Torsion box

[Randy Rizzo]

I'm in the planning stage for constructing a torsion box for a work surface. I've done some research, but can't find anything that addresses what a maximum grid dimension should be. Larger I assume yields less rigidity, but how large can you safely go? I planned on a 4'X8' surface. 3/4" MDF for outside framing, 1/2" MDF for grid, 1/2" MDF top/bottom skins, height of grids 3". If I divide the interior dimensions evenly I wind up with a grid 11 1/4" X 11 3/8". Anyone done something like this and how did it work out?

[Philip Rodriquez]

More grids = more rigidity and less flex. For a 4x8 box, make the grids as small as you can make them. Typically, they are just big enough to fit your nail gun in. Don't worry about exact spacing because you always have to fit the last row and you'll end up covering everything.

[Jeremy Greiner]

Marc over at thewoodwhisperer has an excellent video on a Torsion Box table top that sounds almost identical to what you are going to make. The video is at http://thewoodwhisperer.com/episode-...e-torsion-box/ and he talks about the dimensions he uses etc..

-jeremy

[Harvey Pascoe]

Torsion boxes are very forgiving on the grid sizes unless you are calculating for VERY heavy weights. The size you indicated should be adequate for very heavy service if you use 1" grids of pine. Gluing MDF to MDF is not particulary strong in my experience unless you have a large glue surface. As for 1/2" MDF grids, that sounds a little weak to me because of the lower glue strength and glue surface of 1/2" material. I'd keep the grid large and go for minimum 3/4" pine webbing. It will be much stronger. Just my opinion, no engineering facts to back it up.

[John TenEyck]

There was an article in FWW many years ago that I used when I designed a torsion box bed. If I remember correctly, the spacing I used was pretty small. The cross grid pieces are just stapled in from the top and bottom, so you don't need to worry about getting your gun inside. I used 3/8 or 1/2" thick pine, about 2" wide, and 1/4" plywood skins on both sides. It's amazingly stiff, just like a hollow core door.

[Scott Vigder]

Check this out. I built my torsion table three years ago and it is still my go-to table. I built mine so if the top ever got mutilated I could turn it over and have a brand new top....no need yet!

http://www.sawmillcreek.org/showthre...nty&highlight=

[Brian Schroeder]

Randy,

Here is what I have planned for my torsion box.

Assembly Bench1.jpgAssembly Bench2.jpgAssembly Bench3.jpgAssembly Bench4.jpg

Basically same idea as your, I will have about a 4' x 8' table. Plan on using 1/2" MDF with the spacing being about 4" and about 1 1/2" deep. I plan on notching the MDF using dado cuts and then gluing the assembly. Once I get the jig setup for doing the dados, it shouldn't take too long to punch it out.

[Mike Henderson]

Your box will be stronger with wider webs (meaning a thicker box). The problem with spacing the webs far apart is that the skin bends (depresses) between the webs. That's the main reason you want your top skin to be fairly thick. But the webs don't have to be very thick. I usually make them out of 1/4" hardboard or MDF - it's plenty strong and reduces the weight. Make sure you glue the webs to the skins.

Mike

[Alan Schaffter]

Before you dive into making a torsion box, you need to decide what you want it to do and how you will use it. The reason to build a torsion box is typically you want a flat, lightweight surface that resists bending. But if you need it to do more, then you need to understand the hows and whys of torsion boxes- what the skins do, how the height of the web affects the stiffness of the box, etc., etc.

If all you need is a lightweight, flat bench, you can make the entire thing from 3/16" or 1/4" hardboard, using 2-1/2" tall web pieces spaced 6" apart. I made one 8' long by 18" wide with this material and yellow glue for an article I wrote for American Woodworker last year. In the second photo it is supporting over 300 pounds of bricks at mid-span, but deflecting less than 1/2" at that point!





I used it for a scaffold when I washed my porch ceiling and it easily supported my svelte figure.

If you plan on clamping things to the torsion box or pounding on it - applying point loads to the surface, then you need to increase the thickness of the skin and/or reduce the web spacing. The second torsion box I built for the article was designed to be an assembly table so I used 1/2" MDF for the skins and web. The web was 2" tall and spaced 7-1/4" apart (for convenience and a single setup I usually choose a simple square web spacing between 6" - 7", and let that determine the overall size of the table). The table was dead flat, could absorb point loads, and while heavier than the hardboard table, was still no where near as heavy as a table with a solid top. The method I used to cut and assemble the web was much quicker, easier and precise than the method used by David Marks and the Wood Whisperer (see photo below). The two-part article which appeared in the Dec 09/Jan 10, issue #145, of AWW has a lot more info about torsion boxes and construction.





Don't waste your time fitting and piecing the web together. Use "halved" joints cut on your tablesaw with a dado blade and a simple indexing jig for perfect fitting web pieces. You don't need to staple or brad nail the web together, either; glue is just fine.

[Randy Rizzo]

I'd like to thank you all for the responses. I've got a much better idea of how to proceed. The photos and links were great. Looks like a good time to be doing it too, the thermometer hit -26F here in Frostbite Falls a few nights ago. That shop heater we put in last summer is earning it's keep!