Mechanics

    The 3d Printer is essentially a system of screws and belts, all run precisely with stepper motors. The screws and belts control the x, y, and z axis of the printer.The x and y axis are most often controlled by stepper motors attached to a belt system, and the z axis is controlled by a screw.
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   Stepper motors are brushless DC motors that are characterized by small, precise, radial movement. This component is the heart of the 3d printer for it allows very controlled movement on each axis, making the resolution of the physical object very high.

http://www.seeedstudio.com/wish/wp-content/uploads/2011/09/nm17b12s_w400.jpg

Just as the belts and screw systems are controlled by the stepper motors, a processor controls the stepper motors. When the computer breaks down the digital object into slices, it is actually assigning a value for the x and y motors for each point on the now sliced object. The z coordinate corresponds with each layer or slice. Once the x and y motors complete each coordinate, the z motor moves to the next plane and the process begins again. The layers can be clearly seen in the printed whistle to the right.               http://www.cnccookbook.com/img/OthersProjects/3DPrinting/RepRapWhistle.jpg