Vacuum Plenum

[Bill Keehn]

Hi all.

So about 4 years ago I picked up a used PRT Alpha 48x96. I didn't care for the vacuum table the previous owner had made, so I tore it down and started over. I made a 10 zone vacuum table that I rather like. My only concern was that I made the plenum out of MDF. I was never confident on how well it sealed with the polyurethane, and it was not the least bit durable.

So here we are 4 years later and I have noticed the plenum is starting to lift away from the base at the edges. I am guessing it absorbed some of our Florida humidity. I normally keep the humidity at 50%, but sometimes my shop dehumidifier can't keep up.

So I was trying to find something better to use than MDF. At first I thought about melamine covered particle board and sealing the edges and plenum area with paraffin or plasti-dip. Then I noticed my local home depot has 4'x8'x1/2" sheets of PVC in the building supplies area. It's flat on one side and has simulated wood grain on the other. It's about the same density as schedule 20 PVC.

So what do you all think? Has anybody worked with this product? Will I get a good plenum or will it melt when cut it? Will it ruin my V-bit or foul my dust collector?

[Rob Gunn]

Bill, I cut PVC all the time. It cuts easy and wont hurt the cutter at all. Its just soft plastic... The one and only issue is electrical static, you want to have a good ground on your machine or you may start having toolpath issues during the cut.

[Brady Watson]

You want either Medex moisture resistant MDF or Extira for your grid layer. It should be liberally glued to an MDO support board. Then attach your ultralight on top. I don't like plastic plenums because they expand & contract differently than the wood sheet goods and are difficult to properly glue down or hold down the bleeder board.

-B

[bleeth]

Bill:

I have had the same problem down here. Florida moisture swings can be a pain. When it happens I wedge it open, liberally insert some epoxy, and then clamp. Gets me another couple, few years. I've been using regular density MDF and sealing after cutting it with two good coats of shellac. Also always do the edges of it and the spoilboard. Extira would be better but I hate machining that stuff as it is so toxic.
My sub base under the plenum is actually two layers: one of 3/4" cabinet ply and it is well screwed to a bottom layer of 3/4 MDF that has p-lam on both sides, both also well sealed at the edges. The plenum is Tite-bond glued to the ply.
I wouldn't use 1/2" PVC. The interior is pretty soft and what Brady said as well.

[Mark Farris]

Bill,
I would never dispute Brady or Dave as I know they have been at this as long as anyone. I chose 3/4 King Starboard for my plenum and love it. I have a double layer of 3/4" Baltic Birch for a base. My grid is set up for Allstar gasket, and up to 12 zones. The starboard is held down with a 6" grid of recessed 1/4" nylon machine screws ran into threaded inserts in the top layer of base. I also have inlayed T track for other applications. It's only been down for 6 months but so far I am very happy with it. I also am in Florida but have my shop air conditioned so I don't have moisture to deal with.

Mark

[gbradley]

Mdf is not good to use and pvc is inconstant. Go with the King Star Board or G-10. I ran into some issues with a sheet of mdf that was glued to the plenum:confused:on a machine. I would run a gasket channel around the perimeter of the plenum and put the spoil board on top.

[Bill Keehn]

Mark that was one thing I wished early on that I had done differently. I used MDF as the base.

I've attached a picture of my plenum. Sorry it doesn't show the separation at the corners. One thing about this design is that you realize quickly how much harder it will be to maintain. If you damage it, it's harder to replace than a flat surface. The little frusta (even more rarely used plural of a rarely used word - frustum :) ) are also easily broken off, so you can't set stuff on it or even flatten it after its installed.

Brady, the more I think about it, I think it's time to switch over to your BradyVac II system. I almost went that route the first time. Earlier on I was working with a lot of sheet goods, but now I'm going to be doing electric guitar parts primarily. I intend to be using a lot of purpose built vacuum fixtures and would like to keep the flexibility of using a bleeder board too.

I think I'll tear it down and start over. I'll keep the positioning of the vacuum ports where they are, since I'm happy with it. My vacuum table is 10 zones, split into two groups of five. Each group is powered by it's own Fein Turbo III vacuum. There is also a valve that connects the two groups. I've attached a picture of the manifold too for the curious.

I'm in the process of building the vacuum press from Joe Woodworker and was thinking about using it for vacuum clamping as well, so I think I want a smooth sealed surface. My current thinking is a double layer of plywood like Mark did with a good smooth coat of poly on top. I might go ahead and use the T-tracks too. It would add flexibility and I could divide the surface into smaller panels that can be replaced/repaired more easily.

[Bill Keehn]

I dunno, George. Seems like G10 would cost more than I'd want to pay. I'd be afraid to use it lest I put a scratch on it. I assume you are suggesting this as a plenum and not a flat surface. The King starboard might be a better answer for me. Maybe I can get both materials at my local surplus store for cheap. I'll keep it in mind.

How would you fasten it? Screws into the bottom?

[Bill Keehn]

Mark, actually I had AC installed in my garage shop last spring. I got the sub panel and power drops and vacuum duct work done about the same time. Before that I was in a non-climate controlled warehouse. It's probably more likely my table started to move a while ago and I'm only just now noticing it.

[Bill Keehn]

Shoot. I just realized my two MDF layers are glued together. All the bolts are held captive. I wonder how hard it will be to recover those. Hate to waste them.

I'm also remembering what a hassle it was to find the coordinates for all those mounting holes. The holes in the steel supports on the table didn't seem to be laid out identically from one piece to the other. Oh well, I've been wanting to adjust things anyway. Maybe I'll find a way to make things line up better.

I want to flip the Y car around so that home is at the other end of the table, where the vacuum manifold is. I also want to shift the table closer to one end so I have more room off table near the home position. I want more room to add a fixed zeroing point off table, possibly a 4th axis, and possibly a finger joint/dovetail fixture.

Then again, the MDF base hasn't been a problem. Maybe I'll leave that part alone.

[Mark Farris]

Bill,
I fastened my Starboard with 100+ 1/4" x 20 nylon machine screws in recessed holes into threaded inserts installed in the top Baltic Birch sheet. Drilled all the holes (inserts, through hole and head recess in the Starboard) off of 1 Aspire layout and 3 spiral ramping toolpaths so everything lined up just perfect.I oversized the through hole slightly and used low profile pan head machine screws snugged but not over-tightened to allow slight movement. One word of caution, its almost impossible to glue your PVC ports into the Starboard. I did find a glue with a primer at the orange big box store that worked...so far. It may not hold up forever. I am powered by 4 independently controlled 220V Lighthouse motors in my version of the familiar black box.

Mark

[Bill Keehn]

No worries about the ports. They are PVC couplings friction fit into precision holes and milled flush to the surface of the base. They don't extend into the plenum. The holes in the plenum are precisely sized for schedule 20 pipe. I used short pieces of pipe to align the plenum to the table base.

My intention was to have purpose built vacuum fixtures for my guitar bodies. I'd have a form fit recessed cavity for the part with vacuum directed where I need it for maximum holding force. The plan was to use the pipe to plug into the vacuum port and direct it up past the plenum and into the fixture base. I was also planning on having the fixtures be self aligning by making the fixture cover two zones and using two vacuum ports. The prototype worked nicely. I sanded and rounded over the pipe ends for easy insertion and removal from the couplings. They also remove from the fixture for storage purposes.

If I use Starboard for a flat table surface (ala BradyVac II) I'll refit the holes for the PVC couplings to accommodate a different material. I think a couple sheet metal screws through inside of the couplings into the subtable and sealed with silicone caulk will keep them in place nicely.

[Bill Keehn]

Here is a better look at what I just described.

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