"Jon Elson" wrote in message
...


HeffaLump wrote:
I've developed a problem when facing on my lathe, I do a lot of small dia
disks (20-40mm) in 6082 Aluminium and when it comes time to polish the
face I'm noticing concentric circles appearing on the finished surface.

They're not to pronounced, I'm able to polish them out easily enough, but
I'm worried it might be indicating a developeing problem.

I've tried cutting at 0.5mm down to 0.1mm with little differance both
manually & on powerfeed, I've adjusted to gibs to the cross slide (tried
it loose, spot on & too tight), varied the RPM's, differant cutting
tools/holders and I can see that the saddle isn't moving from the DRO.

I've put a dial guage against the surface with no discernable movement
when rotating !

The circles are easy to see but impossibly difficult to photo, you have
to catch the light just right but they appear to be 2-3mm apart. I cannot
tell if they are truly concentric or spiralling.

I'm thinking 2 things . . . either play in the head bearings or the
crossfeed leadscrew. Please don't let it be the head bearings but I can't
see how the leadscrew can affect the finish if the gibs are correctly
set????

Any ideas?

Could be half a dozen things. It can be built-up edge of workpiece
material on the cutting edge, variations in feedrate from your hands (try
using the power feed at a very low advance rate). Oh, OK, I see you've
already tried that! It could be looseness in the gibs allowing the cross
slide to weave back and forth slightly as you turn the crank.

If the spacing of the rings match the thread pitch of your crossfeed
screw, then the connection is nearly certain. remeber that no machine is
totally rigid, everything is slightly flexible. And, even with the gibs
"too tight", there is still an ability of the upper slide part to rock
just a little. It is possible there is a misalignment between the screw
and the nut that is applying sideways forces on the nut, thus rocking the
slide as the screw turns.

Jon

It seems possible that the leadscrew may be slightly bent. This can easily
rock the saddle enough to form noticeable rings.

Don Young