Plastic or HDPE vac plenum?

[EricSchimel]

Hey all, in searching around the forum it looks like some of you have used plastic, PVC or HDPE vacuum plenums for your vacuum setups.

I have a 4x8 machine with 8 zones. The work that I do is really varied so having a universal vacuum that I can switch from the full table all the way down to a small 24" x 24" zone works well for me.

You can see my setup here: https://goo.gl/photos/MUq1RJaKRV2xxrFk7

I'm getting near the point of having to replace my spoilboard again and I'm thinking of switching my plenum out for something that's made out of plastic. My current MDF plenum has several coats of glue and Shellac on it, but I can't help but think that totally non porus plastic will work even better.

For those that use plastic plenums, what did you use exactly (PVC, HDPE, something else) and if you did switch from MDF, did you notice a good performance boost?

[knight_toolworks]

you would not want hdpe it is so heavy and so slippery it would be horrible. sintra would work but still attaching the top to it will be a pain. I use Baltic birch of apple ply.

[EricSchimel]

I was thinking HDPE for the plenum, and then gluing LDF (Trupan) to the top of it. HDPE is heavy but I deal with it all the time so lifting it on to the table is no problem...

[jerry_stanek]

I have a 1 inch thick sintra top on mine. I counter sunk some screws around the edges and then cut my plenum and drilled holes and counter sunk screws in the high blocks. I then surfaced it to give it some bite for glue. I then surfaced one side of my trupan spoil board then flipped it and glued it down with tite bond III

[EricSchimel]

Did you have an MDF plenun before? I'm wondering what the vacuum increase (if any) there would be..

[knight_toolworks]

I was thinking HDPE for the plenum, and then gluing LDF (Trupan) to the top of it. HDPE is heavy but I deal with it all the time so lifting it on to the table is no problem...


How are you going to attach anything to it? there are a few glues that may work but maybe not.plus iti s not really all that stable when you start removing material from one side.

[EricSchimel]

I was thinking about building this like a traditional ShopBot vacuum system.

I'd have a bottom layer of plywood bolted to the frame, I'd cover that with glue and then glue my HDPE or PVC plenum to that, and then the LDF to the top of the Plenum, like this:

https://goo.gl/photos/2Y2gQi9u4mQbJiA1A

[jerry_stanek]

I was thinking about building this like a traditional ShopBot vacuum system.

I'd have a bottom layer of plywood bolted to the frame, I'd cover that with glue and then glue my HDPE or PVC plenum to that, and then the LDF to the top of the Plenum, like this:

https://goo.gl/photos/2Y2gQi9u4mQbJiA1A

That is how mine is but I glued and screwed the PVC down If you surface the PVC in grabs the glue better. Also I found Tite bond III is better than Tite Bond II for PVC

[EricSchimel]

Thanks for the tips Jerry. How do you find the PVC working? Better than MDF?

[tomkk]

Hi Eric
I am setting up my new PRS Alpha 5 x 10. In another thread I was asking for advice on using 3/4 Phenolic for my table and plenum. I think this should minimize leakage and be resistant to humidity.
Sure its heavy, but I only have to do it once. Any thoughts?

Tom

[EricSchimel]

I know I have my MDF plenum sealed up pretty good, but I'm depending on a thin layer of glue/shellac. That's why I'm thinking that PVC/HDPE would make a good plenum because it's way more airtight. Do I know it will work better? No, that's why I'm asking here! :)

[tomkk]

I have zero experience with PVC/HDPE. How does it compare to phenolic as far as weight, price, machinability?

[mark_stief]

Just throwing this out there. Why glue it down if you would just take wood dowels and pin it on all 4 corners so it couldn't slide when sliding sheet goods on the vacuum would hold both pieces down. I'm looking for a piece of plastic for my table and that was my theory if I find one for the right price

[tomkk]

Mark

I was thinking along the same lines. I guess the only disadvantage would be if you were running something without vacuum. But WOW it sure would make spoilboard replacement a breeze!

