Needing a small vacuum table

[hrdwrkr]

I have read about every thread here and on a couple of other sites trying to find how to build a good reliable vacuum table. I do a lot of work with 3/4" glued up pine panels roughly 28"x40". These have a stong tendency to warp as they are carved. As of now I have always made the panels larger and screwed them to my table but I am now needing a faster method of turning my product around. Does anyone have plans or a material list for a table of this size. I have seen suggestions from shop vacs to 25hp pumps and MDF to acrylic. My eyes are starting to bug out of my head. If anyone has some suggestions or have built a table similiar to this I would love to hear your suggestions. My biggest concern is holding down the warping pine.:confused:

[dana_swift]

Dennis- if you are cutting this material already, you know how often you cut through the material. That is the key factor in vacuum table design.

If you never cut through the material, you can get high-vacuum hold downs with low flow for relatively little money. The quality of the hold is very good with 14psi pressures times the surface area. The clamping force on a 28x40 sheet (1120 sq in) can reach over 16000 pounds! That is EIGHT TONS of clamp force. That can be achieved with a very modest air-conditioner type vacuum pump. No wood warp in 3/4" can resist that kind of flattening force. The key to making this type of design work is gasket tape (wonderful stuff!).

However if you cut through the material at all, the previous wonderful numbers change radically. The is where "reality strikes". As usually we cut through our materials. Then it takes a lot of horsepower and a clean design to accommodate the leakage flows. The key here is CFM and your bleeder-board. With the same surface area the hold forces can drop to two tons, 4000 pounds. Fein vacs can provide the CFM and pressures when you have that much surface area and still get reasonable holding forces.

With cut-through it only takes a few square inches of through-surface area to drop your vacuum down below 3psi or so at 100CFM. You will learn from experience just what your number will be with your bleeder-board. I use 3 psi as the point where I expect hold-failure to be almost guaranteed. When my vacuum drops below 5psi I am looking around for masking tape to cover the cut-through holes so the pressure can go back up.

All these number get worse as the size of the sheet you are cutting goes down. Then think big HP blowers that can maintain the necessary psi with large CFM's.

Never forget that screws work, are very very cheap, and that moment of time you are saving may be replaced with a lot of time tinkering with a vacuum system.

I work both ways, and like both systems. I pick the table I mount on my bot based on what I am going to be cutting.

Hope that helps-

D

[hrdwrkr]

Dana, thanks for the reply. The panels that I am working on for this particular customer are carved only. I do not cut through the wood. That being said i do got a lot of material with small open knots (again it is pine and aspen). some of these are not "holes" until the carve gets to that point and the heart falls out or it just deteriates. I would love to make a table that can be moved on and off of my bot table when it comes time to produce for this customer. i do have a small window A/C unit that has only been ran a couple of times. is this large enough for this type of system? would it need to have multiple zones? MDF? again i have read so many threads i am more confused now than i was before i had this brain storm.

[dana_swift]

Given the size of what you are making and the non-cut through (mostly) you can get away with inexpensive. Consider a Fein vacuum, or a vacuum pump for air conditioning use. The air conditioner itself is of no value in this application, other than to keep the cool of the operator..

Air conditioner parts suppliers sell the vacuum systems and the oil you must use with them. Vent the output from the pump outdoors. It will contain oil particles that are explosive if left indoors. Bad idea..

Any cheap vacuum pump should work. The secret is to make an accumulator. I used a scrap 20 pound freon bottle at first. (Now I have a 100# and the original 20#).

The accumulator builds up vacuum when the pump is first turned on, so when you open the valve to your table there is a place for the air between your part and your table to go. The pump has to suck down and seal the edges against the gasket tape. Small pumps cannot remove the air fast enough to work without the accumulator.

You will find several designs on here, including photos of several of mine. Brady Watson has several excellent vacuum designs published here.

As with anything else "shopbot" ingenuity goes a long ways to saving money. The best part about getting a humble pump (1/4 HP or so) is that it can work well and not cost a fortune. Then you get experience with which to design a bigger system later if you need one.

Good luck-

D

[curtiss]

I found my vacuum pump at a small vacuum repair shop. It was a re-built three stage, draws about 7 amps and came out of a vacuum that was used at a car wash. New bearings and brushes, costs $60 works fine for most things.

Not sure what vac will hold down warped pine, seems if all the stock is basically the same size, you could rig up a jig/ clamp/ vise? which you could change out pieces fairly quickly.