General Tolerances for Straightness and Flatness
Straightness and flatness are included in the general tolerances found in standard ISO 2768-2:1989. They define how much an edge or surface can deviate from a perfect form when there is no specific geometric or dimensional tolerance given.
- Straightness refers to the deviation of an individual edge or axis from a straight line. With general tolerance, it’s ensured that, for example, a shaft or a machined edge remains straight enough for practical use.
- Flatness describes how much an entire surface is allowed to deviate from an ideal plane. This is especially important for mating surfaces, where even slight unevenness can affect how parts fit together.
On drawings, specific markings for straightness and flatness are not given when general tolerances are applied. Instead, an indication like ISO 2768-mK is enough, meaning all such properties are automatically subject to the standard’s allowed deviations.
General Tolerance Table Straightness and Flatness
| Tolerance Class | Straightness and Flatness Tolerances by Nominal Length Ranges | ||||||
| Code | > | 10 | 30 | 100 | 300 | 1000 | |
| ≤ | 10 | 30 | 100 | 300 | 1000 | 3000 | |
| H | 0.02 | 0.05 | 0.1 | 0.2 | 0.3 | 0.4 | |
| K | 0.05 | 0.1 | 0.2 | 0.4 | 0.6 | 0.8 | |
| L | 0.1 | 0.2 | 0.4 | 0.8 | 1.2 | 1.6 | |
The values in the table are given in millimeters (mm)
