How to Build a Torsion Box

http://americanwoodworker.com/blogs/techniques/archive/2013/03/07/how-to-build-a-torsion-box.aspx

How to Build a Torsion Box

It’s amazingly strong, light. . . and cheap!

By Alan Schaffter

Torsion Boxes–the Real Story

“How can something made from such thin wood be so stiff?” That’s what everybody asks when they first meet a torsion box. The engineering principles behind a torsion box are pretty simple; even so, there’s a lot of misleading information about torsion boxes, particularly on the Internet. Without getting too technical, here’s what you should know:

• A torsion box works like an I-beam (see right). It’s almost as strong as it would be if it were made from solid material–but it’s much lighter and less expensive.

• The thicker the torsion box, the stronger it will be–by a lot! If you increase a torsion box’s thickness by 25%, for example, it will be 100% stronger. If you doubled the thickness, it would be eight times stronger. Strength increases by the thickness cubed.

• The stiffness of the skins isn’t that important. A thick material, such as 1″ plywood, doesn’t necessarily make a stiffer box than using thin material, such as 1/4″ plywood. Thickness does matter in another way, though: thin material will deflect more easily where it’s unsupported, in the spaces between the web pieces. If you’ll be pounding on a torsion box, clamping things to it, or setting heavy objects with narrow feet on it, a thick skin is better than a thin skin. Or you could space the web pieces closer together.

• The thickness of the web pieces isn’t all that important, either. They can be relatively thin and lightweight, as long as they resist stretching and compression. For an MDF box, such as the one in this article, I usually space the web pieces 6” to 8” apart. The web pieces must be well-glued to the skins to prevent the skins from buckling, though. That means that the web material shouldn’t be too thin–it must be thick enough to have a sufficiently wide glue surface. You don’t need to use a special glue to assemble a torsion box–a PVA (yellow) glue works fine. And to clear up two misconceptions on the Internet: the webs don’t have to look like miniature I-beams, nor do you have to cut dados in the skins to receive the webs. Torsion boxes don’t need to be that complicated!

Click any image to view a larger version.

Strong, but light

This huge torsion box is 8 ft. long, but it easily holds 300 lbs. of bricks and deflects less than 1/2″. It weighs only 30 lbs., and is made from inexpensive 3/16″ hardboard.

How an I-beam works

When you press down on a solid beam, you actually compress its top section and stretch its bottom section. These compression and tension forces are neutral at the beam’s center. In an I-beam, most of the center is removed, to save material and weight, but the forces still act in the same way.

A torsion box is a remarkable piece of engineering. It’s stable, light and uses a minimum amount of material, yet it’s extremely strong. The idea has been around for years, and you probably have one in your house: hollow-core doors are torsion boxes.

Making a torsion box doesn’t require any specialized equipment or skills–a guy with a small shop and a decent tablesaw can easily handle the job.

When I needed a flat, solid, durable and inexpensive top for an assembly table, I planned on making it as a torsion box. I researched the subject and found a lot of conflicting information. I also talked to some experts–and learned a lot.

In this article, I’ll show you how I built that assembly table top, but you can use this method to build a torsion box of any size or composition. If you stretch your imagination a bit, you can find many ways to use torsion boxes in furniture making: they can be desk or dining table tops, pedestal ends, shelves and, of course, doors.

Materials

A basic torsion box is composed of two types of parts: skins (the top and bottom) and webs (which form an internal grid, and include the sides of the box). Skins are usually made from an engineered material, such as plywood, MDF or hardboard. The webs and sides may be made from engineered material or solid wood. The web pieces do not need to interlock, as mine do, but it is quicker and easier to make the grid that way. Plus, the box will be stronger.

I used 1/2″ MDF for all three parts. Though heavier than plywood, MDF has a more consistent and uniform structure. It is flat and stays flat, if properly stored. It has no internal stresses, resists compression and tension along the surface, machines easily, holds glue relatively well, and is relatively inexpensive. Using the same material for all the parts minimizes the amount of sheetstock you have to buy, and gives you more flexibility in cutting it up.

MDF that’s 3/4″ thick would work, too, but there’s no significant advantage to it (see Torsion Boxes–The Real Story, above). Material that’s 1/2″ thick strikes just the right balance, I think–it’s thick enough so you can safely nail into its edges, but thin enough to make a large table that’s light enough to move.

