Help with vac table layup.

[woodshopcnc]

Rebuilding the VAC table on a PRS alpha 4 x 8. Table on this used machine when purchased: a bottom sheet of plywood bolted to metal, 1/2" sheet of white plastic with a grid machined into it with and outer gasket screwed to the plywood, then a piece of ldf or mdf (flat on both sides). Previous owner used 1 1/2" thick mdf (not 3/4). Also has 2 fein vacs (1 turbo II and 1 turbo III), with a 4 zoned pvc under table.

The majority of my work is no larger than 24" x 96" in hardwoods, I almost never cut sheet material though I do have a job inhouse for a paint grade cabinet that I could cnc or just table saw.

Anyway, how should I reconstruct this? I looked at the brady vac I and II but just want to make sure I don't screw something up.

So, do I go with 3/4" ply at the base, glue (not screw) a layer of mdf (or similar), then put a lfd sheet on top, probably not a full sheet for my smaller areas. If you don't screw the 2nd sheet to the plywood should I run the carriage bolts through both layers or just come up through the bottom into the ply with a insert/bolt? Is it best to grid the 2nd layer plenum or just mill the top spoilboard (grid side down) as with a brady vac I or grid both?

I also have a pod system with a gast pump that could be used.

There is a 3/8" sheet of sintra at the shop as well if i need to layer it in there somewhere.

Are there any + / - on using a 1 1/2" thick sheet of mdf over a 3/4" sheet? I have several sheets of both.

Stephen

[knight_toolworks]

if your doing solid wood the vac table is of limited use. only if the wood is really flat will it actually work. so putting mdf over the ply would be the way I go so I could nail or screw or make a high vac table.

[beacon14]

My personal preference (and everyone here has their own) is to bolt down a sheet of the best 3/4" plywood you can find (I used Baltic Birch plywood, and laminated both faces with brown backer laminate in the vacuum press first). Then I would glue the plenum layer down (I used regular 3/4" MDF). The key is you can't expect to get a good glue bond using any combination of weights, screws, and/or clamps. Over such a large surface the only realistic way to get a uniform, tight glue joint is to use vacuum. And it's easy to do - you are about to rout holes in the table for your vacuum plumbing anyway, right? I just sized the holes at first to be a good fit for the hose that came with the Fein vac, hooked the Fein up to the plywood base layer, spread the glue, set the plenum layer, and covered the whole thing with a sheet of melamine to contain the vacuum. I did the glue-up in 4 pieces but you could do the whole table at once pretty easily.

I routed grooves in the plenum layer and then laid the LDF (Trupan) spoilboard on top. I only sealed the perimeter of the plenum layer - most people go to a lot of trouble to seal the entire plenum surface but I figure the bottom can't leak since it's glued to the base layer, and the top is designed to collect as much air as possible, so why seal it? As long as the bottom and edges can't leak you won't loose any vacuum. (As I was gluing the plenum down in 4 sections I put PVC edgebanding as an air-dam between zones.)

I explained the process in this article (http://www.shopbotblog.com/index.php/2006/10/vacuum-table-ultra/), but most people don't seem to get the concept of using vacuum (even just a shop-vac) to glue the layers of the table together. I hope it makes sense.

As for using 1 1/2 material for the spoilboard, as long as you have enough suction to pull air through it you should be OK, and you'll be able to go a long time between spoilboard replacements. It is common practice to surface the "skin" off of both faces when putting a new spoilboard into service.

BTW I routinely use my table (formerly with two Fein vacs, now with two 220V vac motors) to machine solid wood parts. You do have to have a reasonably flat, smooth bottom surface and/or mask around the part to contain the vacuum but with a little effort you can get great results.

Whatever you do don't listen to anything Joe Crumley tells you about vacuum tables. Signmaking, yes, but not vac tables.

[woodshopcnc]

Looks like a great setup David. I try that approach.

What is brown backer laminate? (I really never work with sheet goods so sorry to ask). To laminate the two sheets to the plywood you put it in a vac bag and pull it down?

