Here's the rub: each tooth needs to be cut at a very slightly different angle in order to "zip" together properly. Thought you could rig your shopbot to do this for you? Nope, sorry. This process needs a full-fledged 5-axis CNC, or a 5+ axis industrial robot arm, or some crazy experimentation with an open saw blade. Not your usual fabber's tools. Here's how Schindler Salmeron illustrate it:
This instructable is the first in a planned series, the ultimate goal of which will be to make zipshape accessible for all with a regular 2-1/2 axis CNC (Ie, using only profile and stepped fixed-depth cuts). I will be developing browser-based online tools to handle the geometry of it, and I'll be inventing a new form of zipshape which makes it possible to cut it using only straight orthogonal cuts. Follow my series! By the end, you'll be able to draw a curve, download a DXF file, cut it on your shopbot, and glue it up into a beautiful piece of furniture. No molds required! Here's a rough table of contents for the series:
Part 1: Introduction to zipshape, my own experience with zipshape, prior work that's been done in the area, the problem to be solved, the proposed solution, tests proving the concept.
Part 2: Refinement of the tooth geometry, a more in-depth look at the geometry and math that make zipshape work. The building blocks of the future web-app to bring the process out of Rhino and into Chrome.
Part 3: To the CNC! Prior tests will have been lasercut in 2 dimensions; In part 3 we'll be moving to the CNC and fixing any bugs that come up with the shift to more furniture-like parts.
Part 4: Design of a finished piece, and Introduction of the web-based tool. Since it will be built on fundamentally different platforms, further 2D tests will be done to verify that the geometry is true to the original method. A final piece of furniture will be proposed, but for all those following along, this is the point where you can design your own.
Part 5: Construction of the new & improved 2-1/2 axis zipshape furniture (type to be determined in part 4).
Step 1: My experience with zipshape
We replicated the zipshape techniques from basic geometric principles, creating a grasshopper definition for Rhino 3D which generated tooth-geometry from an arbitrary curve. The geometric problems turned out to be the least of our worries: actually fabricating these parts was the real mess and took us several months to figure out. Initial attempts to use a Kuka 5-axis robot arm with a beefy router bit turned out not to be precise enough for tight-fitting parts, and we had to move to a much larger and even more expensive setup with a 5-axis CNC gantry; I understand there are only 3 comparable setups available in the North-Eastern United States. We got the parts made, but now that I've graduated I couldn't do it again the same way.
The reason such complex equipment is needed is because each tooth needs to be cut at a unique angle. It implies a completely different type of equipment from the more typical shopbot that can do simpler straight-up-and-down cuts: profiling, pocketing, contouring, and such. My mission here is to fix that, and make a soft rocker more makeable.