Geometrical tolerancing and datums

This chapter presents the advantages of using geometrical tolerances in engineering drawings and demonstrates how their correct use ensures that the communications between the drawing office and the workshop are complete and incapable of any misinterpretation. Geometrical tolerances are used over and above normal dimensional tolerances when it is necessary to control more precisely the form or shape of some feature of a manufactured part, because of the particular duty that the part has to perform. Geometrical tolerances are used to convey in a brief and precise manner complete geometrical requirements on engineering drawings. They should always be considered for surfaces, which come into contact with other parts, especially when close tolerances are applied to the features concerned. No language barrier exists, as the symbols used are in agreement with published recommendations of the International Organization for Standardization (ISO) and have been internationally agreed. The use of geometrical tolerances does not involve or imply any particular method of manufacture or inspection. A datum is designated by a capital letter enclosed by a datum box, which is connected to a solid or a blank datum triangle. A datum surface on a component should be accurately finished, since other locations or surfaces are established by measuring from the datum.