Cut the web pieces

1. You will need only two sheets of 1/2″ MDF to make the skins (A), web pieces (B and C) and sides (D and E). Break down the sheets into manageable pieces (Fig. E), then cut all the parts to exact dimension (see Cutting List, below). Rip the web, sides and leg socket reinforcing parts (F) at the same time to ensure they’re exactly the same width.

2. Set up your tablesaw to cut halflap joints in all the web pieces (Photo 1). First, adjust the width of a dado set to just a little bit over 1/2″, or whatever it takes so the parts will easily slip by each other. Make an indexing jig to space the notches (Photo 2 and Fig. B). The jig is just a fence for your miter gauge with a small notch cut into it, plus a pin to fit the notch. Cut the notch after you’ve adjusted the width of the dado set. Set the distance between the indexing pin and the blade (for this web, it’s 7-1/4″). Raise the blade to cut a notch 1/32″ deeper than half of the pieces’ width (for these 2″ wide pieces, make the notches 1-1/32″ deep). You don’t want the notches bottoming out when the web is assembled.

3. Cut the half-lap joints (Photo 3). Make the first cut with one end against the alignment pin. Slip the first notch over the pin and cut the next one. Repeat the process until all notches are cut in all web pieces. The distance between the last notch and the end of the web piece may be slightly different than the spacing between the other notches, but this is not a problem. Mark an “X” on all pieces at the end you started cutting from. Place all the “X” ends in the same direction when you assemble the web.

Build a construction platform

4. To make a torsion box that’s flat, you must build it on a flat surface. The best strategy is to make a temporary construction platform that you can adjust to become absolutely flat (Photo 4). All you’ll need is a pair of sawhorses and some straight and knot-free 2x4s (Fig. C). Joint one edge of all the 2x4s and run them through the planer so they’re all the same width. Place the sawhorses approximately 3′ apart. Secure them to the floor with screws, nails, hot melt glue–whatever–so they won’t move. Add bracing, if necessary, to make them stable and rigid.

5. Place the long 2x4s on the sawhorses, then use hot-melt glue to attach the short 2x4s on top. Make a pair of winding sticks by jointing and ripping two thin boards that are 2″ to 3″ wide by 4 ft. long. Paint one board white and the other black. Place the winding sticks on the first and last supports. Sight across the winding sticks (a white backdrop helps) and insert shims under the long supports until the top edges of the winding sticks are parallel. Without disturbing the platform, securely attach the long 2x4s to the horses with hot-melt glue.

6. Place both of the skins (A) on the platform–this will give you a large, flat area to glue up the web and sides. Cover the topmost skin with thin poly sheeting to protect it from glue drips. Stretch the sheeting tight and anchor it with tape or thumbtacks.

Assemble the web and sides

7. Join one short and one long side (D and E) with glue and brads (Photo 5). Align and clamp these two sides to the edges of the skins. Place the long webs on the skins with the notches facing up. Slip one or two short webs in place to prevent the long webs from falling over.

8. Glue the web, starting from one end (Photo 6). Make sure the tops of both pieces are flush at each joint. Continue working towards the opposite end; apply glue and add one short web at a time. Place weights on the web as you complete each row.

9. Once all the webs have been glued, slide the grid away from the sides. Apply glue to the ends of the webs and position the grid back against the sides. Shoot a few brads through the sides and into the ends of the webs. Attach the remaining two sides with glue and brads. Remove any glue from the top of the webs, check that the grid is square, and let it dry.

Build leg sockets

10. This is an optional step. There are many ways to attach legs to a torsion box top–I chose to house them in sockets built right into the top. Begin building the sockets by adding reinforcing pieces (F) to the web’s sides (Photo 7).

11. When the glue is dry, temporarily place a leg in the socket and glue and clamp the remaining reinforcing pieces (Photo 8). Remove the leg before the glue dries.

Add the skins

12. Draw layout lines on the bottom skin for attaching it to the grid with brads (Photo 9). First, remove the grid and both skins from the platform. Replace the skin with plastic on it and put the grid on top. On the grid’s sides, mark the centerlines of all the web and reinforcing pieces. Mark the centerlines of each leg socket on the sides as well (you don’t want to shoot brads here). Place the remaining skin onto the grid and align two adjacent edges of the skin with the sides of the grid. Transfer your marks from the sides onto this skin, then use a straightedge or large square to connect the marks. This skin will be the bottom of the torsion box.

