![]() 4-5 are attached to actual features and are acceptable as datum features. It is not clear whether the outside edges, one of the hole patterns, or the slots are the features that determine these center planes. Datum feature symbols must not be applied to centerlines, center planes, or axes.ĭatum feature symbols B and C attached to the center planes in Fig. The datum feature symbol is used to identify physical features of a part as datum features. Datum features may be designated with any letter of the alphabet except I, O, and Q. That is, if a feature is oriented or located relative to one or more surfaces, each surface must be identified with a datum feature symbol, and the letters in the datum feature symbols must appear in a feature control frame in their proper order of precedence or importance. The left edge might have been selected as the secondary datum feature if it were a mating surface.ĭatum features must be identified with datum feature symbols or datum target symbols and specified in a feature control frame. The bottom edge of the part was selected as the secondary datum feature because it is larger than the left edge. The locating datum features are selected because of their relative importance to the controlled features: the holes. Datum feature B is specified as the secondary datum feature, and datum feature C is specified as the tertiary datum feature in the feature control frame. The hole locations are dimensioned from the bottom and left edges of the part. Datum feature A makes a good primary datum feature for the four holes because the primary datum feature controls orientation, and it is desirable to have bolt holes perpendicular to mating surfaces. The designer selected the back of the part as the primary datum feature, datum feature A, because the back of the part mates with another part, and the parts are bolted together with four bolts. Selecting datum features is the first step in dimensioning a part. If parts are symmetrical or have identical features making identification of datum features impossible, the datum features must be physically identified. Surfaces of sufficient size to allow repeatable measurementsĭatum features must be easily identifiable on the part. When selecting datum features, the designer should consider the following characteristics: The tertiary datum feature on the part contacting the simulated datum reference frame eliminates one degree of freedom, that is, it eliminates translation in the X direction.Ī datum feature is selected on the basis of its functional relationship to the toleranced feature(s) and the requirements of the design. The secondary datum feature on the part contacting the simulated datum reference frame eliminates two degrees of freedom, translation in the Y direction, and rotation around the Z-axis. The primary datum feature on the part contacting the simulated datum reference frame eliminates three degrees of freedom, translation in the Z direction, and rotation around the X-axis and the Y-axis. Finally, the part is slid along the first two planes until it contacts the third plane with a minimum of one point of contact. The secondary datum feature is pushed up against a second plane with a minimum of two points of contact. In order to properly place an imperfect, rectangular part in a datum reference frame, the primary datum feature sits flat on one of the planes with a minimum of three points of contact, not in a straight line. A part can move back and forth in the X direction, in and out in the Y direction, and up and down in the Z direction and rotate around the X-axis, around the Y-axis, and around the Z-axis.Ī part is oriented and immobilized relative to the three mutually perpendicular intersecting datum planes of the datum reference frame in a selected order of precedence, as shown in Fig. ![]() Parts are thought to have six degrees of freedom, three degrees of translational freedom, and three degrees of rotational freedom. Position-general functions and location applications.Orientation-perpendicularity, parallelism, and angularity.Form-flatness, straightness, circularity, and cylindricity.Dimensioning and tolerancing fundamentals. ![]() Detailed drawings and examples illustrate each concept. You will get clear definitions along with detailed discussions on how each geometric control is specified, interpreted, and inspected. Written in accordance with the latest revision to the GD&T standard, ASME Y14.5-2018, Geometric Dimensioning and Tolerancing for Mechanical Design, Third Edition, shows, step by step, how to improve quality, lower cost, and shorten delivery times. This thoroughly revised engineering textbook teaches the principles and practices of geometric dimensioning and tolerancing in a straightforward, easy-to-follow manner. A fully updated guide to geometric dimensioning and tolerancing.
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