Need Help!- HELP WITH THREADING S.S 400

[Muzzy]

I'M TRYING TO THREAD SOME S.S 400 TYPE STEEL. THE THREAD IS A 750-20 UNS-3A. I HAVE TYED ISCAR TOPING INSERT-16ERM-20 UN IC908. THIS IS THE THREAD CALL OUT. MAJOR 746-7384 PITCH 713-7108 MINOR DIA IS 6851 MAX. THIS IS THE ISUE. THE FUNCTIONAL DIAMETER CAN NEVER BE LESS THE THE PITCH DIAMETER. IT SEEM WHEN I'M RUNNING THIS PART THE PITCH DIA IS SMALLER THEN THE FUNCTIONAL DIA BY .0002 TO .0004. IS THERE A WAY TO RUN THIS WITH SOME CONCENTENTSE . RPM IS 750 TAKING .001 PER PASS WATCH IS ABOUT 16 PASSES. ANY HELP

[mrainey]

These are the values I calculate for .750-20 UNEF-3A (it's not a UNS thread)

major = .7500/.7419
pitch = .7175/.7142
minor = .6959/.6817

Where did you get your numbers?

[Muzzy]

I got my numbers from the print. this part will get plated so the size is going to be smaller. the company wants me to hold the tol to 713-7108 for plating. the thread is 750-20 uns 3A I copyed it from the print. your number are right also.

[mrainey]

The major and minor diameters change .0020 for each .0010 of plating buildup, but the pitch diameter changes by .0040 on a 60-degree thread.

You might need to get away from the topping insert and use a separate OD turning tool for the major. This way you can hold all diameters where you need to.