311 Chapter 8 Position Tolerancing Fundamentals Copyright Goodheart-Willcox Co., Inc. Coordinate measurements may be compared to the basic (true) position coordinates to determine the amount of variation that exists in each produced feature location. The difference between the basic position coordinate value and the measured value is the amount of variation from true position. The vari- ations from true position may be referred to as the delta coordinates, or delta X (ΔX), delta Y (ΔY), and delta Z (ΔZ) values. See Figure 8-20. The delta coordinates may be used to calculate the diameter of the positional variation for each hole as explained in the next section. These calculations may be performed manually or automated. Software for these calculations is commercially available. The table in Appendix A1 may be used to assist in the review of measurement data. The delta coordinates .376 .031 MMC LMC = + = .028 + .003 = .031 Produced pin dia Allowable position tolerance .379 .378 .377 .375 .028 .029 .030 .032 Produced size – Additional tolerance Specified MMC size Allowable tolerance Additional tolerance Specified tolerance .379 – .376 = .003 Virtual condition Produced size = Allowable tolerance .407 – .376 = .031 – Goodheart-Willcox Publisher Figure 8-19. The MMC modifi er, when used on the position tolerance for an external feature (pin or shaft), can signifi cantly increase the allowable tolerance on the produced parts. Measured hole data Orthographic views Produced part Variation Diameter Hole # X Y X Y .191 .192 1 2 .506 .374 .500 .375 .006 –.001 .500 .875 .496 .878 –.004 .003 –.006 .004 .002 .002 .869 1.504 .377 1.502 .875 1.500 .375 1.500 4 3 .191 .191 Y X Y X Variation Diameter Hole # Measured location Basic dimension Measured location Basic dimension Goodheart-Willcox Publisher Figure 8-20. Design requirements and measurement data are used to assess whether or not the variation in a produced part is acceptable.