ok, I'll bite - where did those magic numbers come from.... let's see

60 degree threads, so the height of 60 deg triangle is sine (60) times the
length of any side (since for a 60 deg triangle all sides are equal) - the
sine of 60 is .866.

So, if I have a 1 inch diameter bar and I want 8 thread per inch, I should
have a minor diameter of 1" - .866/8 = 1"-.108. = .89"

or for 1/2-13 it should be 1/2-.866/13=.4333

none of this matches up with the tables
(http://www.efunda.com/DesignStandard...s/screwunc.cfm) - I know I
didn't account for the clipping of the top of the thread form, but that
factor shouldn't account for the difference - what am I doing wrong?



"Robert Swinney" wrote in message
...
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