You probably need to concentrate on holding the delrin more accurately
before you do any machining. Perhaps the chuck jaws are not staying
parallel to the part's surface as you tighten them.

At any rate, given a set of chuck jaws in reasonably good condition, and
fitting in the chuck okay, an inch or two of the delrin in the jaws should
provide plenty of stability to machine. It's often interesting to indicate
a piece just snug in the jaws, and then after tightening. (Surprising how
much they move sometimes)

Good luck!


"Clayton E. Cramer" wrote in message
...
Thanks to all for the many suggestions. This is a 2.375" diameter, 3"
long piece of Delrin. Yes, it is a bit slippery, and I suspect that the
rotation is causing it to slip a bit.

Yes, I have faced both ends, and when I do so, the workpiece length is
accurate enough that I can say that opposite sides of the cylinder are the
same length within a few thousandths of an inch.

The 3 jaw chuck seems to be accurate enough that I can't blame it; using a
micrometer, its runout is at most a few thousandths of an inch--not enough
to explain the problems that I am having--especially because I can turn
short pieces (1/2" long) and get acceptably accurate centering--good
enough that my cheap micrometer is less accurate than the centering
discrepancies.

Sherline does make a steady rest--but it only goes to 1.75" diameter, so
that's not an option.

Would it make more sense to mount the drill chuck in the headstock, so
that what is rotating is the drill, not the workpiece? Of course, that
means that I need to find a way to mount the chuck in the tailstock
instead, and I don't know if a Sherline lathe can do that.

Clayton E. Cramer