Sunday, April 21, 2013

First All-Nighter at MITERS: Casting the Urethane Wheel

I've been procrastinating the molding of the orbit wheel in favor of machining for too long now. I need at least 24 hours for the mold to set. Starting three days before TechFair.

I poured a test mold before pouring the real mold to ensure the urethane had a good hardness and would stick to the metal. I was kind of worried about how well the urethane would grip to the small water jetted rim pieces. The test mold will also give me some experience when it comes time to do the final mold.

The test mold was a success. There are some visible air bubbles trapped inside, but it still has a good durability/hardness (shore). It also sticks to the metal pieces well, so I won't have a problem with it gripping on the wheel rims. 

Now for the real mold. 


I did some calculations and got a volume number, but there weren't any graduated cylinders at MITERS to measure out the correct volume. So, I ran across the street to a gas station and bought a seven ounce fruit cup which Kramnik gladly ate. Once I had my form of measurement, I could pour out the correct proportions of the A and B liquids.

Ready to pour. (I put tape around the sides to prevent spills just in case I over estimated with my math). 

Pouring the urethane had some specific "destructions" to ensure safety and a strong mold. 

I tried pouring the urethane through one of the rim holes to let the urethane seep in and fill the mold without creating any air bubbles. However, the pot life of Econ 80 Urethane Rubber is only 13 minutes. So I poured it around the top when it started thickening, and put it on a shaker plate. Now all it needs is time. 

Walking to East Campus in the snow after a long night's work. (It's about 0730 the day before TechFair). 


After a long 20 hours of sitting in the mold, the urethane wheel was ready to come out. 

The 80A urethane rubber is pretty durable especially after putting up with prying off the first half with flat head screw drivers. 

Removing the top half of the mold was relatively easy. I designed this feature in my CAD. However, the bottom half is a different story. I was hoping the urethane wouldn't stick to the plastic as well, but it did.

Unfortunately, I had to destroy my 3D printed mold (which Charles probably won't be too happy about). With a few handy pliers, flat screw drivers, and persistence, I managed to break apart the mold (one piece of plastic at a time). This might be preventable in the future if I get some more chemicals from Smooth-On. 

I cleaned up the edges with a belt sander. The urethane rubber was bumpy on the top side, so I just sanded it all the way down to the aluminum to get a nice finish. 

The 3D printed cast left a little 3D printed-like texture on the urethane wheel. This will probably wear off after repeated use, but it looks pretty cool.

Here are some pretty pictures of the CADed urethane wheel:

Sunshine at MITERS... What?

It fits nicely around the frame, and it has enough clearance to lean without scraping the front and back plates too much. The urethane wheel was the last part to get finished. Then it was just a race against time to assemble it all before TechFair. (Notice the sunlight shining on the orbit wheel; its early in the morning).

Connecting all the internal wires without shorting the pack was probably the most difficult part of assembling the wheel. Especially with the adrenaline and exhaustion from working all night right before the revealing.  

After getting the battery wires and the three motor wires connected internally, the rest of the assembly was just putting in screws. 

Done! Well that's what I thought anyway. I took it for a quick test run in the MITERS hallway, and made it about 10 feet before the spur gear started slipping on the motor shaft. 

I decided to take it to TechFair and fix it there.

I had to break it open again, but fortunately I managed to take out the inside plate assembly without disconnecting all the internal wires.

I took a small file to the motor shaft and made the flat better. I also added some of DGonz's red loctite to the set screw to make sure it wouldn't come loose.

This temporary fix worked well for the duration of TechFair. I was able to ride them around all day on one charge.

TechFair was pretty fun. We had multiple electric vehicle parades (MITERS and MIT's Electric Vehicle Team). TechFair marked the end of the building process for my first Electric Orbit Wheel prototype. I've got plans for another. 

1 comment: