Wheels - The Making of Blaster Shark (Sumo)
May 10, 2010
After the design of the body and motor mounts I can get an idea of what kind of wheels I need. The things that are important to consider is the width and diameter. Both are restricted by the dimensions of the the minisumo. In my case, since the motor shaft is offset, I can go with a smaller diameter wheel and not worry about the bottom clearance of the robot. But since the motors are somewhat long, I can't go with a really wide wheel. With this in mind I figured that a wheel diameter of 40mm and a width of roughly 20mm would suffice.
I found some 1" acrylic rod lying around and figured that it would be perfect for the wheel hubs. From previous experience I found that the silicone mold compound I was using was slipping off the hubs, so I threw the rod on the mill to cut channels in for the mold rubber to flow into and grip while the wheel is turning and stay on the hub. After the channels were cut I put it on the lathe to drill the axle hole and part in to the correct lengths.
Now that the hubs are finished, I needed to have a mold to keep the shape and diameter of the wheel. I went to the local hardware store and found 1.5" inner diameter ABS plumbing tube. 1.5" is slightly less than 40mm which was ok because I still had the bottom clearance I wanted. I cut two pieces from the tube to serve as the molds, but I also needed something for the bottom so that the rubber wouldn't just flow out everywhere. I just happened to have a large diameter Delrin rod which I cut two pieces out of and throw onto the mill. I then faced the pieces with the fly cutter to get a nice smooth surface and decreased the diameter on the top portion of the disc to fit inside the ABS pipe. Put it on the lathe to drill a hole and viola two wheel molds. (I didn't actually use a scroll saw to cut the pipe and rod, I used a chop saw but didn't have my camera with me at the time)
Making the Wheels
Each hub is securely fastened into the molds with bolts and nuts. The mold rubber I am using is Smooth-on Mold Max 30. I measured the quantity needed as per instrunctions on the box and used a syringe to inject the rubber into the mold. Normally you would have to vaccuum evacuate the mold rubber before pouring but I don't have and expensive vaccuum chamber. The syringe works just fine and I can see that it is removing the trapped air bubbles from the inside no problem. After the molds are filled, wait about a day and you can demold the wheels and bask in thier glory.
As a last note, if you happen to spill any of the mold rubber on the floor, just leave it there to cure. It will make a bigger mess if you try to clean it up when it is in its liquid form.
You can just pick it up when it is cured