This is an altered version of the freeware gearbox model available from the "ShapwaysMechanical" page. My version uses a ribbed housing in order to save material (and make the model more complex looking).
No assembly is required. Please note the model CANNOT be taken apart. After printing, simply snap the middle axle into position and the gears will become engaged. This was built into the design because the gears require a large gap during printing, but need very small gaps when actually engaged together.
This makes an excellent 3d pringing demo, but please handle with caution; the gear shapes are strong, but their axles are small in comparison, and could be damaged. If this occurs, superglue could mend the fracture.
Note: the model has small gaps which cannot be printed in certain materials. I recommend the strong-flexible or detail family. Avoid the ceramics and metal materials.