Vacuum Pump - How Powerful ?

[Michael Mendelsohn]

I'm looking to build a vacuum chuck for my lathe and would like to buy a decent pump that won't break the bank. I see that many folks are happy with the Gast 523 pumps but they seem to sell for $300 or more. I'd like to spend about half that for the pump.

I've seen a rebuilt Rietschle Thomas 3.15 CFM pump for about $100 (VeneerSupplies.com) and am wondering if this would be adequate. It is a double piston oil-less design and is rated for continuous duty. It is rated for 24" of hg.

Would this be adequate for most turning?

Any other suggestions?

Mike

 

[Tom Coghill]

Mike,

I broke down and purchased new, however many others have been good going used - then again I have heard some bad stories too...

Anyway, you are asking about vacuum force. Most of the time I use 10 - 20 inches of vacuum to hold my turnings (depending). Since you will also be using a bleed valve to control, I believe the volume / vacuum curve is important. Make sure your pump will draw a fair volume too (I believe mine draws about 4 CFM at 12 inches Hg - but I could be wrong there).

Back to vacuum forces ...My pump will draw about 24 inches of vacuum. Each inch of vacuum equates to .491 PSI of force. Figure out that AREA of your chuck, multiply it by the PSI and you will get an idea of the force (this is in effect replacing your tailstock).

A 3.5 inch diameter chuck has 9.621 square inches of area
A 5.5 inch diameter chuck has 23.76 square inches of area

How much pressure do you put on your tunings near completion?

Using a 5.5 inch chuck, 15 inches Hg of vacuum results in 175 pounds of force. So if your turning is on the thin side of normal, this will probably break it.

I set up a table for these two chucks that I use so that I would keep myself out of trouble (on both the low end and high end). It's a quick thing in Excel.

Hope this helps. If you want me to send you a copy of my table, send me a PM with your email address.

Hope this helps you.

Tom

 

[Mark Mandell]

Michael,

The CFM rating on your pump becomes an issue in the context of the wood you're turning. Solid dense wood that is round and makes a good seal on the chuck needs less CFM because the are fewer leaks. If, however, you're turning porous wood (burls, etc) that may have less than a good seal and/or your "system" (hoses,chuck, etc.) is not super tight, more CFM is needed to compensate for the loss of vacuum.

Inches of Mercury (Hg) is also relative to the size of your turning. I've turned an 18" platter on a 12" dish chuck using only 7" of Hg. More and I would have broken the piece. Good general rule is the smaller the turning, the more vacuum you need to hold it.

 

[Bill Boehme]

I have one of the 523 series Gast vacuum pumps -- the dry vane type which is preferable to the version that is oiled. I got mine on eBay for around $50 from a company that makes orthotics. The open port flow rate is about 4.5 CFM. Open port flow means means that it is open to the ambient atmosphere with no flow restrictions at either the inlet or outlet. That is a number that is used as part of designing a system and not something that you are going to see in a complete system which adds a lot of restrictions to the flow even when fully open. The other design parameter is blocked port vacuum which mean the maximum vacuum that can be achieved with zero flow -- in other words, a perfectly sealed system. Just as in the case of open port flow rate, you won't see that in the real world. You do not really need to do any engineering design to set up a vacuum chucking system as long as you have a pump with specs that are somewhere in the neighborhood of what the Gast 523 series has.

In a practical system, you ought to be able to achieve a vacuum greater than 20 in-Hg (inches of mercury) which is more than enough. Just as important is the diameter of your vacuum chuck for the piece that you are turning. How much vacuum depends on several things such as system leakage (you add some leakage on purpose), leakage around and through the wood, condition of the pump, and the altitude at your location.

I have seen many design of vacuum systems where the vacuum gauge is not optimally placed in the system which leads to having an erroneous indication of the vacuum at the chuck. Optimally, the gauge should be located as close to the vacuum chuck as you can -- within reason, of course. Close to the rotary coupler would be a good location. A really bad location would be downstream of the manifold, filter and bleed valve.

 

[Michael Mendelsohn]

Thanks for the advice. Decided not to skimp on the pump and get something decent. I found a rebuilt 1023 Gast pump on eBay that I bought. This is basically the same as the 523 with a higher cfm rating and a 1/2 hp motor. This should hold everything but my wife's colander.