13. Glue the skin to the grid (Photo 10). To begin, remove the skin and place masking tape inside the leg sockets, to prevent glue from sticking here. Apply a liberal amount of glue to the edges of all sides, webs and reinforcing pieces. Work quickly and don’t be concerned about drips or using too much glue. Place the bottom skin on the grid and align the same sides you used when laying out the nailing lines. Shoot 1-1/2″ long brads, spaced every 3″, along the layout lines and along the sides (Photo 11). Place weights on the skin to keep it flat.

14. Drill and rout the skin to open up the leg sockets (Photo 12). The top I’m making also required additional pieces (G) to accommodate bolts that secure a set of leg braces. To install these anchor pads, turn over the top and glue them to the bottom skin. After the glue is dry, drill holes through the skin and pads and install T-nuts in the pads.

15. Remove the plastic from the top skin and mark it using the same procedure as you followed for the bottom skin. Glue the skin to the grid. Use a router and flush-trim bit to make both skins flush to the sides all around.

Add the facing and laminate

16. The torsion box is basically complete at this point, but I added hardwood faces (H and J) to the sides and plastic laminate (K) to the top. You can butt the faces together, miter them, or make box joints, as I did. I attached the facing to the torsion box with glue and brad nails (Photo 13) and trimmed it flush to the top.

17. Glue on the plastic laminate (Photo 14). If you want to avoid working with laminate, you could make the top skin from 1/2″ MDF with one melamine face. It’s not quite as durable as laminate, but better than plain MDF.

18. Sand and finish the facing and the bottom skin.

Add the legs

19. The legs that I designed for this table are removable. They’re fastened to the top with bolts that pass through the box’s faces and reinforcing blocks. The bolts thread into T-nuts on the back side of the leg. Make a drilling jig (Fig. D) to position the bolt holes and to ensure that the holes are perpendicular to the legs. The jig has two holes, one for each side of the leg. Before drilling, mark the holes’ locations. Counterbore the holes so the bolts’ heads won’t protrude. Then drill the holes for the bolts, going into the legs (Photo 15). Finish drilling the holes through the legs at the drill press.

Cutting List

Fig. A: Exploded View

Fig. B: Web Notch Layout

Fig. C: Gluing Platform

Fig. D: Drill Guide

Fig. E: Plywood Cutting Diagram

1. Inside this torsion box there’s a grid of interlocking web pieces. Adjust the width of your dado set to make these pieces easy to fit together.

2. Make an indexing jig to space the web piece’s notches. Adjust the distance from the jig’s pin to the dado set, then fasten the jig to your miter gauge.

3. Cut equally spaced notches on the web pieces. After cutting each notch, pick up the piece and reposition it on the indexing pin.

4. Make a dead-flat assembly platform to build the torsion box. Use a pair of winding sticks to check for twist. If the top edges of the sticks are parallel, you’re good to go.

5. Place the top skin of the torsion box on the platform and cover it with plastic, which prevents glue from sticking to the skin. Assemble the box’s frame with a brad nailer.

6. Construct the grid by gluing one row at a time. Place weights on the completed sections to ensure the grid stays flat as the glue dries.

7. This torsion box has builtin sockets at the corners for removable legs (see Adjustable Height Assembly Table, page 48). Reinforce the corners with additional pieces of hardwood.

8. Temporarily set a leg in each corner. Fit additional reinforcing pieces around the leg and glue them in place. Remove the leg before the glue hardens.

9. Place the bottom skin of the torsion box on the grid. Mark the centerlines of the web pieces inside the box to guide your nailing in the next step.

10. Apply a liberal amount of glue to the entire grid, the frame and the leg reinforcing blocks. Work quickly so the glue does not harden before you install the skin.

11. Place the skin on the grid, align its edges and attach it with brad nails. Using a router and flush-trim bit, trim the skin so that it’s even with the sides.

12. Open the leg sockets. Drill an access hole, then use a router with a flush-trim bit to define the edges. Square the corners with a chisel. Turn the assembly over and glue and fasten the top skin.

13. Fasten hardwood faces on all four sides of the box. Use a block to align the top edge of each face with the skin. Trim the faces flush with the bottom skin using a router and a flush-trim bit.

14. Apply plastic laminate to the top skin. This makes an ideal working surface– laminate is smooth, resists scratches and is more durable than melamine. You can pop off dried glue from it with ease.

15. Drill holes through the faces and reinforcing blocks for bolts that will fasten the legs to the top. The bolts thread into T-nuts in the legs.

This story originally appeared in American Woodworker December/January 2010, issue #145.

December/January 2010, issue #145

Purchase this back issue.

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