[Maderhobby]

Hi,
i'm in the process of making a vacuum table for a specific application and have several problems.
The first problem regards the material it's made of: i sourced a piece of HDPA, 20mm thick and threaded a 1/4 npt on one edge. I faced the top surface in the machine to get it flat. Cut the outline channels and grid on the top surface and fitted a gasket all around.
Now is so far from flat that is basically unusable!
I don't want to spend a fortune and use aluminum for it, so what other plastics should i consider in order for it to stay flat once machined?
The other problem i'm having is that the part is not rigid (a guitar top), so it does not press the gasket on the outline all the way down, even though i left it sticking out very little; i tried to make an MDF 'in between' board, attached it and faced it flat, even sealed it's edges, but now the pump can't suck it through hard enough... It's a membrane pump: good depression but low airflow.
I appreciate any help and suggestions to get this thing moving in the right direction!
Thanks,
D.

[mfgbydesign]

try one or two of these, works on compressed air:
V-Clamp, Vacuum clamps powered by compressed air.

[BAMCNC.COM]

Delrin

[Maderhobby]

Re: V-clamp, i'm trying to setup to upload a pic; it's not a rectangular piece and gaskets don't work: it's too thin and flexible.
Re: delrin: i've read about it and it sounds good, but the prices i've seen are very high; i need 1 piece 1"x18"x24" and one 1/2"x18"x24", and it would be a few hundreds... If i only knew it then worked :-)

[CaptainVee]

Can you describe how and in which manner un-flat is?
If I am correct it may be down to tension inside the material that might be evened out after facing the other side too.

[Maderhobby]

Here are a couple of pictures of what i did; might make it more clear...

[mfgbydesign]

Nice looking fixture.

Maybe you could put a few more screws thru it in the center to hold it down to the flat piece underneath. If it's not flat build up the low spots with teflon tape (or duct tape if tolerance allows).

Add a manifold & pull vacuum from more than one spot.

Also try one of the vacuum generators the run off compressed air. Sure they consume a lot of air & are loud but pull a decent vacuum & no worries about sucking chips into your pump.

[HelicopterJohn]

Hi,

Here is a video you may want to look at.

If you go to 23.37 minutes on the time line you will see my plexiglas fixture setup.

I surfaced my router table to ensure flatness. Then I surfaced the back side of the fixture. Then I turned it over and surfaced the top side of the fixture and also machined the vacuum port inlet and associated grooves. I made them with a 1/4 inch end mill about .100 DOC. I also machined a groove just inside where the profile cut would be about .270 wide. I used gasketing material that was 1/4 X 1/4 with adhesive on the bottom side. By cutting the slot .270 it allowed me the extra room to install the gasketing material without it trying to stick to the sides of the slot during gasket installation. I purchase the more flexible gasketing material and it easily compresses down to the .100 DOC that was milled for the gasket material and provides an excellent seal with GREAT vacuum as shown in the video.

[ame=http://www.youtube.com/watch?v=4WQ8KcTSKyQ]Shopbot PRT Electronic and Software Retrofit - YouTube[/ame]

Let me know if you have any further questions.

John

[Dylwad]

Corian works good too, and is cheap/free if you ask a shop for their scraps. I glue/screw it to a piece of mdf.

Heres an old lithophane fixture I havent used in a few years. Gets located to the table with 1/4" pins, and held down with t nuts.

[HelicopterJohn]

Hi Dylwad,

What type of gasketing material are you using on your Corian fixture. Kind of looks like felt in the pictures.

I love working with Corian. It machines like butter.

John

[Dylwad]

Its weatherstrip from ace hardware

[Maderhobby]

Thanks John and Dylwad; i will try to source some of those materials and see what i come up with that does not cost a fortune: at 17"x24" i doubt it will be free scraps :-)
Meanwhile i got a couple of offers: cast nylon, very expensive and teflon, ridiculously so.
I will be left with a bigger problem though: the guitar top, being thin, flimsy and somewhat permeable to the air, will not be able to lay FLAT with any gasket. I tried soft, thin, sticking out as little as possible, to no avail. I have to come up with a different method.
I have a couple of different 'interface' materials coming in next week; the idea is to make a table with holes, not channels, with a chamber underneath, and have a full sheet of thin material in between the table and the guitar top to act as a gasket.
Hopefully some of the stuff i'll get will be working...
For other parts, thicker and stronger, gaskets are working great.

[txcncman]

You will have to make a mold first, but I recommend you look into pourable polyurethane. It will conform to the shape of your mold, which will be the negative of your finished piece. You sink a plywood plenum box into the polyurethane and then after it sets and you remove it from the mold, you drill vacuum holes through the polyurethane where needed into the plenum.

[Dragonfly]

My experience is quite humble but I thinks bending is due to surface tension. I have noticed that a visually straight sheet of plastic visually becomes bent after a layer has been removed while surfacing. My explanation is that there are residual tension forces in the material due to the manufacturing process (extrusion?) and removing the surface layer frees them asymmetrically. Next idea is to remove a thin layer on the other side but haven't tried it myself yet.

Recently I came across (I think it was here on this forum) a discussion about annealing plastics. It was the first time I hear about heat treatment of plastics for annealing.

[HelicopterJohn]

Hi Maderhobby,

Are you just doing a profile cut? I so, you might want to try using tabs and forget the vacuum for this task.

Just a thought?

John

[Maderhobby]

txcncman, mi workpiece is flat both before and after working it; i want to be able to keep it flat during machining. I suppose the polyurethane can lay flat when poured, yet it seems overly complicated for just getting a flat surface and not having to conform to a more complicated shape. Maybe just try a flat and thin polyurethane sheet?
John, i would like to FACE the top once held down, as to thickness it with a router bit. Not so important on soft tops, more so with harder backs. I would also like to rout the rosette channel, and while i can hold down the top somehow without vacuum, using it has it more stable and allows a cleaner and safer cut.

[CaptainVee]

As far as I can see it seems PE is the cheapest material. Maybe use a thicker piece and not surfacing it?

I may at some point want to have a big vacuum table and would probably use PE (Poly Ethylene) plate made from PE scrap (has flakes in it).

[txcncman]

txcncman, mi workpiece is flat both before and after working it; i want to be able to keep it flat during machining. I suppose the polyurethane can lay flat when poured, yet it seems overly complicated for just getting a flat surface and not having to conform to a more complicated shape. Maybe just try a flat and thin polyurethane sheet?
John, i would like to FACE the top once held down, as to thickness it with a router bit. Not so important on soft tops, more so with harder backs. I would also like to rout the rosette channel, and while i can hold down the top somehow without vacuum, using it has it more stable and allows a cleaner and safer cut.

Was just offering a suggestion as an alternative to your way since your way does not seem to be working. Polyurethane eliminates the need for gaskets since it is the gasket.

For cost, I would actually consider some over-press clamps. It is not like you will be needing to make 100's of these per day.

I have also edge trimmed sheet materials using gravity and friction as the clamping method by placing bags of lead shot on top of the material to hold it in position.

Then finally you have the option of having the material blank to be slightly oversize and drilling holes at the edges and using screws to hold it in position.

[Dragonfly]

One trick I did is to fix the upper plastic plate with the grooves to a thick MDF one as a base. Used screws and those special woodworker t-nuts

Vacuum is fed under the upper plate through channels in the MDF which I painted to prevent losing vacuum. When the sandwich was assembled I surfaced the upper plate to get parallel surface and then cut the grooves for the rubber rope (found here 4 x 6 mm rectangular), the grooves in my case are in a grille manner for different stock sizes.