Introduction: Bench-Go-Round

About: I run Neal's CNC in Hayward, CA, an expert CNC cutting and fabrication service, easily findable by Google search. I'm a founding member of Noisebridge, a hackerspace in San Francisco, and Ace Makerspace (forme…

The Bench Go Round re-imagines public seating to create playful connection between strangers.

It was commissioned for the Market Street Prototyping Festival in San Francisco, CA in April 2015. George Zisiadis came up with the initial concept and and I designed and fabricated it. We installed the piece together and watched in delight as the public enjoyed spinning on the bench.

Step 1: Materials

The Bench-Go-Round was created from the following items:

  • 3 sheets of 3/4" 13-layer birch plywood
  • 6' of 2" mild steel square tubing
  • a couple square feet of 3/16" mild steel plate
  • 3' of 1" mild steel square tubing
  • the lower half of a rotating office chair
  • two non-rotating caster wheels
  • 6" of 2" mild steel round tubing
  • 3' of 1/2" mild steel dowel
  • 6' of 1/2" threaded rod
  • 7 1/2" nuts
  • 5 1-1/2" bolts
  • 2 sheets of 3/4" shop plywood
  • a lot of small attachment plates and screws

The tools used were

  • CAD software
  • CNC router
  • metal chopsaw
  • angle grinder
  • MIG welder
  • power drill
  • hand tools - wrenches, tape measure, screwdriver, marking implements, etc

Step 2: Design and Prototyping

From the basic idea, a bench that spins, I developed two initial directions. George talked to people and got a lot of good feedback which helped us decide on one design - luckily the one we both liked best too. I drew the idea in detail in a CAD program (I used Moment of Inspiration although most 3d drawing tools could be used). Next I sliced the model up to get cross sections that could be cut with a CNC machine.

I tested the idea with a 1/12 model prototype of half of the bench, cutting cardboard on a laser cutter. I took pictures of it facing in both directions and photoshopped them together to get a quick idea of what the whole thing might look like. George and I were both happy with this so we then focused on the next big problem, how to make it spin.

We brainstormed a number of ideas, such as finding a junked playground merry-go-round, using many large ball bearings, or hacking apart a car axle. The solution we eventually decided on was the bottom of a cheap used office chair. We added support legs on wheels under the bench ends as we expected the bench plus sitters to be heavier than the chair was designed for. However, this was a prototype and we were more interested in getting feedback from people on whether they liked the concept or not, than making production level fabrication processes - that would come later if at all.

Here's a video of George and me deciding that yes, the idea is a fun one, and yes, the chair will work:

Step 3: Fabricating the Bench Skeleton

The bench is supported by a backbone of 2" steel tube that runs lengthwise through the base of the bench. In the middle of this tube I welded a 3/16" steel plate to match the plate on top of the chair base. These plates would be bolted together during assembly. Since the chair could recline and also be raised and lowered, I welded the base to prevent these activities. To stop it reclining, I welded the hinge base to the plate on both sides. To stop it lowering (it needed to be full height), I cut a steel tube in half lengthwise, put the vertical chair tube in between, and welded the pieces back together - see pics.

The support legs I fabricated out of 1" steel tube and more of the 3/16" sheet. The legs had wheels at one end, attached via a welded plate similar to the main base. At the top end, I designed a clamp structure to fit tightly around the central square tube backbone. This clamp had two prong with holes near the end so as to be bolted tightly together around the backbone when assembled.

The bottom of the chair had five legs on which press-fit plastic casters were inserted. I removed these and found that the center tube of the chair was now a half inch longer than the legs, and the chair would not stand solidly. Also, the press-fit holes were not well suited as leg ends. I solved this problem by designing a pentagonal plywood base with a central hole to fit the center tube, and appropriately spaced holes for the ends of the legs. I welded bolts into the press-fit holes, and screwed nuts through the bottom of the plywood pentagon to attach it all together.

The ends are held in place by threaded rod. The center bar is 6 feet, and the pieces when all in place stick out about a quarter inch beyond it. To provide tension there are small circles outside of this, and the threaded rod runs all the way through with space for nuts. So the threaded rod needed to be about 6 feet 3 inches, and it came in 6 foot lengths, so I simply cut it in the middle and welded 3 inches of dowel to get the ends of the threading out where I needed them.

Here's a little stop-motion video of me welding up the legs.

Step 4: Fabricating the Bench Seat

I designed the seat for an optimal combination of strength and lightness. I sliced the model crosswise in 3/4" sections and determined that two sections of every three would be spacers and one section would be actual bench cross-section. This way, the seat is not solid wood (very heavy), but the pieces are close enough together that it is not troublesome to sit on. The result is a visually pleasing, airy-looking structure that can nevertheless easily support the weight of sitters.

