Torsion Box Construction Question

[Howard Rosenberg]

I seem to remember reading somewhere that recommended using styrofoam insulation panels inside "skins" to form a lightweight torsion box.

Has anyone here done this?

If so, can you add any particulars about its construction and long term performance.

Thank you.

Howard

[Mike Armstrong]

Don't know about styrofoam, but a guy at the woodworking school I go to was making something along those lines using corrugated cardboard, I guess like the stuff you find inside hollow core doors. I don't recall off hand where he got it, but it did come in different sizes/thicknesses. In his case, he was working with 24x24x1 panels of the stuff, set inside a frame and veneering both sides.

Mike

[Mike Henderson]

How deep (thick) are you making the torsion box? For the last couple I made, I used 3/16" tempered hardboard and cut them so they'd fit together. But the glue-up takes a long time. I used 60 minute epoxy and needed every minute of it.

This one (actually I made two) is 6" deep but I'm using this as a veneer press so I needed a lot of strength - and for it to stay flat when I have clamps around the sides. I used tempered hardboard because I wanted the boxes to be light enough to be handled by one person. The skins are 1/2" baltic birch.

Mike

[Howard Rosenberg]

Hi Mike -
Thank you for these photos.
Are you gang-cutting the hardboard? On the tablesaw?
Thank you.
Howard

[Mike Henderson]

Hi Mike -
Thank you for these photos.
Are you gang-cutting the hardboard? On the tablesaw?
Thank you.
Howard
Howard, I'm not sure whether you mean cutting the hardboard to width, or cutting the slots in the hardboard so that the pieces will fit together.

Regarding the first, I cut all the pieces at one time, with one table saw setting - this includes the baltic birch pieces on the sides. It's important that everything be exactly the same width.

Then for the slots, I cut those on the table saw also, using a sled, with stop blocks to make sure that I cut each piece in the same place. The cut is over 3" so I cut the hardboard laying flat on the sled. For narrow grid pieces, you could cut them with the hardboard upright, in the same way you cut finger joints. Since the slot is too narrow for a dado blade, I make one cut on all the pieces with a stop block, and cut to the middle of the piece (just a bit past the middle). Then I tap the stop block to make the slot a bit wider, and use a piece of scrap to get the right width (cut and try). Once I have the stop block set, I can cut all the pieces (for one slot).

I repeat that process for each slot.

Once I have all the hardboard cut, I put the pieces together and fit it to one of the skins, leaving room for the side pieces. I then use a pencil and mark the location of the hardboard - this is so that I can then put glue on the skin in the right places. I do this for both skins being careful to keep the proper orientation of the ribs.

When glueing, the reason it takes so long is that I glue all the intersections of the hardboard, then put glue on the skins in the places where I marked, and on the top and bottom of the hardboard grid. The strength of a torsion box comes from having the grid glued well to the top and bottom skin. I put the side pieces in and hold them in place with a brad nailer (I also glue the side pieces to the top and bottom skins - there's no need to glue them to the grid - it doesn't add any strength).

I clamp with cauls which have a slight camber. Because of the grid, the box comes out very flat.

Hope that explains everything. A torsion box is essentially I-beams at right angles to each other.

Mike