Fourteen-year-old Lego hacker Leon Overweel built this impressive Lego Mindstorms NXT printer, AKA PriNXT.
PriNXT has three motors: two control the X and Y motion, and the third lifts the pen.
The first motor, the one in the middle, is very straight forward – it has two long axes coming out of it that connect to two tiny gears that can drive around on a series of gear racks, moving the contraption about the Y axis. The second and third motor are a bit more complex, since they both point inward and have to move their power around a lot.
The left one, which controls motion on the X axis, basically just gears its power to a really long row of worm wheels, which, when spun, slowly move the part that holds the pen from left to right. The third motor similarly transfers its power to the outside, after which it moves a long rod up and down, pulling the pen along with it no matter where it is.