Martin Perry wrote:

Thanks for your help and advice. A few follow up notes:

I did not use a tailstock centre when turning the two inch bar. The
last couple of cuts were 0.001 so I thought that this would keep
deflection to a mimimum.

The bed is worn in the area close to the chuck, 4 or 5 inches along
the bed from the chuck.

The bed is mounted on jacking screws so that I can twist the bed and
compensate for some of the wear. But it's a bit of a compromise.

I was happy with the results I got when turning the outside diameter
but I was very supprised with the results from the internal turning.

I used a 0.75 diameter boring bar for the internal cut and the last
couple of cuts were 0.001 to try and keep deflection to a minimum.

The headstock should be in perfect alignment because it mounts on the
bed ways, there are no adjustments. The taper bearing are in good
condition and correctly adjusted.

The lathe has seen a lot of heavy work over the years and I am
probably expecting too much from this "small lathe" (similar to a
Southbend).


Turning unsupported work 8" long on a light lathe will give you these
sorts of results. Mostly, you have probably demonstrated spindle flex.
A .001" cut on a nice aluminum alloy with a brutally sharp tool will not
need
much infeed force. Depending on the workpiece material and the sharpness
and profile of the cutting tool, you may or may not get highly accurate
cutting on very fine cuts like this. You can also have false edge buildup
on the cutting tool and thermal expansion of the workpiece to confound
you. Whenever you rough a piece, you need to allow the temperature to
return to ambient before making that last finish pass. If the part is
still cooling off from the roughing passes when you make the long, slow
finish cut, you will also get taper. It is pretty amazing how much a
1" diameter part expands when hot, when you are using a tenth-reading
mike.

Jon