According to Clayton E. Cramer :
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.
Hmm ... if the facing is not totally square (easy to have
happen, as the first face is probably a saw cut), then it will cause the
workpiece to tilt in the second holding so it is parallel to the first,
but not square to the axis of the workpiece.
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-
How do you measure the runout with a micrometer? A dial
indicator is the better tool for this.
And lacking a steady rest, what I would do is start the spindle
turning slowly (IIRC, the Sherline has a variable speed motor), and
measure the runout at the free end of the workpiece. Loosen and tighten
the chuck as you make adjustments until it is at a minimum. Keeping the
speed slow, use the center drill to make a center in the end while it is
running slowly so it does not slip in the chuck. Then bring a live
center up to support that while you face as much of it as you can manage
before you hit the center.
Then swap ends, and you should have less trouble getting it
centered, as the faced ends will fit on the steps of the reversed chuck
jaws (they must be reversed to hold something that big in diameter).
Center drill this side too (after making sure that the runout is
minimal, support with a live center and face as much as you can. Then
withdraw the center and face the remaining part, and reverse the
workpiece again to complete the facing of the first side. (Hold it
centered with the live center while you tighten your chuck this time.)
At this point, you can drill it and tap it. Do you need the
threaded hole to go through the full length? If not, it is better to
drill and tap each end separately, instead of drilling it all the way
through, because drill bits tend to wander drilling deep holes. That
could be the cause of your error if it was in the end held in the chuck,
instead of the end facing the drill bit and tailstock.
-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.
Again -- how are you measuring the centering with a micrometer?
(Unless you have one of the multi-anvil ones used with a round anvil to
measure the distance from the hole to the outside diameter.
Sherline does make a steady rest--but it only goes to 1.75" diameter, so
that's not an option.
Thus you have to go through more steps, as described above.
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.
It depends. I know that the tailstock on my old Unimat had an
external thread which matched the spindle nose thread -- but that is
rather uncommon.
And I don't think that will really help you. Again -- do you
need the threaded hole to go all the way through? That is a long
distance to try to drill without a gun drill. If you can drill each end
separately, that will probably be better.
Enjoy,
DoN.
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