A while back, I got a free coffee table that I decided to use as a desk for the free computer* I found not long after (it was a really good week). Unfortunately, all the chairs I could find were too tall to comfortably sit at the table. So I decided to make a better sized chair!
I'll note that there's definitely way more efficient ways to make a cardboard chair that use significantly less material, but I wanted specifically to try out this method of "slicing" a solid model.
*Before you get too jealous, the computer was so old that it couldn't open any website with HTML5 or run most programs... but it worked great for watching movies!
Step 1: Tools & Materials
Lasercutter (unless you're a glutton for punishment and want to cut it all by hand)
Step 2: Designing the Solid Model
You can make the chair pretty much any shape you feel like. I used a couple of measurements from my coffee table and existing chairs, to get what I wanted the shape of the chair to be in Solidworks. I wanted the chair to be slightly curved on the seat and the back for it to be more comfortable - however, these details turned out to be a bit of a waste of time. Because the resolution of the final model is limited by the number of cardboard cut outs used, these details were mostly lost but took more effort to make since every single "slice" of the chair was slightly different. Keep that in mind when you're designing your own chair.
Step 3: Figuring Out the Tab Sizes
Before you go ahead and slice the whole model up into 2D parts, you should make some samples to figure out what size slots you need for a good press fit. Since every laser cutter is a bit different, this is something you'll just have to do experimentally. I made a two sample pieces with slots of increasing thickness and tested them out to figure out which had the best fit. You want it to be tight enough that the pieces won't come loose on their own.
You can also add rounded corners (bevels) to the top of each slot to make it easier to slide the pieces together. I opted not to because I didn't like the way it would look for the final piece.
Step 4: "Slicing" the Model
I used Autodesk 123D Make, which makes 2D slices out of 3D models. At the time I made this (about a year and a half ago), 123D Make was relatively new, so there wasn't much in the way of documentation or tutorials, something which has changed sinse. There was also little ability to customize where the slices go (all the slices have to go in the same direction, as in the images above). The problem was that I wanted my slices to curve around with the profile of the chair. What I wound up doing to work around this was creating three individual models with different slice patterns, with the intention of concatenating them after.
Step 5: Vector Model
My next step was to modify the files in Illustrator. I wound up manually overlaying the different slices from 123D Make. This was not especially accurate, but fine for the purposes of this design. Depending on how you decide to "slice" up your chair, you can avoid this step altogether by keeping the slice pattern simpler.
Step 6: Lasercut the Parts
Make sure before you cut the whole thing that you've tested out the width of the slots for the press fit! It took a while to cut - about 5 minutes per sheet of cardboard, and I used 13 sheets. (As mentioned earlier, this is not actually a very efficient use of cardboard!)
Step 7: Assembly
Assembly took longer than expected because the press fits were just a bit too tight. Although I'd tested it earlier, I hadn't thought about the fact I would be trying to align multiple tabs at once. This made pushing the parts in very difficult - and also pretty much irreversible.
Because I was in a bit of a rush to finish the entire project on time (it was for a class), I didn't really do any analysis on how structural the chair would be. It turns out, it's very structural along the long axis, but not on the horizontal. After using it for a while, the back of the chair started to bend every time someone leaned back. So if you're going to make a chair like this, I'd thicken up the back of the seat.
Also in case you're worried that the seat won't be comfortable because the bottom has this criss-cross section of cardboard, it actually wound up being very comfortable!