In CNC milling machines, the movement of the cutting tool is controlled by three axes: the X, Y, and Z axes. These three axes define three planes of motion: the X/Y plane, the X/Z plane, and the Y/Z plane. The operator can choose which plane the machine will work in and set the work coordinate system (WCS) accordingly. This is often done using probing, an edge finder, or dial indicators.
However, as the number of axes on a milling machine increases, the complexity of the machining process also increases. For example, a 6-axis CNC machine has additional rotation movements that must be taken into account. It's important for the machine to know which plane it will be working in and for the operator to set the WCS correctly, as this information is used to calculate offsets and determine where the cutting tool should move. Once the machine is set up and the WCS is dialed in, the machining process can begin. This process of setting the WCS is commonly referred to as "zeroing" the machine.