ 50mm Handle No. 1 ~ 7 40 ~ 65mm Handle No. 8 Single ended handle 65 ~ 80mm Handle No.9 for Go gauge Handle No.10 for No Go There are two types of replaceable handles : (1) the taper lock (used for gauges up to 1.5 in. in diameter) and (2) the trilock (used for gauges above 1.5 in. in di ameter). The former has a gauge member made with a taper shank that fits tightly into a mating taper in the handle, while the latter has a reversibly gauging member, which has a central hole and three adjacent grooves. A screw passes through the gauge and fastens this to the handle while at the same time ensuring a snug fit by forcing three wedge shaped prongs into the corresponding grooves of the gauge. Plain Plug Gauges Generally the gauging members of the plain plug gauges are made of suitable wear resisting steel and the handles can be made of any suitable steel e.g. handles may be made of light metal alloys for heavy plain plug gauges, or suitable non -metallic handles may be provided for smaller plain plug gauges. The gauging surface of plain plug gauges are normally hardened to not less than 750 H.V. and suitably stabilised and ground and lapped. The plain plug gauges are normally of double-ended type for sizes up to 63 mm and of single ended type for sizes above 63 mm. The usual way of designati ng the plain plug gauges is by ‘Go’ and ‘No Go’ as applicable, the nominal size, the tolerance of the work piece to be gauged and the number of the standard followed e.g., a double ended ‘Go’ and ‘No Go’ plain plug gauge for gauging a bore of 10 mm with tolerance H7 and if designed according to Indian Standard (IS : - ) shall be designated as : ‘Go’ and ‘No Go’ plain plug gauge 10 H7, IS : . The various types of plain plug gauges in common use are as below : (i) ‘Go’ and ‘No Go’ plain plug gauges for sizes up to 10 mm, (solid type).
• 100. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 227 (ii) ‘Go’ and ‘No Go’ plain plug gauges for sizes over 10 and up to 30 mm (Taper Inserted Type). (iii) ‘Go’ and ‘No Go’ plain plug gauges for sizes over 30 mm and up to 63 mm (Fastened type). (iv) ‘Go’ and ‘No G o’ plain plug gauges for sizes over 63 mm and up to 100 mm (Fastened type). (v) ‘Go’ and ‘No Go’ plain plug gauges for sizes over 100 mm and up to 250 mm (Flat type). This is a shell form plug gauge. Each plug is relieved to reduce weight. For stil l further bigger holes and to restrict weight, use can be made of segmental cylindrical ended gauges. Spherically ended rods are used for very large holes. It would be noted that with such types of gauges the full form of the gauge is lost and the errorsof holes like ovality may not be detected. It is the general practice not to use cylindrical plugs above 100 mm diameter but to use cylindrically ended bars or spherically ended rods. Similarly Go gauges between the sizes of 100 and 200 mm diameter can take the form of a cylindrically ended bar. The plug gauges in order to protect them against climatic conditions. In order to prevent damage in handling and transit, these are packed in suitable cases. It may be mentioned that gauges with the gauging portion integral with the handle are now becoming obsolete and gauges with renewable ends are gaining popularity because of the following advantages : (i) Worn or damaged end can be replace conveniently. (ii) In the event of scrapping of gauge, handle can be used for other gauge. (iii) To reduce the weight, handle can be made of plastic, which also facilitates in handling the gauge, reduces cost and minimises risk of heat transfer. For smaller through holes, another useful renewable end plug gauge is the progressive type of gauge in which both the GO and NOT GO gauging members are provided on same side separated by a small distance. First Go portion is inserted in the hole, which would be further obstructed by NOT GO portions if hole is of tolerable size. A9.3l Gauge marking The plain gauges are marked with the following on their handles for their identification :
• 101. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 228 (i) Nominal size. (ii) Class of tolerance. (iii) The word ‘Go’ on the ‘Go’ side. (iv) The words ‘No Go’ on the ‘No Go’ side. (v) The actual values of tolerance. (vi) Manufacturer’s name or trademark. The ‘No Go’side is always painted with a red band. A typical example of the marking is shown in Fig. It is usual practice to apply suitable anti-corrosive coating to A9.3m Essential features These gauges are easy to handle and are accurately finished. They are generally finished to one tenth of the tolerance they are designed to control. For example, if the tolerance to be maintained is at 0.02 mm, then the gauge must be finished to within 0.002 mm, of the required size. These gauges must be resistant to wear , corrosion and expansion due to temperature. The plugs of the gauges are ground and lapped. The Go end is made longer than the ‘No Go’ end for easy identification. Sometimes a groove is cut on the handle near the ‘No Go’ end to distinguish it form the‘Go’ end. The dimensions of these gauges are usually stamped on them. ¨ Surface finish Ra --- 0.04 – 0.16 mm ¨ All gauges and gauge members shall be demagnetized ¨ Groove depth 0.3mm X 2mm width. ¨ Taper used is 1:50 A9.4 Sub-Zero Treatment Introduction of one or more cooling periods at a temperature well below the freezing point of water, in the normal heat treatment process of steels is called sub-zero treatment. Generally sub -zero treatment is carried out for gauges, as maintaining their dimensional stability is very important. It’s objective is to ensure complete transformation of austenite into marten site structure which is much compact and hard than austenite. Cooling at 100°F is sufficient for plain carbon and low alloy steels. But for high alloy steel, several cooling may be required, accompanied by tempering and air-cooling cycle between each.
• 102. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 229 On performing this treatment on hardened work pieces, further hardeningtakes place by the conversion of austenite to martensite. The deeper the cooling, higher the hardening effect. Characteristics: ¨ Even on cooling to cryogenic temperature, the austenite does not completely change to martensite. ¨ Repeated heat treatment at sub-zero temperature causes additional transformation of austenite. ¨ Variations in quenching rate within limits have virtually no effect on austenite transformation. ¨ Holding the hardened steel at +20°c for more than 15 minutes or heating to 150~170°F before next cooling before 0°c can cause (sat) stabilization of austenite. The degree of stabilization depends upon degree of alloying or time of holding. But practically it cannot be completely transformed to martensite. Sub-zero treatment produce high hardness, higher stresses. This increases deformation and risk of cracking in sharp tools. A9.5 Maintenance, Safety & Storage Gauges should be used and cared for properly to ensure their maximum useful service life. Some suggestions for their use and care are: ¨ Master, inspection and working gauges should be applied only to the uses for which they are intended, i.e., a master gauge shall be used to check inspection and working gauges, an inspection gauges will be used to check the finished product and a working gauge should be used to check the product as it is being manufactured. ¨ A plain cylindrical gauge should be cleaned and a thin film of light oil should be applied to the gauging surface before it is used. The work should also be cleaned. Then the gauge should be aligned with the hole to be measured and given a forward motion combined with a slight rotation. The ‘go plug’ gauge will enter the hole if the latter is of correct size but if not so, then the gauge will not enter it. Keep the gauge moving into and out of the hole when the fit is closed, to prevent seizure of the parts. The same sugges tions are applicable to the use of plain cylindrical ring gauges. ¨ Don’t force a snap gauge over work, because forcing will cause the gauge to pass oversized parts and it may also spring the frame of the gauge. In fact force should be avoided in any gauging operation as it tends to harm the gauge, the work or both.
