Re: [CAD_CAM_EDM_DRO] Re: Vacuum Table

> Correct until the work deflects. Once that happens, the table
> must
> be able to support the loading of the atmosphere (about 15 PSI) or
> the part will be distorted when the vacuum is released.
>
>
> David G. LeVine
> Nashua, NH 03060

Nope, I disagree, all that the atmosphere is doing is clamping the sheet to the table. There is going to be 15psi pushing against the bottom of the table remember. If the mat'l is flexible it will deflect down to the table surface where it will be pushed into intimate contact w/ the surface. Now, if that surface can't carry the weight of the MATERIAL then it will deflect, the pressure of the atmosphere will NOT deflect the table.

Lets reverse this problem. Say I've built a table [and I didn't do a very good job] and its has 1/4" "valley" in the center. I know my table will hold 500lb's and not fail, as long as its not all piled in one little spot. So I load on a 1" x 4ft x 8ft pc of 6061. Now, the 6061 is stronger than my table, it doesn't deflect like the 1/4" plywood table would. I add my vacuum to the table which pulls the outside edges of the alum tight against the table. Now, I've got a vacuum system which will pull 15psi [amazing I know :-)] and so what happens, well.. the 1" 6061 plate won't deflect very blame much, but the 1/4" plywood table will.. and it will get sucked UP TO the plate! Now once its in contact w/ the plate, does it keep going and lift the plate and the table in the air? No, there is a net pressure differental across the table and the mat'l of ZERO [ 15psi down on the mat'l and 15psi up on the underside of the table]

So now, I put a pc of 1/8" plywood on my table and apply the vacuum. what happens? the thinner plywood will flex down to the table [matching my 1/4" valley or sway in the table] and it stops, why? Because there is a net ZERO pressure differental across the table, 15psi pushing down on the mat'l and 15psi pushing up on the underside of the table. It would not keep going and crush the table or apply any more weight to the table than what a basic pc of 1/8" plywood weighs.

Atmosphereic pressure is the same pressure that keeps sheets of paper together [making them hard to pull appart] or the same thing that keeps sheets of plywood hard to pull apart. We've all had that, get a stack of plywood and try to lift the top sheet off, it's like its stuck down to the sheet below. However, once we get that corner lifted up it breaks the pressure that is clamping down on the sheet and we're able to lift it [because the pressure then has a net differential of zero].

Jarrett