Tips on selecting a lathe -will it thread

In a recent discussion, someone inquired about the threading capability of the new South Bend 8 x 18 lathe. The following is based on my response, with additional details included.

One crucial consideration when purchasing a lathe is its threading capability. You need to understand how the lathe establishes the correct ratio between the lead screw and the headstock.

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Quote:
Why does the SB need quick-change gears if it says that it can thread?
I have never threaded and do not know anything about the QC gears.
Can someone enlighten us?

Click to expand...

Yes, it will indeed thread! In the early days of screw threading lathes, the only way to change the pitch of a threaded screw was to open the headstock cover (if applicable) and swap out the gears to adjust the ratio between the headstock spindle and the lead screw. These gears are typically provided as a set with the lathe, hence the term "change gears" or "change gear set." A complete set allows for the creation of all common threads—be it imperial, metric, or both—depending on the lathe.

Modern production lathes come equipped with a gearbox featuring a series of levers or knobs that enable quick adjustments of the headstock gearing ratio. This gearing also determines the feed rate for automatic feeds.

Contemporary hobby lathes may be hybrids. While they allow for quick thread pitch changes using levers or knobs, their capabilities may be limited to either imperial threads or certain feed rates. To switch modes, you may need to open the box and change gears.

The traditional method saves on production costs but is time-consuming for gear changes. A fully-featured quick-change gearbox is a premium option. However, in a job shop where requirements change rapidly, it can save production time and setup costs.

The hybrid approach provides a balance, making threading setups easier and faster while maintaining reasonable costs. Some smaller hobby lathes, such as Taig and Sherline, lack headstock gearing entirely—a feature that can be added as an option.

Single-point threading was one of the fundamental skills I learned in tech school on a lathe. As a hobbyist, one must decide the importance of this skill and the threading capabilities of their lathe.

Important Considerations

When it comes to metric versus imperial, most lathes are designed to function either as metric or imperial machines. The lead and feed screws are manufactured according to these standards. Thus, if you have an imperial standard machine and wish to thread, you will need a transposition gear of 127 teeth or a 21-tooth gear for the 7 x mini lathes, which provides a close approximation.

The lesson here is clear: if you intend to do extensive threading, select a lathe that aligns with your work needs or comfort level. Some machines are metric and only approximately accommodate U.S. decimal inches. Thankfully, newer imports have addressed this with inch-based screws.

CNC lathes operate quite differently. With features like spindle sensors, the computer sets the ratio of spindle speed to the lead/feed screw electronically. Thus, you simply program the necessary measurements, switch between metric or inches, and let the computer handle the calculations. Nonetheless, I still recommend mastering threading on a manual machine.

In summary, these are essential considerations when selecting a lathe. Threading capability is one factor among many. Choosing a hobby lathe can be challenging. I learned machining on a traditional South Bend lathe—modern and full-featured with all capabilities. My first personal lathe was a Grizzly 7x10. While I had limited resources, it was beneficial to know what a real lathe could achieve. Choosing a Taig, for instance, would mean sacrificing threading capabilities.

Other aspects to consider include budget, necessary accessories, the size of the workpieces, traditional versus imported machines, swing and center-to-center dimensions, available workshop space, shipping weight, shop location (garage, basement, attic), and various other factors. I hope this information assists newcomers in their journey.

Contact us to discuss your requirements for threading in lathe machine. Our experienced sales team is ready to help you identify the best options to meet your needs.

In my school machine shop, the instructor demonstrated how to grind a tool to fit the fishtail gauge, set it up in the lathe, and how to use the fishtail gauge to position the tool on center using the tailstock center. We learned to set up the quick-change gearbox for our desired thread and proceeded with cutting threads, using only a suitable nut to gauge the fit. I rarely used thread wires as they can be quite inconvenient! For threads that require precise fitting, I prefer using a thread micrometer, as they are easier to work with compared to wires and are often available used at reasonable prices.

Avoid wasting your money on insert threading tools—they tend to break easily. Instead, I recommend HSS tools made by Aloris; they are sharpened only on top and last a long time.

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