When that laminated layer is complete, do you bolt it from underneath the table because your plenum is glued to the top and you need to remove it one day? I read your article, the finished photos showing the grid with the vac ports is the plenum right, not the spoilboard that you would then put on top? So in my situation I would put my spoil board on top of the grid plenum, throw a board on, and mask off as much as I can for best vac.

Alot of what I do has a profiled edge where the the board must be elevated, so I'll suck down a piece of 3/4" melamine, put my pod on top, and suck those down with a separate the dedicated pod vac. I think that will work.

I know the current two fien vacs will pull down a full sheet of plywood without any problem through the 1 1/2" thick mdf, the guy I bought it from showed me when I bought it.

If you can answer some of these questions. I'll also have my partner pm you on some of your sandbox cams, pretty slick.

Thank you for your time.

Stephen

[Brady Watson]

Stephen,
For the work you are doing, you should not have to alter your table layup. Use a good plywood as your support board (I like MDO which has phenolic paper faces) and then I countersink the carriage bolts thru that to tie into the machine frame. I crawl underneath, drill an 1/8" hole up thru each crossmember hole. Then come up top and drill each 1/8" with a 7/8" paddle/Speedbore bit about 3/16 to 1/4" deep. Then drill all the way thru with a 3/8" drill. Pop in the carriage bolts and tighten from underneath. Then put glue all over the top of the support board I just bolted down. You'll use about a quart or so (get 3 16oz bottles) and swirl the glue around in circles all over the entire sheet, and hit the edges really well. Just get the glue on there - I've found that mechanically spreading, rolling etc makes a mess and reduces your open/working time. Then CROWN your MDF or Medex MDF sheet with the crown DOWN use plenty of weights and clamps. Let it sit overnight.

From this point you can either make a dedicated vacuum system, where you machine a grid into the layer you glued on (what some will call the plenum layer - or when vac is not used, your spoilboard)...or you can just leave it as is after you have flattened it.

You can make a BradyVac grid out of Trupan or Ultralight MDF. The idea behind the BV is to maintain your versatility. You can take it off when you need to screw something down without risking plenum damage. For lumber planks, there are a few ways you can go. You can cut a 2.27" hole into the spoilboard to be your vacuum supply that will let you plug the Fein right into it from underneath. You'll want to put it where you think the center of your BV vacuum pod will go. You can use a sheet/strip (lets say 24" wide) to go on top of this hole & it will function as a universal hold down - which means vacuum will leak outside of the area your material to be cut occupies.

You can also plane down smooth on both sides some Veranda composite lumber. Machine a grid into it and hole that intersects the one in your spoilboard and lay a piece of 1/4" UL MDF on top. This can also be used universally - OR use regular MDF or even Sintra/Komatex and make what I call a vacuum mask - which is along the lines of a BradyVac2 setup, where vacuum air is directed right under your parts to be cut. This gives the most aggressive holding solution, but it is really only for production and not one-offs because you have the time & expense of making a mask for each part you make.

Hope that isn't too confusing...that's just how I do it. As David points out - there's more than one way to skin a cat. Whichever way works for you - and the materials you cut - is the right one for you. You should plan on experimenting on your own to find what does & doesn't work for you.

-B

[woodshopcnc]

Thanks Brady,

Between you three, I think I finally have it figured out. Your post cleared up the attach from the bottom thing I wasn't clear on. When the plenum is glued to the plywood base that is bolted down (to replace the plenum if it ever gets screwed up) you simply unbolt the base from the steel stringers and throw away the ply / carriage bolt / plenum in one shot. Got it!

I plan to get this thing going this week.

I also want to incorporate some hold downs, some t-rack with David's cams, and additional hold downs.

If any of you guys know of a good source for t-racks and decent clamps that fit in the rack and can also bite down on the top edge of the board let me know. I like to use the vacs as much as possible but with longer carve times, a bolt down clamp type thing would work out better.

Stephen

[Brady Watson]

When the plenum is glued to the plywood base that is bolted down (to replace the plenum if it ever gets screwed up) you simply unbolt the base from the steel stringers and throw away the ply / carriage bolt / plenum in one shot. Got it!

