310 GD&T: Application and Interpretation Copyright Goodheart-Willcox Co., Inc. Additional Tolerance—Hole Position Tolerance at MMC Figure 8-18 shows the effect of the MMC modi- fi er on the position tolerance for a hole. Limits of size for the hole are .500″ and .506″ diameter. The spec- ifi ed position tolerance of .035″ diameter applies when the hole is at its MMC size of .500″ diameter. The chart shows that for every .001″ increase in the hole diameter, there is a .001″ increase in the allow- able position tolerance. The allowable position tolerance can easily be calculated. The allowable tolerance is equal to the sum of the specifi ed tolerance and the additional tolerance. The additional tolerance is determined by fi nding the difference between the produced hole size and the MMC hole size. The fi gure shows how the allowable tolerance is calculated for a hole produced at a .503″ diame- ter. The difference between the produced size (.503″) and the specifi ed MMC size (.500″) is found to deter- mine the additional tolerance (.003″). The additional tolerance (.003″) and the specifi ed position tolerance (.035″) are added together to determine the allowable position tolerance (.038″). Another way to calculate the allowable position tolerance for a hole is to fi nd the difference between the produced hole size and the virtual condition size. Refer to Figure 8-18. The virtual condition of a hole is its smallest size minus the specifi ed position tolerance. Additional Tolerance—Pin Position Tolerance at MMC Position tolerances applied to external features are affected by modifi ers in the same manner as toler- ances on internal features. See Figure 8-19. The pin in the given fi gure has limits of size from .375″ to .379″ diameter. A position tolerance of .028″ diameter at MMC is applied to the pin. When the pin is produced at the MMC size of .379″ diameter, the allowable position tolerance is .028″ diameter. There is an allowable increase in the position tolerance if the pin departs from the MMC size. The given chart shows that for every .001″ decrease in the pin diameter, there is a .001″ increase in the allowable position tolerance. The allowable position tolerance can just as easily be calculated for a pin as for a hole. The fi gure shows how the allowable tolerance is calculated for a pin produced at a .376″ diameter. The difference between the produced size (.376″) and the specifi ed MMC size (.379″) is calculated to determine the additional toler- ance (.003″). The additional tolerance (.003″) and the specifi ed position tolerance (.028″) are added together to determine the allowable position tolerance (.031″). Another way to calculate the allowable position tolerance for a pin is to fi nd the difference between the virtual condition and the produced size of the feature. Refer to Figure 8-19. The virtual condition of a pin is its largest size plus the position tolerance. Verifying Position Tolerances Produced hole locations may be verifi ed to meet positional tolerance requirements using any of several methods. Measurement machines and inspection software may be used to determine if feature locations are within specifi ed position toler- ances. Functional gages may be used to verify that position tolerances are achieved. Measurements may be made manually. .503 .038 MMC LMC = + = .035 + .003 = .038 Produced hole dia Allowable position tolerance .500 .501 .502 .504 .505 .506 .035 .036 .037 .039 .040 .041 Produced size – Additional tolerance Specified MMC size Allowable tolerance Additional tolerance Specified tolerance .503 – .500 = .003 .503 – .465 = .038 = Produced size – Allowable tolerance Virtual condition Goodheart-Willcox Publisher Figure 8-18. The MMC modifi er, when used on the position tolerance for an internal feature (hole), can signifi cantly increase the allowable tolerance on the produced holes.