"ISO 2768-mK" is the most common tolerance note on machining drawings — and the most misread. The complete f/m/c/v and H/K/L tables, what they cover, and where they stop.
ISO 2768 supplies the tolerances for every dimension the drawing does not tolerance individually — one lower-case letter for lengths and angles, one capital for geometry.
The lower-case letter selects the linear/angular class from Part 1 — f (fine), m (medium), c (coarse), v (very coarse). The capital letter selects the geometric class from Part 2 — H, K or L.
So "ISO 2768-mK" means: medium general tolerances on untoleranced lengths and angles, class-K general limits on straightness, flatness, perpendicularity, symmetry and run-out.
Values per ISO 2768-1:1989 for untoleranced linear dimensions. The row for 6–30 mm is where most turned-part features fall.
| Nominal range (mm) | f — fine | m — medium | c — coarse | v — very coarse |
|---|---|---|---|---|
| 0.5 up to 3 | ±0.05 | ±0.1 | ±0.2 | — |
| over 3 up to 6 | ±0.05 | ±0.1 | ±0.3 | ±0.5 |
| over 6 up to 30 | ±0.1 | ±0.2 | ±0.5 | ±1.0 |
| over 30 up to 120 | ±0.15 | ±0.3 | ±0.8 | ±1.5 |
| over 120 up to 400 | ±0.2 | ±0.5 | ±1.2 | ±2.5 |
| over 400 up to 1000 | ±0.3 | ±0.8 | ±2.0 | ±4.0 |
| over 1000 up to 2000 | ±0.5 | ±1.2 | ±3.0 | ±6.0 |
| over 2000 up to 4000 | — | ±2.0 | ±4.0 | ±8.0 |
| Range | f / m | c / v |
|---|---|---|
| 0.5 up to 3 | ±0.2 | ±0.4 |
| over 3 up to 6 | ±0.5 | ±1.0 |
| over 6 | ±1.0 | ±2.0 |
| Length | f / m | c | v |
|---|---|---|---|
| up to 10 | ±1° | ±1°30′ | ±3° |
| 10 – 50 | ±30′ | ±1° | ±2° |
| 50 – 120 | ±20′ | ±30′ | ±1° |
| 120 – 400 | ±10′ | ±15′ | ±30′ |
| over 400 | ±5′ | ±10′ | ±20′ |
General tolerances widen in steps as the nominal dimension grows. For the ranges relevant to turned parts, the four classes separate clearly.
Part 2 sets general limits for form and position where the drawing shows no geometric tolerance frame. Values in mm.
| Class | up to 10 | 10–30 | 30–100 | 100–300 | 300–1000 | 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 |
| Class | ≤100 | 100–300 | 300–1000 | 1000–3000 |
|---|---|---|---|---|
| H | 0.2 | 0.3 | 0.4 | 0.5 |
| K | 0.4 | 0.6 | 0.8 | 1.0 |
| L | 0.6 | 1.0 | 1.5 | 2.0 |
| Class | ≤100 | 100–300 | 300–1000 | 1000–3000 |
|---|---|---|---|---|
| H | 0.5 | 0.5 | 0.5 | 0.5 |
| K | 0.6 | 0.6 | 0.8 | 1.0 |
| L | 0.6 | 1.0 | 1.5 | 2.0 |
Applied wherever no run-out frame is drawn — worth checking before you assume a concentricity is "uncontrolled".
Remains the working reference for general linear and angular tolerances and is still cited on new drawings worldwide. The f/m/c/v tables above are its complete published values.
ISO 22081 replaces the general geometric tolerances of Part 2 — but deliberately publishes no fixed value tables. The designer states explicit general geometric specifications instead. Legacy drawings citing 2768-K/L remain valid to their original reference, so both systems coexist for years to come.
Class m is our default reading where a drawing carries no note; class f is comfortable on CNC-turned brass. Swiss-turned diameters to ±0.005 mm (Ø 2–32 mm), milled features typically ±0.01 mm — as individual tolerances, confirmed against your approved drawing and documented with an EN 10204 3.1 certificate per shipment.
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