For the Market Street Prototyping Festival in San Francisco we set out to build a robust piece of urban furniture that could withstand the abuse of a dense and dirty urban environment. This Instructable will go through the steps used to build 2 chairs out of High Density Polyethylene designed using Rhino, milled using a ShopBot, drilled with a drill press, and assembled with a wrench and an allen key. It will show ways to reinforce, bend, and press-fit elements into a chair that can be rained on, kicked, flipped, and even nested to be packed away or shipped. Follow the step and I would love any feedback for the next iteration.
Step 1: Designing the Chair
High Density Polyethylene has not pores and is often used for cutting boards. This means it is extremely durable and doesn't stain. It also means that no adhesive works. For this reason the chair was designed to be entirely mechanically fastened. Many iterations were worked through resulting in theses defining elements:
1. 1/2" thick Sidewalls_carries the load of the chair to the ground for a solid foundation
2. 1/2" thick Reinforcing Rails_creates press-fit notches and curvature for ribs and seat cover
3. 1/4" thick Supporting Ribs_creates lateral strength across the chair to prevent racking
4. 1/8" thick Seat Cover_creates shear strength and a nice urine ready surface for an urban seat.
The system can be modified to have deeper pitch, lower back, taller seat, or shorter seat. The system allows for this flexibility as any press-fit construction would and you should have freedom in modifying this sections. The final version I proceed to build was for a specific seat used for pedaling device charging generators for a festival. Yours maybe a lawn chair, for instance and would have a different profile.
In this design, the mechanical fasters are limited to the interior of the chair leaving only the socket heads of the 5/16 18 bolts exposed. This you will see later as a means to assemble the chair for a clean and clear exterior construction.
Step 2: Laying Out the Cut Files
Once the chair is modeled and drawn, the components are arranged on 4'x8' sheet material. In an effort to save material, the shapes are arranged according to material thickness then nested as closely as possible. Some of these shapes could be cut out with a table saw, however the tolerances for bolt holes lining up can be achieved through an automated milling operation much better. For this project I used a standard 4'x8' ShopBot and 4'x8' sheets.
Step 3: Setting Up the ShopBot
Once the Layout files are created you can bring the DXF into a CAM software to set up tool paths. For this project I used V-Carve with a very simple interface for 21/2 dimensional milling. I used the following settings.
1. Internal Drill Holes: 3/8" Single Flute Endmill, 12000 rpm, Pass Depth 3/16", Feet Rate 1.5"/Sec
2. Channel Pocket for Seat: 3/16" Endmill, 12000 rpm, Pass Depth 1/8", Feed Rate 1.5"/Sec
3. Exterior Profile Cutout: 1/2" Endmill, 12000 rpm, Pass Depth 1/4", Feed Rate 1.5"/Sec
4. Sheet Thickness: I measured the material with a caliper and set my material thickness accordingly.
Once the G-Code was produced I opened ShopBot Controller, zeroed my X and Y with the table sensor and zeroed my Z off the top of the material. I did a dry run before setting the tool to work on the Polyethylene. Above you will see the beginning and end of the mill job for the sidewalls. These steps were repeated for all sheets in the layout step.
Step 4: Counter Boring Holes for Socket Bolts
Once the sidewalls were cut out, flipped them over so I could counter bore for the socket bolt. I used a drill press with proper clamps so that I would never get lift up. I measured the head of the bolt and set the stop to only go that far through. If the drill bit goes through then the bolt will have nothing to hold onto and the connection will fail. MAKE SURE TO CLAMP WELL.
Step 5: Setting Up for Assembly
With all the parts cut, now is time to assemble the chair. Above is all the pieces needed.
(2) Support Rails
(17) Support Ribs
(28) 5/16 18 7/8" Socket Head Bolts and nuts (zinc plated)
(12) 5/16 18 1-1/2" Socket Head Bolts and nuts (zinc plated)
(40) 3/8" washer
(12) Stanley Angle Brackest 1.5"x 2.5"x1.8"
(8) Stanley Angle Brackets 1.5"x 1.5"x1.5"
(1) Socket Wrench
(1) 1/4" Allan Wrench
(1) Music playing machine for atmosphere.
*for two chairs double recipe.
Step 6: Chair Profile
Attach the support rails to the Sidewalls. Register bolt holes and insert bolt through 1/2" sidewall, 1/2" support rail, through Stanley angle bracket, through washer, and finally secure with a nut. Repeat until both sidewalls are secured with the rails and angle brackets.
Step 7: Chair Back
Attach the two profiles together along the back panel. Again, register the holes and insert the bolt through the 1/2" back wall, and through the Stanley angle and washer. Tighten well and the chair should now be standing upright.
Step 8: Chair Ribs
Insert the ribs into the slots on the support rails. These should go in depending on the tolerances achieved when machining. I attached the ribs designated as lateral bracing first and bolted and secured them. Then, I slid in the ribs that are only press-fit. Now, you can see the chair coming together well with the substructure complete.
Step 9: Bending the Seat Cover
Bending the plastic was done with kerf bending in which strips of material are removed so that it can flex. It is important to have the kerf on the inside of the bend. It is is bent the wrong way, it will snap. I did multiple tests to get the right depth and frequency for the 2" radius bends. In the end I used the width of a table saw blade at 1/16" deep through the 1/8" sheet spaced at 3/16". I recommend bending it beyond the final form so it will be easier to install in place.
Step 10: Finishing
Since this chair will be going outside, it is important to ease all of the edges so nobody gets cut or scraped. I used a hand router with a 1/4" round over bit around all the edges as well as the interior of the side handles. It resulted in a smooth buttery finish, clean white and somewhat 2001 Space Odyssey.
Step 11: Playing
The chairs were used as a component of the Neighborhood Preparedness Hub where off-grid power and communication is provided at the neighborhood scale for everyday use as well as use during a disaster. More information about the project can be found at urbanrisklab.org
photo credit: Justin Lavallee
Step 12: Transporting Chairs
These chairs were designed to be mobile, nesting on a single pallet for shipping along with the rest of the modular build. They will now be shipped and exhibited around the US and maybe a neighborhood near you. Enjoy!