curlyoak Posted March 22, 2021 Report Share Posted March 22, 2021 I want to build a light weight torsion box. 3' x 8'. In general I like over kill. But this is for a collapsable table and weight is an issue. I'm thinking 1/4" fir for top and bottom. I'm not sure on the grid dimensions. Considering an oak outside frame and plywood for inside the grid. What dimensions should the grid be? And any alternatives to the above ideas. The goal is flat, light weight and durable. Thanks. Quote Link to comment Share on other sites More sharing options...
Chestnut Posted March 22, 2021 Report Share Posted March 22, 2021 With 1/4" fir ply for the top I think a 6x6 grid would work well but i'd have the grid by 1/2" ply just so there is a good amount of glue surface. If you want a bit more rigidity you could do 1/2" ply on the top and 1/4" on the bottom with a 12" ish grid in the middle. I just made a top that was 28" x 70" this way it was faily light but very stiff. 1 Quote Link to comment Share on other sites More sharing options...
BillyJack Posted March 22, 2021 Report Share Posted March 22, 2021 Hollow cores are light. Web frame of cardboard. I'd grab 1/8 masonite for the core.. 1 Quote Link to comment Share on other sites More sharing options...
wtnhighlander Posted March 23, 2021 Report Share Posted March 23, 2021 I think it would take a lot of force to flex a 1/4" skin over 12x12 grid squares. With a hardwood rim, the gridwork could be 1/8" masonite, corrugated cardboard, or even foam board, and still be quite strong, if the correct adhesive is used. Or just make an 'ice cream sandwhich' from layers of styrofoam sheeting and plywood skin. Structural wall panels are sometimes made like that. 1 Quote Link to comment Share on other sites More sharing options...
curlyoak Posted March 23, 2021 Author Report Share Posted March 23, 2021 How wide should the grid members be? Meaning the distance between the 1/4" top and bottom. Quote Link to comment Share on other sites More sharing options...
Chestnut Posted March 23, 2021 Report Share Posted March 23, 2021 I'd go at least 2" for the width of the members possibly 2.5". this is going to depend on the base that you are setting it on. I did a rough calc for sag assuming a 48" span aka 2 saw horses 48" apart with a center load of 150 lbs. If you are more evenly supporting the top or think you can get away with less weight capacity you could shrink that down to 1.5" but i wouldn't go any smaller than that personally. Quote Link to comment Share on other sites More sharing options...
curlyoak Posted March 23, 2021 Author Report Share Posted March 23, 2021 2 hours ago, Chestnut said: I'd go at least 2" for the width of the members possibly 2.5" I got lucky. Looking in my wood stash, I am trying to find the right piece with low waste. Not sure what was required I found a piece of soft maple 5 1/2" wide. After truing I ended up with 2 3/8". Turns out right in your wheel house. I milled it to 11/16" for weight considerations. I found some d. fir 1/2" bc grade and ripped it for the corrugation. My 1/2 work day is over. So tomorrow I will try to complete it. 1 Quote Link to comment Share on other sites More sharing options...
curlyoak Posted March 26, 2021 Author Report Share Posted March 26, 2021 I made it 3 x 7. There is 7, three foot rows 12" apart. 4 of the rows are 12" x 12". 3 rows (every other row) are 2- 12 x 18. The rigidity is more than enough. Not good for truck engines. Good for my needs. 1 Quote Link to comment Share on other sites More sharing options...
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