What is GD&T？
To address discrepancies arising from the manufacturing process, a symbolic language called Geometric Dimensioning and Tolerancing (GD&T) is employed by engineers and manufacturers. GD&T enables better control and communication of these variations.
GD&T serves several purposes:
- It defines the ideal geometry of parts and assemblies, known as the nominal geometry.
- It specifies the acceptable range of form and size for individual features.
- It establishes the permissible variation in orientation and location between features.
Dimensioning specifications focus on defining the intended or as-modeled geometry, often expressed through basic dimensions.
Tolerancing specifications, on the other hand, outline the allowable variation in form, size, orientation, and location of individual features. This includes linear dimensions and feature control frames, which utilize datum references.
By utilizing GD&T, engineers and manufacturers can accurately define and communicate the required specifications, facilitating the production of high-quality and precisely manufactured projects.
| 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 |
 |
Flatness | X | NA | ||
 |
Circularity | X | NA | ||
 |
Cylndricity | X | NA | ||
| Profile |  |
Profile of a line | X | Datums sometimes required | ✓ |
 |
Profile of a Surface | X | ✓ | ||
| Orientation |  |
Angularity | ✓ | Datums Required | ✓ |
 |
Perpendicularity | ✓ | ✓ | ||
 |
Parallelism | ✓ | ✓ | ||
| Runout |  |
Circular Runout | X | X | |
 |
Total Runout | X | X | ||
| Location |  |
Position | ✓ | ✓ | |
 |
Concentricity | X | X | ||
 |
Symmetry | X | X |
| Symbol | Modifier | Notes |
 |
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. |
 |
Statistical Tolerance | Features identified as statistical toleranced should be produced with Statistical Process Control (SPC). |
 |
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. |
 |
Free State | Applies only when part is otherwise restrained. |
 |
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. |
 |
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. |
 |
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. |
| Symbol | Tolerance zone | Notes |
 | Controlled radius | Creates a tolerance zone defined by two arcs that are tangential to the adjacent surfaces |
 | Spherical diameter tolerance zone | Creates a spherical tolerance zone defined by a specified diameter |
 | Spherical radius tolerance zone | Creates a tolerance zone defined by two spherical segments with the minimum and maximum radii mentioned |
 | Diameter | This prefix can be used to signify that a tolerance zone exists of a diameter specified by the numerical value after this prefix. |
