Vacuum Pump

[Ed Jarvis]

What size or hp Vacuum pump do you need for your lathe? I have the Vicmak Vacuum system?

Thanks.

ED

 

[n7bsn]

Ed, the HP is usually quite small, I think the one I use is 1/6HP. But the HP really probably isn't the number you want to worry about

The numbers you probably should look at are CFM and (more-so) inches of vacuum.. The one I use is 2.5 CFM and 29 inches (10 pascal)

TTFN
Ralph

 

[Mike Peace]

The Gast pump model 523 sold by CSUSA and Packard for vacuum pumps move 4.5 CFM. An older Gast model 522 sold used by Surplus Supply and commonly used for vacuum chucking moved 4 CFM. You might get by with less than 4 CFM capacity but your system will not tolerate as much leakage or porous wood.

 

[Ed Jarvis]

How many Cubic feet /min(cfm) is too much?

Ed

 

[hughie]

vac pumps

How many Cubic feet /min(cfm) is too much?

Well, work out roughly how much volume between the vac pump and the vac chuck and this is the volume you have to evacuate.
As you will see it is not much, so you wont need a high volume pump. So we are talking about a portion of a cubic foot.
Perhaps the more important number is the negative pressure number, ie inches Hg 0-32, or % 0-100. If your pump can run around 80% or 25Hg continuously it should do just fine.
You would only look at a higher volume number if your using very porous wood and on a regular basis. As for make and model the Gast range are very good and are local to your area. All else fails check the Vicmarc site for vac pump recommendations etc

Remember as the vac negative pressure rises the atmospheric pressure [14.5lb per sq inch] works with you to hold the job onto the chuck.

 

[Bill Boehme]

Well, work out roughly how much volume between the vac pump and the vac chuck and this is the volume you have to evacuate.
As you will see it is not much, so you wont need a high volume pump. So we are talking about a portion of a cubic foot.
Perhaps the more important number is the negative pressure number, ie inches Hg 0-32, or % 0-100. If your pump can run around 80% or 25Hg continuously it should do just fine.
You would only look at a higher volume number if your using very porous wood and on a regular basis. As for make and model the Gast range are very good and are local to your area. All else fails check the Vicmarc site for vac pump recommendations etc

Remember as the vac negative pressure rises the atmospheric pressure [14.5lb per sq inch] works with you to hold the job onto the chuck.

It does not have much at all to do with the system volume when determining the CFM since that is only the static requirement. Total system leakage is the only significant factor in the CFM requirement. There are plenty of areas for leakage in a vacuum chucking system. First, you will have a bleed valve to act as a vacuum regulator. Next, the rotary coupling will have some unknown amount of leakage depending on its construction along with type of bearings and seals. There is also leakage at the seal between the chuck and vessel. Finally, since wood is porous, air will flow through it. Some woods, especially if green, will leak like a sieve.

A rule of thumb is that the pump should be able to maintain the minimum necessary holding vacuum for whatever you are turning with a flow of 4 CFM which should be an adequate safety margin. Generally speaking, the blocked port vacuum does not need to be any greater than about 26 inches of mercury.

Another rule of thumb is that non-lubricated (aka "dry vane") rotary vane pumps are best suited for woodturning use since they provide high flow rate with moderately high vacuum. Generally speaking, diaphragm pumps suffer from low flow rate and no better vacuum than rotary vane pumps. Many woodturner use them with success, but they may not hold as well with larger turnings. Piston pumps produce the highest vacuum, but have low flow rates and the vacuum level pulses consdiderably if there is much system leakage which can be a problem. The remedy for this is to use an accumulator to smooth out the ripple in vacuum to the chuck.

 

[hughie]

I have a vane single phase vac pump and my leakage via fittings and couplings etc is steady and quite small and the main leakage is often the wood its self. Your suggested figures of 26" Hg or mercury are close enough to 80% and true vane pumps are the way to go. Dont know about in the US but here they will have quite often a small accumulator fitted from new. Depending on the make and quality etc as this will effect the size of the accumulator, for some its no more than a dust collector.

The point I was after, is that often, theres the tendency to go for a much larger vac pump than required and hence up front costs and run costs to consider
Here is Australia vac pumps are an expensive item 3-4 time the price of a compressor. In fact my 240v 1hp single phase new vac pump would have been around $1500+ and its by no means top of the range.

 

[Bill Boehme]

They are not cheap here either, but fortunately nowhere near $1500. I was lucky enough to find a barely used industrial surplus unit from a lab that was upgrading their equipment for less than $50.