• 103. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 230 ¨ A gauge should be cleaned after use and prepared for storage. If it is to be stored for a short time only, it should be coated with rust preventive oil. If it is to be stored for a long period of time, however, it should be dipped in a molten plastic material designed as a protection coating for tools and gauges. A9.6 Numerical Examples ¨ A 25 mm H8 – f7 fit is to be checked. The limits of size for H8 hole are : High limit 25.033mm, low limit 25.000 mm. The limits of size for the f7 shafts are : High limit 24.980 mm, low limit 24.959 mm. Taking gauge maker’s tolerance to be 10% of the work tolerance, design plug gauge and gap gauge to check the fit. Solution Tolerance for hole = H.L. – L.L. = 25.033 – 25.00 = 0.033 mm. Tolerance for shaft = H.L. – L.L. = 24.980 – 24.959 = 0.021 mm. Gauge makers tolerance for plug gauge = 0.1 x 0.033mm = 0.003mm (rationalised) Gauge makers tolerance for gap gauge = 0. mm = 0.002 mm (rationalised) As the work tolerances are less than 0.09 mm, wear allowance may not be provided. (i) Plug Gauge Basic size of ‘Go’ plug gauge = L.L. of the hole (MMC) = 25.000 mm. In unilateral system, Dimensions of ‘Go’ plug gauge = 000.0 003.0 mm00.25 - + That is, High limit of ‘Go’ plug gauge = 25.000 + 0.003 = 25.003 mm Low limit of ‘Go’ plug gauge = 25.000 mm Now, Basic size of ‘Not Go’ plug gauge = 25.033 mm Dimensions of ‘Not Go’ plug gauge = 003.0 000.0 mm033.25 - + (ii) Gap Gauge ‘Go’ side = H.L. of shaft (MMC)
• 104. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 231 = 24.980 mm. Dimensions of ‘Go’ gap gauge = 002.0 000.0 mm980.24 - + ‘Not Go’ side = L.L. of shaft = 24.959mm Dimensions of ‘Not Go’ gap gauge = 000.0 002.0 mm959.24 - + ¨ Shafts of 75 ±0.02 mm diameters are to be checked by the help of a Go, Not Go snap gauges. Design the gauge, sketch it and show its Go size and Not go size dimensions. Assume normal wear allowance and gauge maker’s tolerance. Solution High limit of shaft = 75.02 mm Low limit of shaft = 74.98 mm Work tolerance = 75.02 – 74.98 = 0.04 mm Gauge makers tolerance (10%) = 0.004 mm wear allowance = 0.002 mm ‘Go side’ of snap gauge = H.L. of shaft, (MMC) = 75.02 mm ‘Not Go’ side of snap gauge = 74.98 mm Wear allowance is to be applied first to ‘Go’ side, before gauge maker’s tolerance is applied. ‘Go’ side of snap gauge after considering the wear allowance = 75.02 – 0.002 = 75.018 mm Dimensions of snap gauge are given as: Unilateral System ‘Go’ 004.0 000.0 mm018.75 - + ‘Not Go’ 000.0 004.0 98.74 - + Bilateral System ‘Go’ 002.0 002.0 mm018.75 - + ‘Not Go’ 002.0 002.0 mm98.74 - + Example
• 105. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 232 Calculate the dimensions of plug and ring gauges to control the production of 50 mm shaft and hole pair of H7d8 as per I.S. specification. The following assumption may be made : 50 mm lies in diameter step of 30 and 50 mm and the upper deviation for ‘d’ shaft is given by – 16 D0.44 and lower deviation for hole H is zero. Tolerance unit i (microns) = 0.45 3 √ D + 0.001 D and IT6 = 10i and above IT6 grade the tolerance magnitude is multiplied by 10 at each fifth step. Solution For calculation of tolerance, value of diameter is taken as the mean of range in which it lies. = 38.7 mm value of tolerance unit D001.0D45.0i 3 += 7.38x001.07..0 3 += = 0.45 (3.38) + 0. = 1.521 + 0. = 1. microns = 0. mm. Now hole is of type H and grade 7 IT7 = IT6 X10.2 = 10i x 100.2 = 10i x 1.585 = 15.85i and for H hole, fundamental deviation = 0 and value of tolerance = 15.85 x 0. = 0. mm For hole H7 disposition of work tolerance will be as shown in Fig. For shaft d8 Tolerance = IT8 = 100.2 x IT7 = 1.585 x 15.85i = 25i (approx.) = 25 x 0. = 0. mm. Fundamental deviation for ‘d’ shaft = 16D0.44 = - 16 x 38.70.44 = 16 x 5 = - 80 microns = - 0. mm. Upper deviation = - 0.08 mm X30D == mm68. ==
• 106. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 233 And work tolerance = - 0. mm. Lower deviation = - 0.08 – 0. = - 0. mm. Considering for Gauges (1) Plug gauges According to new system, ‘Go’ gauges are given 1/10 th of work tolerance in the tolerance zone and ‘No Go’ gauges, outside it. Since work tolerance is less than 0.09 mm. Effect of wear in ‘Go’ gauges is not considered. Work tolerance = 0. mm. Gauge tolerance = 0. mm Limits for ‘Go’ gauges are 50. mm. 50. mm. And for ‘No Go’ gauge, dimensions are 50 + 0. = 50. mm. 50. + 0. = 50. mm. (2) Ring gauges The various dimensions of Gap gauges are shown in Fig.4.56 and calculations made in a similar manner as for plug gauges. Dimensions for ‘Go’ gauge are 50 – 0.08 = 42.92 mm and 49.92 – 0. = 49. mm. Dimensions for ‘No Go’ gauge are 50 – (0.38 + ) = 49. mm. And 49. – 0. = 49. mm. Answers : Plug Gauge. Dimensions for ‘Go’ gauge : 50. mm. 50. mm. ‘No Go’ gauge : 50. mm.
• 107. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 234 50. mm Ring Gauge. Dimensions for ‘Go’ gauge : 49. mm. 49. mm. ‘No Go’ gauge : 49. mm. 49. mm. Gauge Design – Explanation for the abbreviations used G = High limit of work piece (Hole) H = Tolerance on cylindrical plug or cylindrical bar gauge Hs = Tolerance on spherical gauges Z = Distance between Centre of tolerance zone, of go gauges for holes and go workpiece limit. K = Low limit of workpiece Y = Margin, out side of the Go workpiece limit of the ware limit of the gauges of holes. a = Safety zone provided for compensating measuring uncertainty of gauge for holes.
• 108. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 235 Y1 = Margin out side of the Go workpiece limit of the ware limit of gauges of shafts. Z1 = Distance between centre of tolerance zone of go gauges for shafts and go workpiece limit. H1 = Tolerance on gauges for Shafts. HP = Tolerance on reference disc for gap gauges. a1 = Safety zone provided for compensating measuring uncertainities of gauges for shafts. T = Workpiece tolerance = G – K ES = Upper deviation of a hole (E’ cart superior) EI = Lower deviation of a hole (E’ cart Inferior) es = Upper deviation of a shaft ei = Lower deviation of a shaft. ¨ Gauges for Holes upto 180mm diameter (IT 6 to IT 16) For all grades of work tolerances, the manufacturing tolerance (H) on the go gauge is placed by an amount ‘Z’ inside the work limits. The wear allowance ‘Y’ is arranged for grades IT 6 to IT 8 outside the work limits and no wear allowance on the gauges is specified for grades IT 9 to IT 16. The manufacturing tolerance is symmetrical for GO gauges and NO GO gauges. For NO GO gauges, manufacturing tolerance is placed above the upper limit of workpiece. ¨ Gauges for holes above 180mm diameter (IT 6 to IT 16) As the tolerance zone above 180mm is comparatively great, it can be afforded to guarantee the work limits and to shift the manufacturing gauge tolerances more inside the work limits. A safety zone ( a) is therefore introduced. This serves also as a safety zone provided for compensating the measuring uncertainties of gauges for holes. The basic size of No Go gauge for all grades of workpieces is reduced bya. On Go side, for grades IT 6 to IT 8, permissible worn limit is increased by an amount a. ¨ Gauges for shafts upto 180mm diameter
• 109. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 236 The manufacturing tolerance H1 on the ‘Go’ gauge is placed by a distance Z, inside the limit of the workpiece. The wear allowance ‘Y’ is arranged outside the work limits for grades IT 5 to IT 8 and no wear allowance are specified for grades IT 9 to IT 16. Manufacturing tolerance for Go and No Go gauge is symmetrically positioned. For No Go gauge, the basic size is the lower limit of workpiece. ¨ Gauges for shafts above 180mm diameter Similar to hole gauges, a safety zone a is provided on workshop gauges, by which the basic size for ‘No Go’ side for all gra des of workpieces is increased above the low limit of workpiece. On the Go side, for grades IT 5 to IT 8, the permissible worn out limit is decreased by an amount a. ¨ Steps in gauge design Gauges for inside measurements (plug gauges) Data given – Dimension of a hole with tolerance. (Ex f 20 H 7) - Find out the value of tolerance for f 20 H7 from standard chart. - Corresponding to ‘T’ the value of the following can be calculated or found out. - Manufacturing Tolerance for gauges (H). From Table (1) corresponding to the shape of the gauge chosen and tolerance grade of the workpiece, the grade of tolerance for size and form of the gauge can be found out. - Form Table 2 the actual value of H, Y and Z, can be taken. - Corresponding to the disposition diagram the formula for Go and No Go gauges can be written. See table 4 for formula. Similar way, the steps can be followed for gauges for shafts. Examples ¨ Design a plug gauge for dia 30 H7 30 H7 E S = +21 microns E I = 0 Tol. = 21 microns K (low limit size of hole) = 30.00 G (high limit size of hole) = 30.021