[tomkk]

Mark

Interstate plastics list 3/4 HDPE at $283.57
3/4 PVC at $384.48
3/4 PHENOLIC At 520.53 all 4 x 8

[mark_stief]

I'm not sure 3/4 would stay level I was thinking 1" or even 1.25" it's the shipping that kills the deal kinda Why couldn't you scuff the edges and use some caulk I have some Solar Seal caulk that will stick to any thing it hits

[tomkk]

I think if you seal the bottom of 3/4 ply then bolt to frame then glue your 3/4 phenolic and machine your plenum it will stay flat forever. I don't know if you have any experience with Phenolic ?
This stuff is like steel plate! I'm not an expert by any means, and hope others chime in with their opinions. Eventually I will just go for it and be the Guinea pig. I salvaged several sheets from a local cabinet shop. So I have nothing to lose but time and effort. I can't help but think that this will also prove Eric's thoughts on improved vacuum!

[EricSchimel]

So I'm back revisiting this... I did some bench tests gluing HDPE to plywood... It doesn't grab very well as expected... I know HDPE can curl if you take a bunch of material off of one side so. If I mill a plenum into it I think it will want to curl up and pull of the base layer....

So, I'm thinking either PVC, or maybe just sticking with my MDF plenum and sealing it again now that I'll have it open again.

[jerry_stanek]

I just looked at this again and I had the old MDF plenum and now the Sintra the Sintra is a lot nicer and I think I do get more holding power

[bobmoore]

I made my router bed with 3/4 hardwood apple ply for a base, layed on a 1/8 mdf as the 2nd layer and milled that flat with the bot. the 3rd layer is 12 millimeter corian with the vacuum grid machined in, then 3/4 inch mdf spoilboard to top it off. It works real nice with 8 -10 horse becker pumps. Bob

[ByronConnDesign]

Hey Eric,
I just replaced my table to PVC. I have baltic ply bolted to the frame and epoxied the pvc to that. I cut slots for aluminum extrusions which I use to hold fixtures and such. Any bleeders or additional plenums I seal with gasket rod and have had much better vacuum with this setup.

2999629997

[EricSchimel]

I saw that picture in another thread, really nice looking setup! What kind of PVC did you use? Sintra or something else?

[waynelocke]

Here is an HDPE vacuum table I recently built. It exceeds all of my expectations - to this point. I use a single Fein and it holds down much better than the shellacked mdf. I use round Home Depot weatherstripping and the T Track as stops. http://www.talkshopbot.com/forum/attachment.php?attachmentid=30007&stc=1http://www.talkshopbot.com/forum/attachment.php?attachmentid=30008&stc=1http://www.talkshopbot.com/forum/attachment.php?attachmentid=30009&stc=1http://www.talkshopbot.com/forum/attachment.php?attachmentid=30010&stc=1

[EricSchimel]

That looks fantastic. I love how you're able to pull up the strips and move them around for different work.

It looks like you've got that HDPE glued down to a base layer of regular plywood. What did you use for that? Wood glue or something else?

[waynelocke]

I did not glue it. I screwed with thee rows in the x direction spaced about 8" (I think) apart.

[EricSchimel]

Not sure where you're located but have you been through some heating and cooling cycles with that setup? Have you seen any lifting or curling?

You've sold me. I think I'll do an HDPE plenum, silicone and screw it down to the base layer and call it good.

Right now my spoilboard is glued down to my plenum. I'm thinking that glue won't grab to the HDPE very well. Perhaps I'll just dowel it in place. Do those gaskets provide a lot of side to side grip as well? I'm wondering if I don't glue my spoilboard down if a lot of sideways force would cause it to slip on the X and Y....

[waynelocke]

My shop has AC and heat so that is less of an issue. I read but don't remember the coefficient of expansion of the hdpe but it seemed small over a 40 degree change in temperature for the 2' X 4' pieces. I put the screws through larger holes in the hdpe so that it can move a bit. It has only been a month so who knows maybe in another month I will be screaming obscenities at it.

[wwaldner]

Eric
What did you end up going with for your plenum MDF, HDPE, or PVC? I am poised on making a new vac table, but undecided on whether to go with MDF for the plenum or just go with PVC. I can get a 1" sheet of PVC that I could cut ttracks into, but a bit worried about how stable it would be. My shop is well heated, but could get hot/humid in the summer. Just curious as to what you ended up doing.

To add the discussion, person sourcing the PVC said they taped their PVC to their table with double-sided tape. Anyone else have any experience with that?
wayne

[EricSchimel]

I haven't done anything just yet. I'm about to replace my spoilboard once again and I think I'm leaning towards keeping my MDF plenum and just re-coating it again...

[Brady Watson]

MDF + epoxy sealed is probably both your cheapest and best solution.

