New table for sb

[hh_woodworking]

I Know that this is a repeat for some ,but we are going to replace the table on the shop bot. It is all out of mdf. In one post Bill said no MDF because if swelling. I will be putting in vacuum system in at the same time. This is what I need to know. We have 48x48 prt, the base will be 3/4 Baltic Birch. Should I use the birch or trupan for the vacuum plenum? How many zones should be set up. We have a 5hp vac pump to use. This Shop bot is in a school so we run a lot of small projects and several large ones.
Thanks
Ed

[billp]

Ed,( and others who are planning a vacuum table),
Whatever you make the plenum out of the major characteristic you want to achieve is NO air "loss/bleed" through the bottoms, edges,or surface, (and grooves) of the plenum itself. This is effect "focuses" the force of the vacuum in the most efficient/direct path.
There have been MANY successful Shopbot vacuum systems built using whatever was at hand at the local lumberyard ( and then sealing the hell out of it after cutting the plenum air channels...).
I am a great fan of good plywood because it seals ( and holds it's seal even after some surface punctures...) much better than the composite boards. So in your case I would suggest a plywood plenum, and then on top of that use the Trupan as your sacrificial "spoil board/bleeder board" to pull the vacuum through.It's the best use of both products. Unfortunately neither of them works well for BOTH purposes so you'll have to get a sheet of each ( although with a 48 by 48 you do get TWO pieces of each .....).
As far as the number of zones, this depends on your applications. You might be better off to start with some 24 x 48 "portable vacuum tables" that you can make from stock pieces of plywood off the rack of the Depot so that you could have either table work on it's own, or pair them together to get your full table covered...
Once you get a sense of your day to day needs you could either modify one (or both), of the jigs to give you as many zones as your job might demand.
To date the most stable, and efficient material I have seen on a user built table is Gary Campbell's 1"thick laminated PVC table down in the Florida Keys. It is absolutely not going to bleed air, curl up from the Florida heat, or absorb any of the accompanying humidity that goes with that heat....
There are some good pix of Gary's table on the Camp Flickr (Florida 2008 Camp)page;http://www.flickr.com/photos/campshopbot/2241548095/in/set-72157603848429474/
It's probably more important to TRY something, and fine tune it, than to spend countless days "engineering" something . I think most people underestimate the variety of things they will attempt when they first get their machine, so inevitably they wind up modifying/rebuilding their system. Sometimes it's done a few times as people's skills advance in different directions. And it's the absolutely perfect application of owning a CNC machine; a tool that can build it's own parts.....