• 110. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 237 Go gauge New = (K + Z) 2 H ± Go gauge worn out = K – Y From table 2 Z = 3 microns Y = 3 microns 2 H = 2 microns Go gauge New = (30 + 0.003) ± 0.002 = 30.003 ± 0.002 Go gauge worn out = 30 – 0.003 = 29.997 No Go gauge = 3.021 ± 0.002 ¨ Design a plug gauge for a hole 1.0 2.0 40+ + K = Low limit = 40.1 H = High limit = 40.2 T = Tolerance = 0.1 From table 2 For T = 1 and size = 40 2 H = 0.002 Y = 0 Z = 0.011 Go Gauge New = (K + 2) 2 H ± = (40.1 + 0.011) ± 0.002 = 40.111 ± 0.002 Go gauge worn out = K – Y = 40.1 – 0 = 40.1
• 111. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 238 No Go gauge = G 2 H ± = (40.2) ± 0.002 = 40.2 ± 0.002 ¨ Design a plain ring gauge for diameter 40 K7 For diameter 40 K7 e s = +27 micron = 0.027 e i = +2 micron = 0.002 G = 40.027 K = 40.002 T = 0. From table 3 002.0 2 H1 = Y1 = 0.003 Z1 = 0. From table 4 Go gauge new = (G – Z1) 2 H1 ± = (40.027 – 0.) ± 0.002 = 40. ± 0.002 Go gauge worn out = G + Y1 = 40.027 + 0.003 = 40.030 No go gauge = K 2 H1 ± = 40.002 ± 0.002 ¨ Design a snap gauge for diameter 260 – 0.05 G = 260 K = 259.95 T = 0.05
• 112. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 239 From table 3 2 H1 = 0.006 Y1 = 0.006 Z1 = 0.008 a1 = 0.003 Go gauge new = (G – Z1) 2 H1 ± = (260 – 0.008) ± 0.006 259.992 ± 0.006 Go gauge worn out = G + Y1 - a1 = 260 + 0.006 – 0.003 = 260.003 No Go gauge = (K + a1) 2 H1 ± = 259.95 ± 0.006 TABLE-1 MANUFACTURING TOLERANCES FOR GAUGES Tolerance Grade for workpiece IT4 IT5 IT6 IT7 IT8 to IT10 IT11 to IT12 IT13 to IT16 Size IT Form IT Size IT Form IT Size IT Form IT Size IT Form IT Size IT Form IT Size IT Form IT Size IT Form IT Tolerance Grade for Cylindrical Plug Gauges 0+ 0+ 1+ 1+ 2 1 3 2 3 2 5 4 7 5 Tolerance Grade for Cylindrical Bar Gauges - - - - 2 1 3 2 3 2 5 4 7 5
• 113. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 240 Tolerance Grade for Spherical Plug or Dist Gauges - - - - 2 1 2 1 2 1 4 3 6 5 Tolerance Grade for Spherical Ended Rod Gauges - - - - 2 1 2 1 2 1 4 3 6 5 Tolerance Grade for Cylindrical Ring Gauges - - - - 3 2 3 2 4 3 5 4 7 5 Tolerance Grade for Gap Gauges - - - - 3 2 3 2 4 3 5 4 7 5 Tolerance Grade for reference Disks for Gap Gauges - - - - 1 1 1 1 2 1 2 1 3 2 Tolerance Grade for reference cylindrical Setting Plug Gauges - - - - 1 1 1 1 2 1 2 1 3 2 Tolerance Grade for reference cylindrical setting Ring Gauges - - - - - 1 1 1 2 1 2 1 3 2 Table – 2 Gauge tolerances & their location for gauges. For inside measurements (Holes) All values are in micron (um) Nominal sizes mm Symbols Work Tolerance Grades as per ISO Over Upto & Incl. 6 7 8 9 10 11 12 13 14 15 16 - 3 T 6 10 14 25 40 60 100 140 250 400 600 H/2 0.6 1 1 2 5 5 Y 1 1.5 3 0 0 0 0 Z 1 1.5 2 5 10 20 40 3 6 T 8 12 18 30 48 75 120 180 300 480 750 H/2 0.75 1.25 1.25 2.5 6 6
• 114. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 241 Y 1 1.5 3 0 0 0 0 Z 1.5 2 3 6 12 24 48 6 10 T 9 15 22 36 58 90 150 220 360 580 900 H/2 0.75 1.25 1.25 3 7.5 7.5 Hs/2 0.75 0.75 0.75 2 4.5 4.5 Y 1 1.5 3 0 0 0 0 Z 1.5 2 3 7 14 28 56 10 18 T 11 18 27 43 70 110 180 270 430 700 H/2 1 1.5 1.5 4 9 9 Hs/2 1 1 1 2.5 5.5 5.5 Y 1.5 2 4 0 0 0 0 Z 2 2.5 4 8 16 32 64 18 30 T 13 21 33 52 84 130 210 330 520 840 H/2 1.25 2 2 4.5 10.5 10.5 Hs/2 1.25 1.25 1.25 3 6.5 6.5 Y 1.5 3 4 0 0 0 0 Z 2 3 5 9 19 36 72 30 50 T 16 25 39 62 100 160 250 390 620 H/2 1.25 2 2 5.5 12.5 12.5 Hs/2 1.25 1.25 1.25 3.5 8 8 Y 2 3 5 0 0 0 0 Z 2.5 3.5 6 11 22 42 80 50 80 T 19 30 46 74 120 190 300 460 740 H/2 1.5 2.5 2.5 6.5 15 15 Hs/2 1.5 1.5 1.5 4 9.5 9.5 Y 2 3 5 0 0 0 0 Z 2.5 4 7 13 25 48 90 Nominal sizes mm Symbols Work Tolerance Grades as per ISO Over Upto & Incl. 6 7 8 9 10 11 12 13 14 15 16 80 120 T 22 35 54 87 140 220 350 540 870 H/2 2 3 3 7.5 17.5 17.5 Hs/2 2 2 2 5 11 11 Y 3 4 6 0 0 0 0 Z 3 5 8 15 28 54 100 120 180 T 25 50 63 100 160 250 400 630 H/2 2.5 4 4 9 20 20
• 115. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 242 Hs/2 2.5 2.5 2.5 6 12.5 12.5 Y 3 4 6 0 0 0 0 Z 4 6 9 18 32 60 110 180 250 T 29 46 72 115 185 290 460 720 H/2 3.5 5 5 10 23 23 Hs/2 3.5 3.5 3.5 7 14.5 14.5 Y 4 6 7 0 0 0 0 Z 5 7 12 21 24 40 45 80 100 170 210 a 2 3 4 4 7 10 15 25 45 70 110 250 315 T 32 52 81 130 210 320 520 810 H/2 4 6 6 11.5 26 26 Hs/2 4 4 4 8 16 16 Y 5 7 9 0 0 0 0 Z 6 8 14 24 27 45 50 90 110 190 240 a 3 4 6 6 9 15 20 35 55 90 140 315 400 T 36 57 89 110 230 360 570 890 H/2 4.5 6.5 6.6 12.5 28.5 28.5 Hs/2 4.5 4.5 4.5 9 18 18 Y 6 8 9 0 0 0 0 Z 7 10 16 28 32 50 65 100 125 210 280 a 4 6 7 7 11 15 30 45 70 110 180 400 500 T 40 63 97 155 250 400 630 970 H/2 5 7.5 7.5 13.5 31.5 31.5 Hs/2 5 5 5 18 20 20 Y 7 9 11 0 0 0 0 Z 8 11 18 32 37 55 70 110 150 240 320 a 5 7 9 9 14 20 35 55 90 140 220 Table – 3 : Gauge tolerances & their location for gauges for outside measurement (Shafts ) All values are in micron Nominal sizes mm Symbols Work Tolerance Grades as per ISO Over Upto & Incl. 5 6 7 8 9 10 11 12 13 14 15 16 - 3 T 4 6 10 14 25 40 60 100 140 250 400 600 H1/2 0.6 1 1.5 1.5 2 5 5 Hp/2 0.4 0.4 0.6 0.6 0.6 1 1 Y1 1 1.5 3 0 0 0 0 Z1 1 1.5 2 5 10 20 40 3 6 T 5 8 12 18 30 48 75 120 180 300 480 750
• 116. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 243 H1/2 0.75 1.25 2 2 2.5 6 6 Hp/2 0.5 0.5 0.75 0.75 0.75 1.25 1.25 Y1 1 1.5 3 0 0 0 0 Z1 1 2 3 6 12 24 48 6 10 T 6 9 15 22 36 58 90 150 220 360 580 900 H1/2 0.75 1.25 2 2 2 7.5 7.5 Hp/2 0.5 0.5 0.75 0.75 0.75 1.25 1.25 Y1 1 1.5 3 0 0 0 0 Z1 1 2 3 7 14 28 56 10 18 T 8 11 18 27 43 70 110 180 270 430 700 H1/2 1 1.5 2.5 2.5 4 9 9 Hp/2 0.6 0.6 1 1 1 1.5 1.5 Y1 1.5 2 4 0 0 0 0 Z1 1.5 2.5 4 8 16 32 64 18 30 T 9 13 21 33 52 84 130 210 330 520 840 H1/2 1.25 2 3 3 4.5 10.5 10.5 Hp/2 0.75 0.75 1.25 1.25 1.25 2 2 Y1 2 3 4 0 0 0 0 Z1 1.5 3 5 9 19 36 72 30 50 T 11 16 25 39 62 100 160 250 390 620 H1/2 1.25 2 3.5 3.5 5.5 12.5 12.5 Hp/2 0.75 0.75 1.25 1.25 1.25 2 2 Y1 2 3 5 0 0 0 0 Z1 2 3.5 6 11 22 42 80 50 80 T 13 19 30 46 74 120 190 300 460 740 H1/2 1.5 2.5 4 4 6.5 15 15 Hp/2 1 1 1.5 1.5 1.5 2.5 2.5 Y1 2 3 5 0 0 0 0 Z1 2 4 7 13 25 48 90 Table 3 Continued Nominal sizes mm Symbols Work Tolerance Grades as per ISO Over Upto & Incl. 5 6 7 8 9 10 11 12 13 14 15 16 80 120 T 15 22 35 54 87 140 220 350 540 870 H1/2 2 3 5 5 7.5 17.5 17.5 Hp/2 1.25 1.25 2 2 2 3 3 Y1 3 4 6 0 0 0 0 Z1 2.5 5 8 15 28 54 100 120 180 T 18 25 40 63 100 160 250 400 630
• 117. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 244 H1/2 2.5 4 6 6 9 20 20 Hp/2 1.75 1.75 2.5 2.5 2.5 4 4 Y1 3 4 6 0 0 0 0 Z1 3 6 9 18 32 60 110 180 250 T 20 29 46 72 115 185 290 460 720 H1/2 3.5 5 7 7 10 23 23 Hp/2 2.25 2.25 3.5 3.5 3.5 5 5 Y1 3 5 6 7 0 0 0 0 Z1 4 7 12 21 24 40 45 80 100 170 210 a1 1 2 3 4 4 7 10 15 25 45 70 110 250 315 T 23 32 52 81 130 210 320 520 810 H1/2 4 6 8 8 11.5 26 26 Hp/2 3 3 4 4 4 6 6 Y1 3 6 7 9 0 0 0 0 Z1 5 8 14 24 27 45 50 90 110 190 240 a1 1.5 3 4 6 6 9 15 20 35 55 90 140 315 400 T 25 36 57 89 140 230 360 570 890 H1/2 4.5 6.5 9 9 12.5 28.5 28.5 Hp/2 3.5 3.5 4.5 4.5 4.5 6.5 6.5 Y1 4 6 8 9 0 0 0 0 Z1 6 10 16 28 32 50 65 100 125 210 280 a1 2.5 4 6 7 7 11 15 30 45 70 110 180 400 500 T 27 40 63 97 155 250 400 630 970 H1/2 5 7.5 10 10 13.5 31.5 31.5 Hp/2 4 4 5 5 5 7.5 7.5 Y1 4 7 9 11 0 0 0 0 Z1 7 11 18 32 37 55 70 110 145 240 320 a1 3 5 7 9 9 14 20 35 55 90 140 220 FORMULAE FOR GAUGE DIMENSIONS GAUGES FOR GAUGE SIZE NORMAL SIZE UP TO 180 mm. ABOVE 180 mm. GAUGES REE GAUGE GAUGES REE GAUGE BASIC SIZE MFG. TOL. BASIC SIZE MFG. TOL. BASIC SIZE MFG. TOL. BASIC SIZE MFG. TOL.
• 118. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 245 INSIDE MEASUR- EMENTS NO GO G 2 HS OR 2 H ± ± NOT PROVIDED * 2 H OR 2 HS G ±-a NOT PROVIDED GO (NEW) K + Z 2 H ± K + Z 2 HS OR 2 H ± ± WEAR LIMIT K – Y - K–Y + a - OUTSIDE MEASUR- EMENTS WEAR LIMIT G + Y1 - G + Y1 2 HP ± G+Y1-a1 - G+Y1-a1 2 HP ± GO (NEW) G - Z1 2 H1 ± G - Z1 2 HP ± G - Z1 2 HP ± G - Z1 2 HP ± NO GO K 2 H1 ± K 2 HP ± K + a1 2 H1 ± K + a1 2 HP ± 2 H* SHOULD ONLY BE USED WHEN SPHERICAL GAUGES ARE NOT USED.
