1
$\begingroup$

Consider an almost-rigid part on a vacuum table. There is a flat gasket underneath the part that exposes some portion of the bottom side of the part to vacuum. Let's say 50% of the bottom surface is exposed to the vacuum. Assume the body is flexible enough to seal against the "islands" in the gasket, preventing the vacuum from reaching those areas. To a first approximation, how much downward force does atmosphere apply to the part? Is it different than if the entire bottom surface was exposed to the vacuum?

vacuum gasket for holding part

The picture shows a custom gasket. I'm curious about this issue not only for my custom gaskets, but there is also a more extreme case.

A vendor sells what they call "tile gaskets", which are rubberized gaskets with 1/4" holes on a 1" triangular grid (they say roughly 95% of the board is covered with gasket). Their claim is that if we cover the entire porous spoilboard with these tiles, it limits the loss of vacuum on the non-covered parts of the table and improves hold-down. The loss of vacuum thing is true, but my question is, "How much downward force on the part is there when only 5% of the bottom surface is exposed to vacuum?"

I've used the tile gaskets and, in many situations, they seem to do a good job when compared to a bare MDF spoilboard. Part of that, of course, is due to the sponginess and grip of the gasket material.

I've been searching for months for insight to this question. Anyone want to give it a shot?

UPDATE - Thanks for the answer.

The clamping requirements for CNC routers depend on the depth and width of cut, the material, the bit, etc. Although I'm not a machinist, I believe the requirements are significantly lower than for steel machining. Soft metals like Al and brass are easily machined on a router using vacuum only.

The biggest issue is the size of the part - 3"x3" parts are kind of hard to hold with no gasketing, especially on an MDF spoilboard. MDF is used for spoilboards due to its porosity - the vacuum essentially flows right through it, but leakage becomes an issue as soon as the entire spoilboard is not covered. Thus the tile gaskets I mentioned above.

A lot of the parts we might cut on the router are large, though. An 8"x14" door panel is easy to hold with vacuum. A decent vac pump can easily get 10psi on the part, resulting in over 1000 lbs of downforce.

$\endgroup$
1
  • $\begingroup$ See update in my answer. $\endgroup$
    – DKNguyen
    Feb 23 at 15:49

1 Answer 1

1
$\begingroup$

It should always be atmospheric pressure x area exposed to the free space of the vacuum.

If MDF is as porous as you say then the MDF itself is acting as a breather material. Incidentally, open-celled foams allow gas to pass through them so if these rubber tiles are made of open-celled foam, these would behave similarly. The area exposed to the free space of the vacuum would be reduced compared to if the materials were not there, but not as much as it might appear.

I think the closed cell foam relies on the longer distance in the lateral direction through the material to reduce leaks, well as compression under vacuum to reduce leaks around the footprint of the material, but otherwise allows the vacuum to pass through to the workpiece.

However, I have had cases where I was too lazy to cut a proper ring gasket for vacuum pot lids and instead just placed a sheet of This was a closed cell foam (that should not have allowed gas to pass through) across the entire opening and placed the hard lid on top. I wasn't sure if this would work at first but it did, though I can't confirm if the seal was any weaker.

$\endgroup$
0

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.