"Peter Grey" wrote: (clip) Thoughts and suggestions welcome.
^^^^^^^^^^^
I am not a machinist--I am a woodturner. Vacuum chucks are becoming very
popular on wood lathes, and I use one a lot, so I will tell you what I can.
First, the amount of vacuum you need obviously depends on the area holding.
If the contact area is several inches in diameter, you can probably get by
with less than 26" Hg. However, I believe the tool forces in metal turning
are much higher than wood, so I think you will have to experiment. It is
tricky to get a good vacuum seal, which means you will be fighting leaks.
If your pump has limited volume, small leaks will reduce the vacuum, and
drive you nuts. My pump is a 3/4 HP Gast vane pump--I can be fairly casual
about leaks, 'cause the pump is that large.

In wood turning, we use foam gaskets to form the seal to the vacuum chuck,
but I am afraid that if you tried that on a metal lathe, the deflections due
to compression of the gasket would destroy your accuracy. If you can use a
machined fit between the vacuum chuck and the work piece, possibly with a
little wax or grease to help with the seal, I think you will be good to go.
Another possibility would be to use hot glue to seal leaks.

It is good to have a close-fitting ridge to hold the workpiece against
radial slippage--the vacuum force is good for holding the piece ON, but it
isn't too good for holding it CENTERED.