Some SumoBot Stuff

1 March 2011

About a 1-minute read

I belong to LVL1 Hackerspace in Louisville. We have a SumoBot team competing in a Hive13 competition coming up, in Cincinnati. Searching for wheels for a Mini-class bot, I happened across the 1.25” rubber-tired wheels from Solarbotics (the RW2s). I like the small diameter and grippy material, but I have their GM8 gearmotors with double-flat axles. To work both into my design, I decided I would make a 3D model for a rim that would both fit in those tires and work with the GM8-style output shaft.

After four iterations, the plastic wheel does what I want. It’s intended for printing on a MakerBot or similar plastic-enabled printer, specifically in ABS. I tried PLA and the close tolerances were a bit off, but I’m sure a bit of variable-tweaking would fix that. I printed mine in black ABS, and they fit the GM8s just right. I hope these rims are useful to other makers, and I’ve uploaded the designs to Thingiverse. See after the break for details.

The wheel had several design requirements:

  1. Fit on the GM-style double-flat axle tab
  2. Fit in the RW2 tires from Solarbotics
  3. Be printable on the MakerBot

I designed the wheel in OpenSCAD and compiled and printed it with ReplicatorG.

Measuring the tab came first. It’s 7mm in diameter and 5mm wide between flats, with a 7mm diameter of curvature on the rounded ends. I made a cylinder of the same curvature and intersected a rectangular prism of 5mm width with it.

The original wheels are cylinders with three ridges, one on each end and one in the center. The center ridge is 2mm high by 1mm wide, and the outside ones are 1mm by 1mm. The diameter of the rim without ridges is 17mm, and the wheel width is 13.2mm. A 2mm-tall center ridge doesn’t print well on the MakerBot, so I compromised and made the ridge 1.75mm.

The outboard side of the wheel has a conic inset, a shape the MakerBot can print under the rest of the wheel. The wheel has a screw hole 2mm in diameter for the GM screws, though it fits quite snugly on its own. =

A flag in the OpenSCAD file disables the tab inset so you can print freewheeling rims as well.

See the thing on Thingiverse and the originals on Solarbotics.

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