ISO 2768-fH general tolerance and the calculations
On the design drawing of a watch case or a precision aerospace component, the title block reads ISO 2768-fH. The inspector has to apply the fine class ‘f’ to every linear dimension and the strict geometric class ‘H’ to flatness and perpendicularity. The limits have to be worked out carefully to avoid over-inspection.
Applying ISO 2768-fH and its deviation tables
| Size range (mm) | Fine class ‘f’ (mm) | Flatness class H (mm) | Perpendicularity H (mm) |
|---|---|---|---|
| Over 0.5 up to 3 | ±0.05 | 0.02 | 0.02 |
| Over 3 up to 6 | ±0.05 | 0.05 | 0.05 |
| Over 6 up to 30 | ±0.10 | 0.10 | 0.10 |
| Over 30 up to 120 | ±0.15 | 0.15 | 0.15 |
| Over 120 up to 400 | ±0.20 | 0.25 | 0.20 |
All values are given for the reference environmental conditions of 20 °C.
Linear limits for the fine class f
For any dimension not explicitly toleranced on the drawing and between 6 mm and 30 mm, the fine class f gives a tolerance of ±0.10 mm. A 20 mm dimension with no tolerance of its own therefore has a lower limit of 19.90 mm and an upper limit of 20.10 mm. To compare these values with the medium class m, read our article on the ISO 2768-mK tolerance at 250 mm.
The geometric requirements of class H
The capital letter H governs form and orientation. For a 50 mm dimension, the maximum flatness allowed on the seating face is 0.15 mm. Perpendicularity between two adjacent faces must not exceed 0.15 mm. These limits apply to every precision-machined surface.
Example: inspecting an instrument plate
A technician inspects an 80 mm long brass plate dimensioned under ISO 2768-fH. The linear measurement between two shoulders gives 80.18 mm. 1. ISO 2768-1 fine range (30 to 120 mm): ±0.15 mm. 2. Allowed limits: [79.85 mm; 80.15 mm]. 3. Actual reading: 80.18 mm. Verdict: the measurement exceeds the upper limit by 0.03 mm. The plate is declared non-conforming.
Pitfalls in applying ISO 2768-fH
The first pitfall is applying the fine class f to cast or fabricated parts, where class m or c is required. The second is forgetting the tolerances on radii and chamfers. The third comes from general tolerances adding up along an assembly stack. To automate reading the title block, CoteCote relies on automatic dimension detection.