Jim Wilkins wrote:

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

My 1965 lathe is very far from perfectly rigid.

The concept of perfect rigidity was used purely to illustrate the point.
Like the concept of an infinitely fat man it does not exist in reality I am
sure.

[...]

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.

I think I have to learn to distinguish when to switch to such "finishing
tool". I have had some bad experiences with tools that took too much of the
metal.

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.

Now I have a proper grinding set up I can have a go at making all sorts of
different shapes.

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.

Two days ago I saw a dream - 30" Russian lathe. Slightly bigger than our
living room.

The 6" lathe I bought first was a mistake.

Care to expand on that?

--
Michael Koblic
Campbell River, BC