On Dec 31, 8:15*pm, 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

We kicked this one around a while ago. I measured my 5C set to make
the closer for a drill grinding fixture and found a range of over
0.005", so I cut the thread to fit the largest collet, ~1.245 IIRC.
http://picasaweb.google.com/KB1DAL/T...98039281271986
The 1/2" bushing is to center the shank of an S&D bit.

Cutting in a calculated distance from the initial contact only works
if the OD or ID is correct and the tip of the tool is right for that
thread pitch. If you round the tip properly you need a separate tool
bit for each pitch, PITA.

I don't usually bother. The external threading bit tip is eyeballed to
shape for about 8TPI and the 29 degree infeed on my spreadsheet
threading chart is thus too small by up to about 0.005", a reasonably
safe value. I finish the thread with a tap or die if possible, or cut
internal threads to fit after making and measuring the external one.

The formulas on my chart are
29 Infeed = 0.75/Pitch for nominal tip flat width
Root diameter = OD - 2*0.6495/Pitch.
Tip flat width = 0.125/Pitch, wear limit is 0.25/Pitch
They are pretty much from Machinery's Handbook, slightly adjusted so
mistakes leave metal.

Root diameter is listed as the -offset which I subtract from the
measured OD. 0.6495/Pitch would be the minimum amount to feed in for
the ending groove, but the crossfeed dial is zeroed for the threading
bit instead of the cutoff tool.

jsw