Vacuum hold-downs and rotational problem about the z axis

[cpcp]

I am making a vacuum hold-down utilizing a low flow solution i.e. my vacuum machine is a laboratory type and machining operations will not break the seal -nor is the workpiece porous. My vacuum source easily reaches 30" Hg -in fact its' pull is better measured in the Torr range.

I am finding you can not pull a workpiece off in the z axis direction but it will slide around in the x and y which is a problem.

When I look at state of the art vacuum hold-downs it seems that many of them of course employ some sort of polymer gasket but often this is not just around the edge of the seal but islands are placed inside the vacuum area itself -why?

The issue I am trying to understand is what is the interplay between the gasket material and the bed of the vacuum plate. That is if you just suck an item onto the gasket material alone then the elasticity in the gasket material gives a strong but rubbery hold in the xyz directions so logically (a little logic and no knowledge is a dangerous thing!) I need to suck the workpiece down hard enough that the frictional forces of the bed hold it in the xy directions.

When I do suck it down to the bed it still slips. Do I need to roughen the bed a bit? If I did I may at some point risk damaging the surface of the workpiece.

My workpieces are Acrylic about 25mm thick and 400mm x 1000mm however I would prefer the vacuum hold downs to be much smaller than that.

Thank you for any knowledge



BTW: This is just beautiful: http://youtube.com/watch?v=mJkdk50zjuE

[HuFlungDung]

I would try using an adhesive backed sandpaper. You could apply it to the workpiece, or to the table.

If that is not suitable or does not work, what about adding some low height stops to the table. You would need some sort of fixed fences to bear against. Some thin sheet metal strips could be cut to fit the space between the part and the fences. The vacuum would hold down the sheet strips and the part.

[Geof]

It is possible to hold down with vacuum tight enough to machine completely around the perimeter of a piece of plastic. We machine 10" by 12 " pieces of 1/4" polycarbonate held down onto a polycarbonate base with an O-ring seal that is 9" by 9". The only contact is between polycarbonate and polycarbonate or polycarbonate and O-ring material. Provided the vacuum is better than 15 inches, i.e. around 7 psi clamping effect the parts do not move under a fairly decent cut around the perimeter taking off up 3/8" full thickness at around 30 ipm.

Holding down your pieces you have potentially many times the holding area; 600 sq inches versus about 80 so you should be able to do it. Are you sure you are getting a good vacuum over your entire holding area. The vacuum base needs channels so the trapped air can escape to spread the vacuum over the whole area. If it is just a flat surface with a perimeter seal and the vacuum outlet in only one place there will be a pressure gradient with the vacumm diminishing almost to nothing at the perimeter which is where you need it most to counter the lifting action of the tool.

[Oldmanandhistoy]

I would try using an adhesive backed sandpaper. You could apply it to the workpiece, or to the table.

Hi,

Not sure this will be any help but some months ago now I was working on a vacuum bed for my machine. It was a simple affair using a workshop vacuum and a table with a matrix of holes. Had the same problem plenty of vertical holed but no good as the part would slip. The solution I came up with which may or may not be an old idea was to use latex rubber painted onto my table. It was a bit more complex than explained but it works very well.

John

[One of Many]

When I look at state of the art vacuum hold-downs it seems that many of them of course employ some sort of polymer gasket but often this is not just around the edge of the seal but islands are placed inside the vacuum area itself -why?

The issue I am trying to understand is what is the interplay between the gasket material and the bed of the vacuum plate. That is if you just suck an item onto the gasket material alone then the elasticity in the gasket material gives a strong but rubbery hold in the xyz directions so logically (a little logic and no knowledge is a dangerous thing!) I need to suck the workpiece down hard enough that the frictional forces of the bed hold it in the xy directions.

When I do suck it down to the bed it still slips. Do I need to roughen the bed a bit? If I did I may at some point risk damaging the surface of the workpiece.

My workpieces are Acrylic about 25mm thick and 400mm x 1000mm however I would prefer the vacuum hold downs to be much smaller than that.

Thank you for any knowledge

BTW: This is just beautiful: http://youtube.com/watch?v=mJkdk50zjuE

At 400mm x 1000mm(15.75" x 39.37") you have 618sq inches of surface. At 15psi or 30Hg, you should have 9271lbs of force holding that 1" thick piece down. If it is sliding, I'd suspect you may be floating on an a thin air cussion leaking under your work piece or there is not much of this surface acting on the vacuum. This material wouldn't still have the protective covering on it as shipped, would it? Are you using any seal cord?

The grid or islands as you call them are to support the material and offer slots for changing the configuration of the seal material for smaller parts. In addition to what was in Geof's post, the finer the grid, the more surface of the work piece is exposed to the vacuum holding it down.

DC

[cpcp]

Gentlemen,

Thank you for the advice. Success!

If I may answer my question (from your added knowledge), the main hold is achieved via the friction on the surfaces within the vacuum area. The trick is to find the correct (1) height and (2) width and (3) material type of gasket.
Of course the three factors interplay under vacuum along with (4!) "island" surface area.

That is, if you see the hold-down area as a sea filled with islands then it is the island surfaces that provide the frictional stability -the trick is to seal those islands correctly.

To repeat, the gasket material has to be tunned in three aspects. The first two are height and width as when they are sucked down you don't want excess gasket material being squeezed on to the ""beeches" of the islands. If that were to happen then then the workpiece will only float on the gasket giving a somewhat strong but spongy hold in the xyz directions -no good.

To my mind the third aspect (gasket material) seems the most important. You need a "closed cell" material e.g. Neoprene -as simple sponge will leak air. It is also important that the material be firm; under vacuum you want the cells to collapse and draw the workpiece nearer the bed in the z direction BEFORE being pulled towards the beaches in the x and y directions. It does not matter if, once the workpiece is fully in contact with the "islands", that the gasket material sucks in between the outer island seas a bit.

So with that in mind start with the gasket material. How much will it compress under pressure? What is its elasticity? Get that knowledge first and then design the "island" heights/sea depths\widths.

Once done then sit back with a Singapore Sling and watch your mill hog out chunks you can scarcely believe -or is that eat hunks of Hawaiian hog you can scarcely believe -oh dear I've had too much sun and its getting a bit late on this side of the planet; good night and thanks!