On Dec 8, 1:11*am, Ned Simmons wrote:
...*How about something like the 27 TPI common for microphone
stands? *Pretty difficult to use with a rule graduated as you suggest.
Even with 64ths graduations (which you don't mention) you wind up with
2.3704 64ths -- and it is pretty difficult to distinguish from a 1 mm
thread (27 TPI winds up at 0.9407 mm).

The difference between 5/27 and 3/16 is approximately 2 thou.
Ned Simmons-

If the original needs replacing the crests probably aren't sharp
enough to read that closely. I have an old transit with a 27TPI tripod
mount thread that was quite difficult to measure due to its condition.
You can hold the screw and sawtooth gage up to a light and easily see
a slight mismatch between them.

jsw