Re: [CAD_CAM_EDM_DRO] Re: Vacuum Table

Dave, What your saying is correct in the sense that the table must be able to carry the distributed weight [per sqr/ft] of the mat'l or it will deflect. This is correct. However if the table is designed to carry a 1000lb's and you load a 4 x 8 x 1" 6061 plate on it, the table will not deflect do to vacuum. Period. The table will deflect due to the weight of the material placed on it yes but it would deflect regardless whether there is a vacuum or not! Having a vacuum will distribute the pressure of the part more evenly and if you have a flimsy enough structure to not be able to support the weight of the part it will deflect. But just because you can pull 10psi of vacuum doesn't mean your table will have to support 46600lb's when you lock that 4x8 sheet of aluminum down to it!
You table must be able to support the weight in total and per sqr ft of the mat'l and thats it.

I've used a vacuum in making parts from a composite mold, the mold was fairly flimsy and would easily have deflected had there been any extra pressure, the part came out perfect w/ no twists or unplanned contours, therefore the mold did not deflect due to the vacuum placed on it.

Agreed?

Jarrett

----- Original Message -----
From: turbulatordude <dave_mucha@...>
Date: Thursday, February 15, 2007 9:16 am
Subject: [CAD_CAM_EDM_DRO] Re: Vacuum Table

> --- In CAD_CAM_EDM_DRO@yahoogroups.com, H & J Johnson <hjjohnson@...>
> wrote:
> >
> > No extra bracing will be required for a vacuum table [over a
> non-vacuum] as there is no 'atmoshereic pressure difference' between
> the two. As was mentioned the pressure is applied both to the top and
> bottom of the surface so no 'net change' occurs. The beauty of a
> vacuum is the immense pressure which can be obtained WITHOUT have to
> build the table extra heavy.
> >
> > FWIW
> > Jarrett
>
>
> yes and no.
>
> With a shop vac, about the best you can get is about 10 inches of
> WATER, or about 1/3 of a PSI. 1 psi = 27.72 inches of water.
> 1 atmosphere = 14.7 psi or 29.92 inches of mercury.
>
> with a tight seal, and a vac pump, theoretically you can get the
> pressure about 40 times stronger, or 14.7 PSI.
>
> If you are putting a sheet of plywood over your aluminum table, the
> ply will bend like a reed in the wind.
>
> however, if you are putting a 3/4" thick sheet of aluminum over your
> plywood table, your table will bend like a reed in the wind.
>
> considder the material, and the pressure and the supports. to hold
> paper or whatever the film is made of in a graphics shop, they only
> put in one tiny hole. the film would bend if the hole were too big.
>
> Just common sense.
>
> Dave
>
>
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