5.3 ISO tolerance ranges

Tolerance bands need to be defined which can be related to functional performance and manufacturing processes. The ISO has published tolerance ranges to help designers. Examples of these tolerance ranges are shown in Figure 5.4. This table is only a selection from the full table given in ISO 286 2:1988. The full range goes up to IT18 and 3m nominal size. The tolerance ranges are defined by IT ranges as shown in the diagram from IT1 to IT11. The range given in the ISO standard is significantly more complicated than the extract in Figure 5.4. It should be noted that the range increases as the IT number gets larger and the range increases as the nominal size increases. The latter is fairly logical in that one would expect the tolerance range to be larger as the diameter increases because the precision that can be achieved must be relative. The ranges were not chosen out of the blue but empirically derived and based on the fact that the relationship between manufacturing errors and basic size can be approximated by a parabolic function.