What is Datum
A datum refers to a defined point, line, or plane that serves as a reference for locating the pattern of layers during the manufacturing or inspection processes. It is an essential concept in geometric dimensioning and tolerancing (GD&T) that establishes a framework for measuring and controlling the dimensions and tolerances of a part. It is selected based on the functional requirements of the part and can be either a functional datum or a manufacturing datum. Functional datums are chosen based on how the part interacts with other parts and are not necessarily used in the manufacturing process. On the other hand, manufacturing datums may differ from functional datums to optimize cost, process speed, and repeatability.
A datum reference frame is typically a 3D coordinate system composed of three planes, such as a face side and two datum edges. These planes are labeled A, B, and C, and their selection as primary, secondary, and tertiary datums is based on the degrees of freedom that need to be controlled. For example, Datum A may control translation along the Z axis and rotation about the X and Y axes, while Datum B may control translation along the Y axis and rotation about the Z axis. By establishing a datum reference frame, engineers can accurately measure and control the dimensions and tolerances of a part, ensuring its proper functionality and compatibility with other components. Tolerance analysis may be required to convert between functional datums and manufacturing datums, and specialized software can assist in dimensional analysis.