GD&T Position Tolerancing Calculations 5 — Questions and Answers
Question 1: A CMM report shows a hole axis deviated 0.18 mm in X and 0.24 mm in Y from true position. The diametral position error is:
- ⌀0.42
- ⌀0.60 (Correct answer)
- ⌀0.30
- ⌀0.50
Correct answer: ⌀0.60
Radial = √(0.18² + 0.24²) = √(0.0324 + 0.0576) = √0.09 = 0.30; diametral = 2 × 0.30 = ⌀0.60.
Question 2: Which of the following best defines 'resultant condition' for an external feature (pin) at MMC with position tolerance?
- MMC + position tolerance
- LMC − position tolerance
- LMC + position tolerance (Correct answer)
- MMC − position tolerance
Correct answer: LMC + position tolerance
Resultant condition for an external feature = LMC + position tolerance, representing the worst-case outer boundary at the smallest actual size.
Question 3: When calculating position for a slotted hole (elongated feature), GD&T allows specifying two position tolerance values. What do the two values control separately?
- Orientation and size
- Tolerance in the length direction and tolerance in the width direction (Correct answer)
- Runout and straightness
- Cylindricity and flatness
Correct answer: Tolerance in the length direction and tolerance in the width direction
Two values allow a larger position tolerance along the slot's length direction and a tighter tolerance across its width, reflecting functional assembly requirements.
Question 4: A part has four tapped holes (fixed fastener situation) to mate with a cover plate. The clearance holes in the cover are ⌀6.5 mm (MMC) and the screws are ⌀6.0 mm (MMC). What position tolerance should each part receive?
- ⌀0.5 each
- ⌀0.25 each (Correct answer)
- ⌀1.0 each
- ⌀0.5 for the tapped part and ⌀0.0 for the cover
Correct answer: ⌀0.25 each
Floating fastener: T_each = (6.5 − 6.0) / 2 = 0.25 mm; each part gets ⌀0.25 position tolerance.
Question 5: A tolerance analysis requires that the total positional contribution of a feature be within ⌀0.8. If the feature is at actual size 20.3 mm with MMC = 20.0 mm and stated position of ⌀0.3 (M), does the feature pass?
- No — the actual position error exceeds ⌀0.3
- Yes — total allowable is ⌀0.6, which is within ⌀0.8
- Yes — total allowable is ⌀0.6, within ⌀0.8 requirement (Correct answer)
- Cannot be determined without CMM data
Correct answer: Yes — total allowable is ⌀0.6, within ⌀0.8 requirement
Total allowable = ⌀0.3 + (20.3 − 20.0) = ⌀0.6; since ⌀0.6 ≤ ⌀0.8, the feature passes the stack requirement.
Question 6: The 'simultaneous requirement' rule in GD&T means that position callouts sharing the same datums in the same order:
- May be inspected independently at separate setups
- Must be verified simultaneously as a single pattern (Correct answer)
- Apply only to internal features
- Require a separate composite frame
Correct answer: Must be verified simultaneously as a single pattern
Under the simultaneous requirement, feature control frames with identical datum references in the same order are treated as one pattern and must be verified in a single gauge setup.
Question 7: A true position tolerance of ⌀0.2 is specified for a hole. The CMM calculates a diametral position error of ⌀0.19. The hole is at LMC. Which conclusion is correct?
- The hole fails because it is at LMC
- The hole passes because ⌀0.19 ≤ ⌀0.2 regardless of size (RFS) (Correct answer)
- The hole automatically passes due to bonus tolerance
- The hole fails unless a projected zone is specified
Correct answer: The hole passes because ⌀0.19 ≤ ⌀0.2 regardless of size (RFS)
With no modifier shown (RFS default), the tolerance is ⌀0.2 at all sizes; since ⌀0.19 ≤ ⌀0.2, the hole passes.
GD&T Position Tolerancing Calculations 5
A CMM report shows a hole axis deviated 0.18 mm in X and 0.24 mm in Y from true position.
The diametral position error is: