In article t,
Roger Hull wrote:
The workpiece slides in (or out, depending on which way I'm cutting) on my
Sherline 3.5 x 17 with the 3-jaw chuck. It just started this and I've had it
four years or so. Plus my much older Clausing 12 x 24 never does this. I've
cleaned the chuck and there is no oil on the jaw faces. I'm very puzzled.
Even taking just 0.005 cuts the piece just wont stay still. This project
involves cutting a ball, so a tailstock & center isn't the answer.
You don't mention whether this happens at any other size of
workpiece or just this one size. One thing which I would try is to
*remove* the workpiece, and see whether the jaws can be set smaller than
the current workpiece diameter. It might be a chip jammed in the scroll
or in one of the teeth near the outside of the jaws which is preventing
full tightening -- you are tightening against the chip, rather than the
workpiece.
If it fails this test, then it is time to totally disassemble
it, clean the parts individually, inspect under magnification for
embedded chips, and reassemble. If it passes, then the other suggestion
-- that of the jaws being sprung, and only gripping at the inner end
could be the case. If so, then re-grinding the jaws (as already
described in the other followup) might be the best bet.
If it is badly enough sprung, however, a new chuck may be
necessary -- e.g. if the ways in which the jaws move are now warped.
It is generally bad news to try to clamp a short piece at the
tips of the jaws with a lot of force, as this is the kind of thing which
springs the jaws.
One thing which might be worth considering is a 3-jaw for a
Taig. The spindle thread is the same, IIRC, and the Taig has two-piece
jaws, with the top jaws soft, so you can machine them at need to a
precise fit for a project which needs it (e.g. a step behind the
workpiece, to limit the travel into the chuck).
Good Luck,
DoN.
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