78 Geometric Dimensioning and Tolerancing This chapter is designed to help you identify and read information related to datums on drawings. This chapter also covers the specifications for properly placing datum-related symbols on drawings. This information is covered without regard to specific inspection and tooling techniques. Advanced instruction is recommended after you gain a solid understanding of the basic fundamentals presented here. Datums Datums are planes, points, lines, axis, or a combination thereof. A datum is theoretically exact and is derived from the theoretical datum feature simulator. A datum feature is identified on the drawing with either a datum feature symbol or a datum target symbol. Datum features are used to establish the datum reference frame. Examples of physical datum feature simulators are machine tables, surface plates, gages, mandrels, specifically designed fixtures, or a set of points established on the datum feature using a coordinate measuring machine. These are commonly referred to as processing or inspection equipment. A simulated datum is a plane, point, line, axis, or a combination thereof similar to, or derived from the processing or inspection equipment. The datum reference frame is a set of three mutually perpendicular intersecting datum planes. It exists only in theory and not on the part itself. There are many concepts to keep in mind when selecting datum features, including the function of the part or feature, manufacturing processes, methods of inspection, the shape of the part, relationship to other features, assembly requirements, and design requirements. Datum features should be selected to match on mating parts, to be easily accessible, and of adequate size to permit control of the datum requirements. Datum features should not be removed by later processing. Datum Feature Symbol The datum feature symbol is placed on the drawing to identify the features of the object that are specified as datums and referred to as datum features. The datum feature symbol identifies physical features and shall not be applied to centerlines, center planes, or axes. This symbol is placed in the following locations on a drawing: • On the outline of a feature surface in the view where the surface appears as an edge. • On a leader line directed to the surface. The leader line can be shown as a dashed line if the datum feature is not on the visible surface. • On an extension line projecting from the edge view of a surface, clearly separated from the dimension line. • On a chain line next to a partial datum surface. • On the dimension line or an extension of the dimension line of a feature of size when the datum is an axis or center plane. • On the outline of a cylindrical feature or an extension line of the feature outline, separated from the size dimension, when the datum is an axis. • Above or below and attached to a feature control frame.