• 119. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 246 FOR VALUES IN MICRONS REFER TABLE
• 120. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 247
• 121. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A9 – GAUGES JIGS, FIXTURES & GAUGES 248 SUMMARY: Manufactured parts must be checked to determine whether they are according to the specifications or not, and also to control their dimensions. The measure dimensions are compared with the standard specified dimensions to decide whether the components are acceptable or not. In mass production, where large numbers of similar components are produced, to measure the dimensions of each part will be a time consuming and costly process. Therefore, gauges can check conformance of the part with tolerance specification. Gauges are scale less inspection tools at rigid design, which are used to check the dimensions of manufactured parts. They also check the form and relative positions of the surfaces of parts. Gauges are easy to employ and can be used in many cases by unskilled operators. Gauges are differing from measuring instruments. Gauges find wide application in engineering particularly for mass production. QUESTIONS: 1. Explain clearly what selective assembly means. Give one practical example. 2. What are the various types of plug gauges? Sketch any four of them and state their specific applications. 3. What are snap gauges? Sketch and describe an adjustable snap gauge. 4. Distinguish between measuring instrument and a gauge. 5. What are the essential considerations in selecting the material for gauges? List some of the materials commonly and explain the manufacture of gauges. 6. State and explain the “Taylor’s Principle of Gauge Design”. 7. Explain the following in connection with the gauge design: (i) Gauge maker’s Tolerance (ii) Wear allowance 8. Differentiate between ‘Workshop Gauges’ and ‘Inspection Gauges’. 9. Explain Taylor’s principal & justified wear allowance is provided on Go gauges. 10. Explain about sub zero treatment of gauges. 11. Define a gauge & state the purpose of gauge. 12. Sketch and explain the use of limit gauges in mass production. 13. Explain the following gauges. a) Plug gauge b) Ring gauge c) Snap gauge d) Receiving gauge
• 122. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A2 – JIGS & FIXTURES JIGS, FIXTURES & GAUGES 4 Introduction Purpose of jigs and fixtures 1) Automobile Industries 2) Air craft industries 3) Fixtures in numerically controlled machine tools 4) Fixtures for flexible manufacturing systems 5) Other application of jigs and fixtur es (Ex. Plastic, textile, consumer products Industries, etc.) Advantages of Jigs & Fixtures - 1) Productivity : By way of eliminating the human effort in marking positioning and frequent checking there is a considerable reduction in machine tool time and human fatigue by using Jigs & Fixtures which ultimately contribute to the increase in productivity. 2) Interchangeability : Jigs and Fixtures facilitate the production of similar components of uniform quality. So much so contribute to the interchangeability. Waiting for assembly is totally avoided. Selective assembly of components is completely eliminated. They help in the maintenance of uniform assembly and unified assembly schedules. 3) Skill Reduction : Skill of the individual is taken over by ji gs and fixtures by simplifying the locating and clamping techniques. There is no need for skillful setting of work or tool. Any average worker can be trained in the skills of using jigs and fixtures. The replacement of unskilled/semi skilled workmen, in place of skilled workmen results in considerable saving in labour cost. CHAPTER OUTLINE A2.1 Introduction of Jigs & Fixtures TOPIC OUTLINE a) Purpose of Jigs & Fixtures b) Advantages of Jigs & Fixtures c) Disadvantages of Jigs & Fixtures
• 123. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A2 – JIGS & FIXTURES JIGS, FIXTURES & GAUGES 5 4) Cost Reduction : Higher production, reduction in wastage, reduction in labour cost, easy assembly reduction in inspection costs etc. affects in substantial reduction in costs by using jigs and fixtures. c) Disadvantages of Jigs & Fixtures 1) Initial cost is high 2) Maintenance of the tool is a problem 3) More premium paid for Jig & Fixture.
• 124. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A2 – JIGS & FIXTURES JIGS, FIXTURES & GAUGES 6 SUMMARY : Jigs and fixtures are the special purpose tools used to facilitate production of similar components with greater accuracy and productivity. With the development of technology the application of jigs and fixtures is made even in batch production, where quality and reliability are more important than the cost. The industrial applications of jigs and fixtures is gaining importance in various industries like Machine Tools, automobile, aircraft, Textile, Plastics, etc. With the use of jigs and fixtures the present and future production processes can be made more versatile with greater productivity. QUESTIONS : 1) Explain advantages & disadvantages in using jigs & fixtures? 2) Explain purpose of jigs & fixtures? 3) What do you understand by the term jigs & fixtures? 4) What is the difference between jig & fixture?
• 125. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A5 – JIGS JIGS, FIXTURES & GAUGES 137 A5.1 Introduction Jigs and fixtures are special purpose tools used in mass production. They provide a means of manufacturing interchangeable parts as they establish a relationship with predetermined tolerance between the work and the tool. This apart they eliminate the necessity of a special set up for each individual part. Jigs are the devices that hold, locate and guide the cutting tool on to the job. They are classified into boring and drilling jigs, which are further classified as template jigs, plate jigs, box jigs etc. Fixtures are devices that hold and locate a work piece for a specific operation but do not guide the cutting tool. They are identified by the operation they perform such as assembly fixtures, welding fixtures, milling fixtures etc. Some aspects rel ating to the use, classification and identification of jigs and fixtures are discussed in this unit. CHAPTER OUTLINE A5.1 Introduction A5.2 Function of jig & fixture A5.3 Factor characteristic in jig design A5.4 Jig support A5.5 Jig bodies & rigidity A5.6 Classification of jig A5.7 Types of jig & their description A5.8 Maintenance, safety & storage TOPIC OUTLINE A5.3a Tool design A5.3b Design procedure A5.3c Determining dimensions A5.3d Initial Jig design A5.6a Angle Plate jig A5.6b Leaf jig A5.6c Template jig A5.6d Table jig A5.6e Sandwich jig A5.6f Box jig A5.6g Channel jig A5.6h Plate jig A5.6i Trunning jig A5.6j Multistation jig A5.6k Indexing jig A5.6l Universal jig A5.6m Boaring Jig
• 126. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A5 – JIGS JIGS, FIXTURES & GAUGES 138 A5.2 Functions of Jigs & Fixtures Jig is a special device used in production to hold, support, locate the work piece besides guiding the cutting tool as the operation is performed. A fixture, on the other hand, is a production tool that locates, holds and supports the work securely to accomplish the machining operation. Fixtures also help to simplify metal working operations performed on special equipment. Essential difference between jig and a fixture is that the former incorporates bushes that guide the tools employed whilst the latter holds the component, with the cutters working independently of it. A5.3 Factors Characteristic to Jig Design The factors of design previously discussed are applicable to both jigs and fixtures, since the function of a jig differs from that of fixture, there are some practices, which are characteristic to the design of jigs and others which apply to fixtures. A5.3a Tool Design Tool design is the process of designing and developing the tools, methods, and techniques necessary to improve manufacturing efficiency and productivity. It gives industry the machines and special tooling needed for today’s high speed, high volume production. It does this at a level of quality and economy that will insure that the cost of the product is competitive. Since no single tool or process can serve all forms of manufacturing, tool design is an ever changing, growing process of creative problem solving. A5.3b Design Procedures Once the tool designer decides that a template jig is the best choice for a particular job, the design process begins. Following the planning processes outlined, the tool designer assembles and evaluates all the necessary data. The part is a flat disc, 2.56 inches in diameter and 75 inch thick, with a 1.000 inch hole in its center. · The material specified is steel. · The only operation required of the jig is to drill two holes .19 inch in diameter 1.770 inches apart. · The blank part received for drilling is faced, drilled, and reamed to the specific dimensions.
• 127. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A5 – JIGS JIGS, FIXTURES & GAUGES 139 A5.3c Determining Dimensions When the preliminary sketch is completed, sizes and proportions for various parts of the tool, and some dimensions are added. The exact dimensions are usually computed when the final tool drawing is made. A5.3d Initial Jig Design After calculating the locator and bushing values, the designer is ready to plan the rest of the tool. This first step in this initial design is rough sketching the part. Since the butt plate is a flat disc, only two views need to be sketched, Figure. Starting with the top view, the designer sketches in the rough outline of the jig plate, Figure. To avoid confusion, draw the part in red and the tool sketch in black. This allows for easy identification of the part at all times and prevents confusion where several lines lie close together. Once the jig plate is sketched in, the designer adds the dimensions. Onto the front view of the part, the tool designer sketches in the front view of the jig plate. In this view, the designer must decide how long the locator pin should be. To avoid sticking and jamming, make the locator no longer than one half the part thickness. In this case, the pin should be .38 inch long. To maintain the proper alignment between the holes, a pin must be placed in the first hole after it is drilled. The designer should specify a standard jig pin to keep the tool cost as low as possible. · Before the design is started, the tool drawing must be studied to obtain all necessary information about the part. · Careful calculations must be made to determine the exact size and location of the locators and bushings. · Each design should start as a sketch. - Sketching helps to formulate the design. - Sketches help reduce problems and show relationships. - Once the design is set and the problems solved, the final design drawingis prepared. A5.4 Jig support
• 128. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A5 – JIGS JIGS, FIXTURES & GAUGES 140 Common practice in jig design is the used of supporting feet for the jig. On cast iron jig bodies the feet may be cast as parts of the body, o r they may be a detachable type. Detachable jig feet, usually made of hardened steel for wear resistance, may be purchased as standard jig accessories. In some cases a hard alloy is welded to the jig feet for wear resistance. Jig feet should be placed so as to give adequate and level support. As a rule, a jig has four feel so that, a chip gets caught under one of the feet, the jig would rock. A three legged jig would not rock with a chip under one of the feet. This might result in inaccurate drilling. Supports should be provided under points where the cutting tools exert pressure, so that the jig and the work will not tip or spring under pressure. A5.5 Jig Bodies & Rigidity The design of the body of a jig or a fixture depends upon the locators, the clamps, or other fastening devices and accessories already planned for the jig. Bodies and body constructions are taken from standards. The completed sketch includes a suitable method of latching the leaf in place in a level position. The welded body design also includes satisfactory feet for this drill jig. Other features are added which will provide a proper functioning tool. A5.6 Classification of Jig Jigs are broadly divides into two classes : 1. Drill Jigs 2. Boring Jigs Jigs Template jig Sandwitch jig Box jig Leaf jig Trunnion jig Multistation jig Plate jig Angle plate jig Channel jig Indexing jig Pump jig Combination jig
• 129. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A5 – JIGS JIGS, FIXTURES & GAUGES 141 1. Drill jigs are used to drill, ream, tap, chamfer, co unter bore, counter sink, reverse spotface or reverse counter sink as shown in fig. 2. Boring jigs are used to bore holes which are either too large to drill or must be made on off size.
