Robert Roland on Sun, 02 Sep 2018 16:46:29 +0200
typed in rec.crafts.metalworking the following:
Where on the threads would I need to measure to find the nominal size
of a bolt?

If I measure the major diameter of an external thread (bolt), the size
is always considerably smaller than the nominal size. I.e. if I
measure an M12 bolt, it will measure around 11.8 mm or so. Similarly,
if I measure a 1/2-inch bolt, it will measure around 12.5 or 12.6 mm,
where 12.700 would be exactly half an inch.

The problem occurs when I want to make an external thread on the
lathe. So far, I have always turned the stock to the nominal size (12
mm for an M12 thread), and then cut the treads. This consistently
produces terrible results, where the nut will not fit until the thread
is cut way too deep.

So, I suspected that maybe the nominal thread diameter would be
referenced from the theoretical point where the flanks of the treads
would meet to a sharp point. To check this, I drew a quick thread in
Fusion 360 (it has built-in threads), and did some measurements there.
Alas, it still does not work out. In this case, the size comes out too
large, by about the same amount it was small before.

The nominal size is what the bolt started out as. E.G., the M12
bolt started as 12 mm diameter shaft. (Same with wood sizes - a 2"x4"
board _started_ life as a 2x4 inch board, but then was faced, and
reduced in size).
The problem is, that 12mm bolt has to fit into a 12mm dia hole.
Which leads to the question of "class of fit" (sloppy, normal, or too
tight), maximum material conditions and is the hole "in spec" but too
small for a bolt also "in spec". (also known as "2+2=5 for large
values of 2 and small values of 5.")

So, measuring across the threads gives you a number "close" to the
actual nominal size, and round up. Always up - unless you're
measuring the inside of the nut. Then you round down.

tschus
pyotr

--
pyotr filipivich
"With Age comes Wisdom. Although far too often, Age travels alone."