 

[Richard Coers]

Thanks for the advice. Decided not to skimp on the pump and get something decent. I found a rebuilt 1023 Gast pump on eBay that I bought. This is basically the same as the 523 with a higher cfm rating and a 1/2 hp motor. This should hold everything but my wife's colander.

That makes me chuckle. You decided not to scrimp and then you bought a rebuilt pump from a stranger. Hope he knows how to rebuild them properly, or even did rebuild it. I've been burnt on eBay, I really don't consider that a safe source of critical components.

What's wrong with the oil bath pumps? If misting is a problem, you can add a $4 gas filter from the auto parts store and eliminate that.

10cfm may not be enough when talking about porous wood. That's why you have a gage on it, so you can safely know when you have adequate vacuum.

 

[Larry Komroff]

Michael,

I think you will really like that pump. I have a 1023 purchased off ebay also. As far as 10 cfm goes, I didn't see a pump under $1000 with a higher rating and with that rotary vane pump pulling 26 in. Hg. you should not have any problems at all. When you get it setup, let us know.

Larry

 

[Bill Boehme]

.... That makes me chuckle. You decided not to scrimp and then you bought a rebuilt pump from a stranger. Hope he knows how to rebuild them properly, or even did rebuild it. I've been burnt on eBay, I really don't consider that a safe source of critical components......

We're also chuckling because we have gotten great vacuum pumps on eBay at great prices. I wouldn't buy from just anybody, but it is pretty easy to discern which sellers are reputable and even easier to tell which aren't.

.... What's wrong with the oil bath pumps? .....

Wood dust mixed with oil is the problem. The oil is a lubricant, but mixed with wood dust, it is abrasive ... a very gentle abrasive, but still an abrasive. After a while the sludge that is made up of wood dust and oil prevents the vanes in the pump from sliding properly in their slots which means reduced vacuum. This means more frequent maintenance to disassemble and clean out the pump.

.... If misting is a problem, you can add a $4 gas filter from the auto parts store and eliminate that.....

Have you actually tried that? How long does the gas filter go before it is saturated with oil and either clogged or blowing it out to the open air?

 

[Mark Mandell]

What's wrong with the oil bath pumps? If misting is a problem, you can add a $4 gas filter from the auto parts store and eliminate that.

Richard, a $4 filter won't do; won't allow the air flow and clogs quickly which then backs up into pump. Been there and tried. The Coalescing filter is needed unless your pump is outside. A descent filter will set you back $100+ parts to hook it up. I made the mistake of getting a rebuilt 6CFM oil-lube pump. It needs to be cleaned and the oil changed after 5 hours of use to prevent the sludge even though I have a good filer in the line.

Pump's fine (pulls 28"), but not for turning. I'll sell it shortly and get an oil free unit.

 

[Richard Coers]

I guess I've had the filter on the outlet of the pump for about a year. I don't use it every week, but has worked well for me. Dumb luck I guess, but since I have good dust collection around the lathe, I don't expect the pump will eat a lot of dust. Since Mark says I need a $100 filter, I guess I can use it for another 25 years and still spend about the same amount of money. I'm 61, so if I can use this system till I'm 86, I guess I will be about done turning by then. Thanks for telling me it won't work Mark, I would have not know that without your advise.

 

[Mark Mandell]

Thanks for telling me it won't work Mark, I would have not know that without your advise.

You're welcome, Richard. Guess I should have qualified that it wouldn't work with my model pump?

When I hooked a fuel filter up (the pump's discharge port is 9/16" diameter) the oil quickly got the paper filter soaked and "appeared" to cut the air flow way down so the pump was working against the restriction. I was then told by the tech that I needed a coalescing filter rated to exceed the 6 CFM air flow rating of the pump. Perhaps the Robinair puts out more of the oil than yours. I do have to empty my catch bowl about every 4 hours of operation.

Curious as to how you empty the oil in your filter or do you just change it out after a few hours?

 

[Richard Coers]

Honestly, I didn't see any oil mist before I put the filter on, and don't see any oil in the filter now. I don't run it for hours, just clean up the foot on bowls. It is a Chinese pump, puchased as a scratch and dent special at a HVAC supplier. I was told to release the vacuum before shutting it off. If I didn't do that, it would "use" more oil. Maybe it will change with age, but really happy right now.

 

[DOCworks]

I don't know if this is applicable here but I've put together two systems for my 3520b using a Thomas pump; here are the specs:
115VAC Air Piston Pump/Compressor, used, tested, good conditon.

