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?