In engineering drawings, what is a datum used for?

In engineering drawings, what is a datum used for?

• A. It is the true center of gravity of the part
• B. It is used only for labeling parts
• C. It serves as a reference point from which distances are measured
• D. It determines surface finish requirements

Answer: (C)

Datums provide a stable reference for geometry on engineering drawings. They establish a coordinate system by selecting one primary datum feature and, if needed, secondary and tertiary features. With a datum in place, distances, angles, and tolerances are specified relative to that reference, so features can be located and inspected consistently during manufacturing. This isn’t about mass properties or simply labeling; it’s about where measurements start from and how features relate to each other in space. For example, a datum plane on a top surface and a datum axis through a central hole help control the position of other features. Surface finish is specified separately with finish symbols and roughness values, not determined by datums.

Datums provide a stable reference for geometry on engineering drawings. They establish a coordinate system by selecting one primary datum feature and, if needed, secondary and tertiary features. With a datum in place, distances, angles, and tolerances are specified relative to that reference, so features can be located and inspected consistently during manufacturing. This isn’t about mass properties or simply labeling; it’s about where measurements start from and how features relate to each other in space. For example, a datum plane on a top surface and a datum axis through a central hole help control the position of other features. Surface finish is specified separately with finish symbols and roughness values, not determined by datums.