How is a datum reference frame established?

How is a datum reference frame established?

• A. By selecting three mutually perpendicular datum features in any order.
• B. By selecting three mutually perpendicular datum features in a defined order: primary, secondary, and tertiary.
• C. By using two datum features and a gauge block.
• D. By selecting a single datum feature and rotating the part.

Answer: (B)

A datum reference frame is established by choosing three distinct, mutually perpendicular datum features and applying them in a defined order: primary, secondary, and tertiary. This creates a fixed 3D coordinate system the tolerances are referenced to. The primary datum fixes location and orientation along one plane and one axis, providing the baseline. The secondary datum, perpendicular to the primary, further constrains orientation about the primary and defines the second axis. The tertiary datum completes the frame, fixing the remaining axis so the part can translate and rotate only within the limits set by all three datums together. This defined order ensures a stable, repeatable frame for interpreting all related tolerances. Using fewer than three datums or choosing an unordered set would not fully constrain the part, and a gauge block is a measurement aid, not a datum feature, so it doesn’t establish the frame on its own.

A datum reference frame is established by choosing three distinct, mutually perpendicular datum features and applying them in a defined order: primary, secondary, and tertiary. This creates a fixed 3D coordinate system the tolerances are referenced to. The primary datum fixes location and orientation along one plane and one axis, providing the baseline. The secondary datum, perpendicular to the primary, further constrains orientation about the primary and defines the second axis. The tertiary datum completes the frame, fixing the remaining axis so the part can translate and rotate only within the limits set by all three datums together. This defined order ensures a stable, repeatable frame for interpreting all related tolerances. Using fewer than three datums or choosing an unordered set would not fully constrain the part, and a gauge block is a measurement aid, not a datum feature, so it doesn’t establish the frame on its own.