Geometrical Tolerances—Separate, Combined or Simultaneous?

Abstract

The 14 geometrical tolerances defined in ISO 1101 are supplemented by the alphanumerical symbols defined in this standard and some other standards. The symbols CZ (combined zone), SZ (separate zones) and SIM (simultaneous requirement), which are crucial for the development of robust measuring programs for coordinate measuring machines, were introduced at different times or in different standards. It is shown that the symbol definitions are not always complete. Sometimes there are no univocal rules for their use, which, in some cases, leads to ambiguity in the specifications given by a designer. It is also pointed out that certain functional requirements can be controlled by different symbols, and it is not always clear if the indications are equivalent. This makes it difficult to understand and interpret a drawing by a metrologist and, thus, may lead to uncertainty in the assessment of product conformity regarding specifications. The identified ambiguities and problems in the specification of functional requirements are shown in several figures. Corrections and additions to current standards are proposed.