What size vacuum pump

"Dave W" wrote: (clip)What I can't seem to find is the minimum cfm needed.(clip) ^^^^^^^^^^^^^^ The capacity of the pump affects two things:

1.) How fast does the system reach a usable vacuum? b.) How much leakage will the system tolerate, and still work?

If you use a pump with more than enough capacity, you will be able to turn wood without spending lots of time sealing leaks. You will have a bleed valve, which will usually be partly open, so you don't get TOO MUCH vacuum.

If you go for a pump with very little volume flow, the slightest leak will get you in trouble. I say there is no lower limit, but I advise you to get the biggest pump you can find and afford. Mine is a 3/4 HP Gast, and I am very happy. (It's a bit noisy, though.)

Dave I built Vacuum System for my Lathe and the details are on my Web Site where there is a two part article in the Hints and Tips Section.

cfm of the Vacuum pump in reality only needs to overcome the amountof leakage you may get on the system, but this is difficult to predictas seapage through certain woods can occur. Best advise is to make the bit you make as Air Tight as possible, I used Vacuum Grease on all Joints and this works well providing joints are not to Loose/Wide.

The other area where cfm has an influence is in the length of time it will take to Evaacuate the Volume between the Pump and the Base of the Piece you are holding.

If you assume the system has no leaks then a 4 cfm Pump would take approximately 1 minute to empty 4cu ft of System which is an enormous Volume, my Pump is rated at 2.4cfm and takes less than 1 second to pull a 26 inch Vacumm.

This is for the System + any Leaks.

Hope this Helps

Richard

Dave W wrote:

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If you're a fairly handy "do it yourself" type, you can build a good system for little cash outlay. Find a wrecking yard that has some 60's and 70's cars left. Find one that has a 2 cyclinder air conditioner "V" style compressor (York). These are a preferred style since they can be easily set up vertically and be stable. Take the clutch assy apart, and use epoxy to fill it, or braze it to make it a solid assembly. Use a metal bracket to mount it, using a 1/4 HP or larger electric motor to turn it. Unless you will be turning something very porous, you will need a bleeder valve to reduce pressure. Too much vacuum will implode a platter or large bowl. Rotary vacuum couplers are available thru the usual wood turning suppliers. Cover the faceplate with a closed cell foam rubber (like a wet suit). A vacuum gauge placed between compressor and lathe is a "nice to have " accessory that allows you to monitor your system. An in-line air filter will assure a clean running compressor.

Ken Moon Webberville, TX

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

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 rat> > From what I have been able to find, a vacuum of 10 inches should be

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

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.

Quite right. Good vacuum system design is to use large diameter tubing. In a practical sense, just make sure your pump has a flow rate much higher than your system's leakage rate. It is easier to use an oversize pump that will keep ahead of the leaks than it is to use a miniscule flow pump and track down leaks. A 1/3HP, 5cfm, pump developing up to 28.5" Hg should be sufficient for anyone's work clamping setup and not be too hard on the pocket book. HVAC repairers use such a pump and they aren't too expensive. Dan

I'm not sure that the Renolds # is applicable to a Gas [Air] only applies to Fluids for determining whether you have Laminar or Turbulant Flow for Thermodynamic Calculations.

Surely once you have a Vacuum the Pipe/Tube is empty so you are Sucking Nothing other than any Air that Leaks in? Therefore Pipe Size/System Volume is probably best kept to a minimum?

=====> This has been the most erudite discussion regarding vacuum pumps and I have gained the following knowledge on how to select one: Get the biggest pump you can, but not too big, or not too small; use the largest hose you can, but not too large or not too small. I'm sure glad that I hadn't read that before I had Bill Noble sell me one of his pumps, as it was it was just the right size and I followed the instructions and used just the right hose, it holds the bowl or blank like it was glued on. Thanks, Bill!!!

Leif