Making the Outer Frame
1. Begin by making the outer frame. With the aid of a short core strip, align the parts flush.	2. Staple the first two parts, one leg of the staple in each piece. Set the staples flat to the surface.
3. Repeat the procedure on the three remaining corners.	4. Holding the two long pieces, carefully turn the assembly over. Align the parts and staple.
Stapling the Long Core Strips
1. Use two of the short core strips to position the first long internal strip.	2. Put in two strips starting from one edge, then two from the other.
3. With the short core piece as a spacer, use your fingers to hold the pieces securely as you staple them.	4. Once the strips are attached to one side, flip the assembly and staple on the other, using spacers.

One of the first steps in building a torsion box is deciding what materials to use.

The Skins

Both skins should be made the same size with their edges parallel and corners square. They serve as the template to which the assembled core is fitted. Make them about 1/32" oversize.

Skin material is either plywood or MDF. The skin’s thickness determines the layout of the core grid: the thicker the skin, the larger the grid. Some rules of thumb: 1/4" skin requires core material 4" on center; 3/8" skin requires 5-1/2" on center; and 1/2" skin requires 6-1/2" on center. A skin thicker than 1/2" would seldom come into play as a furniture part. Using 1/4" for skins best capitalizes on the strength to weight ratio of the system. Generally, you’d use the same thickness material for both skins, although you don’t have to.

The intended finish for the torsion box determines the type of skin material. If you intend to spray or paint, MDF or clear birch plywood is a good choice. If you want a show wood surface, a wide selection of hardwood-faced material is available. However, if you intend to veneer the work yourself, veneer the skins before assembly.

The Core Stock

The core consists of all the pieces captured between the skins and can be made of MDF, plywood or a low-density solid wood such as poplar or softwood.

MDF is the heaviest option, and on a big job the weight can be a negative factor. Gluing MDF on edge always raises the question of how much glue is soaking into the fibrous edge. Also, the parts will be put together under pressure, increasing the concern over glue starvation. The solution is to squeeze a line of glue on the edge, then roll it out to cover the edge. Leave it for as long as five minutes, then squeeze another line of glue on the now partially dried edge and put the skin on.

Plywood is in the middle as far as weight. It cuts clean-edged with a sharp blade, and about half of the edge presents long grain, so it glues quite well on edge.

Poplar or softwood seems counterintuitive in this application since all other components are from manufactured sheet stock that provides dimensional stability. However, the shrinkage that takes place on a core 3/8" thick and, say, 1-3/4" wide is so small as to make little difference. However, use the same species throughout.

The core pieces are held together by wide staples applied on edge across the joint lines, top and bottom. The assembly method, more than strength considerations, determines the thickness of the core. Strength would be perfectly adequate if the core stock were 1/4" thick — or even less. However, it’s not feasible to staple parts together that are less than 3/8" thick. Because 1/2"-thick plywood and MDF are more common, you will probably choose that dimension. Anything thicker is unnecessary. Poplar or softwood 3/8" thick staples well.

Cutting the Core Stock

All core material must be flat, uniformly thick and squarely cut. Cut your material to width on a table saw using a sharp blade that cuts cleanly on the top and bottom edge faces. Then cut your stock to length on a miter saw, or use a miter fence on a table saw. The ends must be square to the face and square to the long edge.

Close attention to dimension and geometry at this stage is paramount, because accuracy of the whole torsion box starts here.

Connecting the Core

Staplers and staples: Drive the staples with a pneumatic or heavy-duty manual stapler. Crown staples are defined by two measurements: the length of the leg and the width of the crown. Use staples with a 3/8" leg and a 1/2" crown. The finer the wire gauge that’s used to make the staple, the better it will drive. If you use a manual stapler and it doesn’t seat the crown flush, finish the job with a hammer.

Start by stapling the four outside pieces, then run all the long strips in one direction, using the short core crosspieces as spacers. When all the long core strips are stapled on one side, turn the grid over and staple the other side. Finally, position and staple the short core crosspieces top and bottom. Note that the sole purpose of the staples is to stabilize the box so it  can be handled until the skins are glued to it.