Datum

Definition

Reference feature used for inspection or machining.

Use the frontmatter definition as the controlled meaning for setup, machining, and inspection decisions.

Formula / Rule

• Lock the primary, secondary, and tertiary datum sequence before offsets or inspection decisions are accepted.
• If the datum changes, re-zero the setup and re-check any dependent dimensions.

Typical Range

• Use the same datum scheme from setup sheet to CMM report so result interpretation does not drift.
• If the drawing switches datums, treat it as a new verification condition.

Shop-floor Scenarios

• Re-establish the datum after reclamping a part before trusting previous offset values.
• Use the datum hierarchy to decide whether an out-of-position feature comes from setup or toolpath.

Common Errors / Troubleshooting

• If measurement results conflict between operators, verify that the same datum feature and probing sequence were used.
• If offsets are repeatedly corrected in the wrong direction, confirm that the setup zero still matches the intended datum.

Related Tools

• Gdt Reference
• True Position Calculator