On Jul 10, 8:58*pm, "Michael Koblic" wrote:
Jim Wilkins wrote:
[fine cuts]

....The second pass will not
follow the path of the first pass exactly ...
I assume this is true even in a perfectly rigid system with an infinitely
sharp tool. Thus the trick is to find out how much the tool will remove
between the first and third pass. Was this what you were referring to in
your first response?

Michael Koblic

I see the same effect but the reduction in diameter is well under a
thousandth.

My 1965 lathe is very far from perfectly rigid. I haven't turned
anything critical since I ground the wear off the bottom of the
compound. Previously the compound was loose in the middle of its
travel and jammed near the ends. In that state carbide often chipped
and repeated passes removed fine chips randomly and changed the
surface finish, using either manual or automatic feed. I thought the
patterning came from wear in the feed gears.

I usually get the most even finish when the bit takes off a fairly
thick chip at a slow automatic feed rate. I think the greater cutting
force removes all play from worn, roughened bearing surfaces. However
I can't control the diameter as closely as when alternately measuring
and shaving off most of the difference.

The old books recommend a wide tool and fairly coarse feed for
finishing. Apparently the long contact line stabilizes and guides the
cutting edge. On my lathe a narrow point leaves a rough finish even at
the slowest feed rate of 0.00078" per rev.

You could experiment with a slanted cutting edge such as the one back
rake creates on the end of a right-hand turning tool. In my limited
experience it may do a good job if the clearance angle below the edge
is small enough to keep the bit from digging in. I don't know the
proper angle, I try for about 5 degrees and then raise or lower the
bit until it cuts smoothly.

Despite its problems the 10" lathe has served me well. 9" to 12" seems
to be a good size for making machine parts for repairs and
experiments. The controls are light and sensitive enough to make tiny
things like #0-80 screws and it's capable of roughing a few inches of
steel off a bar at a reasonable hobby shop rate. The 15" lathes at
work are considerably less pleasant for delicate tasks. The 6" lathe I
bought first was a mistake.

jsw