Subject: UN versus AN threads. Was: hole size for metric taps

From: [email-masked] (Jim Hanrahan)
Date: Wed, 1 May 1996 19:23:08 GMT

>From  my research (use at your own risk) :-)

The difference in thread form between the old American National (AN) and the 
current Unified National (UN), excluding tolerances and allowances is that the 
AN thread form is a complete "V" at the tip. The UN is based 
on a truncated "V" with the root (inside of a external thread) truncated by 
.25*H and the tip (outside of an external thread) truncated by .125*H. Where H 
is the height of a perfect 60 degree "V".

H = 0.5 * lead * 1.73205  Where 1.73205 is tangent of 60 degrees or SQRT(3).
The tangent of 60 degrees comes from doing the geometry, SQRT(3) happens to be 
the same and is often easier when using a calculator.

That is why the previously posted formulas for tap drills have
0.625 = 1-(0.125 + 0.25) for UN threads. Tip and root truncated.
0.750 = 1-(0.25) for AN threads. Only root truncated, ie hole not metal.

ie. the tap drill for UN is

O.D. - (lead * 0.625 * 1.73205 * percent thread)

and AN is

O.D. - (lead * 0.750 * 1.73205 * percent thread)

IMHO, The 12.5% (0.125*H) loss of the tip when using UN instead of AN 
bolts results in very little loss of strength. The hole drilled using the AN 
formula and 75% thread is about the same as the hole drilled using the UN
formula and 90% thread.

I guess my point is to use 92% (or less) with the UN formula, and
75% (or less) for the AN formula when selecting tap drill size.

--------------------------------------------------------------
  Jim Hanrahan, PE, CP-IA         Naval Surface Warfare Center
  [email-masked]      Annapolis, Maryland, USA