Datum Systems & 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 Systems & Reference Frames flashcards as text
What is a 'datum reference frame' (DRF) as defined in GD&T?
Answer: Three mutually perpendicular planes that define the origin for dimensional measurements
A datum reference frame consists of three mutually perpendicular planes forming a 3D coordinate system from which the location and orientation of features are measured.
In the context of GD&T datum systems, what is 'true geometric counterpart'?
Answer: The theoretically perfect form of the datum feature used to establish the datum
The true geometric counterpart (or datum feature simulator) is the theoretically perfect boundary or surface used to establish a simulated datum from the actual imperfect part feature.
Which datum modifier requires the datum feature simulator to expand or contract to make maximum contact with the datum feature?
Answer: RFS (Regardless of Feature Size)
Under RFS (regardless of feature size), the datum feature simulator adjusts to make maximum contact with the actual datum feature surface, with no datum shift allowed.
A secondary datum axis established from an internal cylinder is defined by:
Answer: The largest inscribed cylinder axis
An internal cylindrical datum feature at RFS is simulated by the largest inscribed cylinder (maximum inscribed cylinder) that fits within the bore, defining the datum axis.
When datum targets are used to establish a primary datum plane, the minimum number of target points required is:
Answer: Three
A minimum of three datum target points are needed to define a primary datum plane, just as three points define a unique plane in space.
What effect does applying the LMC modifier (L) to a datum feature reference have?
Answer: Datum shift is permitted as the datum feature departs from least material condition
When LMC is applied to a datum feature reference, bonus datum shift is permitted as the datum feature departs from its least material condition toward MMC.
In GD&T, 'simultaneous requirements' for datum features means:
Answer: Multiple geometric tolerances sharing the same DRF are verified as a single pattern
Simultaneous requirements means that multiple features or patterns referencing the same datum reference frame in the same order are treated as a single pattern during verification.