Pumping Mechanism: Piston
Freeflow: 4.8CFM (135.9 lpm)
Max. Vacuum: 29.0inHg (982mBar)
Max. Pressure: 40PSI (2.8Bar)
Input Voltage: 115VAC, 60Hz
Current Draw: 3.8 amps
Suspension-mounted to reduce effects of vibration on surroundings.

Includes required 15µF capacitor.


Thomas
2669CE37-190A

They sell them at Skycraft for $35.00 each.
I pull about 23 to 26 with most anything I put on it...at least that's what the gauge says anyway and nothing has come off yet. I use it to clean up bottoms also, but I also use it to turn off center pieces like Doug Fisher makes, so it does get a workout. They do have a website, don't know what shipping is cause their store is about 3 miles from work.

 

[Michael Mendelsohn]

I got everything connected and working but I noticed a potential problem. When I turned the pump off, the vacuum disappeared almost instantly. I expected the vacuum to bleed off slowly, not crash. Is this normal for a Gast series 23 pump?

If the power fails, the lathe and pump will both stop. If the vacuum bled off slowly, the lathe would be stopped before the bowl released. If the bowl releases instantly, the results would be more spectacular.

I'm thinking that I need to add a check valve and possibly a reservoir tank to the system to maintain the vacuum for a couple of seconds.

How have others addressed this problem?

 

[Tom Coghill]

Some people put into the system a vacuum reservoir. This will allow you some time to regroup should the power fail.

When I want to remove a piece, I always:

1) Stop the lathe
2) Open the bleed valve completely (the piece will remain stuck as the bleed valve is small compared to the volume being drawn by the vacuum pump.
3) Pull the piece free
4) Turn off the vacuum pump

When I assembled my system I made double sure that I used plumbing tape on all my threaded connections (including set screws on the vacuum chuck). I believe that I have very few unintended losses in my system.

This is what I have experienced. Test your system with scrap wood before mounting up your prized project.

 

[Dwight R Rutherford]

Search this forum for "reservoir ". You can also check out an article in the Feb. 2011 issue of the American Woodturner. Hope this helps.

 

[Bill Boehme]

I got everything connected and working but I noticed a potential problem. When I turned the pump off, the vacuum disappeared almost instantly. I expected the vacuum to bleed off slowly, not crash. Is this normal for a Gast series 23 pump?

It is normal for any rotary vane pump and in the configuration that woodturners use, the kind of pump wouldn't really matter much because of all of the various leakages in the system. First, keep in mind that the bowl is like a bundle of straws and there is a huge amount of air passing through it especially if it is thin. There is also leakage between the seal and the wood. The rotary coupler has leakage, and don't forget that you have a bleed valve to regulate the amound of vacuum. If you think that you can prolong the bleed-down by using an accumulator, keep in mind it will need several things:

• a vary large volume in order for it to be worthwhile
• a very large diameter line in order to have very little flow impedance
• connected as closely as physically possible to the rotary coupler

You could also consider the use of either a check valve that goes between the accumulator, rotary coupler, and vacuum chuck on what would normally be the high pressure side and the remainder of the system on the other side. The check valve would result in some differential pressure loss during normal operation. You could use a powered normally open valve which might have less pressre loss.

Additionally, I think that your concerns may not be as bad as you envision.

• First when reverse turning, the lathe speed needs to be slow.
• Second, even though the vacuum is holding the piece, the tailstock is still needed for several reasons -- to keep the piece from shifting and to reduce the problem that you are primarily concerned about. The only time that the tailstock is removed would be when cutting away the nub at the center.
• If the piece falls off the lathe and gets damaged that would at the very least be terrible if not a true disaster. When finishing the bottom of a turning, the piece is fairly light weight, you are running the lathe very slowly, you have the tailstock in place, you are making very very light cuts, and the odds of a power failure during those few moments is far less likely than winner the power-ball lottery (assuming that you do not live in a "developing nation" where the lottery odds aren't any better, but the power grid might not be so good).

 

[Michael Mendelsohn]

I agree that I'm probably better off worrying about the lottery than a power failure. I'll go with what I have for now and see what I need later on.

Thanks for all the feedback,

Mike

 

[Mark Mandell]

I agree that I'm probably better off worrying about the lottery than a power failure. I'll go with what I have for now and see what I need later on.