• 130. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A5 – JIGS JIGS, FIXTURES & GAUGES 142 The basic jig is the same for all the machining operations but with the only difference in the size of the busing used. Drill jigs may be divided into general types open and closed. Open jigs are for simple operations where work is done on only one sideof the part. Closed or box jigs on the other hand are used for parts which need to be machined on more than one side. The names used to identify the jigs refer to the way the tool is built. Following are some of the common used jobs. a. Angle Plate jig b. Leaf jig c. Template jig d. Table jig e. Sandwich jig f. Box jig g. Channel jig h. Plate jig i. Trunning jig j. Multistation jig k. Indexing jig l. Universal jig a) Angle Plate Jigs These jigs are used to hold parts which are machined at right angled to their mounting locators. Modified angle plate jigs are used for machining angles other than 90 degrees.
• 131. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A5 – JIGS JIGS, FIXTURES & GAUGES 143 b) Leaf Jigs These are small box jigs with a hinged leaf to allow for easier loading and unloading. The difference between the leaf and box jig is the size and part location. Leaf jigs are normally smaller than box jigs and are made some times so that they do not surround the part totally. Leaf jigs are usually provided with a handle for easier movement.
• 132. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A5 – JIGS JIGS, FIXTURES & GAUGES 144 c) Template Jigs Normally used for accuracyrather than speed. This type of jig is not usually clamped but fits over, on, or into the work. Templates are the least expensive and simplest type of jigs to use. They may or may not have bushings. When bushings are not used, the whole jig plate is normally hardened. d) Table Jig The table jig, figure, is basically a plain plate jig with legs. Its main purpose is holding irregular or nonsymmetrical workpieces that cannot be held in other plate jig forms. With this jig, the part is referenced by the surface being machined rather than the opposite side. This surface relationship can be seen in figure. Table jigs can accommodate almost any shape workpiece. Their only limitatio ns are the size of the part and the availability of clamping surfaces. One other important point to consider is the tool thrust. The part is clamped between the jig plate and the clamping device. Therefore, the tool thrust is directed toward the clamps rather than the solid parts of the jig. Clamping devices must be selected to resist this thrust.
• 133. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A5 – JIGS JIGS, FIXTURES & GAUGES 145 e) Sandwich Jigs These are plate jigs wit h a back plate. These types of jigs are ideal for thin of soft parts which could bend or wrap in another type of jigs.
• 134. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A5 – JIGS JIGS, FIXTURES & GAUGES 146 f) Box or tumble Jigs Usually they surround the total part. This type of jig allows the part to be completely machined on every surface without repositioning the work in the jig. g) Channel Jig It is the simplest form of box jig. The work is held between two sides and machined from the third. In some cases, where jig feet are used the work can be machined on three sides. h) Plate Jigs
• 135. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A5 – JIGS JIGS, FIXTURES & GAUGES 147 These are similar to templates but will have built in clamps to hold the work. These jigs can also be made with or without bushings, depending on the number of parts to be made. Plate jigs are sometimes made with legs to raise the jig off the table for large work and are called table jigs. i) Trunning Jigs These are the forms of rotary jigs used for very large or odd shaped parts. j) Multistation Jigs
• 136. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A5 – JIGS JIGS, FIXTURES & GAUGES 148 These are provided with simultaneous location and machining of several parts. While one part is drilled another can be reamed, and yet another can be counter bored. The final station can be used for unloading the finished part and loading the fresh parts. This jig commonly used on multispindle machines. k) Indexing Jigs These are used to space the holes or other machined areas accurately around a part. To accomplish this jigs need to use either the part it self or a reference plate and a plunger. Rotary jigs are the larger versions of indexing jigs.
• 137. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A5 – JIGS JIGS, FIXTURES & GAUGES 149 l) Universal or Pump Jigs These are commercially made jigs for very fast loading and unloading. They are produced as basic units and are adopted to specific jobs. The salient features of these jigs are: i) Rigidity
• 138. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A5 – JIGS JIGS, FIXTURES & GAUGES 150 ii) Low height iii) Ample chip clearance iv) Ease of operation v) One jig can be used for different workpieces by changing or removing the top plate. The moving parts are compl etely protected from chips. The working parts consist of a handle connected to a cam or rack which moves either a bushing plate or a nest vertically in order to clamp a work piece. If the top plate is removable with a handle they are called pump jigs. m) Boring jigs Boring jigs must be fastened to the bed or table of the machine. This req uirement makes necessary a very rigidity designed body which will prevent springing. Often a combination of a drilling and boring jig is required. The design of boring jigs is similar in many respects to that of drill jigs. Bored holes are often reamed as a final operation, the reamers being guided by slip bushings which are inserted in the jig following the boring operations. Both the boring bar and the reamer may be piloted by two bushings, one at each end of the hole. This is known as front and back piloting. It minimizes the springing of the cutting tool and assures alignment and greater accuracy.
• 139. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A5 – JIGS JIGS, FIXTURES & GAUGES 151 A5.7 Types of jig and their description 1. Template Jig A plate having holes at the desired positions serves as a template which is fixed on the component to be drilled 2. Plate Jig or Table Jig Is an improvement of the template Jig by incorportiporting drill bushes on the template 2a. Modified Angle plate Jig A Jig for drilling holes at an angle 3. Channel Jig Is a simple type of Jig having a channel like cross – section. 4. Diameter Jig Is used to drill radial holes on a cylindrical or spherical workpieces 5. Leaf Jig The leaf Jig has a leaf or a plate hinged on the jig body 6. Ring Jig The ring Jig is employed to drill holes on circular flanged parts 7. Box Jig The box jig is of box like construction within which the component is located by the buttons. 8. The vise as a drill Jig By providing attachments for holding drill bushings, the machine vises may be used as drill jigs 9. Indexing drill Jigs Indexing jigs are used for circular hole patterns in which the part is indexed sucessively to the different positions under a single bushing
• 140. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A5 – JIGS JIGS, FIXTURES & GAUGES 152 A5.8 Maintenance, Safety and Storage of Jig Provision for Maintenance Has provision been made for lubricating the tool mechanisms? Have all wearing parts been hardened? Are these parts easily made and replaced? Have correct materials and heat treatment been specified? Has provision been made for easy removal or pressed in parts? Can vulnerable parts be removed and replaced quickly without disturbing the set up of the fixture on the machine? Manufacture and Maintenance Cost Is the cost of the tool been properly related to the quantity and accuracy of the part to be produced? Is it too expensive for low volume production? Are the production requirements high enough to warrant a better class of tool? Standards Have standard, or readily purchasable, parts been specified wherever practicable? Have all parts been designed for manufacture from stock size materials with a minimum amount of machining? If the material is not carried in stock, is the right kind and size readily available? Manufacturing Facilities Can the tool be made with the available tool making labor and equipment? Are the tool dimensional tolerance as wide as possible? Has the design included suitable datum surfaces for tool making operations? Is it easy to set up the fixture for grinding locators or supports which have to be sized in assembly? If so, there plenty of clearance for the grinding wheel and spindle? Has provision been made for easy alignment and starting of pressed in parts which need accurate location? Have blind holes been avoided wherever possible? Would it help the heat treater to drill a small hole in each part, which normally has no holes in it, so that it may be suspended it in a salt bath by a wire threaded through the hole?
• 141. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A5 – JIGS JIGS, FIXTURES & GAUGES 153 Safety 1. Does the fixture design protect the operator from coolant spray or flying chips? 2. Is the designed tool safe to operate with? Handling and Storage Lifting Aids Have lifting lugs, eyebolts, or chain slots been provided for slinging heavy tools? Have lifting handles been attached to all awkward or heavy loose parts of the fixture? Loose Parts If loose parts such as spacing pieces, wrenches, or locating pins are unavoidable, can they be attached to the fixture with keeper screws or light chains to prevent loss in storage? Fragile Parts Is there any fragile part of the jig which needs a protective cover in storage? Is the tool so delicate or highly finished as to require a special case, cover, or box to protect it in storage? Identification Has the tool, and all loose items belonging to it, been marked clearly with identification numbers or symbols? Storage Aids Can the tool be stowed safety without danger of tipping over? Is a special storage stand or rack desirable for safe and convenient storage?
