Here is a little preview of what 125 will be rolling on this year. We have experimented in the past with casting some portions of our wheels, but never used them in competition. This year we decided to save time and effort to cast our wheels out of glass-filled urethane.
The mold was modeled from a previous wheel our team had fabricated. Silicone was poured around the old wheel and allowed to cure for 16 hours. After that was said and done, the old wheel popped out of the mold and left a negative for us to pour urethane into. We take urethane and mix it with chopped fiberglass and spread it through out the mold. 60 minutes later we pop the wheel out of the mold and are ready to go.
The nice thing about this setup is it does not require a machinist to run a machine and devote 100% of their time to. High school students and mentors alike can whip up a batch of urethane and pour it into the mold while other activities are occurring.
Let me know if you have any questions, I’d be happy to answer them.
How are they holding up from an elasticity standpoint (meaning, do they deform under the weight of a robot by much?). They look awesome; however, if you could embed LEDs… (not so sure on how the rules would like that, but it would look cool)
You got it right. This stuff is something like an 85 or a 90 on the Shore D scale, which is very rigid. The fiberglass adds some extra rigidity.
EVERY SINGLE TEAM IN FIRST can do this. For teams that like to use the same type of wheel from year to year this may be something to consider. If you invested in a multi cavity mold, a team could feasibly have all of their wheels for the season in a couple of hours.
Haha, these are 4 inchers, so I don’t think they’d fit what you guys are using. Someone accidentally threw out the red dye :-/. We plan on using some withholding allowance to make some red ones. Oh by the way each one weighs less than 3 ounces.
Where do you get the materials? The silicone and the urethane? What do they cost? Do you cast the wheels with the bearings in the mold or do you machine the wheels later and install them?
Thanks
Bruce
During some pseudo-tests the wheels have performed well. One wheel was able to support 165 pounds of weight bouncing on it (me bouncing up and down). This was in the vertical (weaker) orientation. We’ll be doing more testing this week. The thing the wheels have going for them is the tread will buffer some of the blunt impact force.
The hole pattern on the old wheel we used had clay partially filled in the bolt holes. This allowed for us to essentially have pilot holes that were drilled through on the drill press.
There is a store in Boston that sells www.smoothon.com products. However, you can purchase these online and have them delivered to your shop. To make the wheels you see here it cost us 45 dollars. Silicone is capable of holding a remarkably tight tolerance when using it to create molds. We were able to use the hole in the old wheel which was designed for a press fit (~.874") as is. The bearings were then pressed into the new wheels…simple as that.
I’ve had an inkling for the longest time about crossing the bump. After watching the 118 vid, even thought they’re crossing quite quickly, I’m a little worried about the different loads of landing on the wheels directly and at an angle. It’s going to be rough on the chassis and drive train of everybody out there. How many spare wheels are you guys making, just in case?
Those wheels look sweet, and would be a great off-season project. I looked at the smooth-on site that you mentioned, and found a wide variety of products. Can you tell me exactly which silicone (for the mold) and which urethane (for the wheel) you used? Is it the smooth-cast 305 plastic in the 00M00 30 silicone rubber as shown in their video? Also, what type of fiberglass did you add? Thanks in advance.
How were these wheels in retrospect? Are there any changes you wish you had made, or did they perform pretty much the way you wanted? This is such a neat concept!
The mold was made from Mold Max 30. That is the “multipurpose” silicone, that can be used for pretty much anything. I’ve used other varieties, but the Mold Max 30 is just awesome stuff and super simple to use. The wheel was made from task 9 urethane. I’ve had less experience using the rigid urethanes, like the task series, but from my experiences with it in these wheels, it is also very good stuff.
The fiberglass we used was just simple fiberglass cloth you would buy from home depot/lowe’s and then chopped via scissors into ~1/2" long strands and added to the urethane.
These wheels show a lot of promise in regards to FIRST robots. There are some definite changes I would make to the wheels in regards to geometry, but thats about it. What we are going to do over the summer is machine a block of wax into a wheel with more ideal geometry. We are then going to use that wax wheel to create a new mold. We want to produce some of those wheels from the mold and put them through some tests. If they perform like we expect they will, we want to make a multi-cavity mold. This will allow us to pop out 2-4 wheels at a time and essentially make wheels a standard item for us to design around.
Has anyone else tried this? Any success? Brando, what mods did you come up with for this wheel? Can I get a picture of the actual mold? I don’t quite get how you get the entire wheel molded on all sides without any machining.
Thanks
Bruce
If you don’t mind small imperfections on the face of the wheel then you don’t need to worry about machining. Because you are using silicone as your actual mold cavity, the regular rules of molding can be stretched. Things like undercuts and 0 degree draft can be done because the silicone is compliant and moves out of the way of the way of the part when removing it from the mold.
These wheels are from our 2010 robot, so they were in this past years competition. Changes are in the works to make them even more reliable, cheaper, faster to turnover and easier to make.
So, to make the mold you just laid the master on the bottom of the mold box and poured the silicone over it? The top side of the wheel is as smoothe as the urethane was poured, correct? Or did you pour it high and machine off the excess?
Bruce