Thanks for all the feedback,

Mike

On the other hand, Michael,

Having all the shiny plumbing/engineering work of a reservoir, valves, and actuators, not to mention the UPS battery backup, hooked up to your lathe will impress the hell out of all your friends in much the same way as that set of wheel pullers and your collection of oil filter wrenches you have mounted, outlined, and labeled on the pegboard in the garage.

 

[Charlie Knighton]

any opinions on the Holdfast vacuum chuck system , just needs a standard shop air compressor (minimum 2.5 cfm at 55 -65 psi) ?????????

if as Bill says you are going to use the tailstock except to turn off the nub, this system would seem to work ok at a reasonable cost

item #104-527 from craft supplies

 

[Shawn Pachlhofer]

any opinions on the Holdfast vacuum chuck system , just needs a standard shop air compressor (minimum 2.5 cfm at 55 -65 psi) ?????????

if as Bill says you are going to use the tailstock except to turn off the nub, this system would seem to work ok at a reasonable cost

item #104-527 from craft supplies

according to the Hold Fast website - the vacuum "generator" has 0.6 CFM

not very much volume

you can buy a venturi vacuum generator (basically what Hold Fast is selling) at Horrid Freight for $15.99

 

[Bill Boehme]

What Shawn says and the venturi systems are not tolerant of leaks if they are to maintain a reasonable vacuum. If you want to experiment, get one from Horrible Fright (their stuff really is pretty good especially considering the price).

 

[Mark Mandell]

any opinions on the Holdfast vacuum chuck system , just needs a standard shop air compressor (minimum 2.5 cfm at 55 -65 psi) ?????????

if as Bill says you are going to use the tailstock except to turn off the nub, this system would seem to work ok at a reasonable cost

item #104-527 from craft supplies

Hugely inefficient. Running a 2hp (or more) air compressor to get what a 1/8 hp vacuum pump will do is a waste of money every time you use it.

 

[Patrick Miller]

reservoir

I got everything connected and working but I noticed a potential problem. When I turned the pump off, the vacuum disappeared almost instantly. I expected the vacuum to bleed off slowly, not crash. Is this normal for a Gast series 23 pump?

If the power fails, the lathe and pump will both stop. If the vacuum bled off slowly, the lathe would be stopped before the bowl released. If the bowl releases instantly, the results would be more spectacular.

I'm thinking that I need to add a check valve and possibly a reservoir tank to the system to maintain the vacuum for a couple of seconds.

How have others addressed this problem?

I used 10" of 4" schedule 40 pvc and two end caps make a small vacuum tank- drilled and tapped one end cap and added a 1/4" barb fitting to connect to the system with hose. A ball valve to set the bleed so I don't implode my work- an $85.00 ebay Thomas pump and a gauge completes the set up. works well, didn't break the bank.

 

[Charlie Knighton]

checked hf website, they do not have venturi vacuum generator by that name, did go down to the store, talked to manager, they do not carry it in his store

headed down to l owes to try Patrick's method, thanks Patrick

 

[Shawn Pachlhofer]

checked hf website, they do not have venturi vacuum generator by that name, did go down to the store, talked to manager, they do not carry it in his store

headed down to l owes to try Patrick's method, thanks Patrick

http://www.harborfreight.com/air-vacuum-pump-with-r134a-and-r12-connectors-96677.html

I've got one brand new, in box - I'll sell it cheap.

you do have to modify the fittings to work, though - since this pump is designed to work with AC units.

 

[Richard Coers]

checked hf website, they do not have venturi vacuum generator by that name, did go down to the store, talked to manager, they do not carry it in his store

headed down to l owes to try Patrick's method, thanks Patrick

Here is what Harbor Freight sells for a venturi.

http://www.harborfreight.com/air-vacuum-pump-with-r134a-and-r12-connectors-96677.html

 

[Bill Boehme]

http://www.harborfreight.com/air-vacuum-pump-with-r134a-and-r12-connectors-96677.html

I've got one brand new, in box - I'll sell it cheap.

you do have to modify the fittings to work, though - since this pump is designed to work with AC units.

I think that it is funny (in a sad sort of way) that HF advertizes the venturi for air conditioning system work. In order to remove all traces of moisture, the system needs to be pumped down to a hard vacuum. From a performance perspective, the specified absolute pressure of ~1.5 inches of Mercury isn't even close to being adequate. In the old days, it was common practice to use a "sweep charge", but you can't do that sort of thing anymore nor would you want to pay for the extra wasted refrigerant anyway.