Intro: Zip Tie Lounge Chair
Flatpack design is currently at a crossroads -- popularized by IKEA and perfectly adapted to new manufacturing technologies, it is poised to grab an ever-greater share of the furniture market. CNC routing is a key part of this disruption, allowing for global design and localized fabrication, potentially slashing production costs, boosting regional economies, and opening previously closed distribution networks to a vast guerilla army of designers, makers, and consumers.
I designed and prototyped the Zip Tie Lounge Chair for OpenDesk, a new startup that is launching a distributed manufacturing platform. Cut files for the chair are available for free download on their site; alternately, you can buy the chair as flatpack kit from one of the fabbers on their network and get it shipped to your door, ready-to-assemble.
Instead of IKEA's infuriating barrel bolts, or the sweaty process of forcing friction joints, I used zip ties to fasten the chair together. They are cheap, strong, and require no tools, allowing you to snip it apart and stitch it back together every time you need to move. The Zip Tie Lounge Chair has only nine parts, and can be cut out of a half-sheet of plywood. Ergonomically articulated panels cradle the body like a hammock.
This Instructable is meant to be a guide to the process of iterating prototypes for CNC manufacture. I am heavily indebted to the helpful folks at Baltimore FabLab for their assistance in fabrication and Nicholas Ierodiaconou at OpenDesk for his help vetting the files and managing the distribution.
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You will need these materials:
One 4'x4' piece of 3/4" plywood
One 50-ct. package zip ties, rated to at least 50 lbs. breaking strength
Finish of your choice
You will need these tools:
CNC router, minimum 4'x4' bed size
Step 1: Design
The process began with research, documented on my blog. I found dozens of examples of zip tie joinery around the web, which divide into three basic strategies: surface-on-frame, surface-only, or zip-tie-only. Surface-on-frame uses a dimensional lumber frame to support rigid panels. Surface-only uses an origami-like arrangement of panels to make a tensile net that can support weight. Zip-tie-only are structures composed wholly of zip ties, woven into nets that serve as tents at Burning Man or make good lampshades.
I then started making scale models. Despite the digital nature of fabrication for this piece, I always start designing by hand. It grounds the work in reality, ensuring the construction details will actually work. Digital iteration can also get away from you -- cutting and pasting versions, tweaking one minor detail each time, then repeating, will net dozens of nearly indistinguishable, un-testable versions that don't push the process forward.
I began by making about a dozen half-scale models out of cardboard, concentrating on the chair shell. At first, I tried to make a self-supporting shell that would hold its shape independent of bracing from a base frame. The results of those experiments, while handsome, were derivative of existing designs, difficult to figure out bases for, and composed of too many pieces.
I ended up with a surface-on-frame strategy, wherein a 3-panel "hammock" is slung from a rectangular box frame. I made two full-size cardboard mock-ups. However, cardboard doesn't behave like plywood -- it's too flimsy -- so it was hard to gauge how the seating surface would actually articulate and whether the base would be rigid enough to support weight. So I moved on to a plywood prototype . . .
Step 2: Plywood Prototype 1.0
Taking what I learned from sketching and cardboard models to the garage, I cut pieces for a rough full-size prototype from scrap plywood. The pieces vary in thickness, from 1/2" to 3/4"; for the purposes of this experiment I mixed and matched, and didn't bother with sanding or other niceties.
Each side of the chair is based on an equilateral trapezoid, 22" high, 28" across at the base, and 20" across at the top. The arms are 3" thick, and the legs taper from 3" to 1-1/2" wide. I marked everything out in pencil and cut with a circular saw. At the inside corners, I finished the cuts with a hand saw.
The front of the chair is based on a rectangle, 20" wide and 15" high. The center of the rectangle was subtracted, leaving legs that tapered from 3" to 1-1/2" and a top bar that was 3" wide.
The back is 20" wide by 22-1/4" high, with the same taper to the legs and width of top bar.
The seating surface is composed of 3 panels, all 19-3/4" across by 15", 11", and 8".
Once cut, I eyeballed 1/4" holes up and down the outsides of the legs and where the panels joined. I stitched the based together first, arranging the four pieces into a square. It was somewhat wobbly until the seat panels are added. Next, I stitched together the seating panels with 4 zip ties each, sandwiching the 11" panel with the 15" and 8" panels. Last, I attached the seating surface to the box frame with some mid-panel holes into the top bars of the front and back of the frame. I made sure to leave the seating panel zip ties a little loose to allow for some movement in the seat. Also be sure to only attach the seating surface through the top and bottom panels, allowing the middle one to act as a hinge.
Step 3: To the Machines!
After modeling and testing by hand, I put the design into AutoCAD. It took some tweaking to fit the parts into a 4'x4' footprint -- a design goal I set for myself in the interest of efficiency.
To cut the chair on a ShopBot, you must first translate the AutoCAD .dxf file into PartWorks, ShopBot's proprietary G-code software. It is a pretty simple vector-drawing program, and allows you to see all the critical variables and separate layers into toolpaths. Select a 1/4" bit for all toolpaths -- I used an end mill upcut spiral bit, but there are several router bit profiles that would work just as well. Pick one that best suits your materials.
Set the red layer to profile, cutting to the outside (right) of the vector. Most CNCs recommend stepping the cuts -- removing just an 1/8" of material at a time, over and over, until cut all the way through -- to reduce pressure on the bits, spindles, and motors. Set sacrificial tabs periodically to keep the parts form shaking loose as they are cut.
Set the cyan layer to pocket, cutting 3/16" channels for counter-sinking the zip ties.
Set the green layer to drill.
Set all the spindle speeds according to manufacturer's instructions for the particular kind of plywood you are using.
Secure a half-sheet of plywood to the spoil board on the bed of the machine. The lab I was using didn't have a vacuum bed, so I did it manually, with screws. A vacuum bed reduces the need for tabs and thus post-processing work, and is preferred.
Next, use the manual controls on the keyboard to jog the router bit over to the 0,0 point on the plywood and save it in the machine. Raise the bit, manually jog it elsewhere, then hit "Home" in the on-screen controls to test that the computer saved the 0,0 coordinates correctly. Set the "z" depth by opening the "z" depth dialog on the computer and allowing the bit to descend and just kiss the zeroing plate. Replace the zeroing plate.
Start the cut, pocketing first, drilling second, and profiling last.
Once the cut is done, remove the board form the cutting bed and use a pull saw to cut the sacrificial tabs and free the parts. Sand the faces and edges with an orbital sander, first at 100-grit and then 150. Apply finish of your choice.
To assemble, start by zipping together the two side pieces ( the slightly asymmetrical "U"s on the left side of the cut sheet) with their arms. Next attach the front and rear "U"s to the side "U"s, creating a box. Attach the three seating panels together in size order (S,M,L), leaving the zip ties kind of loose. Then, fasten the panels to the frame using the holes in the middle of the L and S panels. Snip off tails of zip ties with wire nippers or scissors.
Customize by using different color zip ties, or subbing in panels of different veneers or laminate colors. I added some thin foam cushions to mine for long-term sitting. I also discovered that the zip ties int eh seat can degrade over time and eventually snap. Double the ties in the seat, or use thicker (75-100 lb. breaking strength) zip ties for long-term usage.