wrote in message
...
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 ?

Bob Swinney

You don't need to obsess about proper angles in a home shop where the
tools do relatively little cutting, aren't left unattended and can be
reground whenever you want without messing up a long repetitive job.

An HSS lathe bit cutting edge wears by developing a flat below the
edge. Once that flat grows to ~1/32" or so high it skates over the
work, according to some old books from Lindsay. Look at a dulled edge
with a magnifier.

If the angle is too shallow, or vertical, only a little metal has to
rub off to grow the flat. If it's too acute the tool can dig in or the
edge can chip. The condition of your lathe affects how likely that is.
Mine's bad so I keep the front relief angle moderate and sharpen more
often.

You soon learn what cuts easily and lasts long enough by
experimenting. The top can be left horizontal to avoid chasing two
interacting variables.

When you regrind a threading bit part way through the job you can
bevel the starting end with hand feed to confirm that it will still
cut.

jsw