• 142. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A5 – JIGS JIGS, FIXTURES & GAUGES 154 SUMMARY: The use of jigs is extending and developing very fast. The quality, type and complexity of jigs used depend upon the type of job and its method of production. Jigs are used for drilling, reaming, tapping, counter boring operations etc., and are broadly divided into two types names, boring and drilling jigs. Drilling jigs are further divided into open and closed jigs. QUESTIONS: 1. How are the jigs normally classified? 2. What is an open jig? 3. What are the box jigs used for? 4. What class of jig would normally be used to tap holes? 5. Suggest a jig to be used for holding a part for machining angle other than 90 degrees. 6. What are the principles to be followed in designing of jigs? 7. Explain design procedure for jig. 8. How will you classify jigs? 9. Explain any one jig with figure. 10. What are the purposes of using bushes in jigs? 11. What are the rules for selecting clamp of work piece in jigs? 12. Why should a jig have four feet and not three? Explain the reason. 13. What are the main types of jigs? Discuss three with the help of suitable sketches. 14. Sketch the various types of jig feet. 15. What are the checks to be made for jigs for (a) provisionfor maintenance, (b) manufacturing & maintenance cost, (c) handling, (d) loading & unloading, (e) storage, (f) human factors
• 143. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A4 – LIMITS, FITS & TOLERANCES JIGS, FIXTURES & GAUGES 96 A4.1 Introduction In nature two extremely similar (identical) things are difficult to obtain. If at all we came across exactly similar things, it must be only by chanc e. This fact holds good for production of component parts in engineering also. No production process is good enough to produce all items of products exactly alike. Every production process involves a combination of three elements viz, men machines and materials. Each of these elements has some inherent or natural variation as well as some unnatural variations. The natural variations are due to chance causes, which are difficult to trace and control. The unnatural variations are due to assignable causewhich can be easily traced, controlled and reduced to economic minimum. These variables result in the variation of size of components. CHAPTER OUTLINE A4.1 Introduction A4.2 Advantages of Limit & fit A4.3 Tolerance A4.4 Limits A4.5 Fits A4.6 Types & assembly A4.7 Allowance A4.8 Deviation A4.9 Minimum & maximum metal conditions TOPIC OUTLINE A4.3a Tolerance & parts A4.3b Tolerance zone A4.3c Grades of Tolerance A4.3d Unilateral tolerance A4.3e Bilateral tolerance A4.4a Limit of size A4.4b Maximum limit of size A4.4c Minimum limit of size A4.5a Types of fits A4.6a Trial & error assembly A4.6b Interchangeable assembly A4.6c Selective assembly A4.7a Difference between tolerance & allowance A4.8a Actual deviation A4.8b Upper deviation A4.8c Lower deviation A4.8d Fundamental deviation
• 144. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A4 – LIMITS, FITS & TOLERANCES JIGS, FIXTURES & GAUGES 97 For example, suppose a drilling operation is to be performed on castings. The first source of variation is the material itself (some castings may be harder than the others, some of them may have blow holes, cracks etc.). If the operations are done on a mass production by number of workers on different machines, the second source of variation is the machine. The condition of machines may differ. The third source of variation, man, is the most variable of them all. There may be differences in skill, experience of the workers doing the same job. The same person may act in different ways in different psychological conditions and adds to variability in the quality characteristics of the product. If the process is under control, i.e., all the assignable causes of variation are controlled or eliminated, the variations in sizes of similar components will be within reasonable limits. Generally, in engineering the article manufactured consists of assembly of number of components. Thus a component manufactured is required to fit or match with some other mating component. The correct and prolonged functioning of most manufacture d articles depends on the correct size relationships between the various components of the assembly. This means that the component parts must fit in a certain desired way, e.g., if a shaft is to rotate in a hole, there must be enough clearance between theshaft and the hole to allow an oil film to be maintained, for lubrication. Similarly, if the shaft is to be held tightly in the hole there must be enough degree of tightness (interference) between them to ensure that the forces of elastic compression gri p them tightly and do not allow any relative movement between them. However, interference must not be excessive, as it may result in splitting of component containing the hole. Ideally any such condition could be obtained by specifying a definite size for the hole and for the shaft, but unfortunately this is not possible due to inevitable inaccuracy of manufacturing methods. A4.2Advantages of Limits and Fits 1. It is not possible to make any part precisely to a given dimension, due to variability of elements of production processes. 2. Even if by chance the part is made exactly to a given dimension, it is impossible to measure it accurately enough to prove it. 3. If attempts are made to achieve perfect size the cost of production will increase tremendously.
• 145. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A4 – LIMITS, FITS & TOLERANCES JIGS, FIXTURES & GAUGES 98 4. The components will assemble together at random. During assembly, individual fitting is not necessary. This result in reduction in cost of production because the elimination of fitting reduces the time required to build the product. 5. Components can be manufacture in large batches or lots and all treated alike. 6. Machine tools which have been developed for quantity production enable the components to be manufactured more rapidly using cheaper labour. 7. Repair of existing machines or products i s simplified because component parts can be easily replaced. A4.3 Tolerance Tolerance on a dimension is the difference between the high and low limits of size. It is got to be allowed in order to cover the reasonable imperfection in workmanship and the inevitable inaccuracy of manufacturing processes, and varies with different grades of work. It can be unilateral or bilateral. Fig. illustrates the concept of limits of size and tolerances. A4.3a Tolerance of Parts Due to the inevitable inaccuracy of manufacturing methods, it is not possible to make a part precisely to a given dimension and may only be made to lie between two limits, maximum and minimum, the difference between which is the permissible tolerance. For the sake of convenience a basic size is a ascribed to the part and each of the two limits is defined by its deviation from the size (the magnitude and sign of the deviation is obtained by subtracting the basic size from the limit in question). Tolerance is equal to the algebraic difference between the upper and lower deviations and has an absolute value without sign. In the context of this terminology for limits and fits, the difference between the maximum limit of size and minimum limit of size is called the tolerance.
• 146. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A4 – LIMITS, FITS & TOLERANCES JIGS, FIXTURES & GAUGES 99 A4.3b Tolerance Zone In a graphical representation of tolerance, the zone bounded by the two limits of size of the part is called the tolerance zone. It is definedby its magnitude (i.e. tolerance) and by its position in relation to the zero line. A4.3c Grades of tolerance In a standardised system of limits and fits, group of tolerance are considered as corresponding to the same level of accuracy for all basic sizes. It designated by the letters IT followed by a number, e.g., IT01…01 16. A4.3d Unilateral Tolerance In this system, the dimension of a part is allowed to vary only on one side of the basis size i.e., tolerance lies wholly on one side of the basic size either above or below it. Unilateral system is preferred in interchangeable manufacture, especially when precision fits are required, because: (i) It is easy and simpler to determine deviations. (ii) Another advantage of this system is that Go gauge ends can be standardized as the holes of different tolerance grades have the same lower limit and all the shafts have same upper limit. (iii) This form of tolerance greatly assists the operator, when machining of mating parts. The operator machines to the upper limit of shaft (lower limit for hole)
• 147. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A4 – LIMITS, FITS & TOLERANCES JIGS, FIXTURES & GAUGES 100 knowing fully well that he still has some margin left for machining before the parts are rejected. A4.3e Bilateral Tolerance In this system, the dimension of the part is allowed to vary on both the sides of the basic size i.e., the limits of tolerance lie on either side of the basic size; but may not be necessarily equally disposed about it. In this system it is not possible to retain the same fit when tolerance is varied ad the basic size of one or both of the mating parts is to be varied. This system is used in mass production where machine setting is done for the basic size. A4.4 Limits These are two extreme permissible sizes for any dimension (high and low). A4.4a Limits of Size The two extreme permissible sizes between which the actual size is contained. A4.4b Maximum Limit of Size The greater of the two is called the maximum limit. A4.4c Minimum Limit of Size The smaller one of the two limits of size is called the minimum limit.
• 148. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A4 – LIMITS, FITS & TOLERANCES JIGS, FIXTURES & GAUGES 101 A4.5 Fits When two parts are to be assembled, the relation resulting from the difference between their sizes before assembly is called a fit. Depending upon the actual limits of hole or shaft, the fit may be clearance fit, or a transition fit, or a n interference fit. Tolerance is considered on one side only. Actually it is representative of total tolerance. It is done for sake of clarity and simplification. In schematic representation of tolerances, no regard is given to show shafts and holes fully, but only their magnitude and relative position w.r.t. basic size are highlighted. The relationship existing between two parts, shaft and hole, which are to be assembled, with respect to the difference in their sizes before assembly is called fit. A4.5a Types of Fits (Classification of fits) On the basis of positive, zero and negative values of Clearance, there are three basic types of fits : i) Clearance Fit ii) Transition Fit iii) Interference Fit Fits Clearance Fit Transition Fit Interference Fit 1) Slide fit 2) Easy slide fit 3) Running fit 4) Slack running fit 5) Loose running fit 1) Push fit 2) Wringing fit 1) Force fit 2) Tight fit 3) Shrink fit
• 149. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A4 – LIMITS, FITS & TOLERANCES JIGS, FIXTURES & GAUGES 102 i) Clearance Fit In this type of fit, the largest permitted shaft diameter is smaller than the diameter of the smallest hole, so that the shaft can rotate or slide though with different degrees of freedom according to the purpose of the mating members. In this type of fit shaft is always smaller than the hole i.e., the largest permissible shaft diameter is smaller than the diameter of the smallest hole. So that the shaft can rotate or slide through with different degrees of freedom according to the purpose of mating part. Clearance fit exists when the shaft and the hole are at their maximum metal conditions. The tolerance zone of the hole is above that of the shaft as shown in figure. Clearance This is difference between the size of the hole and shaft, before assembly, when this difference is positive. Maximum clearance In the case of clearance or transition fit, it refers to the difference between the maximum size of hole and the minimum size of shaft. Minimum clearance In a clearance fit, it refers to the difference between the minimum size of hole and the maximum size of shaft. 1) Slide fit This type of fit has a very small clearance, the minimum clearance being zero. Sliding fits are employed when the mating parts are required to move slowly in relation to each other
• 150. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A4 – LIMITS, FITS & TOLERANCES JIGS, FIXTURES & GAUGES 103 e.g., tailstock spindle of lathe, feed movement of the spindle quill in a drilling machine, sliding change gears in quick change gear box of a centre lathe etc. 2) Easy Slide Fit This type of fit provides for a small guaranteed clearance. It serves to ensure alignment between the shaft and hole. It is applicable for slow and non regular motion, for example, spindle of lathe and dividing heads, piston and slide valves, spigots etc. 3) Running fit Running fit is obtained when there is an appreciable clearance between the mating parts. The clearance provides a sufficient space for a lubrication fi lm between mating friction surfaces. It is employed for rotation at moderate speed, e.g., gear box bearings, shaft pulleys, crank shafts in their bearings etc. 4) Slack running fit It is obtained when there is a considerable clearance between the mating parts. This type of fit may be required as compensation for mounting errors e.g., arm shaft of I.C. engine, shaft of certifigual pump etc. 5) Loose running fit Loose running fit is employed for rotation at very high speed, e.g., idle pulley on their shaft such as that used in quick return mechanism of a planer. ii) Transition Fit In a fit of this type the diameter of the largest allowable hole is greater than that of the smallest shaft, but the smallest hole is smaller than the largest shaft, so that small positive or negative clearance between the shaft and hole members are employable. Location fits e.g., spigot in mating holes, coupling rings and recesses are the examples of transition fit. Transition fit lies mid way between clearance and interference fit. In this type the size limits of mating parts (shaft and hole) are so selected that either clearance or indifference may occur depending upon the actual sizes of the parts. Push fit and wringing fit are the examples of this type of fit.
