Torsion box height vs. strength

[Stephen Saar]

I am planning on making a 8x4 torsion box for my table saw outfeed table/ general work area. I know this isn't really how most people do it, but for my needs I think it will work best. I'm currently trying to design the table, and I ran across an issue. My table saw (craftsman 10" hybrid saw, can't remember the exact model number) has a plastic door part that swings open to access the motor and inside for cleaning out, etc.... Originally I was going to build the table with the table being 4" thick, the outside layers were going to be 3/4 MDF, and the inside structure was going to be 2 1/4 " wide strips of 1/2" MDF, and the very top of the table would be a free floating .25" MDF piece that has a slick top (I can't remember the name of the material now, but it's common).

The problem is that from the top of the saw table, to the top of the door is only 2 15/16". So I replanned the table to have only ~1 11/64" high 1/2" MDF, but I was curious if this would be enough support for a table this big.

I was hoping to only have 5 legs total. 4 on the corners, and then 1 in the middle, I need the storage space underneath, and I didn't want to build an entire cabinet since that locks how the space can be used.

The other option I could do would be to go with the original 4" design, and then just make a cut out where the door would need to swing open. This seems like a better idea, but if you guys think the table would be strong enough using the smaller size I might go with that to help simplify the build process.

Thanks for any input, and sorry if my description was confusing.

[Alan Schaffter]

I would go with 1/4" hardboard bottom skin, 2" tall or shorter MDF web pieces (1/2" thick), 1/2" MDF top skin, and a replaceable 1/8" hardboard top.

That will be plenty strong for an outfeed and general work table. Here is a 8' long torsion box made completely with 3/16" hardboard. The web is 3" tall. In the pick it is loaded with over 300# of bricks yet is deflecting only 1/2":

http://www.ncwoodworker.net/pp/data/1421/medium/P1040010.JPG

http://www.ncwoodworker.net/pp/data/500/medium/IMG_2579.jpg

I made my right extension and outfeed tables from half torsion boxes- laminate over 1/2" MDF top skins, 1-1/2" high webs but no lower skin:

http://www.ncwoodworker.net/pp/data/500/medium/P5200028.JPG

http://www.ncwoodworker.net/pp/data/500/medium/P5260045.JPG

[Stephen Saar]

Is there any real advantage to using a full torsion box vs. a half torsion box? I would think the only major advantage for a full torsion box would be to give it better strength against racking forces since the bottom is then locked in as well.

-Stephen

[Mike Henderson]

The theory behind a torsion box is the same as for an I-beam. Assuming loading in the middle and both ends supported, the top has to resist compression and the bottom has to resist tension.

A box with both a top and bottom will be stronger (stiffer) than one without a bottom. You need to decide how strong you need it to be.

Mike

[Alan Schaffter]

The theory behind a torsion box is the same as for an I-beam. Assuming loading in the middle and both ends supported, the top has to resist compression and the bottom has to resist tension.

A box with both a top and bottom will be stronger (stiffer) than one without a bottom. You need to decide how strong you need it to be.

Mike

Most definitely stronger with two skins. The skins are what make it strong- like you said the bottom skin is under tension so you want a material that won't stretch, while the top skin is under compression so you don't want it to compress or buckle along the surface (so must be attached to the web well). The web just keeps them apart and handles much less of the load. The real issue is how strong of a table do you need. My test torsion box was just assembled with glue and it is strong!!! I used it last summer as a scaffold to wash the porch ceiling. Check out my article in the Dec/Jan 2010 issue of American Woodworker.

[Matt Schnurbusch]

I would build the torsion box with your original plan and then just move it away from the saw far enough to allow the door to open. Then have a separate out feed that sits on a ledger attached to the table, and legs supporting it next to the saw.

That's what I did, only I was able to attach a ledger to my saw. This gives the ability to work from all sides of the table, and get at the door under your saw. I don't have a close up, or even a good picture for that matter. But here (http://www.ncwoodworker.net/pp/showphoto.php?photo=42009) is a shot of my setup. In my case, the assembly table is set about two feet away, and aligned to the center of of my saw. This gives me the ability to run full sheets of plywood and large stock.


(http://www.ncwoodworker.net/pp/showphoto.php?photo=42009)

[Gerry Grzadzinski]

You don't need 3/4" skins. Do what was mentioned, and go with a 1/2" top, and 1/4" bottom, or both 1/2". You'll get a stiffer box with the taller ribs. Increasing the quantity of ribs will also stiffen it up. Even adding 1/4" ribs between the 1/2" ones will add strength.

And, imo, a half torsion box is not a torsion box at all. It's just a panel with some framing under it. It'll sag immediately without support.

[glenn bradley]

I think Matt is on to something as to the doors importance. I have a red plastic door. I open it about twice a year and so far over the years that has only been to peek inside to see how my DC upgrades have effected the interior.

When I added additional cast iron to the right of the table, I added a pedestal with storage vs. just legs. I could have worked around it but balancing the ratio of activity, I put the pedestal in a position that does not allow the door to open fully. This has yet to be an issue. Worst case; a few hinge screws and the door comes off.

Here's a shot from a couple of renditions ago back in January, 2007. The white panel is just a skin on one side of the frame that supports the storage pedestal. The other skin is 2/3" BB ply that supports the overarm mast and pegs for my ZCI's. The pegboard panel moves on full extension drawer slides.

[Gerry Grzadzinski]

I should mention that on my Unisaw, I have the same situation. With my outfeed table, the door only opens about 4". Just enough to reach in and clear the dust collector opening if any big pieces fall in and block it. With a ZCI, I'd never need to open it.