In GD&T, if a datum feature is designated as primary, one as secondary, and one as tertiary, which order is used to evaluate tolerances?

In GD&T, if a datum feature is designated as primary, one as secondary, and one as tertiary, which order is used to evaluate tolerances?

• A. Primary datum A, secondary B, tertiary C
• B. They are evaluated in random order
• C. The order is determined by the drawing creator
• D. Secondary first, then primary, then tertiary

Answer: (B)

The key idea here is that a datum reference frame (DRF) is created from the designated datums, and all geometric tolerances are evaluated with respect to that fixed frame. The primary, secondary, and tertiary designations guide how the part is located and oriented in the DRF, but they do not mandate a specific sequence for checking tolerances. Once the DRF is established, the tolerance zone is checked relative to that frame. The actual measurement order used to verify a feature against the tolerances can be chosen for practicality or fixture setup; there isn’t a required, canonical sequence. In other words, tolerances tied to multiple datums can be evaluated in any order because they’re all referenced to the same fixed DRF.

The key idea here is that a datum reference frame (DRF) is created from the designated datums, and all geometric tolerances are evaluated with respect to that fixed frame. The primary, secondary, and tertiary designations guide how the part is located and oriented in the DRF, but they do not mandate a specific sequence for checking tolerances.

Once the DRF is established, the tolerance zone is checked relative to that frame. The actual measurement order used to verify a feature against the tolerances can be chosen for practicality or fixture setup; there isn’t a required, canonical sequence. In other words, tolerances tied to multiple datums can be evaluated in any order because they’re all referenced to the same fixed DRF.