Datum Reference Frames Flashcards
7 cards from real GD&T practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Datum Reference Frames flashcards as text
Two parts share the same datum reference frame for mating. Part A's position is called out as ⊕|Ø0.020|A|B|C. If datum B is referenced at MMC and Part A's datum B feature is at LMC, what is the maximum datum shift?
Answer: The difference between LMC and MMC of the datum B feature
Maximum datum shift occurs when the datum feature is at LMC; shift equals the full difference between LMC and MMC of the datum feature.
What is the 'true geometric counterpart' concept as it relates to datum feature simulators?
Answer: The perfect geometric shape (plane, cylinder, etc.) that the simulator approximates
The true geometric counterpart is the perfect geometric shape that an actual datum feature simulator physically approximates to establish the datum.
A drawing specifies a position tolerance with only one datum reference: ⊕|Ø0.015|A. What degrees of freedom remain unconstrained?
Answer: 4 DOF remain unconstrained
A single planar datum A constrains only 3 DOF (1 translation + 2 rotations), leaving 3 DOF unconstrained (2 translations + 1 rotation in the datum plane).
Which condition requires a datum feature of size to be simulated at its virtual condition boundary rather than its MMC?
Answer: When the datum feature has a straightness tolerance applied at MMC
When a form or orientation tolerance is applied to the datum feature at MMC, the datum feature simulator uses the virtual condition (MMC size ± geometric tolerance) to establish the datum.
On a CMM inspection report, the reported 'datum A residual error' is 0.003 mm. What does this indicate?
Answer: The measured surface deviates 0.003 mm from the best-fit datum plane
Datum residual error (also called datum fit error) represents how far the actual datum feature deviates from the perfect simulator plane used to establish it.
Why must datum features be chosen based on functional requirements rather than inspection convenience?
Answer: GD&T defines tolerances relative to datums, so only functional datums ensure parts assemble and perform as intended
Because tolerances are defined relative to datums, choosing non-functional datums creates a mismatch between the inspection result and actual assembly behavior.
A datum reference frame uses two coaxial cylinders called out as a compound datum [A-B]. What is the effect of this designation?
Answer: Both cylinders together establish a single datum axis with higher accuracy than either alone
A compound datum [A-B] treats two coaxial datum features as a single combined axis, improving stability and accuracy of the datum axis compared to using either feature individually.