Torsion Box - flat surface

[John Seiffer]

I'm planning to build a torsion box and they say you need a flat surface to build one - but if I had a flat surface I wouldn't need a torsion box.

[And I know about jointing 2x4 on shimmed saw horses - I've seen the wood whisperer thing but I don't have a jointer and am not good with winding sticks]

I'm planning to follow Jamie Buxton's advice found here:
www.geocities.com/bawanewsletter/jun02/torsionbox1.pdf (http://www.geocities.com/bawanewsletter/jun02/torsionbox1.pdf)
and make the grid out of notched strips (he says it's OK if the cross pieces of the grid don't glue to each other - the strength comes from the glue joint with to the top and bottom skins)

Here's my question - if the grid is cut from strips that are the same width (my table saw can do that) and I glue the strips flush with the skins, won't they automatically form a flat surface?

I'm thinking I'll glue the grid to one skin using clamps and cauls on the flattest table I have (even though it's not as flat as I want) then when it dries the grid should pull the skin flatter than the table and that should be flat enough to glue the other skin on.

Am I headed for disaster with this?

[Thane Duncan]

David Marks has a good description on his website. He just shimmed a base prior to making box on top of the level base.

Thane

[Roland Chung]

John,

I procrastinated building my torsion box because I hadn't yet set up or ever used my planer. I ended up using the jointer and planer and made a reference surface and my torsion box turned out great. I used one of the ideas that I found on this website and others about notching the pieces with a dado blade. Yes, it took a while to make the jig and dial it in (as I had never done this before), but I believe that it ended up speeding up the process quite a bit.

If you don't have a jointer, do you have a router table that you could use to joint the 2x4's?

If you don't and can accept that your torsion box may not be as "almost, perfectly" flat as someone who has used the jointer and planer, then try running some straight 2x4's through the table saw a few times till you end up with 3 or 4 straight and flat 2x4's with a uniform thickness. Do the sawhorse thing and shim as necessary. By the time you throw on the 2 sheets of melamine, you will have a pretty flat and level surface.

Once you have the reference surface, putting together the TB really doesn't take that long. I "bothered" to make my TB really flat - when you put a straight edge on it from corner to corner, it is so flat you can't slide a piece of paper under the straight edge. I was happy assembling cabinets on the Costco folding tables, but I know that it is better now with the new assembly table.

Good luck and let us know how you did it and how things turned out!

[John Schreiber]

I don't think you are heading for disaster, but I wouldn't discount what you can do with a few tools and some cleverness.

Winding sticks are a cinch to use. Mine are 3/4 by 36" aluminum angle from the hardware store. (That's really too long most of the time.) I painted one flat black, but that's a refinement.

An incredibly straight straight edge is available using fishing line with weights (clamps) hanging off at each end. Even more precise than using winding sticks, you can run a fishing line from corner to corner with another on the other diagonal. Using finishing nails as spacers to get the line above the surface, you can shim the assembly to as close to dead flat as I can imagine.

Just some ideas, but when money is tight, there are lots of solutions.

[Jamie Buxton]

...

Here's my question - if the grid is cut from strips that are the same width (my table saw can do that) and I glue the strips flush with the skins, won't they automatically form a flat surface?...



After you glue the first skin on, but before you glue the second one on, you'll have an object which is straight along the ribs, but which can be easily twisted. You must remove that twist before you glue the second skin on. I like the crossed-strings method.

[Jim O'Dell]

How flat is your Table Saw? That's what I used to build my out feed table, and my extension table. Perfectly flat? Probably not, but it's pretty close.
If you lay your bottom skin on something that's not flat, it will try to conform to that. You can glue on the internal spacers. They will follow any deviations in the bottom skin. The top skin will then follow it on the spacers. Once glued in place, that is the shape it will hold. If it is not flat, or is twisted, you will be stuck with it. That is why it is important to start with a flat surface.
I'd try the saw horse method with shims under the bottom skin, using the winding sticks or fishing line (do they make that in colors??) to work it into a flat position. It will take time, but I don't think you would have to have a joiner if you can find some fairly flat 2X material. Or cut some MDF on the table saw and make an L shape with 2 pieces. MDF on edge holds pretty straight if supported properly. Just some ideas! Jim.

[Robin Cruz]

DIYnetwork.com Woodworks-David Marks has a video of his version of a torsion box design and construction