Ratchet Strap Chair

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Introduction: Ratchet Strap Chair

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

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.

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    31 Comments

    Nice bench! This particular design is too complex for a dimensioned layout, but you can print the PDF layouts at full scale at any print shop that does large format printing and use the technique I describe in this Instructable: https://www.instructables.com/id/Digital-Fabricati...

    All you need is a drill and a jigsaw.

    So, here's the thing: I'm a Soldier and I deployed to both Iraq (in '04) and Afghanistan (in '08). Among the egregious incidents of poor planning and spending is the fact that the military will actually haul huge, heavy, expensive pieces of civilian-style office and barracks furniture into war zones. But, we also take enormous amounts of plywood for construction of shelters (barracks, offices, etc.). A suite of simple furniture designs (bunks, desks, shelves, etc.) cut from 3/4" plywood, slot-and-tab assembled, and held together with ratchet straps would be invaluable!

    That's a great idea. People have been making flatpack furniture for long-term camping trips (burning man, etc.) for a while- it's a great way to make sturdy, comfortable furniture without having to ship all that volume. I think the dead-simple design of this project is probably even better for this kind of use: http://www.treehugger.com/sustainable-product-desi...

    All the zoomy lines and fussy details of mine require more time to cut out by hand, not so efficient.

    This is another awesome flat-pack project, housing for disaster areas: http://www.wired.com/2013/11/a-disaster-relief-she...

    The idea is that you show up with a CNC machine and a ton of 3/4" plywood, and you've got all the buildings you need.

    Update on my comment. Did the cardboard thing. Worked after a fashion. Until a 350lb friend decided to plop down in it. Legs folded.

    I was preparing to shoot some when Carl crushed it. The legs were still too pointy and thin. I'm going to put on another layer, or two at the bottom of the legs. Perhaps feet too. I used contact cement between layers, the bond didn't break. The paper did. I went five layers of cardboard, for a thickness of about 3/4". The seat and back held up fairly well. We used it for a bonfire starter. Next time I'm thinking seven layers, 1"+,-. If the prototype holds up I'll spray the pieces with an epoxy paint for durability and water resistance. Got other projects right now, will update in a few months.

    So true. Posture is everything when it comes to furniture, and it's been pretty well figured out by the 20th century masters. The Ratchet Strap Chair is based on the Barcelona Pavillion Chair by Mies van der Rohe.

    This project, by one of our AIRs, is based on the Eames LCW chair and is also very comfortable without any padding: https://www.instructables.com/id/P9L-Lounge-Chair-made-with-CNC-Router/

    do you have pdfs of the pieces rather than fusion 360 format?

    Just uploaded a full size PDF to Step 2. If you ever need to convert a DWG file to PDF, just do it on this site for free: http://www.zamzar.com/convert/dwg-to-pdf/