Vacuum Table for large CNC router

[stairgod]

I have resently purchased a used CNC router. It consists of 4 7'x 8' tables. Each table is divided by a 1.5" steel tube. my plan is to install a vacuum system in the table (there wasn't one before) and divide it into 8
different sections so I can turn a section off if it is not needed. Having no idea of what this machine can do or how to run it my first part is to install this vacuum(or plan to install) From my little bit of research I figure that a 25 hp vacuum with at least 400 cfm can do the trick. However I must built weld a 1/8" steel plate to the underside of my table. My question is how far below the table top do I mount this piece of plate and do I need any deflectors for air flow or does it matter.
Mike

[Geof]

Originally Posted by stairgod ....However I must built weld a 1/8" steel plate to the underside of my table...

This caught my eye. What distance will the 1/8" plate be spanning and what type of stiffening or support to you plan between this plate and the underside of the table. With a good pump and a tight system you may have almost 14 psi across this 1/8" plate; that is around 2000lb per square foot or 112896lbs over a 7' x 8' area. I think you will need to divide the space between the table and plate into many small chambers to support both surfaces against atmospheric pressure when the vacuum is on. Some of the separating walls will be solid to get you zone separation while the others are peroforated for the air flow.

[stairgod]

Thanks for the reply Geof that make sense but how much should the space be from the underside of the table to welded plate. Does it make a difference? Also the table is divided in 2 so it would be 3.5' by 8'. This could also be divided to be 3.5' x 4'. So would the baffles be 1" spacing?
Mike

[Geof]

Around 2 inches should be plenty between the table and bottom plate, maybe less down to about 1-1/2". To stop the bottom plate distorting in too much 6" by 6" cells should be fine, maybe larger would be okay because some distortion does not matter on this side. But how thick is your top; you must be planning on holes and grooves for gaskets to seal around your parts. If the top is less than about an inch you might need cells smaller than 6" to stop it deflecting because you don't want any deflection here. Regarding the airways between connecting cells I would say 1 inch holes every cell wall should be plenty; you are not dealing with huge volume flows. The pump might be rated at 400 cfm but that is at atmospheric pressure, once the pressure is pulled down the air volume being moved is greatly reduced.

[stairgod]

I resently found this old question of mine about cnc vaccumn. The first thing I tried was 2 20 hp vacumns and made the table into 2 parts allow the "Y" axis. I then drilled about 90 1/2" holes along the area I was trying to hold down my 2x10 material for stair stringers. I used a closed cell weather stripping. This did not hold down well because there were to many leaks in the machine body to the table top. I was talking to an old timer one day and he said his vacumn pump on his car was small and it pumped out of a "coffee can" like device and ran his convertible roof as well as everything else on vacumn. So here is what I did. From the 3" opening on the vacumn motors I put on 4" ABS pipe. I ran that pipe up to the center of one of the vacumn tables. I then reduced it to 3" pipe for 12" and then I put a piece of 4"pipe by 16" on it with the one side closed off. I then drilled ( I drilled before I attached) 7 holes and glue 1.5" ABS pipe and brought that to the 7 different 4 x 4 portions of my table. I glue a 2" can( similair to the 4" can to the 1.5 ABS pipe and drilled 8 diferrent holes in the 2" can. I used 3/4" plastic water line with brass fittings and plumbed this to the top of the table. I tried many different gaskets to make a seal between the table and my 2x10 but none worked. I was over in a neighbours shop and he had a material handling boom and I like the way the gasket looked. I order this material from the manufacturer and it worked like a charm. If I put my fingers under the 2x10 when the vacumn was turned on I am sure they would break. I am still working on a system for smaller pieces and have used several different designs but it is not my main focus. The gasket material from Smaltz was very expensive and since I cut into it oftern and had to replace it i now have a similiar product made for me by the 1000 foot rolls.
I don't know why this system works scientifically but it works great for me. I hope this can help someone.

[Peter Schuldt]

Stairgod-
Can you please provide specifics on the seal you are using and how to source it. Also can you post some pictures or a diagram of your vacuum plumbing.
Thank you