Introduction: Ratchet Strap Chair

By Portfolio
About: I'm an inventor / maker / designer based in Portland, OR. My background is in residential architecture, film set design, animatronics, media arts, exhibit design, and electronics. I use digital design and fabr…

This chair is made from a 1/2 sheet of plywood, uses no glue or fasteners, and is held together by a single ratchet strap. I made this chair because I was interested in the forces at play to make a chair stand up: the different parts in tension and compression that make up the structure. It's also part of a broader study I'm up to experimenting with ergonomics- trying variations on the dimensions and posture of Mies van der Rohe's Barcelona Pavillion Chair (1929).

Step 1: 3D Model

I designed this chair in Fusion 360 (it's recently become my go-to modeling program). The file is attached here, please download it and tweak it yourself! 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:

Student/Educator

Hobbyist/Startup

Attachments

Step 2: Tools and Materials

Tools

  • I used a Metabeam Laser Cutter to cut out the pieces, because it's fast and we have one. A Shopbot would make short work of this, and you wouldn't have any burn marks to manage later.
    • IF YOU DON'T HAVE A CNC MACHINE: Before I had access to CNC machines, I did lots of projects that were just as complex using full-scale prints from Kinko's, spray-gluing them to plywood, and cutting them out with a jigsaw. Check out my Digital Fabrication by Hand instructable for more details on doing it that way.
  • An orbital sander, finishing at 180 grit.
  • OPTIONAL: I also used a table router with a 1/4" fillet bit for some nice rounded edges.

Materials

  • 1/2 sheet (4' X 4') of 3/4" plywood.
  • One ratchet strap (1" strap).

Step 3: Cut Out the Pieces

My Metabeam settings are as follows:

Material: "Hardwood .25"

Power: 100

Speed: 40

Step 4: Finish the Pieces

I started at 100 grit to get the burn marks and smoke residue off the wood, then stepped down to 180 grit for a smooth finish.

I used the fillet router bit on all of the edges of each piece.

Step 5: Assemble the Chair

  1. Slot the truss into the back legs
  2. Add the front legs
  3. Add the seat
  4. Slot in the seat back

Step 6: Strap It Together

Each piece has a notch to keep the ratchet strap in place. The ratchet strap is looped around the seat, the front leg piece, the truss, and the seat back. When the ratchet is tightened, the chair becomes rigid.

Step 7: Sit in the Chair

It's pretty comfortable, and super-rigid! I think if I go for another iteration, i'll make the back legs less pointy and try to use even fewer pieces.