I've been printing these custom-designed gears. The hubs have a magnetic backing in them, so they effortlessly cling to any large metal surface and allow for multiple gears to mesh together. I can place the handle into either the medium or large-sized gears, and with an endless amount of colors possible, I haven't made any two sets of gears the same yet.
Love seeing these go to classrooms and STEM labs. Kiddos (and big kids!) love playing with them!