Vacuum table: Which would be better?

[Gerald D]

Hi Mayo,

I don’t follow your reasoning entirely, but your experiments (and feedback) are very useful! We don’t have a vacuum table for our ShopBot, because we cannot decide what a multi-purpose system should look like – so my ramblings are based on theory and experience of “fluid transfer” in other applications.

Any extractor fan (shopvac) has a performance curve depending on the rate of air flow through the “turbine”. The highest pressure (vacuum) is attained at zero flow (left side of graph) and the highest flow rate is achieved with the lowest pressure (least resistance)(right side of graph)


745.jpg

Okay, I stole this curve from a metric website, but let us imagine the vertical scale is “inches water” PRESSURE while the horizontal scale is “cubic feet per minute” FLOW RATE. So here we have a pump that can suck 210” or move free air (no resistance) at a rate of 80 CFM. But it cannot do both at the same time! Some unscrupulous suppliers will show you this curve and tell you that you have a 210” at 80CFM pump. Lies! In this case 150” @ 30CFM would be correct. (or 50” @ 60CFM, or 200” @ 15 CFM)

The two parameters of PRESSURE and FLOW RATE are important to us in the following way:

1. PRESSURE is going to give us our “clamping down force” if we multiply it with the area of the work-piece. So we want nice high pressures. (Okay, we are on the vacuum side of zero (negative pressure) , but we want to be as far away from zero pressure as possible – the terminology can be confusing)

2. FLOW RATE is caused by leakage, and we saw from the curve above that if we have a high flow rate then the pressure drops off. So we want to work at minimum flow rates, least leaks, to give nice holding pressures. (to the left side of the curve)

Generally a higher horsepower vac unit will have a curve lying above and to the right of low horsepower unit of similar construction. This means that a bigger vac unit will manage a higher flow rate while holding the same pressure. ie. More tolerant of leaks. Different types of pump (vane, turbine, piston, liquid ring) have different shapes of curves and you may find some curve crossovers if you compare them – vanes will be higher and to the left – turbines will be flatter and to the right . . . . .

The size of the canister (gallons) attached to the shopvac is not important at all!!! (Except in special cases where the canister can be pre-evacuated and special valving allows a quick suction on the workpiece to create the initial seal . . . . )

Now to come more directly to Mayo’s question (I hope), we must realise that the pressure inside the 8’x4’ box is the same everywhere in the box. Each peg-board hole has the same vacuum pressure at its bottom end as any other hole. The more air that leaks into the box causes us to move to the right on the curve and the pressure in the WHOLE box drops accordingly. Therefore as you cut through the sheet and introduce leaks, everything deteriorates rapidly, especially since the square inches per part also reduces quickly.

The only way I see around this is to go for big pumps (leak tolerant), seal obvious leaks and keep parts tabbed together (let the holding area of the scrap work to your advantage).

Good luck and keep us informed. (PS next week I am in Germany again, so if I go quiet, it is not because I have lost interest)

[Howard Andersen]

Mayo:
For sure, the key is to apply whatever level of vacuum you can generate over as large an area as possible. The maximum possible is 15 lbs per square inch, not per square foot. I've used a 3 hp shop vac and estimated I got 3 to 4 lbs per square inch (it would pull a column of water in a tube up 6 to 8 feet). The problem with dropping a sheet of plywood over any typical port, whether its just the end of the hose or something else is that the plywood tends to flex and create a seal in a small area around the port and not permit vacuum over the entire sheet. The physics says its all about area. The parts I was holding together were rigid enough and only a square foot or two in area so it worked surprisingly well.

My thoughts on vacuum pumps would be to use a shop vac to remove the initial potentially large volume of air (depending on the particular application) then use an old milking machine pump to get up to 12 lbs per square inch of vacuum. Lots of these are available second hand and at a fraction of the cost of most units designed for the wood shop. Even a milking machine pump can handle an occasional leak especially if a good tank is built, even out of wood, to "store" a reserve of vacuum. I would think it should work well. If anybody has tried it, I'd be interested to know if/how it worked.

[sheldon@dingwallguitars.com]

If you can place 3M stickit strips in 100 grit where you know you are not going to be cutting, the grit alows a slight amount of air to move helping to equalize the vacuum and it helps keep things in place too.

[danhamm@abccom.bc.ca]

Check out these easy to make clamps for quick on off, above the table..
http://www.vacuumclamp.com/

[ron_cleaver]

Unless I'm missing something, the vacuum clamp ring system seems to me to have a fatal design flaw. What's to prevent all the 'clamps' from moving?

And $220 for 1 ring. Way overpriced.

[danhamm@abccom.bc.ca]

Ron they clamp both sides to the table and to the
object...

[Gerald D]

I often use V-Rings in my mechanical designs and these seem ideal for making the “vacuum pucks” illustrated above.

A V-ring has a V-cross-section as opposed to an O-ring that is a solid-O. See this link for typical metric sizes. Most seal suppliers selling o-rings and hydraulic seals should be able to help you with V-Rings. There are slight differences in the cross-section shape, but the VA type (as illustrated) would work best.

Turn up the pucks from solid wood or plastic so that the V-rings are slightly stretched to go over.


750.jpg

The lip of the ring is very flexible and the weight of the workpiece will make the initial seal.


751.jpg

These pucks will not work with ShopVacs, the pressure is too low. You will need a high-vacuum, low flow system like an ejector/eductor/venturi or vane type.

Let us know if it works – have never tried it myself!

[Wdyasq@yahoo.com]

Resource Conservation Technology, Inc. Of Baltimore, MD has a fairly complete line of gaskets and seals that could be used with either a low or 'high-powered' vacuum system.

They have 'tubeseals' with tongues to hold the seal in place; 'flipper seals' and 'leaf seals'.

I am planning on building my next vacuum table using their products. I am also going to use the same products to seal some windows and doors I need to build.

And, in answer to the next question, they do not have a website I know of.

Resource Conservation Technology
2633 N. Calvert Street
Baltimore, MD 21218
410-366-1146

Ron

[Howard Andersen]

I have had good success making vacuum "pucks" from short lengths of 3" or 4" PVC waste pipe used in residential plumbing . The drawing would be very much like Gerald's drawings above. The seals were just made of self adhesive "closed cell" (this is very critical)foam tape about 3/8" wide stuck around the top and bottom circumferences of the puck with careful butt joints on the tape. One end seals to the table, the other to the workpiece. Its very easy to do, very inexpensive, and the holding power is very impressive. I was using a venturi pump.

[Gerald D]

Howard, your method of closed-cell foam sounds good, and it provides high friction to prevent the workpiece sliding off the puck.

Of course there is no reason for the "Howard puck" to be round. A little square wood box, with no top and no bottom, self adhesive foam on the edges . . . . . .

Some people may wonder what a "venturi" pump is: Well, it is just a chunk of plastic or metal with 3 holes in it! No moving parts, nothing else except this one-piece block.

To get it to work you need compressed air (sometimes quite a lot). Blow the compressed air into the right hole and a vacuum is created at another hole. The third hole is the exhaust where the compressed air plus the vacuum air escape together (sometimes noisily). There is much science (& luck) involved in the size, shape and orientation of the holes in the block - definitely not a DIY job. The bottom line is that a venturi only works if you have "shop" (pressure) air available.