"Jim Wilkins" wrote in message
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wrote in message
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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.


THat is probably true where you have "built-up edge"., otherwise, there
develops a radius, whereas at some point the tool won't give a clean cut
unless your infeed amount is substantially larger than the radius.


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