On Mon, 30 Nov 2009 21:02:24 -0800, "Bill Noble"
wrote:

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?


You didn't clip the top (and bottom) of the thread. g The external
UN thread has a pitch/8 flat at both the root and crest. The flatted
crest is coincident with the major diameter.

Take 3/4 of your .108 deduction to account for the flats and you're
good.

--
Ned Simmons