PVC Plenum for 4x8 vac table?

[EricSchimel]

Hey all, I'm about to replace my spoilboard again, and before I do I wanted to explore the idea of a PVC plenum. Right now I have a 3 layer table, the first layer being plywood that's bolted to the frame, the second being a sealed MDF plenum, and the third being Trupan LDF.

This setup has worked well for a number of years but I'm now toying with the idea of replacing the plenum with 3/4" expanded PVC.

Has anyone done this? I'm thinking this might work well because it'll be more airtight, and I'd be able to glue it to the plywood base, and the spoilboard to it.

I've thought about using HDPE for a plenum, but it won't take glue of any kind....

This is my current plenum:

33691

[coryatjohn]

It certainly will cost more. Value? I doubt it's going to be any different than MDF plenum.

[EricSchimel]

I'm not too worried about the cost, but if I can get some more suction out of it it'll be worth it.

[waynelocke]

I replaced my mdf plenum a few years ago with an hdpe (maybe pvc - I can't remember exactly) and have never looked back. At the time I was using a Fein vac and I didn't and don't often suck through a spoil board. A conversation with the guy from Allstar CNC Products whose view was that with a minimal vacuum source every little bit helps and that mdf is porous no matter how well it is sealed. convinced me. The hdpe loses no vacuum and with two lighthouse motors will hold anything I throw at it.

I know that Brady does not like this type of table because of movement. I have not resurfaced it or checked it closely in a while but it seems to be flat, judged by cutting. I made the plenum in four zones with a separate piece of hdpe for each zone and allowed for expansion with some oversize screw holes on one side. The plenum holds threaded inserts well for jigs. You might look into King Board as a plenum. I think that it is an extruded plastic.

Also, I hated sealing the mdf plenum with shellac. It was long and laborious. Did I mention I hated it.

[Mark Farris]

Eric,
I made a 3/4" HDPE plenum 6 or 7 years ago and it has worked great for me. I have it set up to use ALL Star gasketing and I have added locator pins to align the ultralite MDF spoilboard. My original intention was to have a removable spoilboard for other applications, but I leave it on all the time now. I made plugs for all the supply holes with an O ring seal, but again I tend to leave all supplies open and simply cover the un-used spoilboard. It has worked great for my purposes. Just thought I would share.

Mark

[evan]

If you're not to worried about cost and you'd like to add some mass and stability to the table you might think about a material like Richlite or Paperstone. Little if any movement. Expanded PVC has a large thermal expantion.

[jerry_stanek]

I have a 25 MM pvc plenum on my table. Used to have an MDF one but I think I get better vacuum with the PVC one

[curtiss]

Have you tapped in vacuum guage into the table / or plumbing just to see how much vacuum you are getting ?

[EricSchimel]

I have, I'm getting about 8HG with a sheet of ply on it, It'll drop to 5 when there are a lot of holes.

[bobmoore]

I used corian for mine and it is very stable. I gained quite a bit of vacuum but I no longer remember exact numbers. I switched to 2 5 horse becker vacuum pumps with 4 zones, the smallest being 36 x 17 inches. For many projects I use a small sheet of 1/4 inch mdf on top of the spoilboard so I don't have to resurface as often.

[knight_toolworks]

How do you stick the pdf down to such materials?

[evan]

Since I don't have strong enough vacuum (just a diy two motor "black box") I glue the MDF along edges and across the bed every 24". When it gets surfaced down to about 5/16" I glue new MDF on top. A couple times a year I pull it all off and remove the residue from the 1.25" x 60" x 120" Richlite Plenum with the 2.5" surfacing bit, and start over. IF I had an industrial vacuum I wouldn't have to glue it down, just hold it in place along with the material being cut.

[knight_toolworks]

Since I don't have strong enough vacuum (just a diy two motor "black box") I glue the MDF along edges and across the bed every 24". When it gets surfaced down to about 5/16" I glue new MDF on top. A couple times a year I pull it all off and remove the residue from the 1.25" x 60" x 120" Richlite Plenum with the 2.5" surfacing bit, and start over. IF I had an industrial vacuum I wouldn't have to glue it down, just hold it in place along with the material being cut.
yes but what did you glue it down with? not much sticks.

[JuniorLee]

Mark how did you seal the connection between the top and the PVC (I assume) pipe?
Jr

[jerry_stanek]

I surface the PVC then use tite bond III It holds to my Trupan. In fact I just replaced the Trupan this week and had trouble getting the drips that were in the groves from last time off the PVC

[evan]

knight-toolworks-

With clear Gorrilla glue... used liquid nails once but decided it was over kill.:rolleyes:

[sbd1]

I've used a sealed MDF plenum under a sacrificial spoilboard for ~8 years, no problems. And while I don't have experience with plastic plenums, I agree with Eval that HDPE has HUGE thermal movement. I've used sheets of it for other projects.

As for gluing to a non-porous material, I'd highly recommend a melamine glue (ie: Wilsonart, RooGlue, etc...).

[sbd1]

I just came across this comment by coryatjohn in this thread (http://www.talkshopbot.com/forum/showthread.php?25402-Plexiglass-dividers-for-voting-booth) that seemed particularly well related to temperature concerns.

"Another interesting thing to consider is if you have your pumps under your table, and they generate a significant heat load, they can actually change the precision of your table significantly, especially if the heating is uneven. Say you have your pumps at one end of the table and they get to 140f. That means there could be a 70 degree gradient over the length of your table. The change in dimensions of your table would be enough to be measurable."