Bent Cardboard II

A few weeks back, I began an experiment, attempting to create curved, structural panels of laminated cardboard. The first shells, adhered with wheatpaste and cured in a mold, came out quite well. I buried some strips of Masonite in the edges so that they would hold a fastener, then trimmed them neat and square on the table saw.

Frame layout. Designed it in AutoCAD, then sketched on paper because I don't have a printer.

Next loomed the question of a frame. Cardboard chairs tend to acquire quite a bit of bulk as they try to solve structural problems with a weak material. I wanted to make a visually light, slim-lined frame that would highlight the cardboard shells, so I turned to plywood. Ken Isaac's book, which I recently wrote about, had a lengthy investigation of plywood stress-skin structures, which gave me some ideas about how to make a stiff structure out of a thin, flexible material. 

Otherwise obsolete hand-drafting skills still come in handy sometimes . . .

I settled on a "box" scheme: two identical parallel plywood frames are braced apart by two "U"-shaped leg units, essentially forming an open box that the shells then fit onto. The frames are composed of two layers of 1/2" plywood, laminated together. The legs interlock into the parallel "L"-shaped seat frames, making one solid unit. The whole system is remarkably strong and light. 

Exploded diagram.

Cutting out seat frames. Hard to tile efficiently into a sheet, unfortunately.

Laminated the frames with glue and staples.

Rabbetin'. Plywood's hell on a chisel, try to avoid it . . .

The shells are screwed in from the back, sitting on rubber shock mounts, just like its Eames predecessors. Once the shells were added, the chair stiffened up considerably. 

The posture is pretty upright: the seat is reclined at 2 degrees off of the horizontal, while the back is 3 degrees off of the seat. Together, the whole assembly gives about 5 degrees of recline. With a flat back and seat, this would be too straight up-and-down. The slight curve and flexibility of the panels mitigates the uprightness of the posture, combining good desk ergonomics with back and butt happiness. Contrasting colors and thicknesses in the frame and the surfaces successfully articulate the shells without running away from their essential cardboardiness. It is super-light and sturdy. Two coats of water-based polyurethane (on legs and cardboard) add a nice, low-key sheen.

Attaching the legs. Where the legs stuck out over the seat frame, I routed it flush after the fact.

Assembly.

Detail of rubber shock mount.

I am not happy with the front and rear elevations. The base looks narrow and knock-kneed, while the legs themselves look too thick. I also should've counter-sunk and doweled the screw joints; instead I used nice brass screws, with careful placement. Even so, a screw head is a screw head, and it looks unfinished. In a few spots, I sanded through the veneer of the plywood in my haste and because of careless glue stains.

Side elevation.

Front 3/4.

Cropped the eyes out because they're so magnetic.

Cheeky. (May be a bit much . . .)

Backside.

Front corner, can see the embedded Masonite and alternating cardboard grain.

That said, I think the design has promise. I will be making more shells, refining the methodology, and working on a new base design. The shells themselves are adaptable to a lot of possible frames -- more plywood, hardwood, and an all-cardboard solution. I would also like to revisit a college-era experimentation with masonite/cardboard hybrid structures, which would be one possible way to cut down on the bulk of a pure cardboard support system. This current plywood design is well-adapted to CNC production, with a little refinement, and it would be great to find a local small-run CNC mill to test it out.

I shall report on these experiments and many others in the future. Now, at least, you will know from what chair these dispatches appear . . .