GD&T Symbols and Rules 5 — Questions and Answers
Question 1: Which of the following best describes the 'unilateral' profile tolerance symbol introduced in ASME Y14.5-2009?
- A 'U' in a circle with a value indicating how much of the zone shifts outside the true profile (Correct answer)
- A single arrow pointing inward on the leader line
- The letter 'U' placed after the tolerance value without a circle
- A dashed outline on one side of the profile
Correct answer: A 'U' in a circle with a value indicating how much of the zone shifts outside the true profile
The unequal (unilateral) profile modifier uses a circled 'U' followed by the amount of the zone that falls outside the true profile, shifting the bilateral default.
Question 2: What is the key difference between concentricity and coaxiality in modern GD&T practice (ASME Y14.5-2018)?
- Concentricity controls derived median points; coaxiality controls the axis using MMC/LMC modifiers (Correct answer)
- Concentricity is used for cylindrical features only; coaxiality for any shape
- Concentricity requires two datums; coaxiality requires one
- They are identical — one term replaced the other
Correct answer: Concentricity controls derived median points; coaxiality controls the axis using MMC/LMC modifiers
Concentricity is an RFS control of derived median points (difficult to measure), while coaxiality uses the axis and allows MMC/LMC for easier functional gaging.
Question 3: A straightness tolerance applied to the surface of a cylinder (not its axis) creates a tolerance zone that is:
- Two parallel lines on the surface in the direction of measurement (Correct answer)
- A cylinder coaxial with the feature
- Two coaxial cylinders
- A 3D envelope around the entire feature
Correct answer: Two parallel lines on the surface in the direction of measurement
Surface straightness controls each line element of the cylinder surface within two parallel planes (lines in 2D), independent for each longitudinal line element.
Question 4: Which GD&T symbol is used to indicate that a dimension is for reference only and does not govern production or inspection?
- Parentheses around the dimension value (Correct answer)
- A rectangular box around the dimension
- The letters 'REF' underlined
- An asterisk next to the value
Correct answer: Parentheses around the dimension value
Reference dimensions are enclosed in parentheses to indicate they are informational only and are not to be used for manufacturing or inspection purposes.
Question 5: The 'statistical tolerance' symbol (ST) in a feature control frame indicates that the tolerance:
- Is valid only when statistical process controls are in use during manufacturing (Correct answer)
- Is calculated from a statistical distribution of measurements
- Applies to a sample of parts, not every part
- Is a bonus tolerance granted automatically
Correct answer: Is valid only when statistical process controls are in use during manufacturing
The ST symbol means the tolerance is valid only if the manufacturing process is controlled using statistical methods (SPC), allowing slightly larger individual tolerances.
Question 6: In a GD&T position callout for a pattern of holes, the 'simultaneous requirement' means:
- All features in the pattern are located relative to the same datum reference frame in one setup (Correct answer)
- Each hole is inspected separately in its own setup
- The position tolerance applies simultaneously to both MMC and LMC
- All holes must be drilled at the same time during manufacturing
Correct answer: All features in the pattern are located relative to the same datum reference frame in one setup
The simultaneous requirement means that all features sharing the same datum reference frame are treated as a single pattern and inspected in one setup.
Question 7: Which of the following is the correct GD&T symbol for 'total runout'?
- Two arrows pointing in opposite directions along a surface line (Correct answer)
- A single curved arrow
- A circle with a cross inside
- Two concentric circles
Correct answer: Two arrows pointing in opposite directions along a surface line
Total runout is represented by two arrows pointing in opposite directions (a double-headed arrow), distinguishing it from circular runout's single-headed arrow.
GD&T Symbols and Rules 5
Which of the following best describes the 'unilateral' profile tolerance symbol introduced in ASME Y14.5-2009?