A tolerance designation has two parts
ISO 286 is the international standard for limits and fits of cylindrical features. In Ø20 h9, the letter h gives the position of the tolerance zone relative to the nominal size (the fundamental deviation) and the number 9 gives the width of the zone (the IT grade).
- Fundamental deviation (letter)
- Places the tolerance zone above, below or across the nominal size.
- IT grade (number)
- The width of the zone. A smaller number means a narrower zone and more precise processing.
- Nominal size range
- For the same IT grade, the permitted width grows with diameter. Actual limit deviations are tabulated by size range.
Upper case for holes, lower case for shafts
In ISO 286, capital letters (H, F, K and so on) apply to internal features such as holes and lower-case letters (h, g, k and so on) apply to external features such as shafts. The same letter in upper and lower case places the zone differently, so the two are read separately on a drawing.
Hole-basis and shaft-basis systems
- Hole basis: the hole is fixed at H (lower deviation zero) and the fit is set by changing the shaft deviation. It reduces the number of hole-making tools and gauges and is common in general machinery.
- Shaft basis: the shaft is fixed at h (upper deviation zero) and the hole is varied. It suits cases such as several parts fitted with different fits along one shaft.
Why bright bars are specified with h tolerances
With an h tolerance the upper deviation is zero, so the actual diameter does not exceed the nominal size. Specifying drawn or ground bright bars in an h grade keeps the stock from exceeding the stated diameter when it enters a chuck or guide bush, and pairs naturally with an H hole as the reference fit.
The grade range depends on the processing route. Bars finished precisely, for example by grinding, are often specified in grades with smaller numbers (narrower zones). The grade actually applied is set by the product standard and the drawing.
About the numerical table
Specific limit deviations by size range are defined in the tables of ISO 286-2. When ordering, confirm them from the tolerance symbol and deviations stated on your drawing and applicable standard.
| Nominal size (mm) | h8 | h9 | h10 |
|---|---|---|---|
| over 3 to 6 | 0 / −0.018 | 0 / −0.030 | 0 / −0.048 |
| over 6 to 10 | 0 / −0.022 | 0 / −0.036 | 0 / −0.058 |
| over 10 to 18 | 0 / −0.027 | 0 / −0.043 | 0 / −0.070 |
| over 18 to 30 | 0 / −0.033 | 0 / −0.052 | 0 / −0.084 |
| over 30 to 50 | 0 / −0.039 | 0 / −0.062 | 0 / −0.100 |
Each cell shows the upper / lower deviation in mm. The values above are reference values: figures that agreed across two published manufacturer catalogs, checked against the ISO 286-1 tolerance-unit calculation. Grades and size ranges not in the table, and the final limit deviations, are confirmed against the drawing and the applicable standard.