How to Choose Datums in GD&T

Good datum selection makes a drawing easier to manufacture, inspect and assemble because it gives everyone the same coordinate system.

Think about how the part is constrained

Ask how the real part seats in the assembly. A broad mounting face often makes sense as a primary datum because it establishes orientation; a pin, edge or hole pattern may then locate the remaining degrees of freedom.

Primary datum

The primary datum normally provides the most stable first contact and removes three degrees of freedom in the idealized 3-2-1 concept. Avoid choosing a small unstable surface merely because it is convenient to dimension from in CAD.

Secondary and tertiary datums

The secondary datum removes two additional degrees of freedom and the tertiary removes the last one. The exact surfaces or features should match functional interfaces rather than arbitrary model origins.

Datum features can be holes and patterns

A cylindrical hole, boss, slot or pattern can establish a datum. Feature datums are especially useful where the part is located by pins or fasteners rather than by an exterior edge.

Manufacturing and inspection access

A theoretically functional datum that cannot be reliably fixtured or measured may cause practical problems. Discuss difficult datum schemes with manufacturing and quality teams early.

Common mistake

Do not choose datums solely so that most CAD dimensions look positive or simple. The drawing is a functional contract. The model coordinate system can be convenient without becoming the inspection datum system.