What is the role of a datum frame in GD&T?

• A. It defines orientation and location constraints by specifying the order of datums used to evaluate the tolerance.
• B. It limits the number of datums that can be used in a frame.
• C. It sets the maximum material condition for the feature.
• D. It determines the color coding used on technical drawings.

Answer: (A)

The datum frame provides the reference coordinate system used to evaluate tolerances. It does this by selecting and ordering three datums (primary, secondary, tertiary) to define how the part is oriented and located during inspection. The primary datum fixes the main axis and one orientation, the secondary constrains the next axis perpendicular to the first, and the tertiary locks the remaining degrees of freedom. When a tolerance is applied in a feature control frame, it is evaluated relative to this defined frame, ensuring the part’s features meet the specified orientation and location with respect to the datums. The frame itself isn’t about material limits or color coding; its role is to establish the consistent reference for measuring and interpreting GD&T tolerances.