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What is GD&T？
Due to variations in the manufacturing process, the size and dimensions of the manufactured product may differ from the original CAD model. To better control and communicate these variations, engineers and manufacturers use a symbolic language called GD&T (Geometric Dimensioning and Tolerancing).
GD&T is used to define the nominal (theoretically perfect) geometry of parts and assemblies, the allowable variation in the form and size of individual features, and the permissible variation between features.
- Dimensioning specifications define the nominal, as-modeled, or intended geometry. For example, a basic dimension.
- Tolerancing specifications define the allowable variation in the form and, possibly, the size of individual features, as well as the permissible variation in the orientation and location between features. Examples include linear dimensions and feature control frames using a datum reference.
Common GD&T Symbols
| Symbol | Geometric Characteristic | Feature Modifier | Datums | Datum Modifier | |
| Form |  |
Straightness | ✓ | Datums Not Allowed Form tolerances are defined to limit the deviations of a geometric feature from its ideal form. | NA |
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Flatness | X | NA | ||
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Circularity | X | NA | ||
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Cylndricity | X | NA | ||
| Profile |  |
Profile of a line | X | Datums sometimes required | ✓ |
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Profile of a Surface | X | ✓ | ||
| Orientation |  |
Angularity | ✓ | Datums Required | ✓ |
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Perpendicularity | ✓ | ✓ | ||
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Parallelism | ✓ | ✓ | ||
| Runout |  |
Circular Runout | X | X | |
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Total Runout | X | X | ||
| Location |  |
Position | ✓ | ✓ | |
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Concentricity | X | X | ||
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Symmetry | X | X |
Modifiers
If no modifier follows a datum feature size, the datum feature applies regardless of material boundary.
| Symbol | Modifier | Notes |
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Continuous Feature | This symbol is used to identify a group of features when there is a requirement that they be teated geometrically as a single feature. |
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Statistical Tolerance | Features identified as statistical toleranced should be produced with Statistical Process Control (SPC). |
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Envelope requirement | The upper value of a external feature defines a virtual cylinder that the entire feature has to fit inside. Likewise a internal features minimum value defines a cylinder that has to fit inside the hole. |
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Free State | Applies only when part is otherwise restrained. |
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Independency requirement | This principle sets no limits to the number of errors of form possessed by individual features of a work piece. is the default for ISO tolerancing. For ANSI/ASME tolerancing this has to be specified. |
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Least Material Condition (LMC) | Useful to maintain minimum wall thickness. |
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Maximum Material Condition | Provides bonus tolerance only for a feature of size. |
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Projected Tolerance Zone | Useful on threaded holes for long studs. |
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Tangent Plane | Useful for interfaces where form is not required. |
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Unequal Bilateral | Appears in the 2009 version of the standard, and refers to unequal profile distribution. |
Tolerance Zones
Specifies the type of zone applied to the tolerance.
| Symbol | Tolerance zone | Notes |
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Controlled radius | Creates a tolerance zone defined by two arcs that are tangential to the adjacent surfaces |
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Spherical diameter tolerance zone | Creates a spherical tolerance zone defined by a specified diameter |
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Spherical radius tolerance zone | Creates a tolerance zone defined by two spherical segments with the minimum and maximum radii mentioned |
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Diameter | This prefix can be used to signify that a tolerance zone exists of a diameter specified by the numerical value after this prefix. |
