GD&T Datum Reference Frames 3 — Questions and Answers
Question 1: What is a 'datum target' in GD&T?
- A bullseye symbol indicating inspection priority
- A specified point, line, or area used to establish a datum on irregular surfaces (Correct answer)
- A tolerance zone centered on the datum axis
- A datum used only for angular measurements
Correct 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.
Question 2: Which symbol designates a datum target area in GD&T?
- A circle divided by a horizontal line with the target ID (Correct answer)
- A triangle pointing to the surface
- A square frame around the dimension
- An X inside a circle
Correct 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).
Question 3: How does a tertiary datum typically constrain degrees of freedom in a 3-2-1 locating scheme?
- 3 translational DOF
- 2 translational DOF
- 1 rotational DOF (Correct answer)
- 1 translational and 1 rotational DOF
Correct answer: 1 rotational DOF
The tertiary datum in a 3-2-1 scheme constrains the last remaining rotational degree of freedom (clocking the part).
Question 4: When referencing a datum feature of size at RFS (regardless of feature size), datum shift is:
- Equal to the feature tolerance
- Equal to the MMC bonus tolerance
- Zero — no datum shift is permitted (Correct answer)
- Calculated from the LMC boundary
Correct 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.
Question 5: A part drawing calls out datum reference frame [A|B|C] on a position tolerance. What does the order of datums indicate?
- The order datums were measured on the CMM
- The precedence (primary, secondary, tertiary) of datum constraint (Correct answer)
- The alphabetical sequence of datum labeling
- The order in which tolerances are stacked
Correct 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.
Question 6: What happens to the datum reference frame if a datum feature has form error beyond its tolerance?
- The datum is automatically rejected
- The datum feature simulator contacts the high points and the datum is still established (Correct answer)
- The part must be scrapped
- The datum shifts to compensate for the error
Correct 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.
Question 7: Which of the following is NOT a valid datum feature type in ASME Y14.5?
- Planar surface
- Cylindrical feature of size
- Width feature of size
- Theoretical perfect plane in free space (Correct answer)
Correct 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.
GD&T Datum Reference Frames 3
What is a 'datum target' in GD&T?