What is the purpose of Geometric Dimensioning & Tolerancing?
Geometric dimensioning and tolerancing (GD&T) is a systemof symbolsused on engineering drawings to communicateinformation from the designer to the manufacturer through engineering drawings.
GD&T tells the manufacturer the degree of accuracy and precision needed for each controlled feature of the part. GD&T is used to define the nominal geometry of parts and assemblies and to define the allowable variationof features.
In this article:
Modifiers
Tolerance ZonesFeature Control Frames
| Symbol | Geometric Characteristic | Feature Modifier | Datums | Datum Modifier | Bonus Tolerance | ||
Form |  | Straightness | ✓ | Datums Not Allowed Form tolerances are defined to limit the deviations of a geometric feature from its ideal form. | NA | ✓ Only at MMC or LMC | |
 | Flatness | X | NA | X | |||
 | Circularity | X | NA | X | |||
 | Cylndricity | X | NA | X | |||
| Profile |  | Profile of a line | X | Datums sometimes required | ✓ | X | |
 | Profile of a Surface | X | ✓ | X | |||
Orientation |  | Angularity | ✓ | Datums Required | ✓ | ✓ Only at MMC or LMC | |
 | Perpendicularity | ✓ | ✓ | ✓ Only at MMC or LMC | |||
 | Parallelism | ✓ | ✓ | ✓ Only at MMC or LMC | |||
Runout |  | Circular Runout | X | X | X | ||
 | Total Runout | X | X | X | |||
Location |  | Position | ✓ | ✓ | ✓ Only at MMC or LMC | ||
 | Concentricity | X | X | X | |||
 | Symmetry | X | X | X | |||
If no modifier follows a datum feature size, the datum feature applies regardless of material boundary.
| 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. |
 | Maximum Material Condition | Provides bonus tolerance only for a feature of size. |
 | Projected Tolerance Zone | Useful on threaded holes for long studs. |
 | Tangent Plane | Useful for interfaces where form is not required. |
 | Unequal Bilateral | Appears in the 2009 version of the standard, and refers to unequal profile distribution. |
Specifies the type of zone applied to the tolerance.
| 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. |
In GD&T (Geometric dimensioning and tolerancing), afeature control frameis required to describe the conditions and tolerances of a geometric control on a part's feature. The feature control frame includes four parts:
- GD&T symbol/control symbol
- Tolerance zone type and dimensions
- Tolerance zone modifiers - such as features of size
- Datum references (if required by the GD&T Symbol)
Combined,the feature control frame provides all the information you need tomeasure the geometric tolerance of the features of the part and determine if the part is in spec.
