I'm making a 5c collet closer, the collet thread afaikt from the internet is 1.238-20.
I'll freely admit, I generally just look up threads in machinery's handbook and cut the
thread.

That one isn't in there.

I'm thinking that boring my chunk of 1 3/8" 4130 tubing to 1.238 - 1/20 is a possible
solution before threading, much like calculating a tap drill. Then using my selection of
collets to check for fit is a plan that may work.

I'd really like to know how to calculate this as in I'll build a spreadsheet for future
oddball threads.

Thanks,

Wes