5 - Limits, Fits and Geometrical Tolerancing

Abstract

This chapter discusses the issue of variations that are to be allowed with respect to functional performance and the selection of a manufacturing process. In any manufacturing process, the tolerance that can be achieved depends on two things. Firstly, the variability caused by the vagaries within a manufacturing process—such as vibrations, discontinuities, and inconsistencies—which produce a deviation of some mean value. Secondly, there is a variation that occurs when a tool wears. Thus, in any accuracy graph or table, there are two factors involved: an increasing trend with wear and the variability scattered around this trend. Tolerance bands related to functional performance and manufacturing processes need to be defined. The International Standards Organization (ISO) has published tolerance ranges to help designers in a full table given in ISO 286–2:1988. The full range goes up to IT18 and 3 m nominal size. The range given in the ISO standard is significantly more complicated than the extract, and the range increases as the IT number gets larger and as the nominal size increases.