A feature for ISO 2768-mk establishes a "medium" precision standard for parts, ensuring they are manufactured within acceptable limits for both size and shape without requiring individual tolerance callouts for every dimension. The designation breaks down into two parts:
- "m" (Medium) – Refers to Part 1 (linear & angular dimensions). This is the most commonly used class for general machining.
- "k" (Klasse) – Refers to Part 2 (geometrical tolerances). This class corresponds to "medium" for form and position.
- For nominal size up to 3: m = ±0.1, k = ±0.3
- Over 3 up to 6: m = ±0.12, k = ±0.35
- Over 6 up to 30: m = ±0.15, k = ±0.5
- Over 30 up to 120: m = ±0.2, k = ±0.8
ISO 2768-1 (m): Represents the "medium" tolerance class for linear and angular dimensions (external sizes, diameters, radii, distances).
- ISO 2768 = standard for general tolerances on linear and angular dimensions without individual tolerance indications; “m” = medium tolerance grade; “k” = coarse/very coarse tolerance grade.
- f (fine) < m (medium) < c (coarse) < k (very coarse); choose based on manufacturing precision and cost: k for rough parts where fit/appearance not critical, m for general machined parts.
- Applies to linear/angular dimensions without specific tolerances; excludes: Geometric tolerances (form/position), holes/shafts with tolerance classes (e.g., H7), surface finish indications.
- Specific tolerance on a dimension takes precedence — ISO 2768 applies only where no specific tolerance is given.
- Surface finish, form and position tolerances are not covered by ISO 2768 and must be specified separately.
Geometric Tolerances (Flatness, Straightness, Perpendicularity)
| Nominal Size Range (mm) | Tolerance ‘k’ (mm) | | :--- | :--- | | Up to 100 | 0.1 | | >100 to 300 | 0.2 | | >300 to 1000 | 0.3 | | >1000 to 3000 | 0.4 |