[Ferrari2007]

I have a built a vacuum table for my CNC router that is 40"x40". Initially I have used a 1 HP rotary vane pump as my vac source. I was not sure if it was large enough to adequately provide enough hold down for for the entire table, but I had it and decided to try it, so I broke the table up into to halves and valved it accordingly so I could shut off half the table essentially. So I have a 1 HP pump on an area
20"x40" the plumbing is 1.5" PVC

It performs very well doing large sheet goods, but does not have the hold down force I need when doing smaller pieces. I generally cut 3/4" wood, some are cut through while others are simply carvings

So my question is this....is there a general rule or calculation for sizing pumps for a given area of vac table. CFM rating??, Inches of Hg??

Thanks

[dodger889]

Why not use thin boards or sheets goods to block of the area not being used.

[Ferrari2007]

I have done some initial testing doing that, blocking off parts of the spoilboard not being used.
I was looking at this from a techinical standpoint...ie how many inches of Hg do you generally want to pull in a system, CFM raitings, etc and how much PSI of force is required to hold most parts.

[tinkeringtechie]

Even a small pump can work for a 40"x40" area IF that area is adequately sealed. What kind of gasket material are you using and what is your spoilboard made out of? If the spoilboard is too porous or the gasket isn't sealing well then you can lose a lot of air that way. I can actually turn my pump off and it will continue to hold the material for about a minute.

[Ferrari2007]

I am using MDF for my spoil board that I have fly cut both sides. I have not sealed the edges of it and plan to do that for the next round of testing. I have also heard different types of MDF will make a difference.

For the gasket material, I am using a closed cell foam.

I am using a centrifugal rotary vane pump, so it will need to run continuously to maintain a vacuum

From what I am reading, it looks like in a perfect world you can never get more than 15PSI of force on a piece. Assuming a perfect vacuum below the workpiece and standard atmospheric pressure above the workpiece. So it comes down to how well you can maintain a high vacuum

[tinkeringtechie]

MDF is actually quite porous. If I had to guess, you're probably losing most of your air right through the spoilboard. I've heard of people sealing the entire MDF surface with something that actually soaks into the wood. Personally I opted for a PVC sheet, but it's significantly smaller than your's (12"x24")

[txcncman]

Vacuum, like electricity, follows the path of least resistance. Think about it. Until you zone your table appropriately (two zones would not be enough control for very small parts), you will always have issues.

[BanduraMaker]

First of all, are you using a flow-through table? i.e. the MDF sits on top of the vacuum table and you suck vacuum through it?