Question: I am planning to build a mobile base for my contractor-style tablesaw with a 50-in. extension table. I intend to construct a large torsion box for the base platform. My question is this: for the "lattice" structure of the torsion box, would it be reasonable to use 3/4-in. plywood or do you suppose I really should use solid hardwood? Obviously, I do plan to use plywood for the top and bottom skins. I am just not sure how 3/4-in. ply compares in stiffness to solid hardwood when placed on edge at, say, 4 in. or 5 in. wide. 
-- Chuck, via Ask The Experts , None

Answer:

The stiffness in a torsion box comes from the two skins being separated by a fairly great distance while still being rigidly locked together by the filler structure. The stiffness of the filler can contribute some additional strength, but it isn't what primarily makes a torsion box stiff. This is why you can build very stiff torsion boxes with cores of cardboard or aluminum honeycombs, even though the honeycombs by themselves are actually flimsy.

So the answer is yes, you can build a core lattice from plywood.  You will lose some glue-joint strength if you use yellow glue because 50% of the edge of the plywood will be end grain. But the strength of the glue joint will still be adequate because of the large overall surface area of the edges. To compensate, you could seal the plywood edges with a primer coat of glue. 

An alternative would be to use a construction adhesive that is made for gluing plywood decks to joists, such as PL400. These thick construction adhesives have good strength even when applied on end grain. PL400 and similar construction adhesives are applied as a bead with a caulking gun, so they are easy to use.

The strength of a torsion box is highly dependent on establishing a good grid-to-skin glue joint over the entire surface of the grid, so you need to nail or screw the plywood skins to the grid right after applying the glue to ensure that good contact is maintained while the glue sets.

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John White is Fine Woodworking's shop manager.

Photo: Lorien Crow


July 6, 2006