GD&T Datum Systems & Reference Frames 2 — Questions and Answers
Question 1: In the three-plane datum reference frame, which datum plane constrains the most degrees of freedom?
- Secondary datum plane
- Tertiary datum plane
- Primary datum plane (Correct answer)
- All planes constrain equally
Correct answer: Primary datum plane
The primary datum plane contacts the part at a minimum of three points and constrains three degrees of freedom (one translation and two rotations).
Question 2: A datum feature simulator for a flat primary datum must be:
- Perfectly parallel to the secondary datum
- Nominally perfect in form (Correct answer)
- Adjustable to fit the part surface
- Oriented to the tertiary datum first
Correct answer: Nominally perfect in form
Datum feature simulators are nominally perfect counterparts of the datum feature, used to establish the datum from which measurements are taken.
Question 3: When a cylindrical feature is used as a primary datum, it constrains how many degrees of freedom?
- Two
- Three
- Four (Correct answer)
- Six
Correct answer: Four
A cylindrical primary datum constrains four degrees of freedom: two translations perpendicular to its axis and two rotations perpendicular to its axis.
Question 4: What does the order of datum references (primary, secondary, tertiary) in a feature control frame determine?
- The tolerance magnitude for each datum
- The sequence of part setup and degree-of-freedom constraint (Correct answer)
- The surface finish requirements for each datum
- The material condition modifiers applied
Correct answer: The sequence of part setup and degree-of-freedom constraint
The order of datum references establishes the hierarchy of constraint, dictating the sequence in which the part is brought into contact with datum simulators.
Question 5: A datum reference frame is established by a minimum of:
- One datum feature
- Two datum features
- Three datum features (Correct answer)
- Six datum features
Correct answer: Three datum features
A complete datum reference frame typically requires three mutually perpendicular datum planes established from at least three datum features to fully constrain six degrees of freedom.
Question 6: In GD&T, what is the term for the actual part surface used to establish a datum?
- Datum reference
- Datum feature (Correct answer)
- Datum target
- Datum plane
Correct answer: Datum feature
A datum feature is the actual feature on the part (surface, axis, or center plane) that contacts the datum feature simulator to establish the datum.
Question 7: Which symbol in a feature control frame indicates that a datum applies at maximum material boundary?
- (L)
- (M) (Correct answer)
- (F)
- (S)
Correct answer: (M)
The (M) modifier indicates Maximum Material Condition (MMC), meaning the datum applies when the datum feature contains the maximum amount of material.
GD&T Datum Systems & Reference Frames 2
In the three-plane datum reference frame, which datum plane constrains the most degrees of freedom?