GD&T Datum Systems & Reference Frames 5 — Questions and Answers
Question 1: What is a 'datum reference frame' (DRF) as defined in GD&T?
- The physical fixture used to hold the part during inspection
- Three mutually perpendicular planes that define the origin for dimensional measurements (Correct answer)
- The set of all datum features on a drawing
- A computer model used to simulate part assembly
Correct 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.
Question 2: In the context of GD&T datum systems, what is 'true geometric counterpart'?
- The actual measured part geometry
- The theoretically perfect form of the datum feature used to establish the datum (Correct answer)
- The geometric tolerance applied to the datum feature
- A duplicate copy of the datum feature
Correct 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.
Question 3: Which datum modifier requires the datum feature simulator to expand or contract to make maximum contact with the datum feature?
- RFS (Regardless of Feature Size) (Correct answer)
- MMC (Maximum Material Condition)
- LMC (Least Material Condition)
- BSC (Basic)
Correct 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.
Question 4: A secondary datum axis established from an internal cylinder is defined by:
- The largest inscribed cylinder axis (Correct answer)
- The surface of the cylinder bore directly
- The smallest circumscribed cylinder axis
- The midpoint between maximum and minimum diameters
Correct 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.
Question 5: When datum targets are used to establish a primary datum plane, the minimum number of target points required is:
- One
- Two
- Three (Correct answer)
- Four
Correct 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.
Question 6: What effect does applying the LMC modifier (L) to a datum feature reference have?
- The datum feature must be at its smallest allowable size for any measurement
- Datum shift is permitted as the datum feature departs from least material condition (Correct answer)
- The datum feature tolerance is reduced to zero
- The datum is established only from external features
Correct 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.
Question 7: In GD&T, 'simultaneous requirements' for datum features means:
- All datum features must be inspected at the same time on a CMM
- Multiple geometric tolerances sharing the same DRF are verified as a single pattern (Correct answer)
- The datum features must have identical tolerance values
- Primary and secondary datums are interchangeable
Correct 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.
GD&T Datum Systems & Reference Frames 5
What is a 'datum reference frame' (DRF) as defined in GD&T?