Independence of metrological operations: the relevance of datum systems in GD&T

Nowadays most of the technical drawings, to be complete, must contain information that allows a correct reading about its functionality, and simultaneously the correct interpretation for manufacturing and assembly operations: It is desirable that information allow a definition of manufacturing and assembly sequences to create conditions that lead to more probabilities to avoid production errors. With the increase of the use of geometrical tolerances, the inspection operations are essentially coordinate metrology. So, as to manufacturing and assembly, also to inspection is imperative the definition of a correct sequence of operations, which are dependent of adequate datum systems, to avoid “circular” references that may be a source of coupled operations. An important condition to produce parts with quality is using the same reference systems (datum system) to all operations in manufacturing, in assembly and in inspection. This is obtained in the design phase and this is possible with GD&T - which is a true engineering language – using the independence concept of Axiomatic Design. In this paper is presented a case that illustrates the application of the independence axiom as a contribute to an adequate application of GD&T, which points to a correct sequence of inspection operations.