Peter Grey wrote:

If one has a piece of aluminum plate that one would like to make round using
a rotary table on a mill, how does one clamp it down so that one can use an
uninterrupted cut?

I can put a small (.200-ish) hole in the center, but that doesn't seem to be
enough to hold a clamp decently. I can clamp it from the outside, but I'd
have to move the clamps as I milled around the piece. I'm assuming that
this wouldn't allow for an uninterrupted cut which I wanted for the best
finish. Is this newbie missing anything? Suggestions?

Easiest way if you have a lathe and start out with a suare work piece is
to drill four holes in the corner, screw the stock to a piece of 3/4"
thick MDF (medium density fibreboard aka Medite). Mount in 4-jaw chuck
or on a faceplate and cut out your circle. If the radius is large
enough, the work thin enough and you have a not-too-high parting tool,
you can use it otherwise you need a trepaning tool. A trepaning tool
looks like a parting/groving tool except that it's curved so the sides
don't rub.

Alternately, you can clamp the work piece to the MDF with a large washer
and an a draw bar through the headstock. This is the best if the stock
is rather thick. Rough it out with a band saw then machine it round.

If you really want to do it on the mill, here's how: My RT has a hole
in the middle to take a MT-2 center. This hole extends right through at
the smaller diameter of the taper. I made a special nut that is longish
and hex at one end and steped down to a round that fits into the hole in
the RT. This nut is threaded for 3/8-16 but could be 1/4 or even 3/16.
The work piece has a hole drilled in what will be the center to take the
bolt and a fairly large washer. The RT needs to be aligned with the
mill. You can see my procedure for this in
http://www.metalworking.com/DropBox/Prop_speed_reducer.pdf
The RT is set up on two parallels so a thin wrench can get under it to
hold the nut while the bolt is tightened. The workpiece is set on a
piece of MDF so I can cut right through the work piece without hurting
the RT. Offset by the desired radius and mill away. Contact me off
list if you need a picture.

Ted