On 2012-05-26, wrote:
K. H. Moltrecht in his book Machine Shop Practice advises: " . . . The
left-hand side-cutting tool should have a 10- to 17-degree relief angle on
the flank below the right side-cutting edge, and an 8- to 12-degree relief
angle below the left-hand cutting edge. . . . ".

Can someone please explain why the 2 respective relief angles should not be
the same ?

Because they are cutting a groove which is slanted relative to a
vertical. And which way they are slanted is based on whether the
threads are right-hand or left hand. Right hand threading tools are
about the same, except that the angles swap sides.

What *I* do is calculate the basic angle by calculating from the
pitch and the circumference. Then I add what feels right (I've used
just 5 degrees to the angle on one side and subtract it on the other
side to produce consistent clearance. This results in a tool which is
just right for a specific pitch and diameter, and *might* work well for
another combination of pitch and diameter, as long as they calculate to
the same angle. The values given in Moltrecht's book are more general
purpose values -- should work with most things, except perhaps if you
are cutting a triple lead thread or something similarly strange which
results in an extreme thread angle.

The time I did this, I was cutting a replacement nut for a log
splitter. Acme, and pretty aggressive cut, so I minimized the clearance
in favor of strength.

Good Luck,
DoN.

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