pic: How 125 is rolling this year...

[Brandon Holley]

Originally Posted by Andy Bradley

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.

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.

Originally Posted by Jamie Kalb

Hey Brandon,

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!

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.

Let me know if you have any more questions.


-Brando

[Bruceb]

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

[r2davis2]

Those look very nice, I'd love to see how well they work out on an actual drivetrain.

[Brandon Holley]

Originally Posted by Bruceb

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

Here is a picture of just the mold



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.

Originally Posted by r2davis2

Those look very nice, I'd love to see how well they work out on an actual drivetrain.

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.

-Brando

[Bruceb]

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

[Brandon Holley]

Originally Posted by Bruceb

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

The mold "box" base was an aluminum plate with the bolt hole pattern drilled into it. This was then used to hold the master (aluminum) wheel down to the plate. I then hot glued a bucket with the bottom cut off around the wheel.

Yes, the top side is as smooth as the urethane is poured. Some of our wheels have just the poured urethane surface on top. As we tweaked the process though, we added more glass to the urethane, which resulted in a "bumpier" surface on the top. To enhance the look of the wheels we did start pouring high and machining the excess away. One of the enhancements to the process we are trying to make is to eliminate that step, although it only takes a minute or 2 for that operation per wheel.

-Brando

[Bruceb]

What is your final chopped glass to resin ratio? Are you still using 1/2 inch pieces of glass or have you gone shorter?
Bruce

[Brandon Holley]

Originally Posted by Bruceb

What is your final chopped glass to resin ratio? Are you still using 1/2 inch pieces of glass or have you gone shorter?
Bruce

Our pieces of glass are between 3/8" and 1/2". Right now to measure the amount of fiber we place in the mold, we place the chopped fibers in a cup and gently tap the cup on a table to flatten the fibers out a little bit. We use the same style cup to measure out the urethane needed for the wheel. We then use a 1:1 ratio of urethane to "tapped down" fiber by volume. Our wheels typically needed about 80 mL to fill the wheel safely, so obviously for the 80 mL of urethane, we needed around 80 mL of tapped down chopped fiber.

The amount of fiber we use is something we still want to play around with. I feel with the geometry tweaks we plan on making that we can reduce the amount of fiber necessary.

-Brando

[Bruceb]

Have you looked into buying precut fiberglass? You can get in in pretty short lengths that might mold better.
Bruce

[Brandon Holley]

Originally Posted by Bruceb

Have you looked into buying precut fiberglass? You can get in in pretty short lengths that might mold better.
Bruce

We used chopped fiber at the time because we had cloth laying around that we could use right away. Certainly playing with the size of the fiber would be another variable to explore, and is something we'll certainly be looking into.

-Brando

[KRUNCH DUDE]

Where did you purchase the urethane? It's amazing how well this could work for other applications.

[Brandon Holley]

Originally Posted by KRUNCH DUDE

Where did you purchase the urethane? It's amazing how well this could work for other applications.

www.smooth-on.com

They have a huge selection of materials to choose from. You can use their data sheets to pretty effectively choose a resin yourself, however their customer service is also super helpful, so if you are not sure, feel free to ask them.

The different resin sizes come in pretty handy as well, where trial sizes are cheap and get you a couple parts, and larger sizes can help get you through a large run.


-Brando

[KRUNCH DUDE]

Originally Posted by Brandon Holley

www.smooth-on.com

They have a huge selection of materials to choose from. You can use their data sheets to pretty effectively choose a resin yourself, however their customer service is also super helpful, so if you are not sure, feel free to ask them.

The different resin sizes come in pretty handy as well, where trial sizes are cheap and get you a couple parts, and larger sizes can help get you through a large run.


-Brando

Do you remember what kind you used?

[Brandon Holley]

Originally Posted by KRUNCH DUDE

Do you remember what kind you used?

We used Task 9 for the wheels, however that may not be the best choice for whatever application you are considering. I highly recommend doing a little research for yourself and coming to your own conclusions so you know for sure that you are using the best resin for your needs (and you'll get to learn a bit too about mechanics of materials and material science!!)

-Brando