Vacuum plenum

[dingwall]

I'm gravitating toward an aluminum vacuum grid table top, but need to build a plenum to support the grid plate.

I'm thinking the plenum needs to be bolted to the table, then surfaced before mounting the grid plate.

I'm thinking 1/2" aluminum plate at the moment. Does anyone have experience using cast aluminum jig plate vs regular plate for large surfaces?

[conceptmachine]

Sheldon,
Cast jig plate will be the way to go but be prepared for the "sticker shock".I priced a plate a few weeks ago for the same idea and if i remember it was around $1600.00.
-shawn

[Brady Watson]

You'd be better off putting the AL plate below the grid & bleeder, than making the plate the bleeder, in my opinion. I hear a lot of guys complaining that they can't keep their table flat. If you glue every layer of your table together you should not be having problems with flatness. I get every season where I live & my table stays very flat. I use 2 sheets of Medex glued together, then machine my parts with or without my BradyVac grid.

-B

[conceptmachine]

Brady,
I'm not familar with medex,is it similar to extira?Do you use the 2 sheet's of medex for the table base and spoilboard?I need to redo my setup the mdf spoiboard warps the day after i surface it.I know it's due to the fact that it's screwed down instead of glued so when i redo it i want to go with a good setup.
thanks
-shawn

[Brady Watson]

Shawn,
You can check out Medex specs here: http://www.sierrapine.com/products/mdf/medex/default.asp

I have been using it from day 1 (5yrs) and it has worked out really well. It is like MDF, but contains plastic fillers that make it more stable. They didn't say it was exterior grade when I 1st bought it, but it did say it was 'moisture resistant', which I felt was what I needed.

-B

[paco]

Does wood fiber and plastic composites glue up well?.. with wood glue?

[Brady Watson]

Paco,
If you are calling Sintra/Komatex PVC a composite...no. You need to A) Scuff the back side of the PVC liberally with 40 or 60 grit sandpaper to create some mechanical tooth & B) Use a polyurethane glue such as Gorilla Glue, which I can verify that it works when gluing MDF to PVC for vacuum. Countersunken holes and Kreg screws also wouldn't be a bad idea if using PVC.

-B

[conceptmachine]

Thanks Brady
-shawn

[eidietis]

Has anyone used a sheet PVC material such as AZEK to contruct a plenum grid? Any thoughts on this idea?

[stockbub]

I have often wondered the same thing. No leaks, Flater. Seen one on expensive table, but where to get a sheet that big?

[eidietis]

AZEK sells sheets up to 1" x 48" x 240"

[billp]

I know that John Murphy of (All Star Adhesives), has worked with a few people to develop something similar. I believe he is using the PVC as his plenum, and then depending on the job at hand using either a ""gasketed vacuum mask" OR using the plenum and gasket combination under something porous like a Trupan spoilboard.
I'll be speaking with him this week and I'll try to find out the details.

[Brady Watson]

Bruno,
Yes...It works very well provided that you really scuff the PVC with 60# paper well to give it some tooth before using Gorilla glue to hold it to the table base plywood.

-B

[tree]

I am leaning toward a 1" cast nylon sheet for my plenum. I thinking the same thing, no leaks, more rigid, better at holding flatness. But, Im open to suggestions

[Brady Watson]

There are a few down sides to using plastic sheet as a plenum:

1) It is difficult to glue down without scuffing liberally with sandpaper.

2) Grid machines fine, surfacing exposes the softer foam portion of the PVC. Care has to be taken when planing it because it will melt if you don't move fast enough...and it gets sticky.

3) It has little screw holding power. If you are planning on screwing down your bleeder (which I never recommend by the way), holding will be limited. If using silicone, it may not bond very well. Top grid/surface should have some tooth to it & bleeder should be 'tack glued' around perimeter & between zones to hold it down. Also, if you need to screw something down (like a 2X4 etc) for machining, there's not much there to bite into...and longer screws can puncture the PVC and cause a leak later on.

-B

[waynelocke]

Brady,
Since I first read your reccomendation of gluing the pvc, etc. to the base layer, I have had a question. I instictively avoid gluing two dissimilar materials into a two ply panel, i.e. I religously veneer both side of a panel. Don't you get distortion due to the different coeficients of expansion?
Wayne

[Brady Watson]

Wayne,
I haven't noticed any. The thing about the glue is that the entire panel is glued down to the sub-board, which in turn is bolted to the steel frame. There is much less tendency for the PVC to move, buckle or warp being glued down than if it was screwed down...that much I do know. The ONLY layer that needs to be perfectly flat is the top bleeder/spoilboard layer. In theory, you could have rocks laminated in between everything as long as the top is totally flat via CR/flattening routine. The beauty of gluing is that moisture, dirt and other debris stays out of the layers and keeps everything flat. Screwing down ANY layer but the 1st layer to the table base is BAD NEWS! ~ and the cause of many problems.

-B