A datum feature is selected on the basis of its geometric relationship to the toleranced feature and the requirements of the design. To ensure proper assembly, corresponding interfacing features of mating parts should be selected as datum features. However, a datum feature should be accessible on the part and be of sufficient size to permit its use. Datum features must be readily discernible on the part. Therefore, in the case of symmetrical parts or parts with identical features, physical identification of the datum feature on the part may be necessary.
4.3.1 Temporary and Permanent Datum Features. Selected datum features of in-process parts, such as castings, forgings, machinings, or fabrications, may be used temporarily for the establishment of machined surfaces to serve as permanent datum features. Such temporary datum features may or may not be subsequently removed by machining. Perma-neni datum features should be surfaces or diameters not appreciably changed by subsequent processing operations.
4.3.2 Datum Feature Identification. Datum features are identified on the drawing by means of a datum feature «vmbol The datum feature «vmhol identifies physical features and shall not be applied to center lines, center planes, or axes except as defined in paras. 4.6.6 and 4.6.7.
4.3.3 Datum Feature Controls. Measurements made from a datum reference frame do not take into account any variations of the datum features. Consideration shall be given to controlling the desired accuracy of the datum features by applying appropriate geometric tolerances. Where a control of an entire feature becomes impracticable, use of datum targets may be considered or a partial surface may be designated as the datum feature. See paras. 4.5.10 and 4.6.
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