GD&T Datum Systems & Reference Frames 4 — Questions and Answers
Question 1: What is the primary purpose of applying the MMC modifier (M) to a datum feature reference?
- To tighten the datum feature tolerance
- To allow datum shift when the datum feature departs from MMC (Correct answer)
- To eliminate the need for a tertiary datum
- To apply regardless of feature size
Correct answer: To allow datum shift when the datum feature departs from MMC
When MMC is applied to a datum feature reference, bonus datum shift is permitted as the datum feature departs from its maximum material condition.
Question 2: Datum shift is allowed when which modifier is applied to a datum feature of size?
- Regardless of Feature Size (RFS)
- Least Material Condition (LMC)
- Maximum Material Condition (MMC)
- Both MMC and LMC, but not RFS (Correct answer)
Correct answer: Both MMC and LMC, but not RFS
Datum shift is available when either MMC or LMC is applied to a datum feature of size reference; RFS requires the datum to be derived without shift.
Question 3: Which of the following is NOT a type of datum target?
- Datum target point
- Datum target line
- Datum target area
- Datum target volume (Correct answer)
Correct answer: Datum target volume
Datum targets are defined as points, lines, or areas; there is no 'datum target volume' in GD&T per ASME Y14.5.
Question 4: In a datum reference frame, what does the term 'degrees of freedom' refer to?
- The number of tolerances on a drawing
- The independent modes of translation and rotation of a part in space (Correct answer)
- The number of datum features required
- The range of acceptable part sizes
Correct answer: The independent modes of translation and rotation of a part in space
Degrees of freedom are the six independent ways a rigid body can move in 3D space: three translations (X, Y, Z) and three rotations (u, v, w).
Question 5: A movable datum target is identified on a drawing by:
- A datum target symbol with a bold border
- A datum target symbol with a triangle below it
- Two parallel horizontal lines below the datum target symbol (Correct answer)
- A datum target symbol enclosed in a rectangle
Correct answer: Two parallel horizontal lines below the datum target symbol
A movable datum target is indicated by two parallel horizontal lines (a slide symbol) beneath the standard datum target symbol to indicate it can be repositioned.
Question 6: Per ASME Y14.5, which letter is typically NOT used as a datum reference letter to avoid confusion with numbers?
- Q, X, Z
- I, O, Q (Correct answer)
- A, B, C
- X, Y, Z
Correct answer: I, O, Q
The letters I, O, and Q are avoided as datum reference letters because they can be confused with the numbers 1, 0, and 9 respectively.
Question 7: When a flat surface is designated as the primary datum, what condition must that surface satisfy for proper datum establishment?
- It must be the largest surface on the part
- It must be within its specified form tolerance (Correct answer)
- It must be perpendicular to the secondary datum
- It must be parallel to the part's centerline
Correct answer: It must be within its specified form tolerance
The flat surface used as a primary datum must be within its specified flatness (or other applicable form) tolerance to properly contact and define the datum plane.
GD&T Datum Systems & Reference Frames 4
What is the primary purpose of applying the MMC modifier (M) to a datum feature reference?