Homebuilt CNC Milling Machine
CNC Mills, or CNC milling machines as they are sometimes called, are a more muscular, and usually more compact version of a CNC router.
Both of these types machines turn a sharpened, fluted cutting tool. A mill uses an “end mill”, that can have two flutes (raised, spiral cutting edges), four flutes, or even more. They have chisel-like cutters on the end of their cylindrical shape, so that they can be plunged into a piece of material, and then started to move laterally for cutting slots and other shapes. The end mills are made with rounded bottoms (called Ball End Mills) for cutting pockets with radiused edges), and are also made with slightly tapered shapes that automatically create “draft”…. or slightly tapered edges for use on molds, so that the finished parts can be removed from the mold without sticking in place.
CNC Mills are typically made from heavy, cast iron, with precision machined moveable tables to hold the workpiece in place as it passes under the milling cutter. They are most always smaller than equivalent CNC routers, because they have to be much stronger to withstand the much higher cutting loads that come from cutting aluminum or steel with heavier cuts. And they are smaller because, frankly, large CNC mills are VERY expensive. A CNC router that has a 2 foot by four foot cutting area might cost $2,000 (for a homebuilt machine). A CNC mill with that same size cutting area might cost $50,000.
One nice benefit for the home hobbyist is that many of the smaller manual mills (typically made in China) can be easily retro-fitted with stepper motors and CNC control. If you don’t need to machine large parts, a small CNC mill can do beautiful, high precision machining of model engine, model railroad, or many other types of small, detailed parts that wouldn’t be possible to be done with a manual machine.