torsion box materials - MDF and wood?

[erikjgreen]

Hi, all;

I'm building a 4x8 torsion box for use initially as a fiberglass molding table and eventually as a CNC router table. I'm doing my best to make sure it's flat and square by following the wood whisperer video.

But I re-watched the video yesterday and found out one of my materials assumptions was wrong. I had thought for some reason the internal grid he used was made of ordinary wood, and his is cut from MDF. My skins are MDF, but for the internal grid I bought pine "quality" grade lumber, which still required straightening in the table saw. After some work I do have enough boards with flat, parallel edges to build this thing. Or should I not?

If I use the pine, will the torsion box not come out right, or will it warp over time? I'd like to avoid dropping $44 on two more sheets of MDF at the moment. If I use the pine I'd build the whole thing flat/square and before attaching the second skin I'd seal the interior grid with polyurethane.

Erik

[ger21]

The pine should work fine. Just make sure when you seal the inside, you leave the glue surfaces clean and raw. The glue bond is what gives the torsion box it's strength.

[erikjgreen]

Thanks, will do.

Or maybe I'll just seal the outside with poly instead of the individual pieces inside, that might be enough to keep moisture constant in there. No problems with the glue to worry about then.

Erik

[morphious69]

you should look for a shop that makes displays as you can pick that MDF there up for next to nothing or if you hit their dumpster it is free. and they often times throw away pieces that are over 8 foot long and a foot wide...by the hundreds