CNC machines machine parts by moving a tool along a predetermined or programmed path based on the work piece geometry. Defining the tool path and part geometry requires a coordinate and axis identification system. A computer numerical control (CNC) machine consists of computer instructions (software or program), the machine control unit, and the processing machine. The standard programming includes the tool path, turning the spindle on and off, the coolant on and off, tool changes, and machine feeds and speeds. The machine interface passes outputs and inputs between the control unit and the processing machine. The primary elements of a CNC machine control unit are an operator interface, machine control unit, and machine interface. Common operator interfaces are computers or keyboards with monitors.