The central difference is whether the process is by addition or subtraction. 3d printers, like the reprap, add material (for example melted plastic) to make an

The central difference is whether the process is by addition or subtraction. 3d printers, like the reprap, add material (for example melted plastic) to make an object. A router on the other hand removes material from an object until it is the required shape. As a result you can use many materials (like wood) that cannot be easily broken up into smaller parts and reassembled.