• 151. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A4 – LIMITS, FITS & TOLERANCES JIGS, FIXTURES & GAUGES 104 In this type of fit the tolerance zones of the hole and shaft overlap completely or in part. iii) Interference Fit In this type of fit, the minimum permitted diameter of the shaft is larger than the maximum allowable diameter of the hole. In this case the shaft and the hole members are intended to be attached permanently and used as a solid component but according to the application of this combinations, this type of fit can be varied. Thus, if in use one of the two members is subjected to wear, it should be possible to drive or force the two members apart for replacement purposes. Examples of this type of fit are be aring bushes which are in an interference fit in their housing e.g. a small end in the connecting rod of an engine. In this type of fit the minimum permissible diameter of the shaft is larger than the maximum allowable diameter of the hole. Thus the shaf t and the hole members are intended to be attached permanently and used as a solid component. Elastic strains developed on the mating surfaces during the process of assembly prevent relative movement of the mating parts. For example, steel tyres on railway car wheels,
• 152. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A4 – LIMITS, FITS & TOLERANCES JIGS, FIXTURES & GAUGES 105 gears on intermediate shafts of trucks, bearing in the gear of a lathe head stock, drill bush in jig plate, cylinder linear in block, steel rings on a wooden bullockcart wheels etc. Interference This is the arithmetical difference between the sizes of the hole and shaft before assembly, when the difference is negative. Minimum Interference It is the difference between the maximum size of hole and the minimum size of shaft in an interference fit prior to assembly. Maximum Interference In an interference or a transition fit it is the difference between the minimum size of hole and the maximum size of shaft prior to assembly. Basic size of a Fit It is that basic size which is common to the two parts of a fit. Variation of Fit This is the arithmetical sum of the tolerance of the two mating parts of fit. 1) Force fit Force fit are employed when the mating parts are not required to be disassembledduring their total service life. In this case the interference is quite appreciable and, therefore, assembly is obtained only when high pressure is applied. This fit, thus, offers a permanent type of assembly, e.g., gears on the shaft of a concrete mixture, forging machine etc. 2) Tight fit It provides less interference than force fit. Tight fits are employed for mating parts that may be replaced while overhauling of the machine, for example, stepped pulleys on the drive shaft of a conveyor, cylindrical grinding machine etc. 3) Heavy force and Shrink fit
• 153. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A4 – LIMITS, FITS & TOLERANCES JIGS, FIXTURES & GAUGES 106 It refers to maximum negative allowance. Hence considerable force is necessary for the assembly. The fitting of the frame on the rim can also be obtained first by heating the frame and then rapidly cooling it in its position. Hole basis system In this system, the hole is kept constant and the shaft diameter is varied to give the various types of fits. The basic size of the hole is taken as the low limit of size of the hole. The high limit of size of the hole and the two limits of size for the shaft are then selected to give the desired fit. It is clear, therefore, that in this system, the actual size of a hole that is w ithin the tolerance limits is always more than the basic size; it can equal the basic size as a particular case but can never be less. In the ‘Basic Hole System’, the holes get the letter ‘H’ and shafts get different letters to decide the position of tolerance zone to obtain a desired fit. Shaft basis system: Here, the shaft is kept constant and the size of hole is varied to give the various fits. The basic size of the shaft is taken as one of the limits of size (maximum limits) for the shaft. The other shaft limit of size and the two limits of size for the hole are then selected to give the desired fit.
• 154. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A4 – LIMITS, FITS & TOLERANCES JIGS, FIXTURES & GAUGES 107 It is clear, therefore, that in this system, the actual size of a shaft that is within the tolerance limits is always less than the basic size. As a particular case, it can equal the basic size but can never be larger. In the ‘Basic Shaft System’, the shaft gets the letter ‘h’ and holes get different letters to decide the position of tolerance zone to obtain a desired fit. From a manufacturing point of view, it is preferable to use the “hole basis” system, because it is economical. This is because a great many holes are produced by standard fixes size tools, such as, twist drills, reamers, core drills, tapes, broaches, etc. The advantages of using fixed size tools is that the machine need not be set up to obtain the proper size of the hole, setting up operations can consequently be made quicker and cheaper. Subsequently, the shaft sizes are more readily variable about the nominal size by means of turning or grinding operation. It is easier and more convenient to manufacture shafts of varying sizes than holes of varying sizes, as given above. The hole basis system is preferred, because it lessons the range of cutting and measuring tools for machining of holes, which are more expensive that tools to machine shafts. Also, the control of the size and shape of holes is more complicated and less accurate than the control of shafts. Applications : Machine and engine building, locomotive, construction. The shaft basis system is more advantageous in certain cases, for example, this system can be efficiently applied for long shafts machined to the same size over their full lengths (smooth drawn shafts, shafts ground on centreless grinding machines etc.), if the shaft is to mate with at least two parts having holes that require different types of fit. Examples of “shaft basis” system are : the mating of a piston pin with both the piston and the connecting rod, and the outer rings of antifriction bearings with various bores in housings, electric motors, power transmission and products made from bright drawn bars. It has been found in practice that a number of different fits of each basic type of fit are required which can provide different degrees of tightness or freedom between the mating parts.
• 155. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A4 – LIMITS, FITS & TOLERANCES JIGS, FIXTURES & GAUGES 108 The most commonly used fits of clearance type are : (1) Slide fit, (2) Easy slide, (3) running fit, (4) slack running fit and (5) loose running fit. 1) Wringing fit A wringing fit provides either zero interference or a clearance. These are used where parts can be replaced without difficulty during minor repairs. 2) Push fit The fit provides small clearance. It is employed for parts th at must be dis -assembled during operation of a machine for example, change gears, slip bushing etc. A4.6 Types of Assemblies There are three ways by which the mating parts can be made to fit together in the desired manner. These are: 4.6a Trial and Error 4.6b Interchangeable Assembly 4.6c Selective Assembly A4.6a Trial and Error When a small number of similar assemblies are to be made by the same operator the necessary fit can be obtained by trial and error. This technique simply requires one part to be made to its nominal size as accurately as possible, the other part is then machined with a small amount at a time by trial and error until they fit in the required manner. This method may be used for “one off jobs”, tool room worketc. where both parts will be replaced at once. A4.6b Interchangeable Assembly When a large number of components are to be produced then it will not be economical to produce both the mating components by the same operator. In addition to economy it is also essential to produce the components within the minimum possible time. This is only possible by mass production system. In mass production system there is a division of labour. The components are produced in one or more batches by different operators on different machines. Under such conditions in order to assemble the mating components with
• 156. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A4 – LIMITS, FITS & TOLERANCES JIGS, FIXTURES & GAUGES 109 a desired fit, a strict control is exercised and the parts are manufactured with specified tolerance limits. When a system of this kind is used any one component selected at random will assemble correctly with any other mating component that too, selected at random, the system is called interchangeable assembly. The manufacture of components under such conditions is called interchangeable manufacture. Production on an interchangeable basis results in increased output with a corresponding reduction in manufacturing cost. Example : Suppose a clearance fit is required between the mating parts with hole, specified as mm00.0 04.0 25- + And shaft mm04.0 02.0 25- - In this case the maximum permissible size of the hole will be = 25.04mm and the minimum permissible size = 25.00mm. The dimensions of the number of holes produced will lie between these two limits. Similarly, the maximum permissible shaft size = 24.94 mm and the minimum permissible size of shaft = 24.96. The dimensions of all the shafts produced will lie between these two limits. Therefore, even if we select any hole at random and similarly any shaft at random with these permissible tolerances they will assemble with each other and give the desired clearance fit. Interchangeable assembly requires precise machines or processes whose process capability is equal to or less than the manufacturing toleran ce allowed for that part. Only then every component produced will be within desired tolerance and capable of mating with any other mating component to give the required fit. ¨ Advantages of Interchangeability (a) The operator is not required to waste his skill in fitting the mating components by trial and error and thus assembly time is reduced considerably. (b) There is an increased output with reduced production cost. (c) There is a division of labour, the operator has to perform same limited operations again and again thus he becomes specialized in that particular work, which helps to improve quality and reduce the time for operations. (d) It facilitates production of mati ng components at different places, by different operators. (e) The replacement of worn out or defective parts and repair becomes very easy. (f) The cost of maintenance and shutdown period is also reduced to minimum.
