Cast your own TIRES
Cast your own TIRES
1. From author
My credo is "to share". My primary goal in robotics is to build active community of hobbyists, who will raise European sumo robots to a new level, close or even higher than Japanese sumo. The only way to achieve this is to share all our "secrets". After having read our tutorials, newcomers could start not from zero, but from our level, saving time and money. They could concentrate their efforts on developing new methods and techniques to build more and more advanced robots. Fact of winning any particular event for me is not so important. What really makes me happy- is that many more advanced robots will emerge with my help. It's like philosophy of life: it's much better to give something to others than to hide it and use it alone.
“Rodman Robotics”, Lithuanian robotic club, has developed tire casting technique for sumo robots. This technique is not our invention, of course. We just combined several casting methods found on the web, and adapted them to our needs. In this article I will explain sumo tires casting process in every detail. Our robots, equipped with such tires, won many national and international events, including BalticRobotSumo-2009, BalticRobotSumo-2010, RobotChallenge-2010 and RoboGames-2010.
Many robot builders have asked me to disclose our “BIG SECRET” of tire material. Actually, there is no secret. All info is available on the internet. After reading many internet articles about sumo tires casting, I have realized what kind of material do I need for sumo tires.
First vital property of the material is hardness. The hardness of plastics is most commonly measured by the Shore (Durometer) test. The ShoreA scale is used for 'softer' rubbers while the ShoreD scale is used for 'harder' ones. Material with ShoreA 20-30 is most appropriate for sumo tires. Resin with lower Shore values (ShoreA 5 to 10) will be too soft, too sticky and will tear off the wheel quickly. Resin with higher Shore values (ShoreA 40 to 80) will be too hard, too slippery and provide less traction for your robot.
Second vital property of the material is its ability to cure at room temperature and to be mouldable without any special equipment, because nobody has degassing or high pressure moulding equipment at home.
RTV(room-temperature-vulcanizing) silicone is the best candidate for casting sumo tires. The only problem with RTV- it’s very hard to get proper one at Europe. I spent two weeks trying to find a supplier, and finally I found Por-A-Mold 2020 silicone at German website. Unfortunately, they do not stock 2020 anymore. Por-A-Mold 2020 is two component silicone, which is mixed 1:1. It takes around 24 hours to cure, but 12 hours is enough if you are in a hurry. Other similar types of RTVs, like Polytek 74-20, are widely available in America, but nobody has heard of them in Europe.
Another difficulty is that RTVs are usually sold in relatively big quantities. For example Por-A-Mold 2020 is sold in 2-litre kit: 1 litre hardener + 1 litre resin, and costs ~60 Euro without shipping. Two litres might be overkill if you want to cast just two small tires for minisumo robot. But for a local robotics club it will be a good deal, because all members could do so-called “group-buy” and cast many tires for many robots from one kit.
Once you have ordered your silicone, it’s time to make a mould. You will need a piece of 10-12mm thick polypropylene sheet. Why exactly polypropylene? Actually you can use any machinable polymeric sheet you can find at local supplier of industrial plastics. Advantage of polypropylene is that it does not require mould release agent. For the first time I used Vaseline to lubricate inner walls of the mould, but later on I stopped to use Vaseline, because it caused some strange chemical reaction with resin. From my experience, it is possible to extract wheels from the mould without using any release agent.
You will need mould to be CNC machined for extra precision. First time I tried to make a mould at home with hand power tools, but result was not acceptable because of lack of precision. Of course it’s up to you what level of precision do you need. In my opinion, it is better to invest in CNC machining to get nearly perfect tires each time, rather than get ugly tires with unpredictable surface every time.
I decided to make the mould of several layers. This allows me to cast different width tires. For example, if one layer of polypropylene is 12mm thick, you can cast 12mm, 24mm, and 36mm width tires using 1, 2 or 3 layers of the mould. This will allow more flexibility casting tires for different robots. One more tip- you can make holes with different diameter in one mould to be able to cast different diameter tires.
