After 16 painful-to-wait hours, I cracked open the mold to check out the first prototype tire. It turned out very nicely, and much better than my previous adventures so many years ago. (Maybe when i get a chance I’ll dig up some of my old stuff to use as a comparison).

The traction on these tires is amazing, and the dark blue colour looks cool, however I think I’ll use a bit less pigment next time. The tricky part will be maintaining a consistent amount of pigment, since I only have one tire mold and will have to mix up a new batch for each tire I make. Making another mold would take valuable time away that I need to work on the chassis and electronics design if I’m to have any chance of finishing in time for the WCRG…

Note the air bubble that caused a missing piece in the tire.. Since this was a prototype tire that I plan to rip apart to test the strength, I wasn’t too concerned about fixing that part while molding it.

It’s been a while since my last post, but I’ve been working away hard at designing and a little bit of machining for my Mini Sumo. It’s proving to be quite a challenge to fit all the electronics and sensors I want into the small package of a mini sumo robot only 25mm tall, but I think I’ve figured out how to shoe-horn the electronics in there.

I’m planning to do some fairly sophisticated sensors, but that’s a topic for another day, and likely after the games as I likely have to run a temporary brain for this competition as time is too tight to get a PCB turn.

I have made decent headway on the chassis, having made more wheel blanks, and made a mold so that I can cast my own polyurethane tires. The mold itself is made from Delrin on the mill, held togethor by four screws, and kept in alignment with 6 dowel pins. The center slug was spun on a lathe, and forms the bottom of the mold, as well as a means to seat the wheel directly in the center of the mold. A top plate from scrap polycarbonate is screwed on to keep the top finish of the wheel nice.

Delrin is a really nice material to work with on a hobby mill and lathe, and it was relatively easy if somewhat time consuming to bore out the 25mm diameter hole for the robot.

6 or 7 years ago was the last time I cast polyurethane for sumo tires, I reckon, so I figured what I had lying around was no longer good. A bunch has changed since then, and a stop to a local mold making shop with some samples had me settled on Smooth-On Reoflex 30.  Other people have used various Reoflex durometers for sumo robot tires as well. Feeling in a spendy mood, I also decided to buy some blue pigment.

Excitedly, at the end of the work day I got home with the polyurethane in it’s shiny box, which I’d bought over lunch hour, and prepared the mold. After collecting all the things I would need to cast my tires, I found that the really Old AIrbrus lying around wouldn’t spray the mold release agent anymore, so I cheated and used a brush instead. After waiting for the mold release to dry, I clamped the mold pieces together, popped in a wheel, and started mixing some polyurethane.

This new stuff works better than what I’d used previously, and was less viscous and as such did not get as many air bubbles trapped in it while mixing. An added benefit was that I could use some over-sized syringes I had lying from an old ink-jet cartridge re-filling kit.

A word of caution when using pigment for polyurethane, however: The “SoStrong” pigment.. Is, just as the name would imply, oddly, So Strong. I used a few drops, which was a few drops too many. The end result was a very dark mix. But, as this is a prototype tire that I plan to rip appart to test strength, I wasn’t too concerned.

Now the tire is sitting in the mold. With a 16 hour de-molding time, It should be ready tomorrow roughly around lunch… We shall see how it turned out then.

UPDATE: Check out Part II of this article here!

For now, here are some other links you may find useful with regards to polyurethane sumo tires:

• Jon Hyland’s Seeker3 Sumo - He has some very good pictures of his awesome robots, including some more details on molds
• Dave Hyland’s “Making Marauder’s Tires” – Jon’s brother has more good details on casting polyurethane
• WCRG’s Molding Tires