• 157. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A4 – LIMITS, FITS & TOLERANCES JIGS, FIXTURES & GAUGES 110 A4.6c Selective Assembly It is sometimes found that it is not economical to manufacture parts to the required high degree of accuracy so as to make them interchangeable. The consumer not only wants quality and precision trouble free products but also he wants them at economical prices. Often special cases of accuracy and uniformity arise which might not be satisfied by certainty at the fits given under fully interchangeable system. For example, if a part of its low limit is assembled with the mating part at high limit, the fit so obtained may not fully satisfy the functional requirements of the assembly. Complete interchangeability in the above cases can be obtained at some extra cost in inspection and material handling by using selective assembly whereby parts are manufactured to rather wider tolerances. In selective assembly the components produced are classified into groups according to their sizes by automatic gauging. This is done for both mating parts, holes and shafts, and only matched groups of mating parts are assembled. It results in complete protection against defective assemblies and reduces matching costs since the parts may be produced with wider tolerances. A practical example of this system is the assembly of pistons with cylinder bores. Let the bore size be 50 mm and the cle arance required for the assembly is 0.12 mm on the diameter. Let the tolerance on bore and the piston each = 0.04mm. Then, Dimension of bore diameter is 50±0.02 mm. Dimension of piston shirt is 49.44 ±0.02 mm By grading and marking the bores and the pistons they may be selectively assembled to give the clearance of 0.12 mm as given below: Cylinder bore 49.94 50.00 50.02 Piston 49.49 49.44 49.90
• 158. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A4 – LIMITS, FITS & TOLERANCES JIGS, FIXTURES & GAUGES 111 A4.7 Allowance An international difference between the hole dimension and shaft dimension for any type of fit is called the allowance. Maximum allowance is obtained by subtracting the minimum shaft size from the largest hole size and the minimum allowance is the differe nce between the largest shaft and the smallest hole size. Thus allowance is positive for clearance fit and negative for interference fit. Allowance : The difference between the maximum shaft size and minimum hole is known as allowance. In a clearance fit, this is the minimum clearance and is positive allowance. In an interference fit, it is the maximum interference and is negative allowance. Positive and negative allowances are shown in Figures respectively. A4.7a Difference between tolerance & allowance Tolerance Allowance 1. It is the permissible variation in It is the prescribed difference between the
• 159. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A4 – LIMITS, FITS & TOLERANCES JIGS, FIXTURES & GAUGES 112 dimension of a part (either a hole or a shaft). dimensions of two mating parts (hole and shaft). 2. It is the difference between higher and lower limits of a dimension of a part. It is the intentional difference between the lower limit of hole and higher limit of shaft. 3. The tolerance is provided on a dimension of a part as it is not possible to make a part to exact specified dimension. Allowance is to be provided on the dimension of mating parts to obtain desired type of fit. 4. It has absolute value without sign. Allowance may be positive (clearance) or negative (interference). A4.8 Deviations The algebraic difference between a size (actual, maximum etc.) and the corresponding basic size. 4.8a Actual Deviation The algebraic difference between the actual size and the corresponding basic size. 4.8b Upper Deviation The algebraic difference between the maximum of size and the corresponding basic size. It is a positive quantity when the maximum limit of size is greater than the basic size and a negative quantity when the maximum limit of size is less than the basic size. It is designated by ES for a hole and es for a shaft. 4.8c Lower Deviation
• 160. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A4 – LIMITS, FITS & TOLERANCES JIGS, FIXTURES & GAUGES 113 The algebraic difference between the minimum limit of size and the corresponding basic size. It is a positive quantity when the minimum limit of size is greater than the basic size and a negative quantity when the minimum limit of size is less than the basic size. It is designated by EI for a hole el for a shaft. Deviation is defined as the algebraic difference between a size (act ual, maximum etc.) and the corresponding basic size. Upper deviation is the algebraic difference between the maximum limit of size (of either hole or shaft) and the corresponding basic size. It is designated by letters ES for hole and es for shaft. It is a positive quantity when the maximum limit of size is greater than the basic size and a negative quantity when the maximum limit of size is less than the basic size. Lower deviation is the algebraic difference between maximum limit of size and the corresponding basic size. It is a positive quantity when the minimum limit of size is greater than the basic size and a negative quantity when the minimum limit of size is less than the basic size. It is designated by EI for a hole and ei for a shaft. 4.8d Fundamental Deviation It is that one of the two deviations which is conventionally chosen to define the position of the tolerance zone in relation to the zero line. Fundamental deviation is that one of the two deviations which is conventionally chosen to define the position of the tolerance zone in relation to the zero line. This may be upper or lower deviation which is closet to the zero line. It fixes the position of zero line. Basic shaft is a shaft whose upper deviation is zero, e.g., shaft ‘h’. Basic hole is one whose lower deviation is zero, e.g., hole ‘H’. For shafts ‘a’ to ‘h’, the deviation is below the zero line and for shafts ‘j’ to ‘zc’ it is above the zero line. For holes ‘A’ to ‘G’, lower deviation is above the zero line and for ‘j’ to ‘zc’, it is below the zero line. Upper deviation for shaft is denoted by es and lower deviation by ei. For holes the corresponding deviations are denoted by ES and EI repectively. In the specifications, formulae are given to determine the fundamental deviation. For example, for shafts, the fundamental deviation (upper deviation es or lower deviation ei) is determined by
• 161. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A4 – LIMITS, FITS & TOLERANCES JIGS, FIXTURES & GAUGES 114 means of the formulae given in the table 4.2 o n page 343. The other deviations may be derived directly using the absolute value of the tolerance IT by means of the algebraic relationship. ei = es – IT es = ei + IT The deviation given in the table is that corresponding in principle to the limitclose to the zero line, in other words the upper deviation es for shafts a to h and lower deviation ei for shafts j to zc. For holes also the deviations are derived from those of the corresponding shafts as follows. The general rule is that hole limits are identical with the shaft limits of the same symbol (letter and grade) but disposed on the other side of the zero line, i.e., EI upper deviation of hole)=es of the shaft of same letter symbol but of opposite sign. Hole Basis System and Shaft Basis Sys tem are defined in Figure for clearance fit, transition fit and interference fit. In hole basis system different clearance and interferences are obtained by associating various shafts with a single hole whose lower deviation is zero (H hole). This is standard practice also as it is very convenient to make correct holes of fixed sizes. Since holes are produced by drilling, reaming, etc. their size is not easily adjustable because size of such tools are standard. On the other hand, size of shaft produced by turning and grinding can be easily varied. In shaft basis system upper deviation of shaft is zero, and different fits are obtained by varying the limits on the holes.
• 162. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A4 – LIMITS, FITS & TOLERANCES JIGS, FIXTURES & GAUGES 115 For any basic size there are 25 different holes. These are obtained by providing a series of holes which are progressively oversize and a series of holes which are progressively undersize. The difference from basic size of the various holes is given by the fundamental deviation and it is these differences in size which give the fit required. The 25 holes are designated by capital letters: A, B, C, D, E, F, G, H, JS, J, K, M N, P, R, S, T, U, V, X, Y, Z, ZA, ZB, ZC. Each of the 25 holes has a choice of 14 tolerances which are designated : IT 01, IT0, IT 1, IT 2 up to and including IT 16. The tolerance grade decides the accuracy of manufacture. The seven finest grades (IT 01 to IT 05) cover sizes up to 500 mm and the eleven coarsest grades up to mm. The tolerance in each grade depends on the size of shaft/hole. Similarly for shafts, for any given size there are 25 different shafts designated by small letters from a to zc. Also each shaft has 14 grades of tolerance grades which are designated as for the holes. The setting of tolerance values is not by itself sufficient to define particular limit, the position of the tolerance zone relative to the basic size of the feature must also be specified. This is done by establishing fundamental deviations which are differences between the basic size and the nearest limit of tolerance. These fundamental deviations are obtained from empirical formulae given in Table. These are designated by capital letters for holes and small letters for shafts. The fundamental deviations for holes A to H correspond exactly in value with those for shafts a to h but are in opposite direction. Hole A and shaft a have the largest fundamental deviations, hole being positive and shaft being negative, and the fundamental deviations for both Hand h are zero. Thus the first eight designations represent a clearance fit system. The remaining groups JS to ZC (holes) and js and zc (shafts) do not correspond in their deviations in quite the same way, they are intended for use in interference and transition fits. The above facts are valid irrespective of any basic size. The values of the fundamental deviations are functions not of the basic size but of the range of sizes in which the basic size falls. Maximum and Minimum Metal Conditions Maximum m etal condition (MMC) corresponds to condition when a part has maximum amount of metal, i.e. corresponding to high tolerance of shaft and low tolerance of the hole. Similarly minimum metal condition corresponds to minimum size of shaft and
• 163. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A4 – LIMITS, FITS & TOLERANCES JIGS, FIXTURES & GAUGES 116 maximum size of hole. MMC has special importance with regard to geometrical tolerancing as it critically affects the interchangeability of manufactured parts, which are to be assembled together. SUMMARY: With all the advancement in the machine tool technology, it is not possible to achieve dimensional perfection due to the following reasons : Temperature changes, tool wear, deflections and vibrations of the machine and the work and human error. Even if the dimension is to be maintained within a very close degree of accuracy, lot of time will be consumed resulting in increased cost of manufacture. In mass production where the work has to be done in a set competitive time, greater variations will result. This fact is recognized and certain variations are allowed in the size of the machine elements or parts. This system of manufacture in which the dimensions of a part lie within some specified limits leads to “interchangeable manufacture”. Interchangea ble part manufacture is a major feature of modern serial and mass production. The parts which go in to assembly have been produced with all their dimensions within their specified limits, need not be made in the same shop or even in the same company. This system of interchangeable part manufacture, that is, the system of limits, fits & tolerance results in the lot of advantages. QUESTIONS: 1. Define the following terms: (i) Limits (ii) fit (iii) Tolerance
• 164. INDO-GERMAN TOOL ROOM, AHMEDABAD TG2CHAPTER : A4 – LIMITS, FITS & TOLERANCES JIGS, FIXTURES & GAUGES 117 2. Why it is necessary to give tolerance on engineering dimension? Give an example of both a unilateral and bilateral tolerance. 3. Explain the unilateral and bilateral systems of writing tolerances with suitable examples. Which system is preferred in interchangeable manufacture? Why? 4. Draw the conventional diagram of limits and fits and explain the terms: (i) Basic size (ii) Upper deviation (ii) Lower deviation (iii) Fundamental deviation (v) Zero line. 5. With the help of neat sketches state the essential conditions for (i) Clearance fit (ii) Interference fit 6. Define fits. Describe the various types of fits in brief. 7. Explain clearly the following type of fits and how they can be achieved: (a) Push fit (b) Wringing fit (c) Force fit (d) Sharing fit 8. Define the terms (i) Allowance (ii) Limits (ii) Tolerance (iii) Fit. 9. Differentiate between Tolerance and Allowance. 10. Describe briefly the systems of obtaining different types of fits, with suitable sketches. 11. Differentiate between ‘Hole basis system’ and ‘Shaft basis system’ of fits. 12. Describe briefly the principal features of the Indian Standard System of limits and fits (IS – 91 g and ). 13. Explain briefly the difference between the Interchangeable manufacture and selectiv e assembly. 14.Study the given drawing. The dimensions stated show the HIGH and the LOW LIMIT of the detail. Which of the following figures is the value of TOLERANCE for the detail? 35.2 35.4