Milling machines are machine tools that are used for a purpose of shaping solid materials such as metal. Milling machines are designed in a couple of different basic forms, one of which is vertical and the other of which is horizontal. These forms refer to the orientation the cutting tools spindle is in. Unlike a drill press, where the work piece is meant to be held in a stationary manner and the drill is the part that is moved vertically in order to penetrate the work piece material, when it comes to milling, the work piece is also moved against the cutter, which is rotating. Here, the cutter is able to create cuts not only on the flanks but also the tip as well. CNC machining centers are the automated, computerized versions of manual milling machines.
In a CNC milling machine, the movements of the work piece and the cutter are controlled precisely to a precision of less than .025 millimeters or 0.001″, typically by means of servo motors and lead screws moving tables and heads on precision ground or linear guide ways. Servo motors provide the “muscle” to push the rotating cutter through the metal in whatever direction the computer control instructs. The instructions are the program the machine is running and that is determined by the part being made.
CNC machining centers and other types of milling machines are capable of performing a wide variety of different operations. Some of these operations can be very complex in nature, such as keyway cutting, slot cutting, facing, die sinking, drilling, reaming and routing and the list goes on. Cutting fluid is typically pumped onto the cutting site as a means of cooling and lubricating the cut, and this also helps to clear away any resulting swarf in the process and extends the life of the cutting tools.
Most types of CNC milling machines or CNC milling centers are vertical mills that are computer controlled. They come in 2 axis or 3 axis varieties, where the third axis is the ability to move their spindle vertically along a Z axis. This extra degree of machining freedom (the 3rd axis) makes it possible for these machines to be used in drilling and boring applications, die-sinking and to produce 3D surfaces as well, such as in the creation of relief sculptures or molds.
Most machining centers differ from simple milling machines in other ways as well. A typical machining center has the ability to change tools using an automatic tool changer and tool carrousel. This adds a lot more productivity to the machine by not requiring an operator stand by to change tools. Other features often found on a machining center include chip conveyors to automatically remove the debris from cutting, programmable speeds, programmable coolant, etc.
CNC machining centers can come in 4 axis and 5 axis varieties as well. These machines can rotate a part and tilt it (in the case of 5 axis) so highly complex parts can be made and multiple setups can be eliminated. A part like a pump impeller can be machined in one setup. Many aerospace parts require 5 axis machining.