Alignment blocks for easy glue-up.
Clamp the completed core atop a plywood skin, and hot-glue alignment blocks to the skin in corner and center core sections.
Spreading the glue. A small roller is a good application tool. Coat every top edge on one side of the core, attach the skin, then flip the assembly and glue on the other skin.
Clamp it up. Combine clamps and clamping cauls to distribute clamping pressure evenly. Make sure to glue up the box on a flat surface such as a workbench top.
picccs from flopping around while you attach rhc skins. The core has ro include whatever ledger pockets arc necessary for joining the box to the building or to other furniture elements. Thus the geometry and dimensions of the core must be worked out on paper at the start of the project. (See Fig. 2.) There is no room for fiddling and figuring as you go along.
Good core materials are softwoods such as pine, low-density hardwoods such as poplar or alder, or strips of ply wood or MDF. The thickness of the core material can be anywhere from '/4 in. to 3/4 ¡n. The width of the core stock, plus the thickness of the two skins, is the finished thickness of the torsion box. The core spacing depends on the thickness of the skins. A good rule of thumb is to keep the on-center spacing of core pieces no more than twelve times the skin thickness. This is good insurance against deflection of the skin between core pieces.
The core material must be accurately cut to length and thickness, and must have dead-square ends. A smooth finish is not important—the material can come straight from the saw. These characteristics make the core an ideal tablesaw product.
With a solid wood core, wood movement might seem to be an issue, but in practice it isn't. When the box is complete, the core is sufficiently isolated from the atmosphere for changes in humidity to have little or no effect.
Staple the core together one side at a time. (See photo, page 68.) Use a 1/2-in-wide crown wire staple. An air stapler works best because it drives the staple into rhc surface of the wood. A spring-loaded staple gun may leave the staples a bit proud, so hammer them down flat. It's important to pull the picccs tight together, and to keep them vertical, but absolute squareness is not important. Make several extra pieces of core material to use as spacer blocks. The spacer blocks make it easy to position the core pieces and keep them vertical while you staple them in place.
The skins of a torsion box are man-made sheet material—plywood or MDF—so there will be no problem with shrinkage or expansion. Since the shape of the skin will be the shape of the finished box, the skin has to be cut accurately. As mentioned earlier, skin thickness determines core spacing. It is generally better to have a thinner skin and a densely gridded core than to design a T-box with thick skins and a core of widely spaced pieces. The dense grid/thin skin combination offers more strength with less weight as a result of the increased glue area provided by the core pieces.
Whatever your core spacing, it's important to avoid inferior skin materials, because if the skins delaminate, the box could fall apart. After constructing the box, you can surface the skin with any wood veneer, or with leather, plastic laminate or paint.
Small locating blocks, hot-glued to the skins, take the grief out of the glue-up, as shown in Fig. 1. Align the core on one end and one edge of the skin and clamp it. Now glue a locating block in each corner, plus a couple in the center. (See top left photo, above.) Then flip the core onto the other skin, align the same end and edge, and add another set of locators. Make index marks on
FIG. 2: DIMENSIONING A TORSION BOX SHELF
STEP 1: DETERMINE THE LENGTH
For a shelf that will be installed on a wood-frame wall, length should be a multiple of the stud spacing (either 16 in. or 24 in.) plus 4 in. to 6 in. This will provide maximum support, especially near each shelf end.
STEP 2: DETERMINE THE WIDTH
What do you want to store on your shelf? The finished width of the box will be the width of the skin plus the thickness of the lipping.
STEP 3: DETERMINE THE THICKNESS
For furniture, the minimum thickness is two skins plus a 3/4-in.-thick core, or about 1 Vg in. There is no maximum.
STEP 4: DETERMINE CORE SPACING
Make core pieces anywhere from '/4 in. to 3/4 in. thick, spaced 2 in. to 4 in. apart. Make the grid spacing square, so it's easy to assemble. The grid should be close enough so the skin doesn't sag, so it will vary according to skin thickness. When in doubt, make the spaces smaller.
SHELF LENGTH & WALL STUD SPACING
Was this article helpful?
THIS book is one of the series of Handbooks on industrial subjects being published by the Popular Mechanics Company. Like Popular Mechanics Magazine, and like the other books in this series, it is written so you can understand it. The purpose of Popular Mechanics Handbooks is to supply a growing demand for high-class, up-to-date and accurate text-books, suitable for home study as well as for class use, on all mechanical subjects. The textand illustrations, in each instance, have been prepared expressly for this series by well known experts, and revised by the editor of Popular Mechanics.