Torsion box design

[Dan Chouinard]

I do have the luxury of making the new box on my current 32" x 8' torsion box.

[Dan Chouinard]

Sorry Dan for the snarky post. I honestly am interested in your project. When you say you want the top to be thin. What thickness are you thinking?

My outfeed table is a torsion box that is 2-1/2" thick, using 1/2" Baltic birch for top & bottom skins & the same for the grid. It is very flat & robust. I use it to clamp to & it stands up very well. Is that the sort of thing you're wanting?

No worries Frank. Its all good. I could have been more clear. Its the overall thickness I was hoping to minimize. I will be clamping to it frequently. The weight of it is less important and I should not have mentioned it at all.

[Jim Matthews]

If you're building this with an air nailer, leave enough space between dividers for the "gun" to fit.

[Jim Becker]

No worries Frank. Its all good. I could have been more clear. Its the overall thickness I was hoping to minimize. I will be clamping to it frequently. The weight of it is less important and I should not have mentioned it at all.

From a practical standpoint for both clamping and use of dogs for alignment purposes, I think you have to expect that probably about 3" is about the minimum you'll get away with. The shorter internal structures will also have less weight deflection capacity where the assembly isn't supported by the underlying lift-table...the overhang, in other words. The more-bigger you make the torsion box for your application, the thicker it necessarily has to be to insure it stays flat around the unsupported edges.

[Robert Engel]

Check Mike Farrington. He has a video on building exactly what you're talking about.

[Dan Chouinard]

Sorry Dan for the snarky post. I honestly am interested in your project. When you say you want the top to be thin. What thickness are you thinking?

My outfeed table is a torsion box that is 2-1/2" thick, using 1/2" Baltic birch for top & bottom skins & the same for the grid. It is very flat & robust. I use it to clamp to & it stands up very well. Is that the sort of thing you're wanting?

Yes Frank that is the sort of thing I have in mind. If I can get away with 2 1/2" thickness that would be great. In all likelihood the torsion box will be 4' x 8'. This will produce an overhang no greater than 12" beyond lift top. I have had very limited exposure to baltic birch ply. I will price it. My previous torsion box was made with 1/2" MDF with a sacrificial 1/8" hardboard on top. What would you recommend for a internal grid layout if I use plywood? Should that layout be any different if MDF is used?

[Dan Chouinard]

I may actually have time to build my new torsion box this weekend. It will be 48 x 96, bolted to scissor lift measuring 32 x 72. I cant imagine ever having more than 350 pounds on it. Its only purpose will be to serve as a flat assembly table for cabinet assembly. I would like to keep it 3" thick, using 3/8 mdf skins. Therefor internal grid will be 2 1/4 rips of 3/4 birch ply or 3/4 mdf.
If this is a viable plan, what would the recommended spacing be for the grid?
Thank you

[Kevin Jenness]

4-5" o.c should work for overall stiffness ,maybe more . Do a small sample to see if the skins deflect significantly under clamp pressure between the grid elements. You could do variable spacing, closer at the perimeter where it overhangs your lift.

[Alan Schaffter]

I wrote a two-part illustrated article about my adjustable height assembly table for American Woodworker many years ago. In the article I discuss a little bit of the theory and construction considerations.

A torsion box has three inter-related properties/goals. They are a lightweight, strong, and have a flat surface. How you build it depends on your needs- do you need a hard tough surface that must withstand hammering and other punishment, will the surface have dog holes, how big must the surface be, how will it be supported, is weight a consideration? If all you need is a flat surface then a torsion box with thin 3/6" or 1/4" skins and internal (shear) webs will suffice. Resistance to bending depends on the lateral strength of the skins so you don't want skins that will stretch in the surface plane. The web only serves to keep the skins apart (it also helps the skins to resist stretching and compression) so it mainly needs to resist edge to edge compression.

There is no need to go to all the trouble making the web like David Marks or Mark Spagnuolo. Instead just make a simple indexing jig to cut half laps in web strips using a dado blade on a tablesaw. No need for precise spacing- the indexing jig will do that for all but the last cell, and it doesn't matter if the last one is a bit smaller or larger than the other cells. Using notched strips ensures the lengths of the web pieces will be the same after assembly (unlike the Marks' crazy piecing method!!!). There is no need to staple the web together, glue will be fine. No need to dado the skins for the web- attach the web to the skins with glue, a few brads, and weights until the glue dries. For heavier duty (and heavier!!!) table, consider 1/2" MDF skins and web. I'm sure my AWW article is hiding out there on the web somewhere. I think I found it, but couldn't view the whole thing.

