Unit 15 Geometric Dimensioning and Tolerancing 269 Copyright Goodheart-Willcox Co., Inc. Notice that the straightness tolerance value in the fea- ture control frame is preceded by the diameter symbol. This indicates that the tolerance zone is cylindrical. As a result, the derived median line must be within a tolerance zone of .015″ diameter. In addition, the part must be within the specified limits of size when manu- factured. When measured, each circular cross section along the cylindrical part must be within the permissi- ble limits of .605″ and .615″ diameter. Flatness Flatness is a condition in which all elements of a sur- face or a derived median plane lie in one plane. A flatness tolerance specification establishes a tolerance zone formed by two parallel planes. The entire surface must lie within the tolerance zone. See Figure 15‑19. A flatness tolerance requirement for a flat surface is shown in Figure 15-20. The figure illustrates that both the size and form tolerance requirements must be met. The flatness tolerance specifies that the difference in height between the high points and low points of the bottom surface must be within a tolerance of .005″. In addition to the flatness tolerance required for the lower surface, a size tolerance of ±.010″ must be held between the upper and lower surfaces. The measured size between the high points of the surfaces must not exceed the high size limit of .635″. Also, the measured size between the low points of the surfaces must not be less than the low size limit of .615″. In summary, when machining both the upper and lower surfaces, two tolerance is to apply to a feature in the free state condi- tion. Examples of parts that may be subject to variation in the free state include parts that bend or flex, such as a rubber gasket. The free state symbol is an encircled F. Form Tolerances Form tolerances are applied to control allowable varia- tions in the form of features. Form tolerances are used when other tolerances do not provide the necessary extent of control. Form tolerances are used to con- trol straightness, flatness, circularity, and cylindricity. Form tolerances are applicable to individual features or elements of features and are not related to datums. Straightness Straightness is a condition in which an element of a surface or a derived median line is a straight line. A straightness tolerance specification for a flat sur- face requires all surface elements to be within a tol- erance zone established by two parallel planes. See Figure 15-17. When a straightness tolerance is applied in this manner, each line element of the surface must lie within the tolerance zone. A straightness tolerance applied to a cylindrical feature of size establishes a different requirement. See Figure 15-18. The straightness tolerance specification in the example shown applies to the derived median line of the cylindrical feature. A derived median line on a cylindrical feature is a line established from the center points of the cross sections of the feature. Drawing Interpretation .003 tolerance zone Goodheart-Willcox Publisher Figure 15-17. Specifying surface straightness.