Thread: Vacuum Pump

Yes it will be suitable. I have a similar refrigeration vac pump as a back up for my Gast oiless vane vac pump.

You might find 3cfm is not enough for porous timbers. I have a 9cfm one that usually does a good job.

Good call Woodturnerjosh. Just checked my refrigeration pump and it's 4.2cfm which works fine on my Stubby. My Gast pump is 4.5cfm and always pulls over 26 hg

Originally Posted by Woodturnerjosh

You might find 3cfm is not enough for porous timbers. I have a 9cfm one that usually does a good job.

Got to disagree. The cfm won't make much difference, unless it is so porous it's like a sieve or has holes in it, in which case even a 9cfm won't hold it.

Originally Posted by dai sensei

Got to disagree. The cfm won't make much difference, unless it is so porous it's like a sieve or has holes in it, in which case even a 9cfm won't hold it.

My own experience (plus things I read when setting up the system) tells me otherwise. I've had timber so porous that putting my hands on the outside raised the vacuum pressure (or decreased - however you want to read) from 20 to 24 inches with the pump running flat out. If I had an even higher flowing pump it would have been better. Glenn Lucas mentions the same thing in his DVD which is why he uses a 14cfm vacuum.

Sorry but all of the above information is a little off, the displacement of the pump does not have to be related to the amount of vacuum the pump can obtain, you really should be looking for the maximum amount of vacuum that the pump can provide and be less concerned about the displacement.

The maximum amount of vacuum a pump can provide can be more looked at as the amount of leakage inside the pump design, less leakage = more maximum vacuum/higher x"Hg, and higher leakage = lower maximum x"Hg.

Lets assume you found a 1/2 CFM displacement pump that managed 40"Hg, it would provide more vacuum than any of the pumps mentioned above, but it would take a bit longer to achieve a full suck down. Its the pressure differential between what the pump can provide and what ever the atmospheric pressure is, that provides the clamping.

Originally Posted by Mick C.

... displacement pump that managed 40"Hg...

Wishful thinking considering the maximum achievable is just under 30"Hg

Ok ok, point taken, example was a bit extreme with the number pulled out of the air, but the principal still stands

Thanks guys, your advice makes sense - but how do I calculate how much vacuum pressure is needed? If I understand you correctly, a higher capacity in terns of CFM is necessary to compensate for any leaks and losses in the system or through the porosity of the wood being used. On the other hand the pump must be able to generate a sufficiently high enough vacuum and hence atmospheric pressure to be applied to the external surface of the bowl to provide the necessary 'clamping' force.

For example I have a small venturi system which when connected to the lathe / vacuum chuck set up is capable of about 27.4 "hg when the lathe is off. (I am assuming that the pressure read from the vacuum gauge which is between the venturi and the lathe is the 'net pressure' and would include the effect of any losses in the system.) It is not possible for me to 'pull' a bowl off the chuck with just my bare hands. But is this affected by centrifigal force when the lathe is spinning at 2000 rpm and I am cleaning up the bottom of the bowl with a gouge?

How much "pressure" needs to be asserted on the bowl as a result of creating the vacuum to ensure that the bowl is unlikely to break free and take off in my direction?

Is there a 'simple' formula to calculate that 'pressure' based on the vacuum ("hg) and diameter of the chuck at the sealing rim?

To what extent is the calculation related on the weight of the bowl?

Thanks,

Bob

Have a look at this long winded demo www.youtube.com/watch?v=PEN5GkoelbY

I labor under the delusion that vacuum chucks are used for finishing. Presumably the inside is finished before reversing on to the vacuum chuck. If there are a couple coats of Tung Oil, Danish Oil, etc. on the inside I would think the piece is not going to leak much air, thereby not needing a pump to pull a lot of cfm.

If you have a loose burl you might need one of these: http://imgusr.tradekey.com/p-4319163...e-diffuser.jpg

Originally Posted by WOODbTURNER

Have a look at this long winded demo www.youtube.com/watch?v=PENS5GkoelbY

Unfortunately that link comes up with "This video is unavailable"

Originally Posted by Paul39

I labor under the delusion that vacuum chucks are used for finishing. Presumably the inside is finished before reversing on to the vacuum chuck. If there are a couple coats of Tung Oil, Danish Oil, etc. on the inside I would think the piece is not going to leak much air, thereby not needing a pump to pull a lot of cfm.

If you have a loose burl you might need one of these: http://imgusr.tradekey.com/p-4319163...e-diffuser.jpg

I thinkI might have give loose burls a miss

https://www.youtube.com/watch?v=PEN5GkoeIbY

Hope this one works!