On Thu, 31 Dec 2009 20:15:48 -0500, Wes wrote:

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

DoN Nichols wrote a little program that John Beidl made usable under
Windows. I've been using that for years.
http://www.metalworking.com/dropbox/...s/Threadsw.txt
The one I use is v5.0.0.0, 2 Mar 99. It's worked on every Windows
machine I've owned, currently running Win XP Pro.

A formula that seems to work for internal threads is:
ID = OD - .975/TPI

I made a spreadsheet with these values and printed a chart that is
taped to my lathe.

It's a guide, not gospel. I've made parts to fit a mating part 1500
miles away (and they did) but with internal threads it's really best
to have a try gage at the lathe if at all possible. Perhaps not so
true with ultra-rigid CNC machinery, I don't know anything about that.