Unfortunately with anything other than phenolic, it just isn't reliable enough for large tables to stay flat and keep the bleeder flat. You can't reliably glue the bleeder down.

US Composites 635 Thin (http://www.uscomposites.com/epoxy.html) is decent and a good value for the money. I've laminated a lot of glass in the past with it. Get the pumps.

-B

[EricSchimel]

That's kinda the vibe I've been seeing Brady, that plastics just don't stay flat. As someone who cuts HDPE all the time I can definitely attest to it curling a bit.

In all honesty my MDF plenum has worked great and remained for the most part very flat. I've coated it several times with Shellac, do you think I can go over the top with this epoxy coat, or should I just Shellac it again?

[Brady Watson]

Eric,
Anything that seals the MDF essentially plasticizes it & keeps moisture and suction in...If you're happy with beetle bellies, go for it.

-B

[EricSchimel]

I'm happy with 'em, I just didn't want to pass up an opportunity to put something better on when I have the spoilboard off again :)

[wwaldner]

Thanks guys
I think I will go with conventional wisdom, especially since this is my first go at a vacuum bed setup.

Brady, you mention coating the MDF with epoxy. Here is what I am planning on doing.
1. Sheet of MDO bolted to frame. Not planning on sealing or coating MDO.
2. Epoxy MDF sheet to MDO and cut plenum. Not sure if epoxy is necessary as this step as the MDO under MDF would seal it. Thoughts?
3. Cut plenum grid and ??
1. (shellac/poly/epoxy) the plenum to seal it. Brady, is this step where the epoxy sealing comes in?
2. I have some epoxy bar top coating left over from another project. Wonder if that wouldn't do the trick in sealing things up.
4. surface another piece of MDF for bleeder and glue that plenum

Thanks for the help

[Brady Watson]

MDO, being Med Density Overlay (with phenolic paper), doesn't need to be sealed on the faces. The edges however wouldn't mind being coated. Scratch the face with the sander a little to help with glueing/bonding.

Yes, the MDF grid and edges should be sealed with epoxy. 2 coats is good, after you flatten the entire thing because clamp pressure will cause low spots. When done sealing, kiss the top flat again before glueing down the bleeder. With so many sharp edges, epoxy is bound to puddle under surface tension on the 2nd coat and create some high spots on the grid squares...or just from drips.

I have no experience using bar top epoxy for sealing so do what you think is best. Shellac or thin laminating epoxy is best because you'll get the MDF to wick up the first coat before it kicks.

-B

[wwaldner]

Thanks for the advice guys. There is a lot of info out there and it helps to get some specific questions answered.

[wwaldner]

Eric,
Do you have a VCarve file for your vac plenum with the 8 zones? Would you mind sharing?

[EricSchimel]

I'm more than happy to share mine, however mine was designed for my "V8 Vacuum"

https://3dwarehouse.sketchup.com/model/3759b8ff-64bb-4418-94d8-494bca1d8a4c/ShopBot-V8-Vacuum-System

Mine's got the holes in different places than normal to make it all work. At the end of the day mine's just an 8 zone vacuum table. Each zone is 24" x 24" and I've got a 1" grid in each zone. Pretty simple to duplicate. I'm more than happy to share my files, but I think you'll spend as much time hacking mine as you would drawing a grid from scratch...

[wwaldner]

Do you know the spacing you used for your grid, between zones and outside perimeter?

[EricSchimel]

Do you have SketchUp? (even the free version on the web) you can look at the model and measure the grid, it's right there.

The grid in the model I did as an "on line" cut with a .25" bit.

[Brady Watson]

Umm....FYI goto: C:\SbParts\VacuumTables & have a look...

-B

[jerry_stanek]

I just used the one that is in the shopbot files

[EricSchimel]

Honestly I think that's what I did too, I just modded mine a bit.

[Brady Watson]

Shoot for leaving a 1 to 1.5" border around all zones and 1" squares. If it's +/- 1/8"-1/4" one way or another, no big deal.

Use a down cut spiral to put the grooves in as it will reduce the chance of chipping out the squares. Lesser quality MDF with an upcut often chips out.

Extra points if you dance to the tune the machine plays while cutting the grid :D

-B

[wwaldner]

thanks guys. The shopbot guide indicates a 1/2" diameter bit for cutting the grid, did I read that right?

[jerry_stanek]

thanks guys. The shopbot guide indicates a 1/2" diameter bit for cutting the grid, did I read that right?

yes that is what I used