CNC GD&T Symbol Interpretation 2 — Questions and Answers
Question 1: What does the profile of a surface symbol (an arc with no leader pointing to a plane) control?
- All points on a surface relative to a true profile (Correct answer)
- Only the flatness of a surface
- The angularity of a surface relative to a datum
- The cylindricity of a curved surface only
Correct answer: All points on a surface relative to a true profile
Profile of a surface controls all points on a feature surface to lie within a tolerance zone defined by two offset boundaries around the true profile.
Question 2: A feature control frame reads: ⌀0.005 M A B. What does the 'M' modifier mean in this context?
- Maximum Material Condition — tolerance may increase as feature departs from MMC (Correct answer)
- Metric units apply to the tolerance value
- Minimum Material Condition applies
- The tolerance is measured at the median plane
Correct answer: Maximum Material Condition — tolerance may increase as feature departs from MMC
The M (circle M) modifier stands for Maximum Material Condition, allowing a bonus tolerance as the feature departs from its MMC size.
Question 3: Which GD&T symbol is used to specify that a hole's axis must lie within a cylindrical tolerance zone relative to a datum axis?
- True position ⊕ (Correct answer)
- Concentricity ◎
- Cylindricity ⌭
- Perpendicularity ⊥
Correct answer: True position ⊕
True position (⊕) locates a feature's axis within a cylindrical (or two-parallel-plane) tolerance zone relative to specified datums.
Question 4: What does a flatness callout of 0.002 mean on a machined surface?
- All surface points must lie between two parallel planes 0.002 apart (Correct answer)
- The surface must be parallel to datum A within 0.002
- The surface height variation must not exceed 0.002 from nominal
- The surface roughness Ra must be 0.002 or less
Correct answer: All surface points must lie between two parallel planes 0.002 apart
Flatness requires all points on the surface to fall within a tolerance zone formed by two parallel planes separated by the stated tolerance value.
Question 5: A drawing calls out perpendicularity of ⌀0.003 A on a pin. What shape is the tolerance zone?
- A cylinder of diameter 0.003 perpendicular to datum A (Correct answer)
- Two parallel planes 0.003 apart perpendicular to datum A
- A cone of half-angle derived from 0.003
- A sphere of diameter 0.003 centered on the pin axis
Correct answer: A cylinder of diameter 0.003 perpendicular to datum A
When a diameter symbol precedes the tolerance value in a perpendicularity callout, the tolerance zone is a cylinder whose axis is perpendicular to the referenced datum.
Question 6: Runout tolerance is always measured relative to what?
- A datum axis established by one or more datum features (Correct answer)
- The nominal size of the part
- The surface finish of the inspected feature
- The largest diameter on the part
Correct answer: A datum axis established by one or more datum features
All runout tolerances (circular and total) are measured while the part rotates 360° about a datum axis specified in the feature control frame.
Question 7: Which symbol controls the median line of a cylinder — not its surface — to be straight?
- Straightness applied to the feature (axis straightness) (Correct answer)
- Cylindricity ⌭
- Circularity ○
- Profile of a line
Correct answer: Straightness applied to the feature (axis straightness)
When straightness is applied to the feature (not the surface), it controls the derived median line of the cylinder to lie within a cylindrical tolerance zone.
CNC GD&T Symbol Interpretation 2
What does the profile of a surface symbol (an arc with no leader pointing to a plane) control?