GD&T considers the function of the part, as well as its fit with related parts. This allows the designer to define the part’s features more accurately, without increasing the tolerances. Geometric dimensioning and tolerancing is a method of defining parts based on how they function. Tolerances can be categorized in terms of form, orientation, runout, profile, and location. The feature control frame is probably the most important symbol in the geometric tolerancing system. It states the requirements or instructions for the features to which it is attached. Tolerances of form include controls for flatness, straightness, circularity (roundness), and cylindricity. Tolerances of orientation enable the clear specification of relationships between part features and acceptable limits of variation. There are three orientation tolerances that control the relationship of features to one another: parallelism, perpendicularity, and angularity. Runout is a tolerance used to express relationships between surfaces of features in the many applications in which acceptable part deviation is best observed in terms of rotation about an axis. Profile tolerancing provides an effective means of controlling irregular lines, surfaces, or unusual part profiles. Tolerances of profile include profile of a line and profile of a surface.
By Gary Conner
By George N Bullen FSME