All layers of the mould must be assembled prior to machining holes for the wheels. CNC operator suggested using four 10mm diameter metal rods to hold all layers together. First, he fixed all polypropylene layers together and drilled four holes for metal rods. Then he machined metal rods to fit tightly and precisely into holes. Later, he machined sides of the mould to make proper nice rectangle. Then he drilled holes for the wheels. At this point we got perfectly machined mould. M8 bolts with nuts hold all layers tightly to prevent silicone leakage between layers.
Now we need to make hubs for the wheels. Same CNC operator machined them for me from aluminum rod. Precisely drilled center hole is crucial, otherwise you will get unbalanced wheel and “jumping” robot. I used M3 internal set screws to fix wheel on the motor axis. Set screw is hidden under the tire to save valuable space. You need to pierce tire rubber with hex key (also known as Allen key) to turn set screw.
Grooves on the outside surface are supposed to provide better adhesion of resin to the hub. As you can see on the picture, some inner material is removed (machined out) off the wheel. This will not only makes a wheel lighter, but also reduces axial load for a motor.
5. Casting process
Now it’s time to start casting your super-traction tires. Assemble your mould with necessary number of layers for desired wheel width. For example, if we need 24mm width tires, our mould will contain ground layer (where hub centering bolts are set) plus two 12mm thick layers with holes for wheels. Push centering metal rods through all layers to align all layers precisely. Now insert and tighten M8 bolts to eliminate any gaps between layers, because liquid silicone could easily leak through these gaps.
Put a small amount of Vaseline into internal set screw hole of a wheel hub. This will prevent silicone from getting into the hole. If you leave hole open, liquid silicone will seal entire hole and you will not be able to turn the screw with an Allen key.
Rub wheel hubs with acetone to remove any traces of dust ant grease. Put wheel hubs onto centering bolts and tighten with nuts. Check gap between hub and mould – it must be equal all around the wheel. Take your time and equalize this gap as precise as possible. Otherwise you will get unbalanced tire.
Calculate approximate volume of a gap between a hub and a mould. For example, if hub diameter is 45mm, tire diameter is 50mm and tire width is 24mm, we need to substract volume of cylinder1 (diameter 45mm, height 24mm) from the volume of cylinder2 (diameter 50mm, height 24mm). It will give us approximately 9ml volume, or 10ml with reserve for spillage. It means that we need 5ml of hardener plus 5ml of resin to cast one tire. Multiply this amount by number of tires, and you will get total volume of silicone needed.
Use syringes to measure correct amount of hardener and resin. This is crucial step, because if you mix silicone in the wrong proportions, tires may become too hard or not cure at all. Pour equal amount of resin and hardener into a measuring cup and stir with wooden stick. Stir carefully to get homogeneous mix. Small bubbles may appear during mixing procedure, but it is normal. Small bubbles will not reduce tire quality. Just take care not to make bigger bubbles.
Do not wait too long after mixture is prepared. Pour mixture into a mould in less than 20 minutes, because silicone hardens quite quickly. Pour mixture slowly, allowing silicone to fill all gaps. Leave mould onto a horizontal surface for at least 12 hours. It is better to allow 24 hours for silicone to harden.
Extracting wheels from a mould is an easy process if your mould is made of layers. Unscrew nuts and remove tightening M8 bolts. Remove centering metal rods by pressing them with a screwdriver. Insert flat screwdriver between any two layers and gently try to separate them. Proceed around the perimeter of the mould with a flat screwdriver, trying to separate layers equally. Do not worry, you will not damage tires while mould layers slide off them. You can unscrew wheel centering bolts before separating the mould, or you can leave them tightened. You can try both ways to find most comfortable for you.
Remove wheel from the last layer of a mould by gently pressing it out. Cut excess of silicone with knife or scissors. Do not use wheels straight away. Allow silicone to dry for at least a couple of hours. As for me, I leave wheels for 12 hours after extracting them from the mould. This allows silicone to dry completely.
You are done! Enjoy your new super-traction tires and do not forget to cast a set of tires for your friends. I wish you many victories and many prizes! "NEVER GIVE UP, NEVER SURRENDER", as says David Calkins, Robogames organizer. You win if you fight to the last Ampere, to the last referee's whistle, even if it seems that there is no chance to win.
I wish your robot to be fast, to be smart, to be the winner!
See you at next Baltic Robot Sumo competition!