[Dan Chouinard]

Thanks for the input guys. I had stated earlier in this thread that I had no interest in 1/4" skins. I am now open to the idea of 1/4" mdf skins. This thing does not need to withstand a beating. It just needs to stay flat and tolerate clamping at the edges. Like my current torsion box, it will have a 1/8" sacrificial sheet of hardboard on top. Four to five inch grid spacing has been suggested. Any other feedback in regards to this would be welcome and appreciated.

[Kevin Jenness]

My old 4"x48"x96" bench surface had 5mm luan skins with a p-lam overlay and 5/16" thick core strips on approx. 4" centers with a 3/4" thick core frame. The box was very stiff torsionally but could have used closer spacing for clamping around the edges. There were a couple of cracks in the laminate from errant mallet blows over 30 years of use. I liked the laminate for glue and wear resistance but if you plan to screw into the surface a waxed hardboard or melamine coated particleboard sacrificial sheet makes more sense.

[Ole Anderson]

My first intro to torsion boxes was when David Marks had a show on TV. Hoping to pull up a YouTube video I only found this one on his website, $4.99. https://www.djmarks.com/store/woodwo...n-4-episode-9/

[Alan Schaffter]

Thanks for the input guys. I had stated earlier in this thread that I had no interest in 1/4" skins. I am now open to the idea of 1/4" mdf skins. This thing does not need to withstand a beating. It just needs to stay flat and tolerate clamping at the edges. Like my current torsion box, it will have a 1/8" sacrificial sheet of hardboard on top. Four to five inch grid spacing has been suggested. Any other feedback in regards to this would be welcome and appreciated.

There is no reason you can't make a hybrid- use more closely spaced, and possibly thicker, webs (1/2" or thicker MDF* or wood) around the periphery where you might be clamping, then 5" to 6" spacing of thinner web pieces in the field. I also used extra pieces to reinforce leg sockets so I could use bolt on legs and removed them if needed. FYI, my article contained a photo of a 30" X 8' experimental torsion box I made using 3/16" hardboard and leftover 3/16" to 1/4" peg board. I used it for a scaffold when cleaning my porch ceiling. For the article photo, I supported the ends on saw horses and loaded it with over 350 lbs of bricks stacked in the center of the span. While it looks greater, deflection in the center was only about a 1/2" (I measured it before and after loading)!! It was strong, rigid, and incredibly light- I could pick it up with one hand.

* I use hardboard or MDF on my torsion boxes because it is dimensionally stable and doesn't warp like wood and ply. I never liked the idea of using hardboard, etc, as a protective overlay, so I applied high pressure laminate. I use thinner, acetone, etc. to remove paint and a cabinet scraper, paint scraper, or chisel to remove hardened glue. It doesn't look pristine any more, but has held up extremely well over the years.

I found an MSWord editor's cut of the article which I can send if you want. I'm not sure my current level of SMC membership will allow me to post photos but I'll try (most are in my media albums on NC Woodworker).

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Torsion box web exploded, FigB.jpg

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[Michael Drew]

I built an outfeed table with a torsion box top. I got the idea from a YouTube video.....(two parts). https://www.youtube.com/watch?v=H6Tc...andWoodworking

You may find his methodology helpful.

[Alan Schaffter]

Boy that is a lot of talk and drawings for a non-problem!! He could have built any thickness torsion box and attached it to the outfeed of his Sawstop using a spacer made from wood to rest on the angle!

Since my extension, outfeed and drop-down outfeed tables are not huge, don't ever support heavy (point) loads (unlike in some shops where they are a catch-all ), and are fully supported at their perimeters, when I designed and built a new mobile base for my tablesaw, I just made the tables from half torsion boxes (no bottom skin). I made the webs from 1/2" oak and the skins from 1/2" MDF. The lower edges of the web pieces provide enough resistance to tension (stretching). I also covered the top with high pressure laminate. All are still good almost 15 years later.

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