On 2009-02-14, Michael Koblic wrote:

"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...

It is tricky doing this. You need a set of parallels of the
right size to support it -- or an aluminum extrusion "spider" which fits
between the chuck jaws and holds itself in place with magnets gripping
the chuck body face.

The spiders are neat -- but are rather expensive for what they
are.

The parallels must be removed before you spin up the chuck -- or
they are likely to hit you somewhere.

The advantage of turning soft jaws for the purpose is that you
will have support over most of the radius of the workpiece. Your
workpieces are fairly thin and would be likely to ring. Hmm ... if you
make the soft jaws from aluminum, you can embed permanent magnets in the
end away from the gripping step. (You will have problems with the chips
from steel piling up on the magnets, however. For brass or aluminum, no
problem. But good strong magnets in the jaws will help to control
ringing, and since you are going to be doing this a lot, it is worth
while making proper tooling for the task.

Of course -- there are also pie wedge soft jaws, which form an
nearly complete circle when the chuck jaws are at the tightest setting,
and those can be turned for your workpieces. You can even turn several
diameters working as steps from shallow at the OD to deep at the ID, so
progressively smaller discs can be gripped.

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.

Your "doughnuts" are actually disks not things thick enough to be
proportional for a doughnut, aren't they? Say an 8" disk perhaps 1/4"
thick or so?

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.

I'll bet that you will find that the magnets are not strong
enough for turning -- the workpiece will spin around the boss.

Good Luck,
DoN.

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