A Manufacturing Post-Processor, sometimes simply referred to as a “post”, is a software application that translates the toolpath definition from the CAM system into a language the CNC machine can read to perform the machining motions necessary to manufacture the part.
Inside the CAM system the programmer first defines a program zero on the part to produce. Then, for each tool to be used (can be milling cutting tool, turning tool, laser etc.), the programmer then defines the trajectory of the tool relative to the part. The object of this toolpath is to remove (or add) the necessary material from a stock material to ultimately end up with a finished part. One key aspect to note here is that the toolpath is defined in part space, i.e. the tool moves around the part.
After the program has been post-processed for a given machine tool (combination of machine kinematics, NC control and customer preferences) the machine tool needs to execute the program in machine space, i.e. by moving linear and rotary axis of the machine. This transformation from part space to machine space is entirely done by the post-processor for the CNC machine. If the machine is equipped with RTCP (Rotary Tool Centre Point) mode then this transformation is partially done by the post-processor and partially done by the NC controller.