Aug. 04, 2025
As long as screws are still present in machined parts, threads remain an important aspect of metalwork. It facilitates the connection of parts at a low cost, making them ideal for use with fasteners, fittings, and connectors.
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Asides from knowing what threads are, it is also important to identify the right type of thread for your project. So, what exactly does the term machining threads mean? what are the types of machining threads? Keep reading as we answer these questions and provide other important information about threading.
Helix Angle: Helix angle in machining is the angle between the thread axis and the helix in a straight thread or the angle made by the conical helical area of a thread for taper threads.
Root: This is the bottom of the threads. It is the adjacent area of the projected cone or cylinder.
Crest: The crest is the top of the threads. It is the farthest area of the projected cone or cylinder. The crest is also the thread surface that joins the flanks.
Flank: This threaded part connects the root to the crest. In theory, the intersection of a flank surface with an axial plane is a parallel line.
Pitch: The diameter of the pitch equals that of the cylinder or cone, in theory. The pitch is the distance measured between areas on adjacent threads. The parallel areas measured must correspond.
There are many types of threads according to different classification standards. For the Unified thread system, the UNF (fine) thread and UNC (coarse) thread are standard. However, the two major types of threads are internal and external threads.
An internal thread, also known as a female thread, undergoes machining using a single-lip threading tool. Aside from this single-lip tool, a traditional threading cap could also help cut CNC internal threading. It is important to note that cutting internal threads occur only on concave surfaces.
So, when is an internal thread needed? You would require one if your workpiece needed screw insertions. Machinists use either hand taps or machine taps for cutting internal threads.
Also known as a screw thread, this type of thread finds application in screws, bolts, studs, and plug gages. Using a lathe is one of the most effective ways of making external threads.
Another method is to use a round die to cut the external thread by hand. The round die used in this case remains in a fixed die stock. Aside from round dies, there are also hexagonal square dies, ideal for interlocking use.
Thread cutting is ideal for making parts with screwed connections. Knowing how to cut threads is beneficial. For instance, with this knowledge, you can manufacture a single-piece screw or nut or repair them if necessary. Also, threading machines may not always be available.
So, here is how to cut threads.
Are you looking to cut an internal thread? First, get the right tools, which include safety goggles, a box column drill (for machine taps), an adjustable tap wrench (for hand taps), an internal tap, a twist drill, and a 90-degree countersink.
After getting the right tools, the next step is determining what diameter of the hole you want to place the thread in. With the diameter determined, knowing the right tap for cutting is the next stage. To determine the core hole diameter, subtract the thread pitch from the Tap diameter.
To start cutting the internal thread, you first center with a punch and then drill a core hole using a twist drill. To make a chamfer into the core hole, use the 90-degree countersink. Cutting the thread involves turning the tap wrench into the core hole with the tap attached.
Tools you need to cut external threads include file, die stock, flat-tip, round die, rod, vise (for gripping), and cutting spray.
To cut external threads, file the edges of the round rod first and then chamfer it at an angle of 45 degrees. Ensure that when compared to the depth of the thread, the chamfer is bigger.
The next step is clamping or gripping the round die to affix it firmly. This prevents unwanted motion since the rod requires a lot of pressure to cut the external thread optimally. Use cutting sprays to improve the quality of the part’s surface.
There are several screw thread machining methods in CNC. These different methods have their features and benefits. Here, we’d discuss the common ones and what makes them stand out.
Tapping holes are ideal for use in producing internal threads. It is an economical as well as efficient threading method. This CNC method is applicable for use in threaded holes with the low accuracy of position as well as holes with a small diameter.
It as a method of screw thread machining has reduced CNC threading machine downtime. Also, the machining structure here is quite simple compared to other methods. Furthermore, tapping is a high-speed cutting process that greatly improves productivity and machining efficiency.
Cutting tools used in this machining method are cheaper, thereby reducing the cost of manufacturing. It is also a versatile process with a wide range of applications.
This is another method used in making screw threads. This process utilizes a milling cutter as well as a 3-axis machining center. It also adopts a circular interpolation of the three main axes; x, y, and z linear feed. Thread milling is ideal for threading large parts or materials of high value.
Thread milling has a fast processing speed, high precision, and efficiency. The milling tool used often contains hard alloys. Also, milling cutter tools are versatile, reducing the need to acquire different tools for different milling processes. For instance, the same cutter can cut the left and right-hand thread using the same pitch of screws. You can adjust the hole diameters, tolerances, and material cut with minimal difficulty.
Threads made using milling have a superior-quality surface with zero burrs. This method is ideal for making thin-walled parts, machining blind holes, and asymmetric/ non-rotating parts.
Thread cutting on a lathe is another common screw threading process with a wide range of adoption. CNC lathes facilitate the production of high-quality screw threads. With this method, machinists can make several tapered threads, lead threads, and thread pitches.
Rigid tapping and single-point threading are two of the most common methods used in CNC lathe threading. Single-point threading uses a tool with an indexable insert with a shape and size that corresponds to that of the finished screw head.
