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The Basics of Geometric Dimensioning and Tolerancing (GD&T)

The Basics of Geometric Dimensioning and Tolerancing (GD&T)

19/06/2023

Geometric tolerance is an important factor in determining the shape and accuracy of a product. While dimensional tolerances specify the allowable variation in the size of a part or component, geometric tolerances specify the allowable variation in the form, orientation, and location of features on the part. 

Geometric tolerancing uses a set of symbols and annotations to communicate specific requirements for features such as flatness, circularity, perpendicularity, and parallelism. By specifying these requirements, manufacturers can ensure that their products meet the technical requirements of their customers and function as intended.

1. Geometric Characteristic Symbols

The geometric characteristic symbols are a set of 14 symbols used in the language of GD&T to describe the geometry attributes of a part. The symbols are shown in Figure 1.

Type

Characteristic

Symbol

Uses a Datum Reference

Definition


 

Form

Straightness

 

Never

The maximum deviation of line straightness, where the nominal line profile is a straight line and the limits are composed of parallel and equidistant lines with respect to the nominal location/orientation.

Flatness

The surface deviation is contained between two parallel planar geometric features.

Circularity (Roundness)

Each cross-section of a circular profile must fit between two concentric circles.

Cylindricity

The surface deviation is contained between two coaxial cylindrical geometric features.

 

Profile

Profile of a line

Sometimes

The line profile is a 2d representation of a 3d feature. The deviation of the cross-section can be controlled by the line profile tolerance, which imposes an upper and lower limit to the deviation, equidistant from the nominal line profile. The upper and lower limits are equal in profile to the nominal profile. The space between the equidistant line profile tolerance limits is considered the tolerance zone.

Profile of a surface

The whole surface profile deviation is permitted to be between two surface profiles of equal properties to and with parallel tangent lines to the nominal surface profile.

 

Location

Position

The deviation from the nominal position is contained within a cylinder whose axis is on the nominal positional location. 

Concentricity




 

Always

The deviation of the axis of a feature is contained within a cylinder, limiting variance by translation, angle, and straightness. 

Symmetry

The surface deviation is contained between two parallel planes symmetrically disposed about a surface datum plane located on the nominal surface.

 

Orientation

Angularity

The deviation is permitted between two parallel planes angled equally with respect to the nominal geometric element, based on the called out datum.

Perpendicularity

The deviation is permitted between two parallel planes perpendicular to the called out datum.

Parallelism

The deviation is permitted between two parallel planes parallel to the called out datum.

 

Runout

Circular runout

Each cross-section, coaxial section or conical section with respect to the called out datum can deviate between two concentric or parallel 2d geometric features.

Total runout

The surface is contained between two concentric, parallel, or coaxial 3d geometric features with respect to the called out datum.

Figure 1 Geometric Characteristic Symbols

2. Feature Control Frame 

A feature control frame is a geometric dimensioning and tolerancing (GD&T) symbol used on engineering drawings to specify the allowable deviation in form and orientation of a feature on a part or assembly. The feature may be a point, line, plane, hole, or other geometric shape. The feature control frame consists of several elements, including the feature control symbol, the tolerance zone shape, the tolerance value, and the material condition modifier.

Figure 2 Parts of a Feature Control Frame

  • The first compartment always contains one of the fourteen geometric characteristic symbols.

  • The second compartment always contains the tolerance information including the tolerance value and modifiers that describe the tolerance condition.

  • The third, fourth and fifth compartment (when used) always contain datum reference information.

 

Geometric tolerance is particularly important in industries where precision and accuracy are critical, such as aerospace, automotive, and medical device manufacturing. By using geometric tolerances, manufacturers can ensure that their products meet the high standards required in these industries and that they function safely and reliably. At Smart Viet Nam, we ensure that all our engineers are knowledgeable about GD&T through training and professional development opportunities. 

For those interested in learning more about GD&T, click here.

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