Once you’ve scanned in or modeled a 3D form, these are the machines that turn it into a real physical object. Milling machines and routers do this subtractively; a block of material is held tightly to the bed of the machine or in a fixture, then rotary cutting tools systematically remove everything that isn’t your part. These tools come in various sizes and types: the most important distinction is between “ball end” tools that are rounded at the tip and “flat end” tools that are not. The former are used for cutting parts that are curved; the rounded end won’t gouge the material and repeated closely-spaced passes of the tool produce a 3D part with a smooth contoured surface. Flat-end tools are used for creating flat surfaces; if these are on different levels, this is called a 2½-D part. Milling machines are generally used for machining metals; routers are usually limited to carving wood, plastics and other relatively soft materials. Aluminum, being intermediate in hardness between these two categories, can be cut with milling machines or the most rigid routers.
A CNC lathe works similarly to a milling machine, except that the workpiece is revolved while a rigidly-held tool moved by the axes of the lathe removes successive layers of material until a radially symmetrical part is produced. All these machines need toolpaths to follow, which are generated by CAM programs based on the tool being used, the material being cut and the fineness of material removal desired.
Additive rapid prototyping machines (3D Printers) work somewhat differently. These machines build up a form by applying material in layers, based on “slicing” software that analyzes your model and generates instructions for the machine to follow in reconstructing each horizontal section one layer at a time. Some do this by extruding molten plastic (FDM), others by exposing a liquid photo-reactive resin to light (SLA). There are also many other methods: binder-jetting, laser sintering, and electron beam melting are just a few modes of production used in this rapidly-developing field.