Each CNC machine has a built-in location that is called machine zero. This point typically is located at the farthest positive direction along the X-, Y-, and Z-axes, and it cannot be changed by anyone after it leaves the original manufacturer. A cutting tool or a worktable can be moved to the machine zero position for the loading and unloading of parts.
In addition to machine zero, each part program sets a starting location called program zero. Unlike machine zero, the programmer selects the program zero for each workpiece. This location acts as the origin from which all the other dimensions are calculated during the program and it is usually located on the edge of a workpiece. The CNC machine then adjusts its calculations to accurately align the cutting tool with the workpiece.