"DoN. Nichols" wrote in message
...
Hmm ... what I would suggest is that you get a three-jaw chuck
with two-piece jaws, then pick up a set of soft jaws to fit in place of
the hardened top jaws.

Tighten it onto perhaps a 1/2" diameter bar, and turn the jaws
leaving a projection a little thinner than the workpiece is to be near
the outside end for the smallest plate which you wish to face. Then you
can accommodate several larger sizes with the same set of top jaws just
by opening the chuck more. (Needless to say, the 1/2" diameter bar does
not remain in the chuck while you are using it -- it is only to set the
position of the jaws while you bore and face them.

I thought I could achieve pretty similar result by shimming the workpiece so
it sits higher up in the jaws. I tried it with the chuck on the table and it
seemed to look OK. Of course looking is one thing and turning is another...

Of course this will mean making a different faceplate for different
doughnuts but so be it. Another option is to look at central holding by a
3-jaw chuck but I suspect it wil not be a free lunch either.

Central holding will be better (once you have the center hole
bored), but it will be difficult to face right up to the chuck jaws.
Maybe a second set of soft jaws, turned to have a step to hold from the
inside instead. This way, the rest of the jaw's surface supports the
workpiece, so it is less likely to ring like a bell while you are
turning.

Magnets are used to hold workpieces which are being ground, but
don't have enough grip to handle workpieces which are being turned. The
forces are a lot higher.

Even grinding I do not rely on the magnets alone. That is where the central
boss comes in. It stops the lateral movement of the piece. The magnets (4 of
them) stop the vertical movement. I was hoping that the combination might
work for facing if the boss is sufficiantly tight inside the doughnut hole.


--
Michael Koblic,
Campbell River, BC