A marble machine based on a third order Hilbert Curve (http://en.wikipedia.org/wiki/Hilbert_curve) It is sized to be printable on a Printrbot Simple Metal. https://youtu.be/y3rZ8ikVzTkPrint the four pieces and clean them up. (Prints well without supports.) (update: wheel gear 5 added and can replace wheel gear 4. It just has more holes for a higher marble flow rate) Drill out the screw and shaft holes to size, if needed. Thread 6-32 setscrew hole on the 12-tooth gear The motor is a 298:1 Micro Gearmotor from Pololu (item #2371) You will need a couple of screws to attach the motor mount, a screw to clamp the motor into the motor mount, and a 6-32 set screw to attach the 12-tooth gear to the motor. You might need to put a screw and washer on the end of the shaft of the large gear to keep it in place, but mine didn't need it. I used 11/32" ball bearings, 8-9.5mm should work fine.