GD&T Symbols and Rules 4 — Questions and Answers
Question 1: A 'basic dimension' in GD&T is identified on a drawing by:
- Enclosing the value in a rectangular box (Correct answer)
- Adding the letters 'BSC' next to the value
- Underlining the dimension value
- Placing it in parentheses
Correct answer: Enclosing the value in a rectangular box
Basic dimensions are shown enclosed in a rectangular box and represent the exact theoretically perfect value from which toleranced dimensions are measured.
Question 2: What is the function of the 'tangent plane' modifier (T in a circle) in a GD&T callout?
- The tolerance applies to a plane contacting the high points of the surface (Correct answer)
- The tolerance applies only to tangent line elements
- The feature must be tangent to the datum
- The surface must be measured with a tangential probe
Correct answer: The tolerance applies to a plane contacting the high points of the surface
The tangent plane modifier means the tolerance controls the orientation of a tangent plane that contacts the high points of the surface, not the entire surface form.
Question 3: Which GD&T characteristic uses two coaxial cylinders as its tolerance zone?
- Cylindricity (Correct answer)
- Concentricity
- Total Runout (on a cylindrical surface)
- Circular Runout
Correct answer: Cylindricity
Cylindricity requires all points of a cylindrical surface to lie within two coaxial cylinders separated by the specified tolerance.
Question 4: The 'projected tolerance zone' symbol (P in a circle) is most commonly applied to:
- Threaded holes or press-fit pins to control mating part interference (Correct answer)
- Flat surfaces requiring parallelism
- Profile tolerances on curved surfaces
- Runout measurements on rotating shafts
Correct answer: Threaded holes or press-fit pins to control mating part interference
Projected tolerance zones extend the position tolerance above a hole to the height of the mating pin or fastener, preventing interference during assembly.
Question 5: In GD&T, the term 'virtual condition' for an external feature at MMC is calculated as:
- MMC size plus the applicable geometric tolerance (Correct answer)
- LMC size minus the applicable geometric tolerance
- MMC size minus the applicable geometric tolerance
- LMC size plus the applicable geometric tolerance
Correct answer: MMC size plus the applicable geometric tolerance
For an external feature (shaft) at MMC, virtual condition equals the MMC size plus the geometric tolerance, representing the worst-case boundary.
Question 6: Which symbol instructs the inspector to measure a feature while it is rotating about a datum axis?
- Circular Runout or Total Runout (Correct answer)
- Concentricity
- Coaxiality
- Cylindricity
Correct answer: Circular Runout or Total Runout
Both circular runout and total runout require the part to be rotated 360° about the datum axis while a fixed indicator records deviations.
Question 7: On a feature control frame, a datum letter followed by a material condition modifier (e.g., 'B(M)') means:
- The datum shift allowance applies based on the actual mating size of datum feature B (Correct answer)
- Datum B is measured at its maximum material boundary
- The datum B reference is optional
- Datum B must be re-established for each measurement
Correct answer: The datum shift allowance applies based on the actual mating size of datum feature B
Applying MMC to a datum reference allows datum shift — additional tolerance based on how far the datum feature departs from its MMC size.
GD&T Symbols and Rules 4
A 'basic dimension' in GD&T is identified on a drawing by: