Intro: Bent Plywood Chair
The Bent Plywood Chair is part of a broader study I'm up to experimenting with ergonomics- trying different materials and construction methods using the dimensions and posture of Mies van der Rohe's Barcelona Pavillion Chair (1929). This one is made using 3/4" plywood and kerf bending for smooth contours.
Step 1: Tools & Materials
•Straight edge (I used a 5' aluminum level)
•1X4'X8' sheet of 3/4" finish grade maple veneer plywood
•Satin spray-on Polyurethane
•Sandpaper, glue, spray adhesive, countersunk screws
•Paper template (4'X8' sheet)
Step 2: Design & Templates
I designed the chair in Fusion 360, which makes it very easy to swing back and forth from 2D to 3D. Fusion 360 is free for students and hobbyists, and there's a ton of educational support on it. If you want to learn to 3D model the kind of work I do, I think this is the best choice on the market. Click the links below to sign up:
When you open the file, you'll notice there are a lot of "sketches" plus the 3D parts. I used multiple sketches to design and alter the parts, then when I was satisfied with the design, I created new sketches derived from the finished parts to make the templates- to do this, you just click "create sketch" and select the face of the 3D part you want to cut out, and there's your template!
For the template layout, I nested the parts so that they would all fit on a single 4'X8' sheet. The solid lines are for cutting and the dashed lines serve as straight-edge guides. The offsets for the straight edge are equal do the distance from the edge of the tool guard to the side of the blade (it's the same on the circular saw attachment and the jigsaw attachment I used). That way I was able to clamp down the straight edge and easily make straight cuts.
Step 3: Cut Out the Pieces
I used super 45 spray adhesive to fix the template to the plywood. 45 is not as strong as 75 or 80, so it's easy to peel the template off without having to scrape or sand the surface.
The layout has the seat surfaces justified to the edges so that there's no need to cut those edges.
Using the circular saw attachment for straight lines and the jigsaw attachment for the curved lines, it took about 4 hours to cut out all the pieces.
I made perpendicular cuts to the tighter curves to keep the jigsaw from binding.
Step 4: Kerf Cuts: Geometry
Kerf bending is achieved by cutting through the plywood so that the last layer of underlying wood is left in tact. With multiple cuts made close together at regular intervals, you are able to bend the wood and create a smooth curve.
I chose to make the cuts at 1/8" intervals. Since the width of the circular saw blade is 1/16", you end up with alternating 1/16" solids and voids. This is very important too keep the kerf cuts from creasing when they're bent.
IMPORTANT: This particular circular saw attachment has a blade that's flat in its cut profile. If you were to use a different circular saw, make sure the blade you use is flat in profile (a Dado Blade for example). Many regular circular saw blades leave a "V" profile, which will make for an uneven distribution of force, and in my experience will make the wood crease.
ALSO IMPORTANT: The template layout I made assumes a crucial fact: that the veneer is oriented along the long side of the 4'X8' sheet. The orientation of the grain of the layers of wood on a sheet of plywood alternates. This means that the kerf cuts are made perpendicular to the last underlying layer of plywood- this also prevents creasing.
For the layout, I simply measured the length of the straight segments and arc lengths of the curved seat surfaces, adding the appropriate lines. In the curved segments, I added cut lines at 1/8" intervals as well as dashed lines outside of the cut areas to serve as guides for the cutting jig.
Step 5: Kerf Cutting: Jig
To keep the depth of the circular saw cut consistent, I made a jig out of some 1/8" thick poplar glued to "U" shaped ends to give me the offset I needed to leave that last layer of underlying plywood.
With a 1/4" gap in the center of the jig, I ensured that the offset from the blade to the side of the jig was exactly 2". That way, my offset guide lines allowed me to easily follow along with consistent offsets.
Step 6: Gluing and Sanding
The sides of the chair are double layers of plywood for stability. In order for the entire chair to be cut from one 4'X8' sheet of plywood, I had to cut one of these layers. As a result, I started with the whole side piece and glued the matching cut pieces to that.
Cutting by hand (at least my hand) doesn't make for particularly clean edges. I first sanded the edges of the whole side pieces, then glued and clamped the cut pieces to the more finely finished side.
Once the curved pieces were properly clamped to the surface, I added the screws in the rounded tabs on the seat and seat back pieces. With proper gluing and clamping, these screws don't really do anything to hold the chair together. Anyway, I like the look of them- they give one the impression that they're holding back the spring action of the bending, which in theory they would.
Once the glue cured, I sanded the entire profile of the double-layered chair side to make for smooth geometry.
Some other cleanup may be necessary at this stage. Make sure the rectangular channel in the chair side pieces are a consistent 1 1/2" so that the chair surfaces fit snugly.
Step 7: Finishing: 2 Options
I made 2 of these chairs, one with Minwax Classic Gray wood stain and spray-on Satin Polyurethane, and one with spray-on Satin Polyurethane only. I like the one with the natural wood color better, but the stain hides some of the imperfections.
The wood stain instructions advise brushing on the stain and leaving it for at least 5 minutes before wiping it off. I decided to wipe it off immediately because I thought this would leave more of the variation in the veneer surface visible. I think this also left me with a brighter color in the end.
On both chairs, I did 3 coats of spray-on polyurethane with 300 grit hand-sanding between the coats. The result is very smooth to the touch and helps a little with the veneer delaminating at the edges, which can happen with plywood that isn't edge-banded.
Step 8: Finished Product
This chair is incredibly sturdy, not too heavy, and pretty slick if I do say so myself. It's also pretty comfortable considering the lack of cushioning due to the posture of the chair.
One chair took about 16 hours total over 2 days taking sanding and finishing into account. It's also great exercise for kerf bending. Now go make one yourself!