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The general idea...A torsion box is simply a grid sandwiched between two outer skins. The grid gives it incredible strength in two planes and keeps it from racking, either front to back, or sideways. The skins add strength in the third and final vertical plane. Once the skins are mechanically adheared to the grid, it becomes extremely ridgid and strong. In our world, plywood or even MDF is a great product to use for building these. Here is a short 8 second video that demonstrates its construction. |
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Materials and cut list...For this project, the torsion box is a 40" x 20" platform for what eventually be my tool cabinet. If you need one of a different size, or require a different grid arrangement for the core, vary the materials list to suit your needs.
** Notes **
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Parts layout...Doing the math on this stuff can drive you crazy. Imagine a 4" board that you want two equadistant dovetail pins on (three equal sections). Now, try to Find a ruler that will let you mark out pins centered at 1.33333". Good luck with that. But, there's an "Old Timer's" geometry trick to divide the width of a board into evenly spaced sections with very minimal math, and it works!
1. Determine how many equally spaced sections you want. |
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Build the grid...This is where I break from tradition. All the "rules" say that the grid should be a series of squares. And, if you're a M.E. designing wings for the space shuttle, do your stress and thermo dynamics homework, draw your blue print, and get a great machinist to build it to spec. And if you require yours to be "traditional," then by all means, it certainly won't hurt anything to build yours that way. However, notice how the cross memebers are offset on the middle set of ribs. For me, assembling it this way, where every other set of ribs is offset, was so much easier. What I realized when I made my two big mistakes is that in addition to an easier assembly, it didn't seem to have any impact on the strength, at least considering the weight load I apply to them. So, now it's incorporated into every torsion box build I make. |
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Bottom skin on, Adding corner braces...This is another break from the "norm." Normally, to reduce the build time, one would turn this thing over as soon as the glue had tacked enough to move it. This would facilitate putting the other skin on much quicker, and getting on with the show. But, in this case, the box will be used as a platform for something else, and wheels will be mounted to it and I like to give the lag bolts used for mounting the wheels a little something extra to bite into. To that end, I can only put one skin on at a time. Once the glue is dry, I can fit and glue some scrap ply into the corners. |
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Clamping the top skin...Time to glue the skin on the other side. I had about three projects going on at the same time and it seemed like I had about six different glue-ups going simultaneously; ran short of clamps. So I put a couple of cauls across the box and clamped them down and other clamps around the edges. But, I wasn't real happy and had this gaping blank spot in the middle. The fix was pretty straight forward... something compact, but heavy in the hole. My very old, cast iron Stanley bench grinder weighs about 80 lbs., and it fit the job perfectly. |
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Trim the overhang...There is light at the end of the tunnel. A router and a flush trim bit cleans up the skins where they over hang the inner grid. Trim it up on both sides, and it's pretty much done. Only a couple of steps left to go. Actually, at this point, the torsion box is technically done. If one were to build this, for example, as an assembly table, it would be as complete as it needed to be. However... |
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Install the wheels...This one will be the platform for my tool box. From the bottom side, I shot some " layout lines for the placement of the wheels. Mark and pre drill some holes for the lag bolts and bolt the wheels on. |
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Add the trim...I always add a little trim to the outside of these things. It's dual purpose; it dresses up the edges a little, and in the case of using MDF, it gives it a little better wearability over the long haul. These things aren't hard to build, but I don't want to build any new ones just for the sake of doing so. I happened to have a little red oak laying around, so that's what I used. I usually cut the trim for these things about " wider than needed. Miter the corners and flush it up at the top, including taking a block plane to clean up any small errors. I never flush up the bottoms to the bottom skin, only to the corners of the miters. Finally, I run a " roundover bit in a laminate trimmer to ease the top and bottom edges. |
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Varnish and done...Finally, a few coats of varnish on the trim and at least a couple on the skins to give it some protection from moisture and humidity. It's ready to go to work. |