GD&T Symbols overview

Geometric Dimensioning & Tolerancing (GD&T) is a language representative of engineering drawings to classify deviations and tolerance of part measurements and geometric analysis. It is an efficient way of communicating measurement conditions and specification of a part. Consisting of symbols, this language is used to define the allowable variations from theoretically ideal geometry.

GD&T Symbols & Terms

Geometric characteristic symbols mentioned in the ASME Y14.5 have been distributed in accordance with the type of application, tolerance, characteristic and the accompanying symbol, to provide user friendly layout and ease of understanding.There are three types of applications including “individual features”, “individual or related features” and “related features”.


Within the ASME Y14.5 standard, each application is broken down into sections accordingly to identify the accompanying symbol. There are also five different types of tolerances highlighted to accompany different characteristics and symbols, to additionally provide ease of understanding.

The following is a brief description of symbol characteristics mentioned in ASME Y14.5-2009:


straightnessStraightness
– Identified with application of individual features. According to the ASME Y14.5 standard, straightness controls the form of a line on the surface of feature.

 

flatFlatness – Identified with application of individual features. According to the ASME Y14.5 standard, this symbol controls the flatness of surface.

 

circularityCircularity – Identified with application of individual features. According to the ASME Y14.5 standard, circularity is used to describe the allowable variations to a true circle.

 

cylindricityCylindricity – Identified with application of individual features. According to the ASME Y14.5 standard, cylindricity is used to describe the allowable variation to a true cylinder.

 

profile of a lineProfile of a Line – Identified with application of individual or related features. According to the ASME Y14.5 standard, profile of a line controls size and form to cross sections of a feature.


profile of a surfaceProfile of a Surface –
Identified with application of individual or related features.  According to the ASME Y14.5 standard, profile of a surface controls size and form to entire surface.

 

AngularityAngularity – Identified with application of related features. According to the ASME Y14.5 standard, angularity controls orientation at a specific angle in accordance with primary datum of datum reference frame.


PerpendicualrityPerpendicularity –
Identified with application of related features. According to the ASME Y14.5 standard, perpendicularity controls the orientation of a feature which is perpendicular to its primary datum of datum reference frame.


ParallelismParallelism –
Identified with application of related features. According to the ASME Y14.5 standard, parallelism controls orientation of a feature which is parallel to the primary datum of the datum reference frame.


PositionPosition –
Identified with application of related features. According to the ASME Y14.5 standard, position controls the location and orientation of a feature in accordance with datum reference frame.


concentricityConcentricity –
Identified with application of related features. According to the ASME Y14.5 standard, this symbol controls concentricity of a surface to central datum.

 

SymmetrySymmetry – Identified with application of related features. According to the ASME Y14.5 standard, this symbol controls symmetry of two surfaced in accordance with central datum.

 

Circular RunoutCircular Runout – Identified with application of related features. According to the ASME Y14.5 standard, this symbol controls circularity of each circular portion of a surface in accordance to coaxial datum.


Total RunoutTotal Runout –
Identified with application of related features. According to the ASME Y14.5 standard, this symbol controls circularity, straightness, coaxility and taper of a cylindrical surface in accordance to coaxial datum.

 

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