Cutting Tool Engineering
August 2012 / Volume 64 / Issue 8

Threading on the manual lathe

By Tom Lipton

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.

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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.

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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.

I can never remember which line on the threading dial to use with which thread pitch. If you’re lucky, it will be marked. When in doubt, just use the same number or line each time. Always use the same number when cutting multiple-start threads.

Do internal threading from the inside out with left-hand tools. You will get less chatter and see what’s happening down the bore. You will need left-hand threading tools, running the lathe in reverse. Remember, it’s easy to pull a rope; it’s really hard to push one.

When you have a choice, fine threads are easier to cut and need fewer passes than coarse ones. The shallower depth on difficult-to-cut materials might save your bacon.

For quick and easy day-to-day threading gages, I keep a complete set of nuts on rings in my toolbox for fitting threads. One ring holds coarse threads and the other holds fine ones. When you thread, be sure to run the nut the full length of the threads. When left to their own devices, machinists tend to cut threads tighter than necessary.

Mating materials in threaded connections are important. If you must use the same material for male and female threads, do yourself a favor and put a few molecules of thread lubricant or antiseize on them before you crank them together.

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A thread file is ideal for straightening the annoying half thread fade at the beginning and end of an external thread.

If you do happen to get male and female threads wedged together in an intimate embrace, a simple trick to separate them is to quickly warm the female portion to 100° F or so, using a propane torch. A quick shot of penetrating lube before you twist might save the work.

When measuring threads, a dedicated thread micrometer is handy and quick to use at the machine. But for the highest accuracy, use the three-wire thread measuring method. This method is more accurate because the wires present a true parallel surface for measuring. If it’s good enough for the gage makers, it’s good enough for me.

A piece of modeling clay or window glazing putty can help hold pesky thread measuring wires. Better yet, buy a set of the plastic holders that fit the micrometer spindle.

Thread files actually work. They are great for straightening the annoying half thread fade at the beginning and end of an external thread. CTE

About the Author: Tom Lipton is a career metalworker who has worked at various job shops that produce parts for the consumer product development, laboratory equipment, medical services and custom machinery design industries. He has received six U.S. patents and lives in Alamo, Calif. Lipton’s column is adapted from information in his book “Metalworking Sink or Swim: Tips and Tricks for Machinists, Welders, and Fabricators,” published by Industrial Press Inc., New York. The publisher can be reached by calling (888) 528-7852 or visiting www.industrialpress.com. By indicating the code CTE-2012 when ordering, CTE readers will receive a 20 percent discount off the book’s list price of $44.95.
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