This is ideal for use in threading hardened workpieces. Two main types of grinding wheels are used for this process; multi-line grinding wheels and single-line grinding wheels. The single-line grinding wheel has a pitch accuracy with grades 5 ~ 6. It also has a surface roughness of R1.25～0.08 μm.
Single-line grinding is ideal for making a precision screw, worm, thread gauge, shovel grinding hob, and small batch threading.
The multi-line grinding is divided into cut-in and longitudinal grinding methods. The major difference between these two methods is the width of the grinding wheel. In the longitudinal method, the width of the grinding wheel is less than the length of the thread being ground. On the other hand, the width of the grinding wheel used in the cut-in method has a greater length than the thread.
After completing this unit, you should be able to:
• Determine the infeed depth.
• Describe how to cut a correct thread.
• Explain how to calculate the pitch, depth, and minor diameter, width of flat.
• Describe how to set the correct rpm.
• Describe how to set the correct quick change gearbox.
• Describe how to set the correct compound rest.
• Describe how to set the correct tool bit.
• Describe how to set both compound and crossfeed on both dials to zero.
• Describe the threading operation.
• Describe the reaming.
• Describe how to grind a tool bit.
Thread cutting on the lathe is a process that produces a helical ridge of uniform section on the workpiece. This is performed by taking successive cuts with a threading toolbit the same shape as the thread form required.
Practice Exercise:
1. For this practice exercise for threading, you will need a piece of round material, turned to an outside tread Diameter.
2. Using either a parting tool or a specially ground tool, make an undercut for the tread equal to its single depth plus .005 inch.
3. The formula below will give you the single depth for undertaking unified threads:
d = P x 0.750
Where d = Single Depth
P = Pitch
n = Number of threads per inch (TPI)
Infeed Depth = .75 / n
To cut a correct thread on the lathe, it is necessary first to make calculations so that the thread will have proper dimensions. The following diagrams and formulas will be helpful when calculating thread dimensions.
Example: Calculate the pitch, depth, minor diameter, and width of flat for a ¾-10 NC thread.
P = 1 / n = 1 / 10 = 0.100 in.
Depth = . x Pitch = . x .100 = . in.
Minor Diameter = Major Diameter – (D + D) = .750 – (.075 + .075) = 0.600 in.
Width of Flat = P / 8 = (1 / 8) x (1/10) = . in.
Procedure for threading:
1. Set the speed to about one quarter of the speed used for turning.
2. Set the quick change gearbox for the required pitch in threads. (Threads per inch)
Figure 1. Thread and Feed Chart
Figure 2. Setting Gearbox
3. Set the compound rest at 29 degrees to the right for right hand threads.
Figure 3. 29 Degrees
4. Install a 60 degree threading tool bit and set the height to the lathe center point.
Figure 4. 60 Degree Threading Tool
5. Set the tool bit and a right angles to the work, using a thread gage.
Figure 5. Using the Center gage to position the tool for machining Threads
6. Using a layout solution, coat the area to be threaded.
Figure 6. Layout
7. Move the threading tool up to the part using both the compound and the cross feed. Set the micrometer to zero on both dials.
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Figure 7. Compound Figure 8. Cross Feed
8. Move cross feed to the back tool off the work, move carriage to the end of the part and reset the cross feed to zero.
Figure 9. End of the part and Cross feed to Zero
9. Using only the compound micrometer, feed in .001 to .002 inch.
Figure 10: Compound feed in .002 inch
10. Turn on the lathe and engage the half nut.
Figure 11: On/Off Lever and Half Nut
11. Take a scratch cut on the part without cutting fluid. Disengage the half nut at the end of the cut, stop the lathe and back out the tool using the cross feed. Return the carriage to the starting position.
Figure 12. Starting Position
12. Using a screw pitch gage or a rule check the thread pitch. (Threads per inch)
Figure 13. Screw Pitch Gage Figure 14. Screw Pitch Gage(10)
13. Feed the compound in .005 to .020 inch for the first pass using cutting oil. As you get near the final size, reduce the depth of cut to .001 to .002 inch.
14. Continue this process until the tool is within .010 inch of the finish depth.
Figure 15. Threading operation
15. Check the size using a screw thread micrometer, thread gage, or using the three wire system.
Figure 16. Three wire measurement
16. Chamfer the end of the thread to protect it from damage.
Reamers are used to finish drilled holes or bores quickly and accurately to a specified sized hole and to produce a good surface finish. Reaming may be performed after a hole has been drilled or bored to within 0.005 to 0.015 inch of the finished size since the reamer is not designed to remove much material.
The workpiece is mounted in a chuck at the headstock spindle and the reamer is supported by the tailstock.
The lathe speed for machine reaming should be approximately 1/2 that used for drilling.
Reaming with a Hand Reamer
The hole to be reamed by hand must be within 0.005 inch of the required finished size.
The workpiece is mounted to the headstock spindle in a chuck and the headstock spindle is locked after the workpiece is accurately setup. The hand reamer is mounted in an adjustable reamer wrench and supported with the tailstock center. As the wrench is revolved by hand, the hand reamer is fed into the hole simultaneously by turning the tailstock handwheel. Use plenty cutting fluid for reaming.
