I always wanted to make a bent plywood chair so I went ahead and made this prototype. I used compression mold to curve the plywood and a water based resin glue to put the bendy ply together.
I decided to make a simple design that would be broken down into 7 different pieces:
• Front leg (x2)
• Back Leg (x2)
• Arm rest -connecting front and back leg- (x2)
The seat is elegantly supported from the edges of the branching legs creating a bridge between the front and the back legs. The ramification of the legs not only split in two directions with the same angle to each side generating a consistent flow in the legs but also creating the armrest. The biggest design challenge was creating the partition in the legs that become part of the armchair and seat. The front legs start thick from the ground and bend forward to become the seat and also backwards to become the armrests which then come down as the back legs. The ramification was the whole concept around the project, which made it an interesting detail.
Material: Poplar bendy ply and red oak veneer
Molds: Random scrap plywood
Strip of plastic (x2) for quick and clean unmold
DAP® Weldwood® Plastic Resin Glue
Biscuit Joints to put the 7 pieces together
Step 1: Testing Ergonomics
Making a full size prototype is helpful to check dimensions and proportions.
Testing the ergonomics to verify if the angles and lengths are comfortable.
Step 2: Testing Material
I tested the flexibility of the bendy-ply to know exactly the minimum radius I could bend the wood. After deciding the minimum radius was 3.2 inches I then went back to the CAD drawing and did some adjustments in the design so I could manufacture the prototype.
The total material for my chair was 2 sheets 8x4 ft. of poplar bendy-ply of 3 mm and 1 sheet 8x4 ft. of red oak paper backed veneer. I got all of my material at Macbeath in San Jose for less than $100.
Step 3: Mold and First Bend
I started doing the male mold for the front leg. I decided not to use a female mold because I was going to be short on time and had to do 7 pieces in total. I decided to clamp the pieces down with just pieces of wood that spread fairly even the pressure of the clamps.
Making the first bend:
In total I used 5 strips of the 3 mm bendy-ply and used regular wood glue and left it for 24 hours to dry. It was an easy and short bend of 90 degrees. As a tip: I put a thin piece of plastic over the mold for two important reasons, first, to make a smooth surface to bend, and second, to make it easier to unmold. Also as it was my first bend I used 13 clamps but after that I realized I could have probably used 8 clamps.
As I was told that regular wood glue will tend to open up after a few months I decided to use a great wood water resin glue called: DAP® Weldwood® Plastic Resin Glue. It is basically a powder that you mix with water and creates a super strong glue. You don’t have too much time to play around (probably 30 minutes) so you need to make the bends and clamp them down fast.
Step 4: Second Bend
Bending the armrest was way harder than what I had expected because it took triple the time than the front leg. Of course, doing the mold was also more complicated. I believe this is due the 2 bends on the single piece. In addition, having long pieces to bend makes the setup and handling of the material more difficult. After I took the piece out from the mold I discovered that the bendy-ply wasn’t very structural for longer pieces. It flexed more than what I wanted and got worried that the whole chair wasn’t going to resist a person.
The Third bend was probably the easiest one. You can check out the last picture.
Step 5: Clamping
Molds need to have parallel surfaces so that you can place the clamps. Clamping really tight is very important. The orange clamps worked better than the rest. Warning: don’t do so much pressure with the same hand because you can get blood blisters in the palm of your hand.
Step 6: Mold: Seat
Making the mold of the seat was definitely harder. I cut some strips of wood over the curve and then put some wood puddy and sand it down smoothly. As a female mold was going to be complicated I kerfed a piece of ¾ fine plywood (only the part of the radius) up to the very last layer, spacing them evenly 7 mm away from each other. This created a flexible piece of plywood that I could easily use as a female mold. To clamp the seat to the mold I had to ask my wife for some help gluing the surfaces of the wood and also clamping it from the other end.
Step 7: Cutting the Pieces and Veneering:
One the pieces were bent I sanded the edges and cut it 2 mm from each side so that it was squared and even. I then put some contact cement on the wood and on the veneer, waited for 20 minutes, and then stick them together. At first I clamped them down but I realized this was not totally necessary. Contact cement is really strong and will stick together and make it almost impossible to remove once it is done. Something tricky but everyone should be aware of. The veneer was cut slightly bigger than the piece and I later cut it with a knife.
Step 8: Prior to Assembly
After I had my 2 sets of legs I aligned them with some clamps and measured it with a 1:1 plan to cut it exactly where I had to cut the legs. I glued the legs together with 6 biscuits and let them dry for 24 hours. Finally I squared the 2 sets of legs so that they were identical.
Step 9: Assembling
One of the key parts of the process was the assembly. I had the two set of legs and had to glue them together with the seat. I measured where I wanted to put the biscuits and did the 3 holes in the back and 1 hole in the front of the legs. It was easy to glue using some parallel woods that helped me keep everything straight.
Step 10: Finishing
I decided to use oil and wax as a surface finish. I heated Boiled Linseed Oil to 140 degrees and then applied it with a brush. I let it dry for a few hours and then remove it with a cloth. Then I added 3 layers of wax to have a nice and protected surface. If you want you can smooth the surface with a 600 sandpaper.
Step 11: Enjoy Your New Chair
Although Bendy Ply is not super strong to make a commercial chair it worked OK for this prototype.