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.
Your 1/4 hp gast pump actually has a pretty good flow rating, refer to
http://www.gastmfg.com/pdf/rotvane/r...ne_catalog.pdf
to see the specifications for your pump - you didn't specify the model. The
0323 series, for example, has a zero pressure flow of around 3 CFM. I use a
3/4 hp pump that has a 0 pressure flow of about 10 cfm - I find it is far
easier to hold objects that have leakage from cracks, worm holes, or just
the pores of the wood. 3 CFM is adequate, but more is better. 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.
"but it's only a matter of waiting a few seconds more to reach max vacuum
than say, a 3/4 hp pump" - this is just plain wrong under many
circumstances. It is true if there is zero leakage. It is probably false
if the total leakage approaches 1 or 2 CFM or so -- referring to the curves
in the manual whose link I provided above, you can see that at a flow of 2
CFM your pump can develop a max of 10 inches of vac, whereas at a flow of 2
cfm leakage, the 1023 series can develop 20 inches or more. You could amuse
yourself by writing out the differential equations for this if you like,
from the provided curves, they are linear in the operational region of
interest, but I don't think it's worth the effort to do that.
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.
"Ken Grunke" wrote in message
...
Dave W wrote:
From what I have been able to find, a vacuum of 10 inches should be
sufficient for a vacuum chuck.
What I can't seem to find is the minimum cfm needed.
Used pumps are fairly easy to find, but I'm not sure what is the minimum
size I need.
Dave in Cherrylog, GA
A smaller passage between the chuck and pump is better than a larger
one, since there's less air to evacuate and you don't have to take into
consideration air flow as with a compressor. With this in mind, cfm may
not be as important as you think--an efficient low cfm pump can be
better than an inefficient hi cfm one.
My 1/4 hp Gast pump has fairly low cfm rating, but it's only a matter of
waiting a few seconds more to reach max vacuum than say, a 3/4 hp pump.
BTW, a good source for closed-cell foam rubber seal material: 2 mm thick
craft foam in 9 x 12 sheets, with adhesive backing. Available at craft
stores and Walfart for about 60 cents each, many choices of colors. Also
great for jam chucks. Moldable around compound curves (doesn't buckle).
Ken Grunke
rural La Farge, WI
http://www.token.crwoodturner.com/
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