Calculating the g6 fundamental deviation and its limits

On the drawing of a pump drive shaft, the sliding guide seat is designated ⌀25 g6. The turner wants the exact numerical limits to set up the lathe. Position g gives a guaranteed minimum clearance with an H7 hole, which prevents seizing under thermal load.

Values of the g6 tolerance position to ISO 286

Nominal range (mm)	IT6 (µm)	Upper deviation es (µm)	Lower deviation ei (µm)	Limits for the range’s upper size (mm)
Over 6 up to 10	9	-5	-14	9.995 / 9.986
Over 10 up to 18	11	-6	-17	17.994 / 17.983
Over 18 up to 30	13	-7	-20	24.993 / 24.980 (⌀25)
Over 30 up to 50	16	-9	-25	49.991 / 49.975

All values are given for the reference environmental conditions of 20 °C.

Upper and lower deviations for g6

For a g6 shaft, the upper deviation es is negative. For the 18–30 mm range, es is -7 µm (-0.007 mm). The lower deviation ei is es - IT6, i.e. -7 - 13 = -20 µm (-0.020 mm). To go back over the general rules for shafts, see our article on converting ISO 286 fits into numerical limits.

Inspection limits for a 25 mm shaft

Max size = 25.000 - 0.007 = 24.993 mm. Min size = 25.000 - 0.020 = 24.980 mm. The tolerance interval spans 13 microns. Any reading outside this range means the shaft is rejected.

Machining example and metrology verdict

A ⌀25 g6 shaft is measured with a micrometer at 20 °C. The reading is 24.976 mm. 1. Allowed limits: [24.980 mm; 24.993 mm]. 2. Measured value: 24.976 mm. Verdict: the part is 4 µm undersize. The shaft is declared non-conforming.

Shop-floor pitfalls when making a g6 shaft

The first pitfall is getting the sign of es the wrong way round. The second is the part heating up on the lathe. The third comes from a micrometer calibration error. To automate your tolerance calculations, discover our automatic fit detection tools.