ISO 1101 symmetry tolerance and how to inspect it
On the drawing of a keyway machined in a drive shaft, the frame specifies a symmetry tolerance of 0.02 A. The inspector has to make sure the median plane of the keyway lies between two parallel planes 0.02 mm apart, placed symmetrically about the median datum plane A. A symmetry error makes the key bed in and deform in service.
Definition and measurement of an ISO 1101 symmetry tolerance
| Feature inspected | Datum feature A | Tolerance zone | Measuring instrument |
|---|---|---|---|
| Median plane of a slot | Median plane of the outside diameter A | Two parallel planes 0.02 mm apart | CMM / gauge pins + block |
| Median plane of a tenon | Locating hole axis A | Two planes 0.015 mm apart | 3D height gauge |
All values are given for the reference environmental conditions of 20 °C.
Calculating the symmetry offset with pins and probing
Insert two pins of identical diameter into the slot. Measure the distance between the edge of each pin and the outer edge of seat A. The difference between the two dimensions measured gives twice the symmetry offset. To go over other orientation tolerances, see our guides on ISO 1101 geometric symbols.
The symmetry tolerance zone and the theoretical median plane
Symmetry controls the position of the median plane built from the two actual faces of the slot. It must not be confused with the position tolerance, which can take the maximum material modifier.
Worked example: inspecting a keyway
10 mm keyway toleranced 0.02 A relative to the ⌀40 mm diameter. Dial indicator readings of the face offsets: left side = 14.992 mm, right side = 15.016 mm. 1. Offset of the median plane: (15.016 - 14.992) / 2 = 0.012 mm. 2. Symmetry value (2x offset): 0.024 mm. Verdict: the value (0.024 mm) exceeds the 0.020 mm limit. The part does not conform.
Practical pitfalls when measuring symmetry
The first pitfall is measuring the draft of the slot walls instead of the median plane. The second is choosing the wrong datum A. The third comes from forgetting the effect of the slot width. To record your results, use the CoteCote quality inspection module.