Datum Reference Frames Flashcards

What is a 'datum target' in GD&T?

Answer: A specified point, line, or area used to establish a datum on irregular surfaces

Datum targets are designated points, lines, or areas on a part used to establish datums on castings, forgings, or other irregular surfaces.

Which symbol designates a datum target area in GD&T?

Answer: A circle divided by a horizontal line with the target ID

A datum target symbol is a circle divided by a horizontal line; the lower half contains the datum letter and target number (e.g., A1).

How does a tertiary datum typically constrain degrees of freedom in a 3-2-1 locating scheme?

Answer: 1 rotational DOF

The tertiary datum in a 3-2-1 scheme constrains the last remaining rotational degree of freedom (clocking the part).

When referencing a datum feature of size at RFS (regardless of feature size), datum shift is:

Answer: Zero — no datum shift is permitted

At RFS, the datum feature simulator expands or contracts to make contact with the actual feature, so no datum shift is allowed.

A part drawing calls out datum reference frame [A|B|C] on a position tolerance. What does the order of datums indicate?

Answer: The precedence (primary, secondary, tertiary) of datum constraint

The left-to-right order in the feature control frame compartments defines datum precedence: first = primary, second = secondary, third = tertiary.

What happens to the datum reference frame if a datum feature has form error beyond its tolerance?

Answer: The datum feature simulator contacts the high points and the datum is still established

Datum feature simulators contact the part's high points (or largest inscribed / smallest circumscribed geometry), establishing the datum despite surface variation within tolerance.

Which of the following is NOT a valid datum feature type in ASME Y14.5?

Answer: Theoretical perfect plane in free space

Datums are established from real physical features on the part; a theoretical plane with no physical basis cannot serve as a datum feature.