position tolerancing—expanded principles, symmetry, and concentricity

9Position Tolerancing—Expanded Principles,

Symmetry, and Concentricity

GD&T: Application and Interpretation © Goodheart-Willcox Co., Inc.

• Explain functional gaging methods for checking hole position tolerances specified at MMC.

• Specify and explain composite position tolerance specifications.

• Explain the effect of using identical datum feature references in multiple position tolerance specifications.


• Specify separate pattern requirements for groups of features not acting as a single pattern.

• Specify position tolerances for in-line holes.• Specify tolerances to control symmetry.• Control coaxial features with position or

concentricity tolerances, depending on the given application.


Symmetry

• Position tolerance • Symmetry tolerance

– Symbol removed from standard in 1982– Symbol reinstated in 1994


Position for Symmetryof Hole Patterns


Symmetry for Features of Size


Concentricity

• Requires derived median line located relative to datum axis

• Always has diameter symbol• Always applied RFS• Must have datum feature reference• Datum reference is always RMB• Rarely used


Concentricity Feature Control Frame


Concentricity Median Line

GD&T: Application and Interpretation

Concentricity Zero Error

© Goodheart-Willcox Co., Inc.

• Even though a feature has surface and size variation, it may have no concentricity variation


Composite Position Tolerance

• One position symbol• Two or more segments• Same datums, same order, same modifiers


PLTZF

• Pattern-locating tolerance zone framework• Pronounced plotz• First segment• Located relative to datum reference frame


FRTZF

• Feature-relating tolerance zone framework• Pronounced fritz• Lower segment(s)• Not located relative to datum reference frame• Constrained in rotation relative to referenced

datums• Must, at a minimum, overlap PLTZF


Composite Position Application


No Repeated Datum References

• Free to float relative to datum reference frame• Features must be able to meet tolerance

requirements in first and second segments


Repeated Primary Datum Reference

• Rotational degrees of freedom constrained relative to primary datum

• Features must be able to meet tolerance requirements in first and second segments


Repeated Primary and Secondary Datum Reference

• Rotational degrees of freedom constrained relative to primary and secondary datum

• Features must be able to meet tolerance requirements in first and second segments


Functional Gaging for Position Tolerances at MMC


Gaging a Composite Position Tolerance

• Similar to single segment position tolerance• Two gages typically required to check composite

tolerances

position tolerancing—expanded principles, symmetry, and concentricity

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