Some people bake during the pandemic quarantine. Some people CAD swerve modules . Following on to the design I posted before kickoff, I wanted to see if I could come up with a design that:
- Uses a 4" colson wheel (because I wanted something that would last all season without maintenance)
- Is compatible with a COTS (in this case the WCP) module as much as possible
- Can be manufactured through relatively accessible* means.
- Total cost per module about $300 / module (about the same as a tank drive gearbox, and significantly less than than other COTS options)
I didn’t get all of the way there, but I got close. The design below has is about $325, including wheel, motors and sensor, if you don’t charge for in-house made parts (big if). That’s still a fair bit cheaper than a WCP or SDS module, so I’m calling it a win :).
The custom parts include:
- a bottom plate, routered from aluminum
- an “azimuth” plate, routered from aluminum
- A top plate laser cut from delrin (or routered from aluminum if that’s easier
- a number of 3D printed parts, including the wheel forks. My hope is that these pieces can be PLA so that they can be made by any team with a 3D printer. To that end, the forks are reinforced with aluminum inserts.
- a vertical drive shaft cut from 3/8" thunderhex. This is probably the hardest part to make if your team doesn’t have a lathe (and a 3/8 hex broach), but I tried and failed to think of a simpler design.
- the tube that serves as a hub for the colson wheel
The Commercial (COTS) parts are listed in the BOM spreadsheet below.
Overall the module is symmetrical, 6" wide x 7" long x 8" tall, can be mounted above or below the frame rails, and it weighs 4.8 lb as shown according to Solidworks. The drive gear ratio can be changed from anywhere between ~12 ft/s free speed (8.4:1) through ~23 ft/s (4.5:1) in roughly 10% increments by changing the motor pinion. Similarly the steering ratio can be changed from 15:1 through 125:1 depending on what stages you choose for the Ultraplanetary.
I’m sure there are lots of ways to improve the size, weight, and cost of this design, but I think it’s a pretty good balance for now. The concept of using a regular old 3D printer to make the forks (as reinforced with aluminum spacers) also needs to be confirmed through testing. I welcome any feedback or suggestions. It looks like we’ve got a few more weeks to tinker with the design before we’d be allowed to build anything.
*“accessible”, in this case, means that all of the parts can be made in house at the HWCDSB shop, which is pretty well equipped. But ideally, I think this design could be modified so that the plates could be outsourced to a waterjet or laser-cutting sponsor which would mean lots more teams could build it.