In the previous blog, we introduced the fundamental concepts of the ISO system of limits and fits, which provides a framework for designing parts with specific fits, such as holes and shafts. In this second part of the series, we will focus on the calculation of tolerance grades, a key element in the system that helps determine the accuracy and precision of manufactured parts.
Calculation of Tolerance Grades
In the ISO system of limits and fits, the values for tolerance grades remain consistent over a range of basic sizes. This range is defined by diameter steps, which represent the minimum and maximum sizes within that range. These diameter steps are crucial in ensuring that the tolerance grades, such as IT 1 to IT 16, can be applied uniformly across various sizes of parts. The values of tolerance grades for a given diameter range are typically multiples of the tolerance unit ‘i’, which is calculated using the R5 series of preferred numbers. For sizes up to and including 500 mm, the tolerance unit ‘i’ can be determined by the formula:
Experience has shown that in manufacturing processes, dimensional inaccuracies tend to be proportional to the cube root of the absolute size. This relationship is reflected in the ISO system, where the tolerance unit ‘i’ is calculated in units of 10⁻³ mm or µm from above equation.
In the ISO system of limits and fits, the tolerance unit 'i' is calculated in units of 0.001 mm (or 10⁻³ mm). The basic size D is the geometric mean of the diameter steps between which a particular basic size lies. This means that for a given size, the tolerance values are based on the average of the minimum and maximum size within the diameter step range, rather than the exact size.
The geometric mean is calculated as follows:
Diameter Steps in 'mm' |
|||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| To and Inc | 1-3 | ||||||||||||
| Over To and Inc | 3-6 | ||||||||||||
| Over To and Inc | 6-10 | ||||||||||||
| Over To and Inc | 10-18 | ||||||||||||
| Over To and Inc | 18-30 | ||||||||||||
| Over To and Inc | 30-50 | ||||||||||||
| Over To and Inc | 50-80 | ||||||||||||
| Over To and Inc | 80-120 | ||||||||||||
| Over To and Inc | 120-180 | ||||||||||||
| Over To and Inc | 180-250 | ||||||||||||
| Over To and Inc | 250-315 | ||||||||||||
| Over To and Inc | 315-400 | ||||||||||||
| Over To and Inc | 400-500 | ||||||||||||
For IT01, tolerance = 0.3 + 0.008 D
For IT0, tolerance = 0.5 + 0.012 D
For IT1, tolerance = 0.8 + 0.02 D
For IT2 to IT4 the values of tolerance grades are placed geometrically between the tolerance grades of IT1 and IT5
The values of the tolerances for tolerance grades IT5 to IT16 are given below :
| Tolerance Grade | IT5 | IT6 | IT7 | IT8 | IT9 | IT10 | IT11 | IT12 | IT13 | IT14 | IT15 | IT16 |
| Magnitude | 7i | 10i | 16i | 25i | 40i | 64i | 100i | 160i | 250i | 400i | 640i | 1000i |
The values of fundamental tolerances are given
The following list will demonstrate where these grades are applied according to their tolerance range in a particular application.
IT01 to IT4 – For the production of gauges, plug gauges, measuring instruments
IT5 to IT 7 – For fits in precision engineering applications
IT8 to IT11 – For General Engineering
IT12 to IT14 – For Sheet metal working or press working
IT15 to IT16 – For processes like casting, general cutting work
Supplements: