Datum

A datum is a reference feature, such as a point, line, axis, or plane, that serves as the geometric foundation for all part dimensions and tolerances on an engineering drawing, establishing the coordinate framework for machining operations. In GD&T, datums are typically the first features machined (primary datum), followed by secondary and tertiary datums that together establish a three-dimensional reference system analogous to XYZ machine coordinates.

Common primary datums include a flat reference surface (face), a cylindrical hole or journal, or the outside diameter of a shaft, each selected to represent how the part will be positioned in its actual assembly. Understanding datums is critical for programmers and setup technicians because CNC machining work offsets and tool paths must be oriented to these same datums to ensure parts are made correctly relative to their design intent.

Many precision machining shops establish datums early in the operation sequence with face milling or boring to create stable reference surfaces.