Ed Huntress fired this volley in
:

The former is much more likely. I think the thing to do is to start at
the tool tip and work your way out from there. Tool nose sharpness?
Rake angle? Relief? Too much of the tool in the cut? Insert clamp?



He's never identified it as chatter. If it's circumferential ridges
spaced out along the work, instead of the common chatter herringbone,
then it may be that there's some slop in the saddle gibs.

Slight warp in the lead screw or longitudinal drive shaft can cause an
eccentric action that, with a loose saddle, will cause a well-defined
ridge a few tenths deep every revolution of the screw or shaft.

LLoyd