Ever heard of Zipshape? It's a technique developed by Schindler Salmeron which lets you to draw a free-form profile curve for (say) a piece of furniture, and then cut a series of teeth into two sheets of thick material like MDF. Those custom-cut teeth are very specifically shaped and matched to each other such that they can "zip" together, but only when the sheets are bent into the curve that you drew at the beginning. The result is amazing: free-form bent furniture parts that you can glue up without molds of any kind. Draw a curve, cut the teeth, slather glue, stick it in a vacuum bag, and let it set. This image, linked from the Schindler Salmeron page directly, illustrates what I'm talking about:
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).
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).
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Signing UpStep 1: My experience with zipshape
My main exposure to Zipshape, aside from its being sort of "in the water" of design school and journals, was through a project I participated in last year while in the master's of architecture program at MIT. Under the direction of Prof. Sheila Kennedy, we made a series of "SOFT Rockers" for MIT's 150th birthday celebration: human-powered solar tracking devices that connect social activity and electricity generation through a unique piece of public urban furniture. This Instructable-series is more about the fabrication process than the SOFT Rocker project itself, so if you want to read more about SOFT Rockers just google them or see links here at engadget and gizmag. Big photos and a more thorough description are also available on my website.
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.
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.
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A large bending (steam?) pot/oven and a large peg table would work for most of these designs, except those curving into mobius strips....
The cost of that CNC device (while quite lovely, very unique, and yes I want one!) could pay for lots of materials and a few large hot tubs and a deck to put them on.
Bent laminations are beautiful and, I suspect you're right, stronger than a method like this. But they require solid-surface molds which are expensive and time-consuming to make. A pegboard is a good solution for eliminating the mold, but then you can't do wider sheets and your bend radius is pretty limited.
I'm interested in finding ways to employ CNC's for more than they are obviously useful for. I'm not imagining a paradigm of "what's the cheapest way to make a particular curved piece" so much as "what are some exciting ways we can push the limits of this new technology?" In the case of this project, I'm proposing that we replace some of the labor ordinarily involved in bent furniture parts with precision made available by the CNC. The most obvious benefits being mold-free construction and easy parametric variation: it's just as easy to make every chair seat fit an individual person as it is to make them all the same.
In the design stage, I'll try to design something that would be hard to make by other means :)
In particular, zipshape is probably cheaper and faster than the conventional method for creation of solid-surface molds for bent laminations! :)
Not only is a bent lamination likely to be stronger, but if you make many pieces of furniture/whatever using a single mold, then there will be less wasted materials, per piece, on average.
I am probably the only person that has actually gotten a chance to play with a CNC for two weeks (at Haystack) and was left cold by what came out - and by what else I have seen. Perfection leaves my eyes no where to rest, I feel like the person who can see the emperor has no clothes. I just don't like what I am seeing being made - yet. It is too static, for now anyway.
This 'ible is wonderfully simple to understand for such a complex project, thank you for sharing.
As a builder without any computer-controlled tools, I wonder if Zipshape fabrication could be accomplished using more traditional methods...
Given a well-tuned table-saw (likely capable of accuracy to ±0.2 mm), do you think it would be possible to slice the requisite 'steps' into a sheet of material? What tolerances are required for your current level of success? What tolerances are you planning to use for your next iteration in attempting to improve curve fidelity?
An unrelated question, have you found that gluing your curves improves their fidelity? It appears to me that the addition of the flexure joint is what's allowing your shape to relax. Do you think that is accurate?
You might consider filling the flexure joint with a a block (or, in the case of a wider panel, a spline) after fitting the halves together. It would make the entire construction tighter, and perhaps truer? If you used the right wood glue to fit the spline, it will swell the surrounding material (think mortise and tenon joinery) and create a tighter joint than could otherwise be achieved.
I'd be most interested to hear your thoughts. I (and my circular saw) might just have to try for a bent-wood chair; no steam-box required!
You have two boards - I'm calling them the leader and the follower. The follower has V-bit grooves cut over the whole surface - each groove is the same depth and they are adjacenct so there are no flats.
