301 Chapter 8 Position Tolerancing Fundamentals Copyright Goodheart-Willcox Co., Inc. tolerance on them. In the given fi gure, rectangles are drawn around the dimension values to identify them as basic. It is also possible to use a general note to state that location dimensions for holes are basic. The basic dimensions defi ne the true positions for the two holes. The true position is exact and is related to the specifi ed datum reference frame. However, a true position does not indicate the hole is required to be perfectly located. There must always be an allow- able amount of position tolerance. The true position is the as-designed location for the hole, the specifi ed position tolerance zone, and the virtual condition. The axis of the actual produced hole may be located anywhere within the tolerance zone. When the toler- ance is applicable at MMC or LMC, the surface of the hole must not violate the virtual condition boundary. Figure 8-8 shows two horizontal dimensions chained together to locate the true positions of the two holes. There would be no difference in the true positions if both holes were dimensioned with base- line dimensions extending from the datum feature. There would not be a difference because there is no tolerance accumulation on basic dimensions. There is no tolerance applied to the basic dimension itself the posi- tion tolerance is applied to the features. Relationship to the Datum Reference Frame Position tolerances must include datum feature references except when the toleranced features act as the primary datum from which other features are located. This exception is explained in more detail later in this chapter. The datum feature references establish the datum reference frame. See Figure 8-9. A datum reference frame provides a known origin and coordinate system for dimensions and toler- ances. Review Chapter 6 for additional information about datum reference frames. Dimensions are applied with consideration given to referenced datum features. The given part shows a position tolerance in which datum features A, B, and C are referenced. The hole pattern is dimen- sioned to locate it relative to these datums. In a solid model where no dimensioned views are created, the hole locations relative to the datum reference frame are established by the digital data in the model. All location measurements for the two holes are made relative to the datum reference frame, and not relative to the part surfaces. There is a difference. The datum reference frame is a perfect coordinate system. The part surfaces have variations. Depending on the magnitude of the part variations, dimensions extending from the datum reference frame could be signifi cantly different from dimensions extending from the part surfaces. The exaggerated variations on the shown fab- ricated part indicate the importance of the datum reference frame. When working relative to the datum reference frame, all measurements are in a single coordinate system. Being able to remain in one coordinate system is unlikely when working off imperfect feature surfaces. Because of angular varia- tion between datum features B and C, measurements made off the two surfaces may not be perpendicular to one another. If the measurements are not perpen- dicular, then an accurate check of the hole positions cannot be made. Effect of Material Condition Modifi ers The three material conditions applicable to position tolerances have distinctly different effects. One of the material conditions is optimum for any given design application. Specifi cation of the correct material condition is relatively easy when the effects are clearly understood. From here Not from here Datum plane B Datum plane C Datum plane A True position Goodheart-Willcox Publisher Figure 8-9. Datum feature references in a position tolerance specifi cation require that locations be determined relative to the datum reference frame.