Plenum Board for Vac

[john_l]

I have read and read and read and read. And I still wonder if I am heading for success or failure.

I bought the 2 vac heads mentioned in the "9-15 open source" thread. I also already have the filters, I want to build a couple boxes and try this out.

I mostly will be cutting the following materials on the bot and would like to make the most use of vacuum where i can .. .040" aluminum, 3-16" acrylic, mdf, signfoam, melamine, and hardboard.

I have an existing table base of 3/4" oak plywood attached to the steel structure with countersunk carriage bolts.

I was going to try 4 zones cut 1/4" deep into 2 sided melamine as per the drawing below for the plenum. Melamine so that I wouldn't have to worry about the underside of the plenum leaking or curling.

After cutting the holes for 2" pvc and the plenum grid, I was going to seal all of the sides of the pvc holes, the sheet edges, and the entire grid with at least 2 coats of bulls eye sanding sealer. After dry, I suppose I would then glue up with titebond and attach the plenum to the table base (grid up) with some cinder blocks for temporary weight to hold it down.

Next, I have some 18MM Trupan I got today. I would remove skin / surface both sides of this for a bleeder. I will also seal the Trupan edges.

In this case, do I glue the Trupan bleeder to the melamine plenum with Titebond?

I sized the grid the way I did because I want to use it for both 48x96 and 49x97 full sheet materials... and well, I guess I was figuring if the bleeder caused a problem with hold down I could always lay the material right on the grid (and if I did, the grid would need the borders as shown).

So someone please tell me if this makes any sense at all before I cut up my stuff. Thank You!



26676.jpg

[john_l]

Puttng these drawings together was making the thread real wide so here's the plenum...

[knight_toolworks]

a faster way to seal is brush on a coat of yellow glue. it does it in one coat and dries faster then shellac.

[john_l]

Thanks Steve, I will use glue instead. Not sure what happened to the drawing of the melemine plenum. But here it is again.


26686.jpg

[Brady Watson]

I'm not a big fan of melamine for this sort of application, but it will work. You seem to have the concept down pat.

-B

[john_l]

Thanks. Good to hear I got a slight grasp anyway.

I was thinking the melamine would help with any leakage or warping issues with the plenum? Maybe I missed the point.

Brady, you still suggest regular MDF for the plenum?

[beacon14]

All those little raised squares of melamine (particleboard) will be weak and want to break off, if you can even machine them without losing some. I suppose once the bleeder is in place it won't matter, but any time you have the plenum exposed you will have to be very careful not to damage it. The same holds for MDF but not to the same extent. You can make the grooves a little farther apart, making the squares larger, that would make them a lot stronger.

I don't see why you have to go to the trouble to seal the entire grid if the bottom is non-porous and the edges are sealed. Also, how are you going to glue the melamine down to the plywood base? Tightbond won't stick to melamine.

[john_l]

I did realize after I wrote that.. that wood glue wont work for the slick melamine surface. I do have some outdoor adhesive that will work, but it skins kinda fast. It's cold here right now so that would probably work anyway.

I havent cut up anything yet. What would be the ideal material for plenum?.. or maybe that's a loaded question.

[Brady Watson]

MDF, MedEx(MDX), Exterra etc all work well and are all gluable. PVC also works well, but it is pricy and in many cases non-gluable.

Vacuum can leak thru the bottom sheet (support board bolted to table supports) so make sure that there aren't any holes in it and that it is not a porous material like MDF or you will lose performance. You can slap a couple coats of polyurethane or shellac on your machined plenum to seal it up - allowing you to use nearly any material.

David brings up some good points about not using melamine...it's a pretty junky product for vacuum - using it for anything other than a quick/temporary vacuum fixture.

-B

[beacon14]

I covered many of these points in my column, here's the link, scroll down to October 2006.