Each of the previous machines is part of subtractive manufacturing. In other words, they remove material to get the desired shape. 3D printers are the relatively new kids on the block and bring with them a unique method for producing objects: additive manufacturing.
With 3D printing, the machine lays down successive
layers of material until the object is created. Each of these layers becomes a
thinly sliced cross-section of the object. As with other CNC machines,
everything begins with a 3D model, and many software tools help you build the
object.