The Overall Plan

See, you can make a box that is light and strong.

It is not that hard to build a really strong bed for a router table. I have seen some torsion box builds but they tend to be way too heavy and not much better than just stacking lumber together into a pile. The mess of plywood and clamps you see here is my little OZ Racer I built from plans designed by Michael Storer. Yes, I was too lazy to go out and clear it off to take a picture of the finished boat. Use your imagination. It is painted bright yellow with red accents. It are a perfect example for what I am planning. The boat was made much lighter than what I will make the table. The boat is mostly 1/8" plywood. I will probably go ahead and go with 1/4" ply for the router bed. It is about the same price as the thinner and I don't have to travel as far to pick it up.

Rib Framework

This is the basic layout I am planning for the ribs for the torsion box for the router table bed. It will be 1/4" ply (Well, actually 5.2mm Luan underlayment). It will be skinned on top and bottom with a full sheet. To join, I will glue 3/4 in. gluing blocks to distribute the glue join. The intersections of the ribs will be glued with thickened epoxy. Specifically, it will be System 3 Gel Magic since I have about a half gallon left over from my last project. I will add an extra sheet of 1/2 ply on the top to give a more solid work surface and skirt it with 1/2 in ply to give me a solid surface to attach external clamping and the guide rails.  The holes and slots to connect the egg crate framework look more complicated than they are. With the pieces all stacked, I can cut the holes with a hole saw through all the pieces at one time. Same for the slots. I might forget the outside pieces in the frame and only use the 1/2 skirt. Depends on how I feel when I am cutting them out. It doesn't really add any strength but it will add material to attach the rails.

Table down and folded.

 
This is where this little exercise is leading. I plan on my table to be able to be tilted up and rolled out of the way against the wall, so I don't have to give all my space up to have a full sized table. The bed will be self supporting for the length and the rails will be attached to the box. Now, the first thing you might think will be: Won't the bed sag? Some but not very much if it is built well. There is a lot of strength and I am not putting a lot of force on it. There is one other thing that is going to make it work. I don't really care if it sags some. The gantry is running on the rails attached to the sides of the torsion box. If the bed sags any, the distance from the gantry to the table top is unchanged. The rails move with the bed. It is the relative distance that matters. The box is much more rigid than anything I would ever put on top of the table and the sheet goods to be cut will conform to the table top. I have built a much thinner torsion box for my sewing machine. The machine weighs about 40lbs and is set into a hole on the table which is 1-1/2 thick with 1/8 ply on the bottom and 1/4 ply on the top. After a couple years, it has not sagged appreciably. I am not worried about how this much stronger box will hold the gantry which will be directly over the trusses and a sheet of plywood that will be distributed over the entire surface.