Re: Vacuum Table

Jon,

Don't forget that the air pressure is acting equally on the back side
of the 3/4 acrylic...

William,

As you can see from the responses, there are a lot of ways to skin
this cat. I agree with Jon, that if these are high vac "lab" pumps
using oil, they would NOT be a good choice for the job. Someone else
mentioned selling them, and using the proceeds for this project
(another good idea, I think). However, if these are rotary vane oil-
LESS vac pumps, they can be made to work well.

The two major solutions to vac clmping are:

1)High vac, low volume. This is used where the part size is small and
where part shape seldom changes. It can be done with weatherstripping
gasket (available at Home Depot, be sure it is closed cell) and the
aforementioned rotary vane pump. As Doug mentioned, be sure you have
the "collector jars" attached to the input (and output, these guys
are blowing out carbon mist as the vanes wear)to keep the coolant (if
used) out. This technique requires that the part outline be "taped"
and if cutting through the material; that a spoil board be used.
Cheap, fast, re-configureable. works well when coolant isn't involved.
This type of pump is available surplus for 50-150 bucks, and new for
200-500.

To work with flood coolant you prob'ly want to make the
standard "grid" table and place rubber hose or proprietary gasket
material in the grooves to isolate the part profile. These work well,
but are a bear to keep chips out of...

Another variation of this technique is to use "pods" first
commercially made available by Carter Industries to the commercial
woodworking world. These are reversible (approx. 4") cylinders which
provide a "circle of vacuum" and can be re-configured as required.
VERY pricey, but work well. try a search on "carter pods".

On the machinist side of this equation, there is the reusable
vacuum "bags" sold by IBAG. The consumable bags are relatively
inexpensive, but the "base unit" and manifold are VERY pricey! Could
be hacked, IMO.

2)High volume, low vacuum. This is used where parts are large, shape
changes often, and part warpage(thin stock, or warped wood/plastic
materials) makes it difficult to get the part "sucked down" with the
lower volume, higher vac approach.

This is also the arena of shopvac "kludges" at the low end (which CAN
work well; check out the info on the shopbot website:

http//www.shopbottools.com
Keep some cooling air going over or through the motor!

At the high end, the large vac pumps and their large prices dominate.
One earlier post mentioned the air permeability of mdf. This fact is
USED by some of these systems to advantage; An entire sheet of mdf is
placed over a "box" which contains the channels and zones of vacuum.
This box may be only an inch or two thick, or several, and it MUST BE
(as Jon said) strong enough to resist the atmospheric atempts to
crush it! (but remember WHERE the pressure is acting FROM. Usually it
is just trying to "compress" the box, NOT "bend"or "deflect" it.)

Anyway, you end up with an "area" of low pressure (sounds like a
weather report <G>) where parts can be placed and will be held DOWN
(Dougs? suggestion to have pins to prevent sideways motion is
important to pay attention to!) The high volume of air being pulled
through the mdf holds the part for cutting.

Lastly, if/when you go this route (pun intended) be sure to consider
your tool path! There are many steps you can take to achieve greater
success by cutting this before that, using "tabs"(not quite all the
way throught the material cutting), and changing cutting direction.

Hope this helps.

Ballendo

--- In CAD_CAM_EDM_DRO@y..., Jon Elson <elson@p...> wrote:
> William Scalione wrote:
>
> > Anyone out there ever built a vacuum table for a cnc router? I am
> > in the process of building a new larger router and would like to
add
> > a vacuum part hold down to it. I picked up a couple of vacuum
pumps
> > at the NC State University surplus today although I am not sure
if they
> > will work for it yet. They are nice units direct drive high
vacuum lab
> > units.
>
> Don't ruin these pumps with vacuum table operation. These are 2-
stage,
> low volume, very high vacuum pumps. they also have extremely fine
> tolarance parts inside, and will quickly be ruined by even a small
amount
> of dust. The inside of these pumps are totally coated with
expensive pump
> oil, and are run under an oil bath. This will trap dust particles
in the
> pump.
> The pump oil needs to be changed frequently even under normal
service, and
> it is really expensive, like $150 / gallon! Also, they really
don't move
> much
> air, just a few liters a minute.
> What you need is a high volume, low pressure differential pump with
no
> close tolerances inside.
>
> > What I need to know is what size and number of holes would
> > I drill in a 4' X 4' table? I will probably add some valves to
section
> > the table off, possibly go back and get another pump and section
it
> > into thirds, one pump for each section. Use only the sections I
need
> > to, depending on the size fo the work. I was thinking of using an
acrylic
> > sheet maybe 3/4" thick for the table. Will it require a large O-
Ring
> > around the edge or will just the holes be sufficent? Any tips
would be
> > appreciated.
>
> IF you are thinking of having the 3/4" acrylic support the full air
> pressure,
> it won't work. You would need something behind the acrylic to
resist the
> air pressure. Remember, only a 1 PSI difference exerts 144 Lbs /
sq Ft,
> so there wouls be 144 * 16 = 2304 Lbs over the 4 x 4' sheet! You
will have
> to build a substantial structure to resist the effort of the air to
collapse
>
> it. You will probably get somewhere around 5 PSI differential,
giving over
> 5 tons of force on that sheet.
>
> Jon