CNC GD&T Symbol Interpretation 3 — Questions and Answers
Question 1: What is the difference between circular runout and total runout?
- Circular runout is measured at individual cross-sections; total runout accumulates error along the entire surface (Correct answer)
- Circular runout includes axial variation; total runout does not
- Total runout uses two datums; circular runout uses one
- Circular runout applies only to cones; total runout applies to cylinders
Correct answer: Circular runout is measured at individual cross-sections; total runout accumulates error along the entire surface
Circular runout checks each individual circular cross-section separately, while total runout sweeps the full surface and captures cumulative form and location errors.
Question 2: A part drawing shows the concentricity symbol ◎ with a tolerance of 0.004 A. What is being controlled?
- The median points of a feature must fall within a cylindrical zone coaxial to datum A (Correct answer)
- The surface of a bore must not vary more than 0.004 from datum A's diameter
- The axis of a feature must be within 0.004 of datum A measured at each cross-section
- The eccentricity of the part's center of mass relative to datum A
Correct answer: The median points of a feature must fall within a cylindrical zone coaxial to datum A
Concentricity controls the median points (derived median line) of a diametric feature to lie within a cylindrical tolerance zone coaxial with the datum axis.
Question 3: On a feature control frame, what does placing a datum reference in the third compartment (tertiary datum) accomplish?
- It constrains the remaining rotational degree of freedom not fixed by primary and secondary datums (Correct answer)
- It provides a backup datum if the primary datum is inaccessible
- It specifies a higher tolerance for that datum's orientation
- It overrides the secondary datum when both conflict
Correct answer: It constrains the remaining rotational degree of freedom not fixed by primary and secondary datums
The tertiary datum eliminates the last remaining degree of freedom (usually rotation about an axis), fully constraining the part for measurement.
Question 4: What does the free state symbol (F in a circle) indicate when applied to a GD&T callout?
- The tolerance applies to the part in its unrestrained, free condition without fixturing (Correct answer)
- The feature is exempt from all GD&T controls
- Measurement must be performed at room temperature (free of thermal variation)
- The tolerance is unrestricted and may float to any value
Correct answer: The tolerance applies to the part in its unrestrained, free condition without fixturing
The free state modifier means the tolerance applies while the part is in its natural unrestrained condition, which is critical for flexible or non-rigid parts.
Question 5: A GD&T callout shows angularity of 0.003 A on an angled surface. What does the tolerance zone look like?
- Two parallel planes 0.003 apart, inclined at the basic angle relative to datum A (Correct answer)
- A conical zone at the specified angle to datum A
- Two planes 0.003 apart always oriented at 90° to datum A
- A cylindrical zone of diameter 0.003 oriented at the basic angle
Correct answer: Two parallel planes 0.003 apart, inclined at the basic angle relative to datum A
The angularity tolerance zone is two parallel planes separated by the tolerance value, oriented at the basic angle specified on the drawing relative to the datum.
Question 6: Which of the following is NOT a valid GD&T form tolerance (applies without a datum)?
- Parallelism (Correct answer)
- Flatness
- Cylindricity
- Straightness
Correct answer: Parallelism
Parallelism is an orientation control that always requires a datum reference; form tolerances like flatness, cylindricity, and straightness stand alone.
Question 7: What does the term 'basic dimension' mean in GD&T?
- A theoretically exact dimension used to define the true location, orientation, or profile of a feature (Correct answer)
- The minimum acceptable dimension on a drawing
- The dimension without any tolerance applied, used for reference only
- The standard nominal size from which tolerances are equally distributed
Correct answer: A theoretically exact dimension used to define the true location, orientation, or profile of a feature
A basic dimension (shown in a rectangle/box) is a theoretically exact value used to establish the true geometry of a feature; the actual tolerance is specified in the feature control frame.
CNC GD&T Symbol Interpretation 3
What is the difference between circular runout and total runout?