GD&T Geometric Symbols & Notation 4 — Questions and Answers
Question 1: What does the 'independency' symbol (circled I) indicate when applied to a feature of size?
- The size and form tolerances are independent and do not interact (Correct answer)
- The feature is inspected independently of all datums
- The feature has an independent datum
- The tolerance applies only once, not repeatedly
Correct answer: The size and form tolerances are independent and do not interact
The independency symbol overrides Rule #1 (perfect form at MMC), allowing the form tolerance to be independent of the size tolerance.
Question 2: A GD&T 'Symmetry' callout controls:
- The location of a feature's median plane relative to a datum plane (Correct answer)
- The shape symmetry of a cross-section
- The equal spacing of multiple features
- The mirror image relationship between two surfaces
Correct answer: The location of a feature's median plane relative to a datum plane
Symmetry (removed in ASME Y14.5-2009, retained in ISO) requires that the median points of a feature lie within a tolerance zone centered on a datum plane.
Question 3: On a GD&T drawing, a dimension value enclosed in a rectangular box (no text) signifies:
- A basic (theoretically exact) dimension (Correct answer)
- A reference dimension
- An inspection dimension
- A tolerance dimension
Correct answer: A basic (theoretically exact) dimension
A boxed dimension value (without the word BASIC) is the ISO method for denoting a theoretically exact basic dimension.
Question 4: The 'Least Material Condition' (LMC) modifier circled L is used to:
- Apply additional tolerance when a feature is at its minimum material condition (Correct answer)
- Reduce the tolerance at the smallest feature size
- Require measurement at the thinnest cross-section
- Control the maximum envelope of the feature
Correct answer: Apply additional tolerance when a feature is at its minimum material condition
The LMC modifier allows a geometric tolerance to increase as the feature departs from its least material condition, useful for ensuring minimum wall thickness.
Question 5: Which symbol on a feature control frame indicates a 'simultaneous requirement' is not in effect and features are verified independently?
- SEP REQT (Separate Requirement) notation (Correct answer)
- Circled S modifier
- A double-headed arrow
- Two stacked feature control frames
Correct answer: SEP REQT (Separate Requirement) notation
Adding 'SEP REQT' (Separate Requirement) to a feature control frame means each callout is gauged independently rather than simultaneously.
Question 6: In GD&T notation, a 'statistical tolerance' symbol (circled ST) means:
- The tolerance is assigned based on statistical process control and may only be used in statistical sampling inspection (Correct answer)
- The tolerance is tighter than standard
- The tolerance is derived from a statistical average
- The feature must be measured using statistical software
Correct answer: The tolerance is assigned based on statistical process control and may only be used in statistical sampling inspection
A statistical tolerance callout indicates that the tolerance is larger than it would be conventionally, justified by statistical process control methods that ensure assembly function.
Question 7: The 'Profile of a Line' symbol controls:
- The shape of a cross-sectional slice of a surface at a single cutting plane (Correct answer)
- The 3D shape of an entire surface
- The straightness of a line element
- The orientation of a line to a datum
Correct answer: The shape of a cross-sectional slice of a surface at a single cutting plane
Profile of a Line (single arc symbol) applies the profile tolerance to each individual 2D cross-section independently, not to the whole 3D surface.
GD&T Geometric Symbols & Notation 4
What does the 'independency' symbol (circled I) indicate when applied to a feature of size?