Mini Sumo seems like one of those hobbies that starts out innocently enough, and ends up with a special room in the house dedicated to it. One day you’re excitedly opening up your first Basic Stamp kit, and the next you’re milling out mini molds on a mini lathe to make mini extra sticky tires.
[Dave] started out trying to find a part from the local big box store that was just a little bigger than the wheel he wanted to rubberize. He set the wheel inside a plumbing cap and poured the urethane in. It worked, but it required a lot of time with a sharp knife to carve away the excess rubber.
In the meantime he acquired a Sherline Mini Mill and Lathe. With the new tools available to him, he made a new mold out of a bit of purple UHMW and some acrylic. This one produced much nicer results. Using a syringe he squeezed resin into the mold through a hole in the acrylic. Much less cleanup was needed.
He later applied these methods to smaller, wider wheels as his mini sumo addiction took a stronger hold on his life.
This is really cool and I like to see guys finding creative solutions for anything. But… Readers might like to know that Tigerbotics, the same people that makes the wheel seen in this hack, also make an injection molded Sticky Tire of equal or better performance for this wheel. These are already available from distributors like Pololu (https://www.pololu.com/product/694) and are very inexpensive. The Sticky Tires have an amazing amount of grip and offer the precision and consistency of the commercial injection molding process. Nevertheless, our Kudos to Gerrit for his clever solution and excellent workmanship.
…Tiger
While I’m all for small business and especially small business catering to niche hobbies, I don’t think this is done in good taste. First, I am not the one who has done the work, that credit goes to [Dave]. This irks me as I feel there was light skim done with a poor understanding of the work or willingness to involve oneself in the community of Hackaday. Rushed too fast to post an advertisement.
Second, while your product does seem to fit this particular widely used part, I think you are severely overselling the “consistency of the commercial injection mold part” Not only would this severely limit the “stickiness” of the rubbers you could use, they also have to be thermoform plastics.
The kind of thermosets that you can buy in urethanes and silicones can reach rather extreme performance levels as far as plastics are concerned. Not only are the polymer chains longer and the curing process more controlled than the quick cool of a thermoform process; resulting in a superior plastic. They can be tuned to practically any desired material property. Which in the case of sumobots is a disgustingly sticky rubber that is, by personal experience, truly awful to the touch.
This method is extremely useful for those wishing to tune their robot to a higher degree than a commercially available part can provide. This site provides a helpful tutorial to get someone starting on learning a skill more valuable than the ability to press buy on a website.
Also, the standard solarbotics plastic tire thing is too thin in width and too large diameter to be considered competitive in higher levels of sumo robots. It is a nice beginner product.
I don’t have a horse in this race. But what does thermoplastics have to do with injection molding? (You could say I know a thing a or two about injection molding… and that it’s got a bit of a long history in my family actually)
You have a die and then a means of injecting whatever, it could be jello if you wish at whatever pressure you wish…
Technically we would call simply pouring molten metal into a die at regular atmospheric pressure “Permanent Molding” as the dies or “molds” were reused vs. sand casting where you destroy the molds after each use (again those processes vary wildly)
In an injection molding die casting machine and we used molten aluminum at thousands of PSI with a dwell time long enough for the molten aluminum to harden. It could have been any material though, as long as it has the ability to set and be ejected from the die at some point. It could have been water as long as the dies were chilled to below freezing…
I’m missing where the material use negates the process of injection molding? As long as the material has a way of hardening, its still a viable process. If getting something sticky out of your die is a worry, you use a proper release agent… I mean uh, think about tires?
You cold inject chemically setting material, such as two-part epoxy and simply dwell until it hardens, you get the same result…. You would inject at high pressure to control porosity, fill and over density of the molded part….
I think you know something about molding, but I think you don’t know as much as you would like to think….
But yeah, you were covering a hack and yes, anyone could BUY something, and that is against the spirit of HaD, but still. Expert of molding, you are not.
Back when I created those molds and the webpage, which was in 2002, there were no commercially available tires (just the rubber bands that came with the wheels).
Being able to create custom tires is still quite useful.
A lot of the top Sumo bots have custom tires that are very wide, tiny in diameter, and very sticky. I don’t think they sell equivalents in stores. It’s a neat method that’s still very relevant. Plus, there are lots of situations other than sumo bots where this can be useful.
In the very high coef of friction department. a quick hack is a layer of Loctite GO2 adhesive. It’s a urethane/silicone hybrid made in Germany. Trigger is moisture in air, so I always give it some heavy breathing :-) You can tool it on surfaces, and they are high friction city in 24 hr. about $5 @ Home Depot and the like.
One of he advantages of being to pour your own is when you can get different durometers of polyurethane.
For example, with Wedgy, it was designed to compete on a steel ring, and used rare earth magnets for the wheel hubs. I tried using the Shore-A20 polyurethane that I used on Marauder, but the magnets were too strong and deformed the tire as the robot was moving, eventually popping the tires right off the wheels. So I needed to change to a harder A-60 polyurethane.
The magnets were strong enough that Wedgy would stay on the ring when the ring was flipped over.
For somebody getting started, I’d definitely recommend going with something cheaper, like the commercially available tires. You also typically have to buy the polyurethane a liter or two at a time, so it helps to have a bunch of robotics buddies who maybe want to share. But when you’re competitive, sometimes the cost is irrelevant :)