Chapter 1 Geometric Dimensioning and Tolerancing Essentials 23 Copyright Goodheart-Willcox Co., Inc. In a positional tolerance for the location of a hole, this 0.7 diagonal becomes a diameter tolerance zone cylindrical in shape through the thickness of the part. The diagonal tolerance zone is acceptable in any direction, thus creating a circular tolerance zone with a diameter equal to the diagonal length. The result is an increase of 57% in permissible area to locate the hole. Example 1-19 shows the relationship between the square coordinate tolerance zone and the round positional tolerance zone. Positional tolerancing increases tolerances for production purposes and the number of accept- able mating parts, which reduces manufacturing costs. Refer to Appendix B8 for infor- mation about analyzing the XY location of a hole and verifying that it is within the positional tolerance zone. Tolerance Buildup In the drawings shown in Examples 1-5 and 1-7, the holes of the face plate are located using chain dimensioning. In chain dimensioning, the dimensions are placed one after the other from one feature to the next. Caution should be used with chain dimension- ing and coordinate tolerancing, because the tolerance of each dimension builds on the next. This is referred to as tolerance buildup, tolerance accumulation, or tolerance stacking. The face plate includes two chain dimensions with individual tolerances of ±0.25. If each hole has an actual size measured at or near the upper (+0.25) limit, the potential tolerance buildup is 2 × 0.25 for a total accumulation of 0.5. See Example 1-20. 57% more area Goodheart-Willcox Publisher Example 1-19. The positional tolerance zone circumscribed about the square tolerance zone produced by coordinate tolerancing.