"william_b_noble" wrote in message
news:1115448791.a702db2e45a5b5805b9a5f6457eee204@t eranews...
allow me to most humbly disagree. a small passage means high reynolds #
loss and inefficiency. A small pump, however "efficient" (whatever that
means) has some maximum flow rate. if you turn a thin walled vessel, or
have any irregularities, warpage, etc, you will have leakage. The small
pump will at some level of leakage, have inadequate flow and will be
unable
to obtain sufficient vacuum to reliably hold the piece. Having a small
orofice like you suggest will aggrevate the problem.



I would say, based on experience and tests using a variety of pumps, that
a
open or zero pressure flow of less than 2.5 to 3 CFM means that the pump
will be marginal as a vacuum chuck - it may be totally fine for many other
purposes, for example evacuating molds, AC systems, etc, but not for wood
turning. A flow of much over 10 CFM is also probably not needed, if you
have that much leakage, the losses elsewhere in the system will prevent it
from working.

It's a bit like deciphering a dust collector rated in HP (or SearsPower)
rather than flow rate @ some pressure, or just flow rate. You've got to be
consistent in your units first, then figure out which are important for what
you do. Have to be aware of some of the tradeoffs as well. Bill's note on
passage size is spot on.

"Excess" capacity on a nice piece of hard maple can quickly translate to a
flying piece of red oak if you're not careful.

Vacuum chucking is something I'll try once the last is out of college - on
my new BIG lathe.