3D Printed Wheel + Axle, Parametric

Here' a basic, parametric definition for toy wheels, created in Rhino + Grasshopper.

The "axle" has a square shaft for assembly into whatever vehicle you dream up. The part is generated with a user-defined gap between the hub and wheel (0.4mm worked well for me). You can also include a through-hole to use a nail or screw for a more durable toy. In the included photo, I've used a small nail to lock in the 3D printed wheels. The nail does two things: (1) reinforces the plastic where the hub narrows to the plastic axle and (2) expands the axle section for a tighter fit into the hole. You could use glue, but I found that the nail does a great job of locking the wheels on. The reinforcing hardware allows you to use a much smaller axle size (that can stand up to enthusiastic toddlers).

I used these wheels on my Modern Diesel Train. After assembling onto the train and hammering in the nails, I had to work the wheels back and forth to snap the 1st layer skin that occasionally binds the wheel in place. After breaking this loose, the wheels spins smoothly, with some resistance. One note: you may need to slightly oversize the central through-hole, in case your printer/slicer isn't completely accurate on ID dimensions.