The leader is cut with a variety of V bits. The goal is to cut the board so that each face (hypotenuse) is the same length, but the angle is controlled by the bit. If the follower is cut with a 90° bit and the master is cut with a 100° bit the assembly will curve at a constant rate. If you use two bits (cut 5 rows at 90° and then 5 rows at 100°) you will have a flat that transitions to a curve. If you "dither" the bits (rows 1,2,4,7 cut with bit 1 and rows 3,5,6,8,9... cut with bit 2 you may get a more continuous transition.
The trick here is that depth of the cut on the leader board needs to vary so that faces retain the same length. After cutting the teeth on the leader board, you want to flip it over and cut the backside so that the board is approximately uniformly thick.
The cut plan will be more difficult to compute but I think this solution will get you most of the effect with a lot less expensive hardware.
That said, I'd be excited to be proved wrong by a test! Would you be willing/able to run a test like this? I'd be happy to help setup the geometry in any way that would be helpful.... PM me if it sounds like something you might be interested in trying out!
Any reason you chose 2 step heights? Why not more, it would represent the original curves more closely.
Will the depth to width ratio of the deep flexture joint you added in each tooth in one side be a problem/slow down production on a 2.5 axis CNC bot?
James
As for the depth of the flexure, it may well slow down production a bit. As I tried it it's about 3/16" thick and 3/4" deep, though the material I'd be using wouldn't be quite that thick most likely. Still, it's probably 3 passes with a 1/8" flat end mill, and that's definitely time. Not as much time as making molds for a single piece, though!
I'm pretty keen on web technologies so let me know if you want any help with the web app or programming side of things, it sounds interesting. PM me if you want my email address to bounce ideas around.
James
I made an account just so I could vote for this Instructable.
James
Use strips instead of surfaces. Use the strips as ridges to support regular flexible plywood (which curves easily in one direction but not the other).
Instead of cutting the strips using a lasercutter, or a CNC machine, use a regular table or drop saw and a single axis control to feed the wood to make your simple one-axis cuts.This will lower the cost of production and enable the widespread use of this technique.
This has the added benefit of not 'wasting' as much material - a lot of wood is excavated in the CNC process and discarded (hopefuly recycled to construct MDF).
With your new method your speed is basically limited by your depth of cut, which is ultimately defined by machine rigidity, material composition, and limits of tooling. It also doesn't require a lot of passes to achieve the shape you are looking for as it is basically composed of straight lines.
However, a way to cut your original tooth form from your first design using a 3 axis mill would be to do it with two tools, the first tool, a regular endmill, would cut to the root of the tooth perpendicular to the plane you want to curve, the second tool would be a ball endmill of the maximum radius that will fit in the tooth gap and keller cut the shape using the tangent point of the radius relative to the surface you want to cut to form the tooth shape. I have used this method to manufacture complex planes in the aerospace industry, no pun intended. Of course, it takes a LONG time to cut parts this way.
I'm curious to learn more about the "Keller cuts" you mention, but a moment of googling doesn't show much. Is there something I could read to learn more?
Thanks for the insightful comment!
"Keller cutting" is a general term referring to the use of a ball nose or other tool with a large corner radius to achieve a complex plane. The reason you would use it in this case is to reduce the number of tools needed. I believe a newer Machinist's Handbook talks about Keller Cutting.
One thing you could do is cut notches from both sides of the strip, but then that strip would be a sort of wobbly piece of spaghetti, i think. Are you imagining a different method for making the strip "rigid" again, and maintaining the curve?
Sorry if the response adds more confusion than it helps explain. These are tricky things to write about without lots of images to point and wave at and draw all over!
I have actually! It's super neat stuff, although does something a little different I think. It's flexible because of the kerfs on one side, but to make it hold a specific shape you'd need to add a rigid structure (ribs, two sheets glued together over a mold, something else). I'm hoping to make a rigid bent sheet with a specific pre-determined shape, without molds, ribs, or any other supporting structure... each individual product could be unique, and there'd never be any overhead associated with the production of molds and setup. But yes -- bending board is an awesome idea!
Thank you for sharing this. It is world class thinking. Love what I seen and can't wait for the next installment.
Regards