The cross sections of the seat parts are L shapes with a square hole at the meeting of the branches (the outermost shapes at each end have an armrest). The spacers are triangular with a square hole. For both types, the square hole starts out aligned with the bottom, and gradually rotates until it is at 45 degrees in the center sections. When the pieces are slid onto the square center bar in order, the gradually rotating shape of the bench appears. The pieces are kept from rotating by the square shape of the center bar and holes. They are kept vertical by axial pressure from a central threaded rod that passes through the square tube. The rod is kept in place by two small circular pieces of plywood that cap the ends of the bar after the last bench pieces. The threaded rod passes through a small central hole, and a nut is screwed on the end to tension the whole assembly.

I cut the pieces on a Komo Innova CNC router, along with the circular end pieces and the pentagonal plywood base. Additionally I cut two rectangular pieces with square corner insets which you can see in some of the pictures. This was to lengthen the wheeled leg pieces by the same amount as the thickness of the pentagonal base, as I had measured the leg length before realizing I needed a base. Prototyping!

Before doing the bulk of the cuts, though, I had made one test triangle with a square hole, in order to verify that it would slide properly onto the square center bar. The cut could not be exactly the same size as the bar's measurement or it would be too tight to slide on. A small offset is needed. For this hole, I used .0015 inches (15 thousandths) which worked well, letting the pieces slide onto the bar without a great deal of difficulty, but still being tight enough not to permit more than a minimal amount of rotation.

After the pieces were shaped on the CNC router, I ran all the L shaped ones through an edge rounding router. The straight router leaves a very sharp edge and I did not want anyone to sit on it! The rounded edge is safer and gives a very nice, finished look to the edges.

The very center of the bench posed a special problem. This is where I had welded on the base plate that bolts to the chair base. With a welded plate attached to the center bar, pieces could not completely encircle the bar to slide all the way in. For the 6 or 7 centermost pieces, I had to slice off part of them along the lower edge of the square hole. But now they could be simply lifted off of the bar as they no longer fully encircled it. I solved this by making two more smaller aligned holes and inserting two pieces of half-inch steel dowel through all of the central pieces and 3 pieces out towards the edge, to anchor them in place.

Step 5: Assembly

The bench was assembled from the center outwards. First I bolted the central bar to the chair base. Then I placed the wooden pieces. The central pieces were placed on the bar over the chair attachment plate (these pieces as noted in the previous step had a cut-off section to allow this): spacer, bench seat, spacer; spacer, bench seat, spacer, etc. Due to the variations in the welds on each side, I needed to do a bit of filing on one or two of these central pieces for them to fit properly in their correct locations. The 3 or 4 next outer pieces, with the extra half-inch holes, slid on next. Then I inserted the half-inch dowels through all the pieces, aligning them and keeping them from popping up (or being pried up). I then continued sliding pieces onto one side at a time in order, spacer, bench seat, spacer, until I got about a foot from the end, ending with a bench seat piece. At that point, I slid one of the wheeled legs' clamps on, pressed towards the center, and tightened the bolts, fixing it in place. The clamps are as wide as two spacers, so the next piece is again a bench seat piece. At the very end, there was one spacer and then the last piece, the bench seat with armrest. At this point the pieces stay on easily enough by themselves, but the armrest pieces can be knocked off if I'm not careful. Finally after doing both sides, I put an end cap and bolt on one end of the threaded rod, push it through the center bar, and end-cap and bolt the other end.

Step 6: Installation

Our assigned location on Market Street for the festival was on the south side between 1st and 2nd Streets, along with several other festival entries. We decided that for stability and a better spinning experience, we should include a larger base big enough for the wheel legs to roll on. I designed this like a pie - a circle about 7 feet in diameter, made from 8 wedge sections. I cut this on the same CNC router as the rest, and bought a bunch of small metal plates and wood screws to fasten them all together. To hold the circle together while fastening it with screws, I used a ratchet strap and carefully ran it all the way around the edge of the circle and tightened it.

Installation was at night so I brought headlamps but the streetlamps were sufficient. We spent about an hour screwing the circular base together and the bench assembly to the circular base, and then as soon as we tried it on site, the wheels started squeaking terribly (which they had never done before). Lithium grease solved this but I had to come early in the morning to apply it as I had not brought any the night before. Luckily this meant I was on hand for the morning visitors to discover the festival and interact with the bench and the other nearby entries!

Step 7: Interaction

Over the three days, there were thousands of people of all ages using it. Once they sat down, they couldn’t help but smile and laugh and have fun.

It's not too hard to figure out what to do with the Bench-Go-Round. We did get a sense that some people were reluctant to actually sit down because maybe it was Art? And we have all been trained to Not Touch. We ended up writing "Have A Seat" in big letters on the base. But after a while there was a line of people waiting for their turn. People sat and spun with their kids, their shopping bags, their dog. A few people even tried to lie down on the bench, which kinda worked if you weren't too big!

We hope the Bench-Go-Round can inspire people to look at the urban landscape a little differently, re-imagining the world as a place for play.