GD&T Datum Reference Frames 4 — Questions and Answers
Question 1: In the 3-2-1 locating principle, which datum provides the 3-point contact?
- Tertiary datum
- Secondary datum
- Primary datum (Correct answer)
- Constrained datum
Correct answer: Primary datum
The primary datum provides 3-point minimum contact, constraining 3 degrees of freedom (1 translation + 2 rotations).
Question 2: A shaft diameter is called out as a secondary datum at MMC (⊕). As the shaft is produced 0.010 smaller than MMC, the datum shift available is:
- 0.000
- 0.005
- 0.010 (Correct answer)
- 0.020
Correct answer: 0.010
Datum shift equals the difference between the actual mating size and MMC; if the shaft is 0.010 under MMC, shift = 0.010.
Question 3: What is the purpose of using multiple datum targets (e.g., A1, A2, A3) to establish a single datum plane?
- To increase measurement speed
- To simulate a datum plane on a non-flat or flexible surface using discrete contact points (Correct answer)
- To allow datum shift in three axes simultaneously
- To create redundant inspection checkpoints
Correct answer: To simulate a datum plane on a non-flat or flexible surface using discrete contact points
Multiple datum targets simulate a plane on irregular surfaces (like castings) by defining exact contact points rather than relying on full surface contact.
Question 4: Which ASME Y14.5 standard section governs datum reference frames and datum feature selection?
- Section 2 — Definitions
- Section 4 — Datum Reference Frames (Correct answer)
- Section 6 — Tolerances of Form
- Section 8 — Statistical Tolerancing
Correct answer: Section 4 — Datum Reference Frames
ASME Y14.5 Section 4 specifically covers datum reference frames, datum features, datum feature simulators, and related concepts.
Question 5: A feature control frame reads: ⊕ | Ø0.010 | A | B(M) | C. What does the (M) modifier after B indicate?
- Datum B is a maximum material condition datum feature reference (Correct answer)
- Datum B must be measured at its mean size
- Datum B is measured independently
- Datum B applies only in the major axis direction
Correct answer: Datum B is a maximum material condition datum feature reference
The MMC modifier (M) after a datum letter means datum shift is permitted as datum B departs from its maximum material condition.
Question 6: When is it appropriate to use a partial datum surface (datum target area) instead of an entire surface as a datum feature?
- When the surface is perfectly flat
- When cost reduction is required
- When the surface is large, warped, or only a specific area is functional (Correct answer)
- When the surface has a roughness value under Ra 0.8
Correct answer: When the surface is large, warped, or only a specific area is functional
Datum target areas restrict contact to a functional zone when the full surface is too large, irregular, or when only a specific region controls part function.
Question 7: In a customized datum reference frame (CDRF) per ASME Y14.5-2018, what is the key difference from a standard DRF?
- CDRF uses only one datum letter
- CDRF can selectively constrain specific degrees of freedom using DOF indicators (Correct answer)
- CDRF eliminates the need for basic dimensions
- CDRF applies only to non-rigid parts
Correct answer: CDRF can selectively constrain specific degrees of freedom using DOF indicators
A CDRF allows designers to explicitly specify which degrees of freedom each datum constrains using u, v, w, x, y, z notation in the feature control frame.
GD&T Datum Reference Frames 4
In the 3-2-1 locating principle, which datum provides the 3-point contact?