Threading on the manual lathe

Screws and screw threads hold millions of things together. Nearly as many types and forms of threads exist as there are products that use threaded fasteners and connections. Equally, there is much confusion and misuse of threads in general among those who aren’t “gearheads.”

From the machinist’s point of view, cutting threads is a satisfying experience. When you’re done, hopefully, you have two parts that mate together with a level of precision and smoothness not found in run-of-the-mill, hardware-grade fasteners. I have always enjoyed cutting threads on the manual lathe and have learned a few tricks over the years.

Courtesy of All images: T. Lipton

Align your threading tool against a freshly faced end or against the side of the chuck.

■ Align your threading tool against a freshly faced end or against the side of the chuck. The little arrow-shaped alignment tools you see are a pain and are only good for gaging hand-ground tool bits.

■ If you do a lot of threading on a manual lathe, invest in a tool that accepts inserts. The inserts are precisely ground and easily changed. One insert cuts dozens of thread pitches.

■ I learned how to thread on the lathe using the compound infeed method. Contrary to popular belief, the compound set doesn’t have to be at half the thread angle. By using what’s called “modified-flank infeed” and changing this angle, you help alleviate threading problems in difficult-to-cut materials.

■ Another advantage to threading with the compound is you don’t have to keep track of the dial position. The cross-feed dial is always zeroed after each pass, so you have less to remember, such as whether the last pass was at 0.030 ” or 0.050 “. The main disadvantage is your Z-axis position changes as you feed in. This is usually not a problem on external threads, but it can be on internal threads that end against a shoulder.

■ Try the following strategies when you are ending threads and the part designer has not specified a thread. When I want to do something with the groove that gets cut at the end of the thread, I usually use the threading tool and traverse a small relief at the end. It saves a tool change and looks OK. If I want a nicer look, I switch to a radius tool. Just be sure the relief is a little smaller than the thread’s minor diameter so the mating part will thread all the way to the shoulder.

Keep a complete set of nuts on rings, with one ring for coarse threads and the other for fine threads.

■ Use a large DOC on your first pass during threading. The point is small; in the first couple of passes, the area of the tool tip engagement is also small. Taper your DOC as you get deeper. On the last pass, feed straight in with the cross-feed at a light 0.001 ” spring cut. This cuts on both tool flanks and removes chatter and tool marks from the thread.