Dimensional tolerances take into account the quality of the part. The quality or quality index is a set of tolerances corresponding to the same degree of precision for any group of diameters. The higher the quality of the part, the lower the tolerance, and the lower the quality index.
The ISO standard distinguishes eighteen grades designated as IT01, IT0, IT1, IT2, IT3…, for nominal dimensions between 0 and 500 mm, with the higher the number associated with the designation, the lower the quality grade. In this way, the following main groups of grades can be distinguished:
– Grades 01 to 3 for shafts, and 01 to 4 for bores, are used for the manufacture of gauges and high-precision parts.
– Grades 4 to 11 for shafts and 5 to 11 for bores are intended for parts to be adjusted.
– Finally, grades above 11 are used for isolated parts or elements that do not require such a fine finish.
The following table shows the fundamental values in microns (µm) for each of the eighteen grades and for each of the thirteen dimension groups of the main series:
