Workholding · Released

Datum Planning Worksheet

Plan locating surfaces and inspection datums before setup. Good datum strategy eliminates stacking errors and ensures the part is inspected the same way it's machined.

Setup checklist

Primary datum (3-2-1): Which surface does the part rest on? Is it flat, clean, and accessible? Secondary datum (2-2-1): Which edge locates X position? Is it a finished surface? Tertiary datum: Which feature locates rotation? Hole, pin, or edge? Inspection datum matches setup datum: Will CMM or comparator use the same datums as the machine? Clamping points avoid datums: Are clamps placed away from locating surfaces? All critical features accessible: Can the tool reach every feature without re-fixturing?

The 3-2-1 locating principle

3 points on the primary datum plane restrict 3 degrees of freedom (rotation about X and Y, and Z translation). The part rests flat on this surface.

2 points on the secondary datum restrict 2 more (rotation about Z and Y translation). This is typically an edge or pin.

1 point on the tertiary datum restricts the last (X translation). This locks the part in position.

Golden rule: machine the datum surfaces first, then use them to locate everything else. Never locate from a rough surface if a finished datum is available. The inspection drawing datums must match the setup datums.