Understanding datums per ISO 5459
Datums are one of the most important concepts in geometric dimensioning and tolerancing (GD&T). When they are poorly or ambiguously defined, manufacturing and inspection are forced to make assumptions about how the part should be located.
For instance, if a functional mounting face is not established as the primary datum, machining may reference a non-functional surface while inspection constrains the part differently. This results in unnecessary and non-functional tolerance stack-ups, inconsistent measurements, false nonconformances, or assemblies that do not fit despite ‘passing’ inspection; ultimately driving avoidable cost, rework, and extended lead times. To fill knowledge gaps and improve understanding, this Mikroniek article aims to cover the high-level basics of datums. (Image courtesy of Firat Büyükcivelek)
