262 Machine Trades Print Reading Copyright Goodheart-Willcox Co., Inc. ■ ■ Profile tolerances are used to control form or a combination of size, form, orientation, and location of a feature. A profile tolerance specifies a constant boundary along the true profile within which all points or elements of the surface must lie. The two types of profile tolerances are line profile and surface profile. ■ ■ Runout tolerances control the relationship of one or more features of a part relative to a datum axis. GD&T Symbols Previous units described the use of symbols adopted by various ASME standards, including the diameter (Ø), radius (R), counterbore, spotface, countersink, and depth symbols. Symbols used in GD&T include geometric characteristic symbols, datum feature sym- bols, and feature control frames. Modifying symbols, such as material condition modifier symbols, are also used and are explained later in this unit. Geometric characteristic symbols have been established for form, orientation, position, pro- file, and runout tolerances. The accompanying chart, Figure 15‑1, shows the 12 geometric characteristic symbols used in GD&T. Datum feature symbols are used to identify datum features. The datum feature symbol consists of a square or rectangular frame containing the datum feature identification letter. See Figure 15‑2. An equi- lateral triangle connected to the frame by a leader identifies the feature. A feature control frame is a rectangular frame with compartments containing the geometric tolerance specification. The geometric characteristic symbol is given first, followed by the allowable tolerance. See Figure 15‑3. When a geometric tolerance specification is related to at least one datum, this relationship is stated in the feature control frame. The datum feature reference let- ter is placed after the geometric characteristic symbol and the tolerance, as shown in Figure 15‑4. A single geometric tolerance specification can have multiple datum feature references. Datum feature references are read from left to right. A feature control frame can be applied to a feature in one of several ways. The application depends on the type of tolerance that is specified and the type of feature to be controlled. An example application for a perpendicularity tolerance is shown in Figure 15-5. In this example, an extension line from the feature control frame extends to the controlled feature. This application indicates that the controlled feature is a surface (the bottom surface). The tolerance specified Type of Tolerance Characteristic Symbol Form Straightness Flatness Circularity Cylindricity Orientation Angularity Perpendicularity Parallelism Position Position Profile Profile of a Line Profile of a Surface Runout Circular Runout Total Runout The American Society of Mechanical Engineers Figure 15-1. Geometric characteristic symbols.