Shop Doc – Dreading the Threading

Today’s Machining World Archive: July 2006 Vol.2, Issue 07

Dear Shop Doc,

For turning external threads on my CNC lathe, I am currently using a laydown threading system. I chose this style of threading system due to the option of using the partial profile or full profile type of inserts. The partial profile inserts give me the capability of achieving a range of threads per inch, so I don’t need an insert for every “pitch” of thread. For a larger production run, I purchased the full profile insert for the specific thread per inch – I wanted the entire job to be very consistent in the quality of the thread. The full profile insert is good because it can cut the major/minor/pitch diameters all at the same time. But at times, I have a problem with smaller diameter or coarse threads. It seems that the insert rubs on one side or the other of the 60° angle. I would guess 65% of the threads are produced with great success, but there are some threads with poor tool life and others that fit loosely on gauges even though the major diameter is correct. What can we do for the tool life and gauging issues I listed? I really think the laydown inserts are a great advantage overall.

Dreading the Threading

Dear Dreading,

These issues can be fixed in a couple of simple steps. You should see included with the holder, an anvil that the insert sets on. This is more than just a seat like a turning insert and holder sometimes has. This anvil is tapered to a certain angle. The standard anvil included with a laydown holder has 1.5° of taper.

These angles correspond with the required helix angle that a threading tool will need to have designed into it. You don’t want it to rub as it travels at the extreme “feed per revolution” to cut a screw thread feature.

For your problem with smaller diameter or coarse threads, the helix angle needs to be adjusted due to these 2 variations of thread dimensions. The helix angle has to be increased the closer the tool gets to the centerline of the machine. It also needs to be increased the more it moves per revolution, as that is required with coarse threads.

There are many angled anvils available that will achieve other helix angles besides 1.5°. The standard anvil, with 1.5° helix, will accommodate about 65% of threads, which is what you have encountered so far. Actually, by reviewing any of the tool manufacturers’ charts that tell you which anvil to use for specific threads, you will see that the 3/8-24 thread is the smallest to use the standard 1.5° anvil. I suggest you review the charts that the manufacturers provide in their technical sections and order a couple of the 2.5°, 3.5° and even 4.5° anvils. You will see a great improvement in the quality of the thread form. The gauging will be better, and the tool life will increase.

Good Luck,

Jim Rowe
Application Specialist / Medical Accounts,
Mahar Tool Supply, Warsaw, IN

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