You should not have to replace the glued up plywood/MDF layup unless you have to completely disassemble the tool to move or something. The layer of MDF is ONLY a plenum if you machine a vacuum grid into it - (plenum - a chamber intended to contain air, gas, or liquid at pressure); Otherwise, the 2nd layer will be your spoilboard.

IF using a plenum layer, you'll also use a 3rd layer which will become your bleeder board - effectively becoming your spoilboard as well. This layer MUST be ultralight MDF. You can machine this down to about 1/4" before you have to replace it. I prefer to lightly glue the entire perimeter (incl. zone boundaries), which lets me take it down closer than a 1/4". Other people countersink screws from the top and fasten it to the plenum that way. Either way you would machine off the old, or unscrew, and then replace the bleeder board ONLY. No need to unbolt the whole deal.

If you are just using the 2 layers with no plenum, you can scarf, scrape, and otherwise SPOIL it as much as you want, then machine it flat. As it starts to get thin, just flatten one more time & glue another sheet right on top of the old one, over and over again, as is required.


-B

[jTr]

Stephen,
Just finished building my new table.
T-Track I learned of in other threads is from 8020inc. Most users advocate 1.5x.75 size, referred to as 1575 in catalog listing. It does instil a certian sense that it may improve/help maintain a rigid base - plus it looks like the same stuff the bot is made with. :cool:
NOTE: The 5/16 t-bolts and knob assortments I'd recieved from those other resources fit this brand.

8020inc has a great store front on ebay - Do not let shipping costs panic you - they shipped 4 1/2 96" sticks to me for less than $170 including shipping. Packaging was impressive. As you're browsing, get some extra oval thread inserts for more versatility.

Jeff

[beacon14]

The "brown backer laminate" is plastic laminate ("Formica") without the color layer; it's sold as an inexpensive option for laminating the bottoms of countertops or the back face of panels that will only be seen on one side. Its advantage (other than low cost) is that you can glue the next layer to it, unlike regular laminate, and it's pretty much impervious to air and moisture. I have a 4x8 vacuum bag and vacuum pump that I use to lay up full sheets. Once the plywood has this stuff laminated to both faces, and the edges are sealed, it's not going to move.

Brady's method of bolting the base sheet down is about the simplest, fastest way I have seen. You should never have to remove the base layer except in the case of a major re-build (like after a fire or flood). The plenum layer is glued on after the base layer is bolted down.

I bought my T-track from t-trackusa.com (http://ttrackusa.com/track_%20systems.htm#1018). They also have various hold-down options. I'd be really careful using any hold-down devices that are taller than the material thickness. Don't ask me why I say that.

"I'll suck down a piece of 3/4" melamine, put my pod on top, and suck those down with a separate the dedicated pod vac." Depending on what you are trying to hold down, and how aggressively you are cutting, you could rout openings in the melamine and use the vacuum to hold both the melamine and your workpiece to the table. A gasket (search for All-Star Adhesives) would help maintain vacuum and minimize slipping.

[michael_schwartz]

I routed grooves in the plenum layer and then laid the LDF (Trupan) spoilboard on top. I only sealed the perimeter of the plenum layer - most people go to a lot of trouble to seal the entire plenum surface but I figure the bottom can't leak since it's glued to the base layer, and the top is designed to collect as much air as possible, so why seal it? As long as the bottom and edges can't leak you won't loose any vacuum. (As I was gluing the plenum down in 4 sections I put PVC edgebanding as an air-dam between zones.)

I explained the process in
this article (http://www.shopbotblog.com/index.php/2006/10/vacuum-table-ultra/), but most people don't seem to get the concept of using vacuum (even just a shop-vac) to glue the layers of the table together. I hope it makes sense.
.

I got a chance to try this when I laid up my plenum, and it worked great. I don't think there is a better way. I agree that speaking in a practical sense your just never going to be able to place enough weight on top of the sheet within the working time of the glue to do much good. This is a fact that has been known for a while when dealing with veneer.

I laid up my plenum board in one piece so it was a good feeling just turning on the vacuum once it was in place and being done without having to run all over the place grabbing clamps and weights.