300 GD&T: Application and Interpretation Copyright Goodheart-Willcox Co., Inc. tolerance zone are defi ned. The tolerance specifi ca- tion has a diameter symbol applied to the .024″ tol- erance value. Explaining the tolerance in terms of the axis of the hole, the tolerance creates a requirement for a tolerance zone that is a .024″ diameter cylinder. The cylindrical zone extends completely through the hole. It terminates fl ush with the two surfaces. The tolerance zone cylinder is centered on the true position. The true position is defi ned by basic dimen- sions that locate the hole. The tolerance zone on this part must be perpendicular to primary datum A. The tolerance value includes the MMC modifi er, so the tolerance zone diameter must be .024″ diame- ter when the hole is produced at its MMC size. Based on the axis method, the axis of the hole (meaning the axis of the unrelated actual mating envelope) must be within the tolerance zone. The preceding description explains the position tolerance requirement based on the axis method. When a position tolerance is applied with the MMC or LMC modifi er, the tolerance requirement may also be explained on the basis of the surface method. The surface method requires that the surface of the hole does not violate a virtual condition boundary. See Figure 8-7. In the example shown, the hole from Figure 8-6 is shown at a larger scale. A boundary known as the virtual condition (Vc) is created by the applicable size minus the position tolerance for the hole. In this case, the applicable size is the MMC. The surface of a manufactured hole must not violate the virtual condition. In the given exam- ple, the virtual condition is a .601″ diameter cylinder. Any hole that is within the size limits does not vio- late the virtual condition boundary and is acceptable. When the MMC or LMC modifi er is applied to the tolerance, the axis and surface methods may give different results when measuring holes that have form variation (imperfect cylinders). When there is a difference, the surface method takes precedence for MMC and LMC applications. Even though the surface method takes prece- dence, the axis method is generally used throughout industry for explaining position tolerances. The axis method is often used for measuring and reporting feature locations because of the ease of performing calculations. Provided the surface form of measured features is relatively accurate, the differences between the axis and surface methods are generally insignifi - cant and the axis method is adequate. The surface method cannot be used when the tolerance is applied RFS. The axis method must be used when position tolerances are applicable RFS. Basic Dimensions and True Positions Basic dimensions must be used to defi ne the locations of features to which position tolerances are applied. See Figure 8-8. The given fi gure shows two holes located by three basic dimensions. Basic dimensions are theoretically perfect and have no Tolerance zone extends the full length of the hole Tolerance zone perpendicular to datum A True position (location) Ø.024 When at MMC Ø.601 Virtual condition Datum A Goodheart-Willcox Publisher Figure 8-7. The surface of a hole must not violate the virtual condition boundary and the axis of the hole must not violate the tolerance zone that is centered on the true position. (holes are not shown) Virtual condition and tolerance zones are centered on true positions Goodheart-Willcox Publisher Figure 8-8. The virtual conditions and tolerance zones for holes are centered on the theoretically exact true positions for the holes.