Introduction: Cardboard Cantilever Chair
This cardboard chair explores variations on some techniques first thought up and worked through here. The structure uses a hybrid of two types of cardboard: traditional corrugated from boxes and sheet stock, and hexagonal-cell cardboard used in packaging of imported stainless-steel sinks. I combined stack lamination with embedded threaded rods to provide clamping pressure, tying the layers together. To further unify the structure and reduce the variations in the layers, the whole thing is wrapped in five layers of paper-mache, which was polyurethaned and waxed.
This chair is unusual in that it is a cantilever chair, a traditional modernist form first pioneered by the Bauhaus. Cardboard can be very strong, but the cantilever form presents several challenges in using a paper material.
First, the entire chair is essentially in tension: the weight and force of the sitter is pulling on all three of the critical joints, at the floor, the knee, and the junction of the back and the seat. Corrugated cardboard is very strong in compression, when the force is aligned with the grain of the corrugations. However, it has very low tearing strength, especially when the force is perpendicular to the corrugations. I partially overcame this by using the hex cardboard, which is strong in all directions because it has no directional grain.
Second, there are no back legs, meaning the force has to find a path to the ground that is circuitous, winding along the seat to the front legs. The runners along the ground also have to be long enough to prevent the chair form overturning backwards when the sitter leans back. Most cardboard chairs have mass positioned directly under the sitter to add support where there is the most weight.
Last, unlike the cantilever chair featured in my other instructable, this chair only has two legs, instead of a continuous face across the front. To overcome all these structural challenges, it is key that every piece of cardboard become a unified whole. There are no continuous pieces of cardboard; the whole thing is made out of strips, as opposed to cutting out big pieces make the whole profile. The strips are not nearly as strong, and are susceptible to tear-out. The threaded rods at all the critical junctions provide pressure to overcome this tendency.
Step 1: Cut Cardboard
I ran corrugated cardboard through a table saw set with a combination blade. I cut strips 5" wide, both with and against the grain of the corrugations. Only the legs need to be with the grain, so I made substantially less strips with that orientation.
Instead of drafting on paper or in the computer, I just measured out a grid on the floor of my basement and sketched it freehand, full-scale, in chalk. The seat comes to about 14" off the ground. The end of the runners on the floor come even with a vertical line dropped from the top of the back piece. They have to be at least this long to keep the chair from tipping over.
I laid strips over the drawing and cut them with a boxcutter to get all the angles. I marked each one, and they became my templates. For strength, you'll want to overlap each alternating layer, meaning each joint should resemble a log cabin in section as you stack the pieces on top of one another. This also means you'll have to make two different back pieces and two different floor pieces, one short and one long. To save time on all the cutting, I bundled strips together with masking tape, scribed the angles from my templates, and cut them all at once with a handsaw. The last photo shows how strong the cardboard is, supported my weight with just ten inches of corrugated cardboard.
Step 2: Laminating
The strength of this chair is mostly derived from the laminations. The chair has to act as a unified whole in order for it to work.
I just used plain 'ol white glue, about three quarts of it. I made a level surface to lay out the layers on, then stacked pieces, brushing on white glue thinned with water. The water makes it spread easily. Make sure all the cardboard is completely covered, with no dry spots. Too much water will make the pieces curl, so be careful about loading the brush too much. The goal is keep the glue as even as possible. Once I had a layer about two or three inches thick, I clamped it with plate weights.
For the leg/side pieces, I used 1-1/2" of corrugated on either side of two pieces of the hex-section cardboard. Then I put two of those together to make each side.
Again with the handsaw, I chamfered the corners. With a regular power drill, I centered 5/8" holes at the five joints.
Step 3: Assembly
I trimmed the threaded rods down to the approximate width of the finished piece and poked them through the holes drilled earlier. Then, I laminated each layer together just like the others. Once assembled, I cranked down on the nuts on the rods and squeezed all the layers tight instead of clamping them. Let it dry for twenty-four hours.
Step 4: Surfacing
To clean up and finish off the surface, I covered it in five layers of paper-mache, the final two being kraft paper that is recycled packing material. My paper-mache recipe is a little different then most: three parts hot, hot water; one-and-a-half parts white glue; and half-to-one part flour. Stir very thoroughly. It should be the consistency of watery syrup, with no visible chunks of flour.
Once dry, I sanded down the surface, first with 100 grit, then 120, wiped it with a clean, damp rag, and put three coats of semi-gloss polyurethane on it. The material is very thirsty, and will soak up a lot on the first coat. Sand with 220 grit in between coats. I gave a final rub down with 0000-grade steel wool, then again with a clean damp cloth, dried it, and put on a coat of paste wax. The final surface is very hard, with a nice sheen to it; I would compare it to burnished leather.
Then I trimmed the threaded rods and tightened all the bolts again. The two threaded rods on the floor have bolts on the interior, which serve to space the legs and keep the thing square and un-twisted. Adjust those bolts to even the space between the legs.
The final product is very comfortable, but this chair was also very time-consuming, and the layers did not register as precisely as I would have liked, giving the final piece an uneven texture that serves to remind you of its handmade origins.