vacuum pump for chucking

[Topcat32]

hi doing a bit of Resin casting and turning, i got a cheap vacuum pot and pump, the pump has already given up the ghost ,looking to replace it with a better quality one and also use it for vacuum chucking, so will have to run constantly i guess, anyone use or got ideas on 1 that wont break the bank, dont really want to go down the holdfast road as then i would need to upgrade my compressor as that's not man enough as well as buying a new vacuum pump the 1 i had was a vevor, lasted about 2 weeks i think from doing a little research i need a 2.5 to 3 CFM one, any help greatly appreciated

 

[SVB]

Have a look as some of Bob’s old articles - they largely remain valid.

http://www.bobchapman.co.uk/html/for_woodturners.html
in terms of pumps, Gast make good pumps that frequently come up on the normal auction sites, worth setting a search reminder.

Simon

 

[chaoticbob]

This comes from experience in chemical laboratories rather than woodworking, so may or may not be relevant.
You can get vac pumps which attach to a water tap - they work on the same principle (Venturi effect) as the Holdfast systems, but are cheaper - for instance this from camlab. The theoretical ultimate vacuum you can get from these things is equal to the vapour pressure of the water at the temperature it comes out of the tap. In practice I've found that they will pull about 85% vacuum, so ~850 grams per square centimetre. I'd think that's OK for a vacuum chuck, but I don't know how much that would be derated by leakage between the chuck, workpiece etc. Might be worth an experiment?

I don't know what sort of vacuum is actually necessary for resin casting, but I'd be interested to hear.
Bob

 

[Inspector]

I don't know what sort of vacuum is actually necessary for resin casting, but I'd be interested to hear.
Bob

For casting you need pressure to keep bubbles from forming. The pressure keeps the air in solution. Vacuum is sometimes used briefly for degassing resin and for stabilizing wood. The wood is submerged in special heat setting resin under full vacuum and when the bubbles stop coming from the wood the vacuum is released and atmospheric pressure forces the resin into the wood. Then it is heated to set the resin. For stabilizing wood you want as close to maximum vacuum as you can get.

Pete

 

[chaoticbob]

For casting you need pressure to keep bubbles from forming. The pressure keeps the air in solution. Vacuum is sometimes used briefly for degassing resin and for stabilizing wood. The wood is submerged in special heat setting resin under full vacuum and when the bubbles stop coming from the wood the vacuum is released and atmospheric pressure forces the resin into the wood. Then it is heated to set the resin. For stabilizing wood you want as close to maximum vacuum as you can get.

Pete

Thanks Pete - that makes sense. I have been flirting with ideas of resin casting (and also resin wood stabilisation) hence my interest in this topic. From what you say casting needs positive pressure and stabilisation needs negative. So I think that maybe Topcat32 is referring to stabilisation of wood prior to turning rather than casting solid lumps of plastic. Apologies if I've got that wrong.

I too had looked at the Vevor single vane rotary pump, but in view of Topcat's experience I'm glad I didn't buy. I did think that £43 for a pump which could pull a 5Pa (that's fifty millionths of an atmosphere!) vacuum was a bit fishy. At work we would pay about £2k for a vane pump (Edwards).

I can see what you mean about getting the maximum vacuum possible to clear the wood pores of air, but I'm sure it doesn't need such a hard ultimate vacuum. And you did say 'briefly'. So I wondered if these fancy pumps might be be overkill and the low-tech water pump could meet the OP's needs - almost certainly for a vacuum chuck I'd think, but maybe 85% isn't good enough for impregnating wood. I'll suck it and see.
Bob.

 

[Inspector]

Yes pressure for casting. Vacuum for stabilizing, as close as can be had to 100% vacuum for 6 to 60 hours depending on the wood, soft or spalted less and hard resiny wood longer. Most of the hybrid blanks have the wood stabilized before casting for colour etc in the voids. Cactus Juice is the gold standard everybody else toys to copy. TurnTex, LLC There s a bunch of information there if you are really intrested. We are still using inches of mercury for the amount of vacuum. 29" or a touch more at sea level is the most one can draw. I have a compressed air powered venturi that can draw 26" at sea level and it is okay for degassing for a short time but not enough for stabilizing. It should work for turning if the seals are good and the losses through the wood is minimal. The water type might be similar.

Pete

 

[Topcat32]

Thanks Pete - that makes sense. I have been flirting with ideas of resin casting (and also resin wood stabilisation) hence my interest in this topic. From what you say casting needs positive pressure and stabilisation needs negative. So I think that maybe Topcat32 is referring to stabilisation of wood prior to turning rather than casting solid lumps of plastic. Apologies if I've got that wrong.

I too had looked at the Vevor single vane rotary pump, but in view of Topcat's experience I'm glad I didn't buy. I did think that £43 for a pump which could pull a 5Pa (that's fifty millionths of an atmosphere!) vacuum was a bit fishy. At work we would pay about £2k for a vane pump (Edwards).

