Dunno, Karl. I think it came from Mach. Handbook. The one for tapping goes like this: Drill for
size for tapping = Nominal diameter - (1.299 X % / # thds.) per inch Where percent is stated as
a decimal, ie. .75, etc.

Bob Swinney
"Karl Townsend" wrote in message
anews.com...

The shaft to be threaded should be turned down to dims re the formula:
Nominal diameter - (.10825 /
# thds per inch) For a 1/2-13 that would be: 1/2 - (.10825 / 13 ) =
0.491 in.

I learned something today. Don't happen often for an old goat like me. Is
this just some sort of emperical formula or is there a logical basis?

Karl