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Foundation 09

Datum Feature Modifiers & Datum Shift

How material-boundary modifiers on datum references can allow functional movement in a datum setup.

A material-boundary modifier after a datum reference changes how that datum feature is simulated. When the drawing permits it, the actual size of a datum feature can create datum shift—controlled movement between the feature and its simulator.

Read modifiers where they appear

A modifier after the geometric tolerance value affects the controlled feature’s tolerance. A modifier after a datum reference affects the datum feature simulator and the way the datum is established. They are different instructions.

Why datum shift can be functional

If a locating pin is smaller than its maximum-material size, a mating part can have clearance around that pin. A datum-feature modifier can model that functional clearance rather than forcing the part into an artificially fixed location during inspection.

Boundary terms matter

ASME uses material-boundary concepts such as maximum material boundary, least material boundary, and regardless of material boundary for datum features. These are advanced topics; follow the exact callout and the applicable standard when making production or acceptance decisions.

Example: modified datum reference

⌖⌀0.20ABM

The circled M after datum B modifies the datum reference, not the position tolerance. It changes how datum feature B is simulated and may permit datum shift.

Common mix-up

Do not assume datum shift whenever you see a circled M. It depends on whether the modifier is attached to the controlled feature or to a datum reference.

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Foundation 09

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