The letters 'm' and 'k' are not arbitrary; they denote specific tolerance classes.
Key Insight: Using ISO 2768-mk tells the manufacturer: "Unless a specific tolerance is written on a dimension, assume the 'medium' class for size and the 'medium' class for geometry."
Understanding where 'mk' fits in the hierarchy helps designers choose correctly. general tolerance iso 2768-mk
| Standard | Class | Precision Level | Typical Application | | --- | --- | --- | --- | | ISO 2768-fH | Fine / High | Very tight | Precision instruments, aerospace | | ISO 2768-mk | Medium / Medium | Balanced | General machining, automotive, industrial equipment | | ISO 2768-cL | Coarse / Low | Loose | Welded assemblies, rough castings |
Why choose 'mk' over 'fH'? Cost. Tight tolerances require slower machining speeds, specialized tooling, and 100% inspection. 'mk' provides sufficient accuracy for 80% of mechanical parts without inflating the budget. The letters 'm' and 'k' are not arbitrary;
Why choose 'mk' over 'cL'? Functionality. 'cL' is too loose for sliding fits, rotating shafts, or aligned mounting holes. 'mk' ensures basic interchangeability.
These apply to features (flatness, straightness, perpendicularity, symmetry, runout) unless a specific tolerance is given. Key Insight: Using ISO 2768-mk tells the manufacturer:
| Nominal Size Range | Permissible Deviation (± mm) | |--------------------|-------------------------------| | 0.5 – 3 | ±0.2 | | >3 – 6 | ±0.5 |
This section covers geometric deviations (form and position) for features without individual tolerance indications. It is divided into three classes:
| 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 |
Note: For symmetry and runout (ISO 2768-2), the ‘k’ class generally allows 0.2mm for most common part sizes.