On Mon, 15 Jul 2013 08:34:06 -0400, "Jim Wilkins"
wrote:

"Ned Simmons" wrote in message
.. .
On Sun, 14 Jul 2013 22:06:34 -0400, "Jim Wilkins"
wrote:

This one is easy to check. I measured a few 1/4" and 3/8" Greenfield
taps at 0.0015 - 0.002" under the nominal diameter.
jsw


Then they won't cut proper UN threads. The *minimum* major diameter
for 1/4-20 UNC - 2B (and 1B & 3B) internal threads is .2500.
Likewise,
the minimum major diameter is equal to the nominal size (the basic
major diameter) for all 1B, 2B and 3B UN threads. Similar rules
apply
to metric threads.
Ned Simmons

OK, many of my tools were second-hand. New 1/4, 5/16 and 3/8 taps
measured 3-4 thousandths over nominal size.
jsw


You got me wondering so I checked some taps I have that are old enough
to be marked "USS." USS threads were superseded by National (NC, NF)
threads, which were in turn replaced by UN threads. Even the USS taps
I checked, which happened to be Greenfield taps, had a major dia
greater than nominal. What I don't know, because the oldest MH I have
covers only National threads in detail, is whether the USS standard
insisted on the oversize major dia.

--
Ned Simmons