torsion box - component sizes

[TomFromStLouis]

It appears I will be building a pair of 5' x 5' torsion boxes (to make a 5' x 10' bed) for a friend's new-to-her printing press. The finished bed will have a sheet metal skin layered on top and the weight to be supported does not appear like it will be much, so I am tempted to build these with the thinnest material that will do the job. Since baltic birch comes 5'x5' I figured four sheets of 1/4" would work for the skins, and perhaps 1/2" MDF would work as the core (~3" high core grid?). Make the grid maybe ~10" squares?

Actually, I suppose the skins could be even thinner, but a little beefiness won't hurt. I may insert some hardwood around the perimeter when done and also where the two boxes attach to each other (so the screws have something to bite into). 

This is my first torsion box build, so I am open to any informed comments.

[32ojdidit]

The skin can be thinner but I like a little beef on the top one. I built one for my router table a few years ago thinking it was a little over kill, but over the years I really have come to appreciate it.

[Mr Eddie]

I recently made a torsion box for my work table top.  For your application I think the 1/2' mdf grid would be plenty strong and 3" is tall enough.  I only made mine 2 1/4".  I agree about making the top and bottom a little thicker just to make it a little more resistant to abuse.  I would also double up the side pieces.  Here's a couple of links to my threads when I was making my top.  A lot of useful info from other members that might be helpful.

https://forums.woodnet.net/showthread.php?tid=7230967

https://forums.woodnet.net/showthread.php?tid=7293484

Lonnie

PS  I think it was pointed out in one of the threads that if you only need to support weight you could go with very 'skinny' setup.  Joe G said he had used a 1" core with only 1/4" skins and was able to support @ 400 lbs.  Not suitable for a workbench but appropriate for the right situation.

[Steve N]

Unless the press will also be on the boxes you are at the perfect point of being able to get by with fairly ultralight weights as far as material to make them. If the metal covering is consistent thickness it will also go to help. I wouldn't make the honeycomb less than 10" squares as that is the strength, but thin materials would be a green light with this build. Personally I would use BB ply for the skins over MDF, especially in 1/4" thickness, not so much worry about it snapping with a 10" honeycomb, but it's edges can be friable, and it sux up moisture like a sponge. Printing press = ink, ink = moisture.

[jteneyck]

Maybe it works fine, but to me MDF seems like a poor choice for the core. You would be gluing to its edge grain, and MDF sucks up glue like crazy on its edges. A poor bond there and your torsion box isn't going to fair well. Also, MDF is heavy. I'd use BB plywood for both the skins and core.

John

[barryvabeach]

I have made a few, and if you don't expect any point loading - and it sounds like you don't, you can go very thin on the top and bottom and still be fine.  I made a ramp for a powered wheelchair - with rider and a person walking alongside,  I estimated 400 to 500 pounds, and I went with luan, 7/32,  and it was fine.  As Steve and Eddie say, the strength comes from the grid -  I would use baltic birch, or regular ply for the grid - since that gives you a much better glue surface than mdf.  IIRC,  I went with a 2 inch deep pieces on the grid, and the depth of those pieces is the biggest factor in keeping the box from bending.   The spacing of the grid, OTOH,  will impact how spongy the top is, but won't play much into resistance from bending.  One test I did was  to cut four strips, then set them up in a box formation that lets you adjust the size, and laid a test piece of the top on it and pressed down in the center.  Then readjust to get a wider spacing and test again. Your actual torsion box will be stiffer due to the glue.   I was surprised how wide the spacing I could get away with.

[TomFromStLouis]

Thanks all of you. Looks like I will be usingbaltic birch for the whole thing.

Appreciate the help!