Which statement best describes the function of a datum reference frame?

• A. It defines the three mutually perpendicular planes used to locate features.
• B. It defines color coding for features.
• C. It calculates manufacturing cost.
• D. It specifies surface finish requirements.

Answer: (C)

The datum reference frame provides a fixed coordinate system for locating and measuring features on a part. It is built from three mutually perpendicular planes (datums) that establish the X, Y, and Z directions and an origin. This frame is what tolerances are applied to and what inspection measurements refer to, ensuring consistent interpretation across design, manufacturing, and quality control. By selecting a primary datum, then a secondary, and a tertiary, you define how the part is oriented and located relative to this reference frame, so every feature can be measured and controlled in a reproducible way. This concept is about establishing where and how things are positioned, not about color coding, costs, or surface finish specifications.