CNC Mills run on G-Code. This code is made up of commands that tell the mill how to cut your part, performing actions like turning on and off, rapid positioning moves, controlled feed moves in straight lines and arcs, selecting tools, turning coolant on and off, and setting spindle speeds.

In G-Code, "type codes" are comprised of capital letters followed by two numbers. These tell the mill what type of information you are giving it. You follow the type code with more characters, specific to the type code, to tell the mill what to do. For example, the code N01 M216 would be used to turn the load motor on and the code N04 T03 selects the third tool from the carousel.

While G-Code can be directly programmed into a text file and read by the mill, it is more common to use Computer-Aided Manufacturing (CAM) programs, like Pro/Engineer, to write G-Code over a much easier to use user-interface.

In order to program the CNC mill with Pro/Engineer, you need to create your part in ProEngineer. (Note: .sat and other types of files cannot be directly used in Pro/Engineer.) You also need to create a "blank," which is a model of the piece of material you will be cutting your part out of. After attaching your part to the blank, and importing a file containing information about the type of machine you will be using (in .gph format) you can define your toolpaths and the process the mill should use in cutting the part. Commonly, both rough and finishing cuts are created for added accuracy and smoothness on edges.