These are sprockets that work with Pololu's rubber tank tracks that are designed for hobby robots. They work perfectly with the Zumo robot as shown in the picture. This is not a clone of the official Pololu product, but a variation that is designed to be very easily 3D printable. There is no need for a support structure during print. Use a 0.4mm nozzle and 0.2mm layer height. The axle holes have a tolerance adjusted for my printer, but the model can be edited to adjust the tolerances or to even fit other sized axles. The fully editable 3D model is hosted at https://cad.onshape.com/documents/73b633234ac34c7b359045c5/w/c3a45c5086297a1687aaf947/e/a170847f715e2e19002cf291 , go ahead and change measurements, add teeth, etc.