Re: Vacuum Table

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Sebastien Bailard
<penguin@...> wrote:

>
> On Monday 12 February 2007 20:28, turbulatordude wrote:
> > --- In CAD_CAM_EDM_DRO@yahoogroups.com, "cncnovice" <crcitser@>

wrote:

> > > Group,
> > > I have a 5ft x 5ft cnc router I built a couple of years ago.

It has an

> > > aluminum table, and I now want to add as a accessory a vacuum

hold down

> > > table of about 2ft(X) by 4 ft.(Y).
> > >
> > > Does anyone have any advice or plans that they would care to

share?

> > >
> > > TIA,
> > > John Champlain
> >
> > It is really simple to make such a table. the underlying table

needs

> > to be flat, and if you check think and tinker, I think they carry

a

> > foam that can be used for sealing the part.
> >
> > A shop vac should be all you need for larger parts, as the total
> > surface area will potentially have tons of force.
> >
> > Even at 10 inches of water (1/3 psi) per sq inch, a 3 sq inch area
> > would have one pound of force. That is about 50 pounds per sq

foot,

> > or for a 4x8 sheet, about 1,500 pounds.
>
> --snip--
>
>
> In a larger CNC router, you might want to have sections with

valves, to only

> apply vacuum to one section of the table.
>
> As Mucha pointed out, you're going to have a ~1500 lb force due to

atmospheric

> pressure (1 atm = 14.696 pounds / square inch) pressing down the

top of your

> vacuum hold down table. You'll need to design internal bracing in

the hold

> down table to resist this, or it will implode (or more likely, sag

a bit).

>
> Regards,
> -Sebastien Bailard
>

If you can make the vacumn connection from below then things become
much easier. Rather than having to make a sealed box with a labyrinth
support structure inside it, you can just make a plate with the
connection to it and as seal around the outside. I used a piece of
UPVC facia with a seal of 1/8" bore silicone rubber tubing glued into
a shallow groove just in from the edge of the board, use RTV silicone
for the glue. The vac pump I use is a refrigerator compressor, used
because I also do vacumn degassing of pourable silicone rubber for
moulds. It gives a higher vacumn than is absolutely needed, you need
to put an air admit valve in the line to release the vacumn,
otherwise you will wait forever and a day before you can lift your
workpiece off. Just remember to top up the oil occasionally if you
use one of these comperssors as they are designed to work in a sealed
system, and will seize if you let them run dry.

Chris Brown