296 GD&T: Application and Interpretation Copyright Goodheart-Willcox Co., Inc. Throughout this chapter and other chapters of this textbook, allowable feature variation is often explained and illustrated showing the requirement in terms of a feature axis within an allowable tolerance zone. The axis method of explaining tolerance require- ments, as described in Chapter 7, may be used for convenience and ease of showing the tolerance effects for position tolerances applied RFS, MMC, and LMC. However, for position tolerances applied MMC or LMC, the surface method takes precedence when there is a different result obtained by the two methods. The axis of an imperfect feature is defi ned as being the axis of the unrelated actual mating enve- lope when tolerances are applied RFS or MMC. When a tolerance is applied at LMC, the axis of a tol- eranced feature is defi ned as the axis of the unrelated actual minimum material envelope. See Figure 8-1. Some explanations for position tolerances applied MMC or LMC show the requirement in terms of an acceptance boundary that the feature surface must not violate. This is referred to as the surface method. The similar term boundary method is sometimes used to mean the same thing. The surface method is only applicable to tolerances applied with the MMC or LMC modifi er. The surface method is not applicable for features toleranced RFS, so the axis method is always used for position tolerances applied RFS. Provided a feature has no form variations, the axis and surface methods show the same result in terms of measured position variation and part accep- tance. When features have form variation, the two methods may give different results, and the amount of difference is impacted by the amount of form vari- ation in the measured feature. When the results of the two methods are different, the surface method takes precedence for features toleranced with the MMC or LMC modifi er. Position Position tolerances are used to specify the required location accuracy for features of size and bounded features. Features of size typically have a size dimension, such as a diameter for a cylinder or a width for a slot. Bounded features are typically enclosed and have a profi le tolerance that creates inner and outer boundaries for the feature. Tolerance specifi cation methods make it pos- sible to control the location of as many features as necessary. One feature control frame may be used to show the position tolerance on a single hole, or it may be used to add a tolerance to a pattern of holes. Any model or drawing that includes position tolerances must include several characteristics. These are listed below and shown in Figure 8-2. ■ Datum features are identifi ed. (Exceptions to this are explained later in this chapter.) ■ Basic dimensions are used to defi ne locations. ■ At least one feature control frame specifi es a position tolerance. In the given example, position tolerances are applied on two groups of holes. Each of these characteristics has a purpose. The datum features serve to locate the datum refer- ence frame from which the hole locations are mea- sured. The basic dimensions defi ne the basic location of each hole. Basic dimensions may be defi ned by the data in a model or they may be shown in a drawing view. The basic location is commonly known as the true position. True position is a theoretically exact location and orientation that is established by basic dimensions. The feature control frame defi nes the size of the tolerance zone that is located on each of the true posi- tions and references the datum features that are used to create the datum reference frame. The given fi gure shows two feature control frames that defi ne the position tolerances for fi ve holes. The feature control frame in the top view is placed adjacent to a noted Unrelated actual mating envelope Axis Position RFS Position MMC Unrelated actual minimum material envelope Axis Position LMC Goodheart-Willcox Publisher Figure 8-1. The axis of a feature is defi ned as the axis of the unrelated actual mating envelope or of the unrelated actual minimum material envelope.