Calculating the H7 and h7 fundamental deviations in mm
On the drawing of a hydraulic pump body, the guide bore is designated ⌀30 H7 while the mating shaft is toleranced ⌀30 h7. The turner at the machine wants the exact numerical limits before setting the finishing tool. Without numbers converted into millimetres, there’s no way to set up the lathe or zero the flat-anvil micrometer.
Calculating the H7 and h7 fundamental deviations to ISO 286
| Nominal range (mm) | IT7 value (µm) | H7 hole (mm) | h7 shaft (mm) |
|---|---|---|---|
| Over 3 up to 6 | 12 | +0.012 / 0.000 | 0.000 / -0.012 |
| Over 6 up to 10 | 15 | +0.015 / 0.000 | 0.000 / -0.015 |
| Over 10 up to 18 | 18 | +0.018 / 0.000 | 0.000 / -0.018 |
| Over 18 up to 30 | 21 | +0.021 / 0.000 | 0.000 / -0.021 |
| Over 30 up to 50 | 25 | +0.025 / 0.000 | 0.000 / -0.025 |
| Over 50 up to 80 | 30 | +0.030 / 0.000 | 0.000 / -0.030 |
| Over 80 up to 120 | 35 | +0.035 / 0.000 | 0.000 / -0.035 |
| Over 120 up to 180 | 40 | +0.040 / 0.000 | 0.000 / -0.040 |
All values are given for the reference environmental conditions of 20 °C.
Limits for an H7 hole
For an H7 hole, the capital letter H sets the lower deviation at zero. This means the actual diameter of the machined part can never be smaller than the nominal diameter, or it is rejected outright.
The upper limit is found by adding the IT7 value from the standard table straight to the nominal size on the drawing.
For a nominal diameter of exactly 30 mm, the size falls in the range from 18 mm to 30 mm inclusive. The matching IT7 interval is 21 microns (0.021 mm).
The lower limit is therefore 30.000 mm and the upper limit 30.021 mm. The tolerance allows no extra material and a bore up to 21 microns larger.
The guide on converting ISO 286 fits into numerical limits lets you compare this position with interference positions such as p6 or m6.
Limits for an h7 shaft
For an h7 shaft, the lower-case letter h sets the upper deviation at zero. The machined diameter can’t exceed the nominal value on the drawing.
The lower limit is found by subtracting the IT7 value from the nominal size.
For a nominal size of 30 mm, the IT7 value stays at 0.021 mm for the 18 to 30 mm range.
The upper limit is 30.000 mm and the lower limit falls to 29.979 mm. The shaft is accepted if its measured diameter lies between 29.979 mm and 30.000 mm.
Because the limits sit asymmetrically about the nominal, tool offsets on the machine tool need extra care.
Applied example and acceptance verdict
An inspector receives a 35NCD16 steel guide pin dimensioned ⌀30 h7 for an aircraft actuator. The part has been ground, then kept in the metrology room for five hours at 20 °C.
The check is done with a steel-anvil outside micrometer set on a 25 mm gauge block. The reading taken across the middle of the seat is 29.975 mm.
- Nominal size on the drawing: 30.000 mm.
- ISO 286-1 size range: from 18 mm to 30 mm inclusive.
- IT7 tolerance interval: 21 µm (0.021 mm).
- Upper deviation es: 0.000 mm (max limit = 30.000 mm).
- Lower deviation ei: -0.021 mm (min limit = 29.979 mm).
- Measured value: 29.975 mm.
Verdict: the measured size (29.975 mm) is 4 microns below the minimum limit (29.979 mm). The part is declared non-conforming, undersize beyond recovery. The pin is scrapped, because hard chrome rebuilding would cost more than a new part.
Classic pitfalls when reading H7 and h7
The first pitfall concerns range boundaries. A nominal size of 30.000 mm belongs to the 18–30 mm range (IT7 = 21 µm). A size of 30.001 mm moves straight into the 30–50 mm range, where IT7 rises to 25 µm. Reading the wrong row of the ISO 286 grid shifts the upper limit by 4 microns.
The second is swapping the letter case when copying into the FAI package. Writing a capital H instead of a lower-case h turns a shaft into a hole. The tolerance interval flips completely about the nominal, and parts judged good at machining all turn out bad at assembly inspection.
The third comes from forgetting temperature. A 30 mm aluminium bore measured at 28 °C in an unheated shop has grown by more than 5 microns. The part measured as conforming on the lathe will turn out under its lower limit once it has cooled to 20 °C in the customer’s laboratory.
To extract fit values from your PDF drawings without errors, CoteCote’s automatic ISO 286 detection turns the designation into numerical limits instantly, ready for your dimensional inspection sheet.