METRIK MFG
GD&T Datums Reference
A practical reference to datum features, primary/secondary/tertiary datum references and datum reference frame concepts under ASME Y14.5.
Datum references establish the basis from which geometric requirements are oriented or located. In manufacturing, the datum strategy should connect design intent to how a part is actually located, inspected and assembled.
Datum reference frame
PRIMARY DATUM ↓ first level of constraint ↓ SECONDARY DATUM ↓ additional constraint ↓ TERTIARY DATUM ↓ complete reference frame
Manufacturing questions
| Question | Why it matters |
|---|---|
| What physical feature establishes the datum? | Connects theoretical datum to a real feature. |
| Can the fixture locate it repeatably? | Datum strategy should be manufacturable. |
| Does inspection reproduce the datum scheme? | Otherwise measured results may not reflect design intent. |
| Are modifiers involved? | Modifiers can change interpretation and allowable variation. |
ASME describes Y14.5 as the authoritative guideline for GD&T design language. The ASME site currently lists Y14.5-2018 (R2024) as the reaffirmed version. ASME Y14.5.
This page is a practical explanation, not a reproduction of the standard. The governing drawing and applicable standard edition control.
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