GD&T Datum Reference Frames 2 — Questions and Answers
Question 1: In a datum reference frame, what is the maximum number of degrees of freedom that can be constrained?
- 3
- 4
- 6 (Correct answer)
- 9
Correct answer: 6
A complete datum reference frame constrains all 6 degrees of freedom: 3 translational and 3 rotational.
Question 2: Which datum feature simulator contact rule applies when establishing a primary datum plane from a flat surface?
- Minimum contact with 1 point
- Maximum inscribed contact
- Minimum circumscribed contact
- True contact with at least 3 points (Correct answer)
Correct answer: True contact with at least 3 points
A primary datum plane is established by physical contact with at least 3 high points on the part surface.
Question 3: A datum reference frame established by [A|B|C] uses datum A as primary. How many degrees of freedom does datum A constrain?
- 1
- 2
- 3 (Correct answer)
- 6
Correct answer: 3
A primary planar datum constrains 3 degrees of freedom: 1 translation and 2 rotations perpendicular to the datum plane.
Question 4: What is a 'basic dimension' in relation to a datum reference frame?
- A dimension with a tolerance of ±0.001
- A theoretically exact dimension used to locate a datum or feature (Correct answer)
- A dimension measured from a datum
- A dimension that requires no tolerance
Correct answer: A theoretically exact dimension used to locate a datum or feature
Basic dimensions are theoretically exact values enclosed in a box, used to define the true position of features relative to a datum reference frame.
Question 5: When a cylindrical feature is used as a secondary datum after a planar primary datum, how many additional degrees of freedom does it constrain?
- 1
- 2 (Correct answer)
- 3
- 4
Correct answer: 2
A secondary datum cylinder constrains 2 translational degrees of freedom (the two directions perpendicular to the cylinder's axis).
Question 6: What does 'datum shift' allow when a datum feature is referenced at MMC?
- The tolerance zone shifts toward the datum
- Additional tolerance as the datum feature departs from MMC (Correct answer)
- The datum feature to move out of the tolerance zone
- The feature to ignore the datum reference
Correct answer: Additional tolerance as the datum feature departs from MMC
Datum shift is bonus movement allowed between the part datum feature and the datum feature simulator when the feature departs from its MMC size.
Question 7: In GD&T, what is the term for the perfect geometric counterpart of a datum feature used during measurement?
- Datum axis
- Datum feature simulator (Correct answer)
- Datum reference plane
- Datum target point
Correct answer: Datum feature simulator
A datum feature simulator is the true geometric counterpart (gauge pin, surface plate, etc.) that makes contact with the datum feature to establish the datum.
GD&T Datum Reference Frames 2
In a datum reference frame, what is the maximum number of degrees of freedom that can be constrained?