GD&T

GD&T (Geometric Dimensioning & Tolerancing) is the modern drafting standard for creating unambiguous documentation of parts. In contrast to traditional tolerancing methods that give acceptable deviation as scalar values attached to measurements, GD&T applies tolerances as deviations from an ideal geometric shape each feature is attempting to achieve. This feature-based tolerancing scheme communicates part intent clearly and improves part acceptance rate through consistent inspection methods.

The GD&T method achieves higher precision and clarity by defining each feature relative to a datum reference frame (DRF), which is a theoretical construct that constrains all six degrees of freedom for the part. Defining every feature directly from the DRF removes the potential for compounded tolerances when defining features off of one another, as in traditional tolerancing. The DRF has a hierarchy, wherein primary, secondary, and tertiary datum features each constrain fewer degrees of freedom than each feature before it. This hierarchy communicates which features are most important to the design, and aids in the inspection process by implying an order in which to position the part.

Quick Example

The classic example of GD&T's value is positional tolerancing for holes. Traditional XY tolerancing relies on defining the positions of features as linear ranges along the X and Y axes of the drawing plane. This results in square or rectangular tolerance zones in which the feature reference point can sit. However, the nature of a rectangle's diagonals in relation to its sides means the maximum deviation of the feature from the ideal position is greater than the tolerance values.

To address this, GD&T instead uses—as the "G" and "T" imply—Geometric Tolerancing, where the tolerance is clarified further with geometric shapes. If the intention of a designer is to allow a hole to deviate from an ideal position by a distance of T, then a circular tolerance zone is more appropriate than a rectangular one. By definition, a circle is the set of all points in a plane each the same distance from a reference point. The ability to define the tolerance zone as a circle rather than a rectangle means the tolerance value accurately describes the needs of the design, and will allow for more parts to pass inspection.