318 GD&T: Application and Interpretation Copyright Goodheart-Willcox Co., Inc. the tolerances. In this particular example with only two holes, it appears that a .015″ square zone can be converted to a .0212″ round zone. For this particular arrangement of two holes, that may be an accept- able conversion, but without knowing the hole sizes, fastener size, and whether a fl oating or fi xed fastener is used, the needed tolerance may be unknown. Further- more, if there are more holes in the pattern, the way the coordinate tolerances work to create the tolerance zone may not be the same as in this fi gure. Conversion from one tolerance zone shape to another is not rec- ommended. Position tolerances should be calculated because there really is not equivalence between the two methods of applying tolerances. Additional Tolerance (Bonus Tolerance) It has been shown that using a round tolerance zone increases the tolerance zone area by 57% in comparison to the area of a square zone. The round tolerance zone of a position tolerance may be further increased to obtain additional tolerance that comes from using the MMC or LMC modifi er. Figure 8-27 shows how the additional toler- ance may be used to increase the acceptable amount of location variation. An example hole specifi cation is given. Just below it, a table compiling inspection data from two holes on a produced part is shown. The resulting position variation is tabulated in the right column. The methods for calculating the diameter of the position variation were previously explained in this chapter. A second table shows one method used to deter- mine the additional tolerance for each of the holes. The additional tolerance on hole #1 is .003″. The .003″ additional tolerance is added to the specifi ed .031″ position tolerance and results in an allowable position tolerance of .034″ for hole #1. The additional tolerance on hole #2 is .004″. This value added to the specifi ed .031″ position tolerance results in an allow- able position tolerance of .035″ on hole #2. The additional tolerance on holes #1 and #2 signifi cantly increases the allowable variation. However, the variation will not prevent the part from assembling properly. The advantage of an additional tolerance for location can only be achieved by using position tol- erances and the MMC or LMC material condition modifi er. There is no way of taking advantage of additional tolerances when using coordinate (plus and minus) tolerances. Coordinate tolerances pro- vide a fi xed tolerance value. It remains unchanged regardless of how large the holes are produced. There are several ways in which additional tolerances may be used to increase producibility. Careful planning can actually let the manufacturing organization take advantage of the additional toler- ance before any holes are produced. For the example Goodheart-Willcox Publisher Figure 8-26. The area of a round tolerance zone is 57% greater than the area of a square zone. For a given hole pattern dimensioned with plus and minus tolerances, it is not recommended to convert from a square tolerance zone to a round tolerance zone. Instead, position tolerances should be calculated. Hole #1 Hole #2 X Y 1.000 2.500 1.009 2.492 +.009 −.008 X Y 2.500 1.375 1.390 2.505 +.015 +.005 Ø.0316 Hole #2 Hole #1 .281 .284 .003 .281 .285 .004 Yes .0316 .035 Yes .0241 .034 Hole #1 Hole #2 Specified tolerance at MMC + Additional position tolerance = Allowable position tolerance Example hole specification Tabulated hole location data Measured diameter - Specified MMC = Additional position tolerance Specified location Produced location Position variation Specified MMC Measured diameter Additional position tolerance Allowable tolerance Position variation Acceptable part Ø.0241 Goodheart-Willcox Publisher Figure 8-27. Departure from MMC results in additional tolerance that further increases the size of the allowable position tolerance zone.