In article , Dan Caster says...

If the top of the thread is slightly truncated because the diameter is
a little less than the nominal diameter, the strength is almost the
same and the effort to cut the threads is greatly reduced. The
Machinery Handbook has tables with dimensions for various classes of
threads.

Another point is, it's a lot easier to avoid drunken
threads from a die, if the stock is turned way undersized
for a length equal to about one diameter or so. This
allows the die to start cutting true, and in most cases
the first bit of undersized stock can be trimmed off
afterwards.

This is especially helpful with large diameter or
coarse threads.

Jim

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