I can see what you mean about getting the maximum vacuum possible to clear the wood pores of air, but I'm sure it doesn't need such a hard ultimate vacuum. And you did say 'briefly'. So I wondered if these fancy pumps might be be overkill and the low-tech water pump could meet the OP's needs - almost certainly for a vacuum chuck I'd think, but maybe 85% isn't good enough for impregnating wood. I'll suck it and see.
Bob.

Bob, Just to clarify, i have a pressure pot and compressor for casting bowl blanks, unfortunately its only a small 1 Max about 10" i wanted to start making larger bowls about 12-13" and a pressure pot this big is in the region of £300, i had a idea of buying a cheap (vevor) vacuum pot and pump about £100, to pull the air out of the resin before pouring it into the blank mould> this seems to work i had lovely bubble free casts, just the pump is not up to the task overheats and falling to bits after 2 castings, i have been onto a specialised shop Hvacstore in Andover, very helpful , they are aware of the problems i have had and suggested a 2 stage pump ant single and a 3CFM 1 is about £150, i am looking also at the holdfast vacuum chucking system and they said this pump would be more than able to cope with this, so i could purchase just the chuck and spindle, i wouldn't need the expensive vacuum generator that runs off a compressor oh stay clear of Vevor

regards Tim

 

[Lefley]

Bob, Just to clarify, i have a pressure pot and compressor for casting bowl blanks, unfortunately its only a small 1 Max about 10" i wanted to start making larger bowls about 12-13" and a pressure pot this big is in the region of £300, i had a idea of buying a cheap (vevor) vacuum pot and pump about £100, to pull the air out of the resin before pouring it into the blank mould> this seems to work i had lovely bubble free casts, just the pump is not up to the task overheats and falling to bits after 2 castings, i have been onto a specialised shop Hvacstore in Andover, very helpful , they are aware of the problems i have had and suggested a 2 stage pump ant single and a 3CFM 1 is about £150, i am looking also at the holdfast vacuum chucking system and they said this pump would be more than able to cope with this, so i could purchase just the chuck and spindle, i wouldn't need the expensive vacuum generator that runs off a compressor oh stay clear of Vevor

regards Tim

the holdfast pump is a Venturi pump. So you need an air compressor to run it. a good one. So it is only good for pulling a moderate vacuum so you can vacuum chuck on a lathe. It is no good at all for pulling a vacuum for resin casting. It will not pull the needed 29 hg.

 

[Lefley]

Yes pressure for casting. Vacuum for stabilizing, as close as can be had to 100% vacuum for 6 to 60 hours depending on the wood, soft or spalted less and hard resiny wood longer. Most of the hybrid blanks have the wood stabilized before casting for colour etc in the voids. Cactus Juice is the gold standard everybody else toys to copy. TurnTex, LLC There s a bunch of information there if you are really intrested. We are still using inches of mercury for the amount of vacuum. 29" or a touch more at sea level is the most one can draw. I have a compressed air powered venturi that can draw 26" at sea level and it is okay for degassing for a short time but not enough for stabilizing. It should work for turning if the seals are good and the losses through the wood is minimal. The water type might be similar.

Pete

Cactus juice is the gold standard at the consumer level For the people that believe it is. The guys that do it professionally use a few different ones. They have different consistencies for different types of wood. The method employed by those guys we can’t achieve at home with a house oven and a pressure pot. They use tons of pressure to force the resin into the wood to get maximum penetration.

 

[chaoticbob]

Pete, Tim, Lefley, thanks for clarifications of the differences between techniques for casting and wood stabilisation.
For the record, I must have made a typo or arithmetical error in saying that the water pumps I described could achieve 85% vacuum - they are much better than that - more like 98.5% ! They will give an ultimate vacuum <16mbar with water at 10 Celsius (50F) and 3.5 bar pressure (source). Taking atmospheric pressure at sea level as 29.92 inches, that's better than 29.4 inches of vacuum - so maybe they are indeed OK for wood stabilisation.
Bob

 

[Lefley]

Pete, Tim, Lefley, thanks for clarifications of the differences between techniques for casting and wood stabilisation.
For the record, I must have made a typo or arithmetical error in saying that the water pumps I described could achieve 85% vacuum - they are much better than that - more like 98.5% ! They will give an ultimate vacuum <16mbar with water at 10 Celsius (50F) and 3.5 bar pressure (source). Taking atmospheric pressure at sea level as 29.92 inches, that's better than 29.4 inches of vacuum - so maybe they are indeed OK for wood stabilisation.
Bob

You realize those are for small scale in a lab and use over 2 gallons of water a minute to operate. For resin you run your pump for hours sometimes days for some woods. There is no way one of those is feasible or environmentally friendly to pull vacuum for a vacuum chamber for resin and wood, and water and moisture is your enemy for resin. Even steam or humidity will wreck your resin.

and by the way the maximum a household tap can put out is usually 2.1 gallons a minute. So watch your water meter spin.

us woodworkers are a pretty frugal bunch, so there has to be other reasons why this 20$ vacuum pump has never been deployed in home stabilization that I am missing.