What is a datum feature?

What is a datum feature?

• A. A theoretical size used to define true position.
• B. A measurement device.
• C. A physical feature used to establish a datum (e.g., a flat surface, a bore axis).
• D. A tolerance value for flatness.

Answer: (C)

A datum feature is a physical feature of a part that provides the basis for the datum reference frame in GD&T. It is something you can touch on the part, like a machined flat surface or the axis of a drilled hole. The datum reference frame—primary, secondary, and tertiary—uses these features to define planes and axes that serve as the reference for measuring and locating other features. In practice, everything is measured and constrained relative to that frame, ensuring consistent interpretation during inspection and production. So the datum feature is the real feature used to establish the datum, not a measuring instrument, a theoretical size, or a tolerance value. The other options describe different concepts: a theoretical size relates to true position concepts, a measurement device is an instrument, and a flatness tolerance is a spec rather than a feature that defines a datum.

A datum feature is a physical feature of a part that provides the basis for the datum reference frame in GD&T. It is something you can touch on the part, like a machined flat surface or the axis of a drilled hole. The datum reference frame—primary, secondary, and tertiary—uses these features to define planes and axes that serve as the reference for measuring and locating other features. In practice, everything is measured and constrained relative to that frame, ensuring consistent interpretation during inspection and production. So the datum feature is the real feature used to establish the datum, not a measuring instrument, a theoretical size, or a tolerance value. The other options describe different concepts: a theoretical size relates to true position concepts, a measurement device is an instrument, and a flatness tolerance is a spec rather than a feature that defines a datum.