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.

It sounds like you've sprung your jaws. Did you have something
held really tightly just before the slipping started? Or, did you have
a crash?

What it sounds like is the jaw faces are no longer parallel, but
would fit a conical taper. I've had this on larger chucks due
to wear at the front of the jaw faces. Anyway, if the rest of the
chuck is still sound, you need to regrind the jaw faces parallel
to the spindle axis. What you do is mount a Dremel tool or similar
grinder to the toolpost. Open the jaws enough so you can fit a small
grinding wheel between the jaws. Use chewing gum, rubber bands or
whatever you can find to pull the jaws outward, against the face
of the scroll, just like it is gripping a part, but leaving the entire
gripping face of the jaw clear.

Pull the crossslide toward you until the grinding wheel just takes the
slightest bite of the jaw at the back. Run the carriage back and
forth, taking a little off the jaws at a time until the grinder cuts
the full length of the jaws. Repeat on all 3 jaws. Then, set the
lathe spinning at the slowest speed, and use power feed to run the
grinding wheel across the full face of the jaws.

When you are done with this, disassemble the entire chuck, clean
and relube. You will need to run cloth or paper towels through
the jaw grooves to clear all the dust out.

After reassembling the chuck, test it with a smooth, hard rod, like
hardened and ground shafting. If you tried this before, you would
have seen the front of the rod move, and you could have slipped
paper between the jaws and the rod at the jaw tips. now, you
should have no movement and no gap there.

Regrinding jaws is a fairly common procedure in the shop, and it can
make a HUGE difference in chuck performance. When the work starts
walking in the chuck, you know it is time.

Jon