Reaming with a Machine Reamer
The hole to be reamed with a machine reamer must be drilled or bored to within 0.010 inch of the finished size so that the machine reamer will only have to remove the cutter bit marks. Use plenty cutting fluid for reaming.
Procedure:
1. Grip the tool bit firmly while supporting the hand on the grinder tool set.
2. Hold the tool bit at the proper angle to grind the cutting edge angle. At the same, tilt the bottom of the tool bit in towards the wheel and grind 10 degrees side relief or clearance angle on the cutting edge. The cutting edge should be about .5 inches long and should be over about ¼ the width of the tool bit.
3. While grinding tool bit, move the tool bit back and forth across the face of the grinding wheel. This accelerates grinding and prevents grooving the wheel.
4. The tool bit must be cooled frequently during the grinding operation by dip into the water. Never overheat a tool bit.
5. Grind the end cutting angle so that it form an angle a little less than 90 degrees with the side cutting edge. Hold the tool so that the end cutting edge angle and end end relief angle of 15 degrees are ground at the same time.
6. Check the amount of end relief when the tool bit is in the tool holder.
7. Hold the top of the tool bit at about 45 degrees to the axis of the wheel and grind the side rake about 14 degrees.
8. Grind a slight radius on the point of the cutting tool, being sure to maintain the same front and side clearance angle.
Grind front Grind side Grind radius
Lathe tool bits are generally made of four materials:
1. High speed steel
2. Cast alloys
3. Cemented Carbides
4. Ceramics
The properties that each of these materials possess are different and the application of each depends on the material being machined and the condition of the machine.
Lathe tool bits should possess the following properties.
1. They should be hard.
2. They should be wear resistant.
3. They should be capable of standing up to high temperatures developed during the cutting operation.
4. They should be able to withstand shock during the cutting operation.
Cutting tools used on a lathe are generally single pointed cutting tools and although the shape of the tool is changed for various applications. The same nomenclature applies to all cutting tools.
Procedure:
1. Base: the bottom surface of the tool shank.
2. Cutting Edge: the leading edge of the tool bit that does the cutting.
3. Face: the surface against which the chip bears as it is separated from the work.
4. Flank: The surface of the tool which is adjacent to and below the cutting edge.
5. Nose: the tip of the cutting tool formed by the junction of the cutting edge and the front face.
6. Nose radius: The radius to which the nose is ground. The size of the radius will affect the finish. For rough cut, a 1/16 inch nose radius used. For finish cut, a 1/16 to ⅛ inch nose radius is used.
7. Point: The end of the tool that has been ground for cutting purposes.
8. Shank: the body of the tool bit or the part held in the tool holder.
9. Lathe Tool bit Angles and Clearances
Proper performance of a tool bit depends on the clearance and rake angles which must be ground on the tool bit. Although these angles vary for different materials, the nomenclature is the same for all tool bits.
• Side cutting edge angle: The angle which the cutting edge forms with the side of the tool shank. This angle may be from 10 to 20 degrees depending on the material being cut. If angle is over 30 degrees, the tool will tend to chatter.
• End cutting edge angle. The angle formed by the end cutting edge and a line at right angle to the centerline of the tool bit. This angle may be from 5 to 30 degrees depending on the type of cut and finish desired. For roughing cuts an angle of 5 to 15 degrees, angle between 15 and 30 degrees are used for general purpose turning tools. The larger angle permits the cutting tool to be swivelled to the left when taking light cuts close to the dog or chuck, or when turning to a shoulder.
• Side Relief (clearance) angle: The angle ground on the flank of the tool below the cutting edge. This angle may be from 6 to 10 degrees. The side clearance on a tool bit permit the cutting tool to advance lengthwise into the rotating work and prevent the flank from rubbing against the workpiece.
• End Relief (clearance) angle: the angle ground below the nose of the tool bit which permits the cutting tool to be fed into the work. This angle may be 10 to 15 degrees for general purpose cut. This angle must be measured when the tool bit is held in the tool holder. The end relief angle varies with the hardness and type of material and type of cut being taken. The end relief angle is smaller for harder materials, to provide support under the cutting edge.
• Side Rake Angle: The angle at which the face is ground away from the cutting edge. This angle may be 14 degrees for general purpose tool bits. Side rake centers a keener cutting edge and allows the chip to flow away quickly. For softer materials, the side rake angle is generally increased.
• Back (Top) Rake: The backward slope of the tool face away from the nose. This angle may be about 20 degrees and is provide for in the tool holder. Back rake permits the chips to flow away from the point of the cutting tool.
1. What is pitch for ¼-20 tap?
2. To what angle must the compound be turned for Unified Thread?
3. Explain why you swivel the compound in Question 2.
4. What is the depth of thread for UNF ½-20 screw?
5. How would you make a left-hand thread? This is not covered in the reading—think it out?
6. What Tool bit do we use for cutting thread?
7. Please describe Center Gage.
8. What do we use to check the thread pitch(Thread Per Inch)?
9. The first and final pass, how much do we feed the compound in?
10. Name four material that use to make Tool bits.
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