GD&T Symbols and Symbology

This information provided by Bryan R. Fischer, of TDP360 LLC.

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GD&T is a symbolic language that is formally defined in the ASME Y14.5 standard. GD&T was developed to help people describe geometry and geometric requirements in a way that depended less on the vagaries of traditional written language. In the past, requirements such as how flat a manufactured surface must be were described in lengthy paragraphs of written text. The requirements were not standardized across industry and were subject to interpretation. A lot of guessing was required to understand what was needed and to manufacture an acceptable surface. GD&T was developed to solve that problem. 

Some symbols are used with dimensions and tolerances, some symbols are used in feature control frames, some symbols are used to define datum features, datum targets, and datum reference frames, and some symbols are used to define the features that a specification applies to, the target set. Target set symbols define the features that must conform to a specification, such as nX (e.g. 2X, 4X, 10X), all around, all over, between, and limited area. Target set symbols are used with many types of specifications to indicate the extent or number of places the specification applies. In some cases, target set specifications affect geometric tolerance zone behavior, such as whether tolerance zones are constrained one another, such as when nX is specified with a positional tolerance. Note that target set symbols may be used with any type of specifications, such as dimensions and tolerances, GD&T, welding specifications, surface texture specifications, painting and coating specifications, etc. For simplicity, target specification symbols are shown with dimension and tolerance specifications below.

The following sections include more information and examples about many of the symbols used in GD&T. There are other symbols used, some from mathematics and science, some from other disciplines such as electrical and electronics, and industry and company specific symbols. The material in this document is focused on GD&T and dimensioning and tolerancing. Information about symbols from other disciplines and practices can be found elsewhere.

Dimension and Tolerance Symbols

Symbols are used with dimensions and tolerances for many purposes, such as defining the nominal shape of the surface (e.g. Ø, R, SR, countersink), the shape of a set of holes (counterbored and counterdrilled holes), the depth of a hole, and many other examples.

Dimension and tolerance symbols are used with directly-toleranced dimensions, reference dimensions, and other specifications. Note that some symbols are also called modifiers. It is not important whether a GD&T symbol is called a modifier or a symbol. It is important that you understand what the symbol means and how it affects the product geometry.

Geometric Tolerance Symbols

Each geometric tolerance is represented by a symbol – the symbol is used in a feature control frame to specify a geometric tolerance. Geometric tolerance symbols appear in the first compartment in a feature control frame. Feature control frames are read from left to right.

The number of geometric tolerances defined in the ASME Y14.5 standard has changed over time. 

There were 14 geometric tolerances defined in the ASME Y14.5M-1994 and ASME Y14.5-2009 standards. There are 12 geometric tolerances defined in the ASME Y14.5-2018 standard. We removed coverage of concentricity and symmetry tolerances. We removed these tolerances because they were often used in cases where they did not control the correct geometry, the requirements they imposed upon the geometry were not functionally important, they controlled many center points rather than an axis or center plane, and in many cases verification of conformance to the specification was time consuming and costly. In almost all cases, a positional tolerance, total runout tolerance, or profile of a surface tolerance should have been applied instead of a concentricity or symmetry tolerance. Thus, to help users avoid specifying the wrong tolerances, and avoid wasting time and money, we no longer support concentricity and symmetry tolerances in the ASME Y14.5 standard.

Feature Control Frames 

Geometric tolerances are specified in feature control frames. A schematic representation of a feature control frame is shown in the figure below. Feature control frames are read from left to right. Every feature control frame contains the first two compartments. Some feature control frames include the datum feature reference compartments, and some do not. Some geometric tolerances cannot be related to a datum reference frame, so the feature control frame they are defined in cannot include datum feature references. Other geometric tolerances may or may not be related to a datum reference frame, such as position and profile of a surface. 

Symbols Used in Feature Control Frames

Symbols are used in three places in feature control frames. See the figure Feature Control Frame – Contents and Purpose.

A geometric symbol is specified in the first compartment

Symbols that define the geometric tolerance zone are specified in the second compartment.

Symbols that define how datum features are simulated and how the datum reference frame is related to the part are specified in the last set of compartments.

Symbols Used in the Tolerance Zone Compartment

Symbols are used in the tolerance zone compartment to define tolerance zone characteristics. 

Symbols Used in the Datum Feature Reference Compartments

Symbols are used in the datum feature reference compartment to define datum feature simulator behavior and other datum feature simulator characteristics, such as how datum features are simulated and how the datum reference frame is related to the part.

Datums and Datum Reference Frames

Geometric Tolerances

Form Tolerances

Circularity

Cylindricity

Flatness

Straightness

Orientation Tolerances

Angularity

Parallelism

Perpendicularity

Positional Tolerance

Position

Runout Tolerances

Circular Runout

Total Runout

Profile Tolerances

Profile of a Line

Profile of a Surface