Unit 15 Geometric Dimensioning and Tolerancing 263 Copyright Goodheart-Willcox Co., Inc. in the feature control frame is referenced to a datum feature (datum feature A). Notice that the datum fea- ture symbol appears on the drawing and identifies the referenced datum feature. The tolerance specification in the feature control frame means that when the part is manufactured, the controlled feature (the bottom surface) must be perpendicular relative to the refer- enced datum feature (datum feature A) within .005″. Stated another way, the surface is required to be within a .005″ wide tolerance zone that is perpendicular to datum feature A. GD&T Terminology The following are several terms that should be reviewed prior to further study of GD&T. Additional terms will be defined as they appear in this unit. A datum is defined as a theoretically exact point, axis, line, or plane. A datum is used as an origin from which geometric controls of features are derived. In reality, a dimension can never be exact because of numerous variables that can cause variations in the manufacturing process. A basic dimension is a numeric value that states the theoretically exact size, shape, location, or angle of a feature or datum target. A basic dimension has no allowances or tolerances applied to its value. In addition, title block tolerances do not apply to basic dimensions. A rectangular frame placed around a dimension value indicates a basic dimension. Basic dimensions indicate a feature size or location to which tolerances are applied. A basic dimension used to locate a feature indicates that the tolerance for the dimensioned feature is defined by a feature control frame tolerance or a note. The term feature is used to describe a physical portion of a part, such as a surface, hole, thread, or groove. The term feature of size is a general term that can be applied to describe a regular feature of size or an irregular feature of size. A regular feature of size is a feature associated with a single size dimension, such as a cylindrical hole. An irregular feature of size is a feature or collection of features that may be nonuniform in shape. The theo- retically exact location of a feature of size defined by basic dimensions is known as the true position. As explained in previous units, a tolerance is the total amount a dimension is allowed to vary. The tolerance is the difference between the lowest allowable value and the highest allowable value. Geometric tolerance zones can be two-dimensional or three-dimensional in shape. Datums It is important to be aware of the different terms asso- ciated with datums and datum feature identification. A datum feature is an actual feature on a part used to establish a datum. As previously discussed, a datum is a theoretically exact entity. It is a theoretical, not actual, entity. A datum feature is an actual physical entity, such as a flat surface, and is identified on the drawing. Datum features can be identified on a draw- ing by using a datum feature symbol, datum target symbol, or note. Datum feature A Datum feature B Goodheart-Willcox Publisher Figure 15-2. Datum feature symbols applied to a part. Geometric characteristic symbol Tolerance Goodheart-Willcox Publisher Figure 15-3. Feature control frame. Datum feature reference Tolerance Geometric characteristic symbol Goodheart-Willcox Publisher Figure 15-4. Parts of a feature control frame. Goodheart-Willcox Publisher Figure 15-5. Application of a datum feature symbol and a feature control frame.