Chapter 1 Geometric Dimensioning and Tolerancing Essentials 21 Copyright Goodheart-Willcox Co., Inc. Positional tolerances, covered in Chapters 8 and 9, define a tolerance zone in which the center point, axis, or center plane of a feature of size can vary from true position. True position is the theoretically exact location of a feature of size as estab- lished by basic dimensions. Compared to coordinate tolerancing methods, positional tolerancing concepts provide some of the greatest advantages to mass production. Coordinate tolerancing limits the actual location of features to a rectangular tolerance zone. Using positional tolerancing, the positional tolerance zone changes to a cylin- drical shape, thus increasing the allowable location of the feature by about 57%. This improves the interchangeability of parts while increasing manufacturing flexibility and reducing the scrap rate. Maximum material condition (MMC), explained in Chapter 5, is the condition where a feature has the maximum amount of material within the stated limits of size. For example, maximum shaft diameter and minimum hole diameter are MMC appli- cations. Applying a positional tolerance at MMC allows the tolerance zone to increase as the feature size departs from MMC. This application also allows greater flexibility in the acceptance of mating parts. Example 1-16 shows locating one of the face plate holes using coordinate toleranc- ing methods. The 14±0.25 and 15±0.25 location dimensions in Example 1-16 establish a total tolerance zone of 0.5 (0.25 + 0.25 = 0.5). This tolerance zone is square, as shown in Example 1-17. The 0.5 square tolerance zone shown in Example 1-17 shows that the actual center of the hole can fall anywhere within the square area and the manufac- tured part is acceptable. Goodheart-Willcox Publisher Example 1-16. Locating a hole using coordinate tolerancing.