[Eric Olson]

I have a vacuum table set up on our 4Axis machine and I'm getting mixed results and performance.
The bed itself is made from 2 3/4" thick MDF boards with a plastic cowling underneath, reinforced my aluminum rails on the outside that the pump hooks up to. The bed has 6 separate tables for a 6'x12' overall size. One board has the typical square pattern cut into it to create a space for suction, the other sheet is screwed and sealed on top for a smooth surface.
The pump is a 25hp Quincy rotary screw pump, when this thing gets turned on its like a jet engine starting up.
Initially I thought this would be a great solution for holding material, 25hp should hold anything right!, but I've run into a couple major problems.
The pump sucks the MDF board down in the middle of the table sections. My Z Zero is different on the outside of the table to the middle of the table. Basically as soon as I turn the pump on my table is not level anymore, its a cereal bowl.
I recently had a piece of 48"x48" Lexan plastic on the table to router out fake student credit cards for a business class.
The pump would not hold the cards in place when the router bit cut the cards. Onsrud suggested I surface the table top to take off the layer of finishing wax on the MDF, but with so much suction that I am distorting the table I question what the real issue is. When I put the sheet of Lexan plastic on the table, it was just the plastic sheet alone, no other material was placed on the table top to "seal" off the vacuum.
So in the process of working on this job, the cards will not stay in place during the cut (I even used bridges)
I have to set the cut depth on .040" thick material to .075" because the middle of the bed is so much lower than the outside edges.
Very frustrating, hoping its a stupid easy fix mistake on my part. Any suggestions? If you need pictures, more info, let me know.

Thank you

[nwrepair1]

Can you reinforce the middle of table? Is this on a home made machine? You are probably losing vacuum everywhere.

[Eric Olson]

I found a great .pdf online that talks about vacuum tables. I shaved off .0625" on each side of the top spoil board, edge banded the sides and drilled the holes in the Plenum larger from .125" to .375"
This helped immensely, I was pulling 25 in. hg. and deflecting the table .065" in the middle, now I'm pulling about 22 in. hg. and deflecting .02", still not great but a big improvement that shows me I'm going int he right direction.
I think now I just need to reinforce the center of the table, but the way the bed was built, it'll take some doing.