Hanry Segerman showed off what you can do with 3D printers and a lot of imagination. The three gears work in pairs and are able to move relative to each other because the movement axis are skewed. There are several contact surfaces and the calculations need to be precise in order to leave enough rolling place.
The world of 3D printing offers many great surprises. See this video on how they created a working wrench that can be used in our workshops. Creating the models layer by layer allows the 3D printer to make precise pieces and take into account the functional relationships between pieces (like if piece A moves somehow relative to piece B). Awesome does not describe what these guys are doing in there.