only large items because most shop vacs pull somewhere between 3 and 5
inches of mercury - that won't hold a small item. There is a handy dandy
table of forces (diameter, vac) in my article on vac chucks that may be
helpful. Figure that a shop vac will get you to 3 or so PSI of holding
force - so with a 12 inch diameter item, that's about 36 square inches, so
about 100 pounds of force - enough to hold a bowl while you lightly clean up
the foot.
With a 3 inch diameter object, that's 9 square inches, and you get around 30
pounds of force - not enough to counter even sanding pressure.

bill
"Owen Davies" wrote in message
...
Among other comments, william_b_noble wrote:

If you will
only turn large items (a foot in diameter or more) a shop vac is
adequate
and has lots and lots of flow.

I'm trying to figure out why only large items. Because it's a weak
vacuum, but the large area of the piece produces enough total pressure
to ensure adequate gripping power?

How large does your work have to be in order to use a shop vac?

Thanks.

Owen Davies