GD&T Datum Systems & Reference Frames 3 — Questions and Answers
Question 1: Datum targets are used primarily when:
- The part has perfectly flat surfaces
- The datum feature surface is irregular, large, or warped (Correct answer)
- The part is made of plastic only
- Multiple parts are stacked together
Correct answer: The datum feature surface is irregular, large, or warped
Datum targets (points, lines, or areas) are used when full-surface contact is impractical due to surface irregularities, large surfaces, or casting/forging conditions.
Question 2: A datum target point is represented on a drawing by:
- A circle divided horizontally with an X (Correct answer)
- A solid filled triangle
- A square with a center dot
- A dashed circle with a cross
Correct answer: A circle divided horizontally with an X
A datum target point is shown as a circle divided by a horizontal line, with an X in the lower half and the datum target identification in the upper half.
Question 3: What does it mean when a datum reference is labeled with [CF] (Continuous Feature)?
- The datum applies only to continuous machined surfaces
- Two or more interrupted features are treated as one datum feature (Correct answer)
- The datum tolerance is continuous across all zones
- The datum must be verified with a CMM
Correct answer: Two or more interrupted features are treated as one datum feature
The [CF] modifier indicates that two or more separated (interrupted) features are treated as a single, continuous datum feature for establishing the datum.
Question 4: In a datum reference frame, the tertiary datum plane is established by a minimum of how many points of contact?
- One (Correct answer)
- Two
- Three
- Four
Correct answer: One
The tertiary datum plane requires a minimum of one point of contact because it only needs to constrain one remaining translational degree of freedom.
Question 5: Which statement best describes a 'customized datum reference frame'?
- A DRF where datum features are measured in any order
- A DRF where specific degrees of freedom are selectively constrained by each datum (Correct answer)
- A DRF used only for custom-manufactured parts
- A DRF established without physical contact
Correct answer: A DRF where specific degrees of freedom are selectively constrained by each datum
A customized DRF (per ASME Y14.5-2018) allows specific translational and rotational degrees of freedom to be selectively assigned to each datum reference using bracketed letter codes.
Question 6: When [BSC] (Basic) appears after a datum reference in a feature control frame, it means:
- The datum feature has basic dimensions only
- The datum feature simulator is at its basic (true) size (Correct answer)
- The datum is established from a basic plane only
- The datum applies only in the basic orientation
Correct answer: The datum feature simulator is at its basic (true) size
The [BSC] modifier indicates the datum feature simulator is held at the basic (nominal) size rather than adjusting to the actual mating size of the feature.
Question 7: A primary datum feature that is an external cylinder contacts its simulator at:
- Three equally spaced points on one end
- The full length of the cylindrical surface
- The maximum inscribed cylinder (inner surface)
- The minimum circumscribed cylinder (outer surface) (Correct answer)
Correct answer: The minimum circumscribed cylinder (outer surface)
An external cylindrical datum feature is simulated by the minimum circumscribed cylinder that perfectly contacts the high points of the external surface.
GD&T Datum Systems & Reference Frames 3
Datum targets are used primarily when: