Moldmaking for spherical wheels

     Molding a new part is a slow, tedious process. You start out with the object or shape to be molded, called the "plug", coat that in whatever material the mold is made out of, disassemble the mold and remove the plug, then refill the mold with whatever material you're using to make the copy. If you're lucky, the object you're copying already exists. If not, you have to sculpt that first.

     I needed to create a spherical tire for one of my robots. It had to be rigid but lightweight with good grip, and about 9" in diameter. If something like this exists on the market, I have not found it. I decided to coat a hollow aluminum sphere with polyurethane. That sounds simple and straight forward. It's not. The first step was to create the plug, which was interesting to say the least. A sphere is mathematically simple, but practically very complex. My first idea was to wrap the aluminum sphere in a sheet of 1/4" rubber, foam, or some other material, but there was no way to ensure it would be smooth all the way around.
     I went with my second idea, which was to take a 1/4" thick sheet of plastic, cut it into little bits, and glue them all over the surface of the sphere (seen right). After that, I coated the entire sphere in plaster just thick enough to cover all the plastic bits. After I baked it dry, I sanded the whole ball down until the plastic spacers were just visible all the way around so I knew the plaster coat was even. Then I painted the whole ball with a very thin layer of plaster and finish-sanded it smooth, producing the final plug. I measured the diameter of the ball using a large caliper to ensure it was as close as possible to round (within about 1mm). This was the most time-intensive part of the whole process. I applied many coats of plaster and did a lot of sanding, correcting imperfections in stages. The thing that makes it difficult is that each successive step includes a full day of waiting. You apply the plaster, bake it dry, measure it, sand it, repeat. It's important that the plug be more or less flawless.

     The second part of the process, which is faster than the first though still not exactly fast, is using the plug to create a mold. For the frame, the mold's body and structure, I used inexpensive sheets of styrofoam. I cut circles in the sheets to make a spherical hole big enough to hold the plug then glued to sheets together into two halves. After the frame was ready, I positioned the plug inside. The plug needed to be centered within the (roughly) spherical hole, so push-pins were put into the styrofoam to hold the plug off the bottom and the sides. The plug sat on top of these pins while I poured in silicone to make the mold. This only had to be done for the first half. For the second (top) half, I clamped the mold halves together, cut a hole in the top, and poured in the silicone. Voila. We have a complete mold. It is important to remember that the plug might float in the rubber. This caused problems more than once.

     After the silicone mold was complete, I took the plug out and broke off the plaster. For the final polyurethane coating, the aluminum ball had to be spaced off of the silicone just like the plug was spaced off of the styrofoam, though this time it had to be much more precise. Again, I put pins in the mold, sticking 1/4" out of the surface, and set the ball on top of them. I also had to put spacer pins in the top half of the mold because, as I mentioned, the ball floats. This was the trickiest part of the coating process, because it takes several hours for the polyurethane to dry and the buoyant ball slowly pushed the upper pins into the mold, moving itself off center. The easiest solution for this was to use lots of upper pins.

   After letting the polyurethane dry overnight, I cracked the mold apart, pulled out the coated ball, and cleaned it off. After shaving off the flash, I rolled the ball around on the floor and saw that, despite all my efforts, it wasn't evenly balanced. The polyurethane coating was slightly thicker on the bottom than on the top, but this was fairly easy to fix. I used a belt sander to remove some of the polyurethane to balance the ball and clean up and imperfections in the surface finish.

     The final product works quite nicely. The ball wheel is light (~4lb), incredibly durable, and well balanced. Total cost: about $450. This includes the first ball wheel, the reusable mold, and enough polyurethane to coat several more balls (which is the next step). Slow and tedious, but successful.

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