Calculating the m6 fundamental deviation and its limits
On the drawing of a dowel or a fixed assembly pin, the dimension reads ⌀20 m6. The setter has to turn the fit into numerical limits. Position m6 makes the shaft strictly larger than nominal, which gives a light interference fit when it goes into an H7 hole.
Values of the m6 deviation to ISO 286
| Nominal range (mm) | IT6 (µm) | Lower deviation ei (µm) | Upper deviation es (µm) | Example limits (mm) |
|---|---|---|---|---|
| Over 6 up to 10 | 9 | +6 | +15 | ⌀10: 10.015 / 10.006 |
| Over 10 up to 18 | 11 | +7 | +18 | ⌀18: 18.018 / 18.007 |
| Over 18 up to 30 | 13 | +8 | +21 | ⌀20: 20.021 / 20.008 |
| Over 30 up to 50 | 16 | +9 | +25 | ⌀40: 40.025 / 40.009 |
All values are given for the reference environmental conditions of 20 °C.
The ei and es deviations for m6
For the 18–30 mm range, the lower deviation ei is +8 µm (+0.008 mm). The upper deviation es is ei + IT6 = +8 + 13 = +21 µm (+0.021 mm). To go over other interference positions, read our piece on the H7/p6 fit and its interference.
Inspection limits for a 20 mm shaft
Max size = 20.000 + 0.021 = 20.021 mm. Min size = 20.000 + 0.008 = 20.008 mm. The size must lie between 20.008 mm and 20.021 mm.
Example: inspecting a locating pin
A ⌀20 m6 pin is measured at 20.014 mm at 20 °C. 1. Allowed limits: [20.008 mm; 20.021 mm]. Verdict: the reading (20.014 mm) sits in the middle of the range. The pin is declared conforming.
Common pitfalls in reading the m6 tolerance
The first pitfall is mixing up m6 with the capital position M6. The second is pressing parts together without checking the lead-in chamfers. The third comes from machining distortion. To make your dimension calculations reliable, use our CoteCote quality inspection module.