Chapter 1 Geometric Dimensioning and Tolerancing Essentials 11 Copyright Goodheart-Willcox Co., Inc. The tolerances allow the dimensions to vary from perfect geometry within specified lim- its. For example, a bilateral tolerance of ±0.5 mm applied to the 70 mm dimension allows the width of the part to be as large as 70.5 (70 + 0.5 = 70.5) and as small as 69.5 mm (70 – 0.5 = 69.5). A bilateral tolerance is a tolerance permitted to vary in both the plus (+) and minus (–) directions from the specified dimension. A specified dimension is the part of the dimension from where the limits are calculated. If the part is larger than 70.5 mm (the maximum limit) or smaller than 69.5 mm (the minimum limit), the part does not meet functional requirements and is rejected. Various factors determine the selected tolerance, such as the desired manufacturing process, the surface finish, and the size of features. Directly toleranced dimensions are most appropriate for defining the size of fea- tures such as chamfers, fillets, rounds, bosses, and holes. See Example 1-6. In the example shown, a general note is used on the drawing to indicate applicable toler- ances. The tolerances given in the general note apply when tolerances are not other- wise specified. Geometric tolerancing is superior to coordinate tolerancing for locating features, describing the form of features, and directing relationships between features because of the ambiguity created by directly toleranced dimensions. Example 1-7 shows the face plate from Example 1-5 defined using GD&T. The face plate example is used in the following sections to compare coordinate tolerancing and geometric tolerancing. Chamfer Round Fillet Hole Boss Goodheart-Willcox Publisher Example 1-6. Directly toleranced dimensions are best for dimensioning the size of features, such as the features shown on this inch unit orthographic drawing. As shown in this example, tolerances may be indicated by a general note.