Every feature on products or parts has a size and a geometrical shape. To ensure that the size and geometry of all features are made as required, we should carefully take care of the tolerancing on the drawing. Nothing shall be implied or left to interpretation in the workshop or inspection department. General tolerances for size and geometry make it easier to ensure that the size and geometry of all features can be done as requested.
What is ISO 2768?
The ISO 2768 series of standards were developed by the International Organization for Standardization to provide general tolerances for linear and angular dimensions without individual tolerance indications on technical drawings. Since individual tolerances aren’t provided, the designer must ensure that products made to the drawings will function properly.
What is ISO 2768-mK Meaning?
ISO 2768-mK means the dimension information for which the tolerances are not specified will be followed according to the m and K class. m class is specified in ISO 2768-1, and the K class is specified in ISO 2768-2, which includes H, K, and L tolerance levels.
The following is a summary of ISO 2768-1 and ISO 2768-2 specifications:
#1 General Tolerances ISO 2768-1
ISO 2768-1 stands for the general tolerances for linear and angular dimensions without individual tolerance indications, ISO 2768-1 indicates the linear dimensions and angular dimensions such as external sizes, internal sizes, step sizes, diameters, radii, distances, external radii, and chamfer heights for broken edges. This standard covers general tolerances in three 4 classes of tolerance:
- M – Medium tolerances
- F – Fine tolerances
- C – Coarse tolerances
- V – Very coarse tolerances
Table 1 – Linear Dimensions
| Permissible deviations in mm for ranges in nominal lengths | Tolerance Class Designation (Description) | |||
|---|---|---|---|---|
| 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 |
For nominal sizes below 0.5 mm, the deviations shall be indicated adjacent to the relevant nominal size(s).
Table 2 – External Radii and Chamfer Heights
| Permissible deviations in mm for ranges in nominal lengths | Tolerance Class Designation (Description) | |||
|---|---|---|---|---|
| f (fine) | m (medium) | c (coarse) | v (very coarse) | |
| 0.5 up to 3 | ±0.2 | ±0.2 | ±0.4 | ±0.4 |
| over 3 up to 6 | ±0.5 | ±0.5 | ±1.0 | ±1.0 |
| over 6 | ±1.0 | ±1.0 | ±2.0 | ±2.0 |
For nominal sizes below 0.5 mm, the deviations shall be indicated adjacent to the relevant nominal size(s).
Table 3 – Angular Dimensions
| Permissible deviations in mm for ranges in nominal lengths | Tolerance Class Designation (Description) | |||
|---|---|---|---|---|
| f (fine) | m (medium) | c (coarse) | v (very coarse) | |
| up to 10 | ±1º | ±1º | ±1º30′ | ±3º |
| over 10 up to 50 | ±0º30′ | ±0º30′ | ±1º | ±2º |
| over 50 up to 120 | ±0º20′ | ±0º20′ | ±0º30′ | ±1º |
| over 120 up to 400 | ±0º10′ | ±0º10′ | ±0º15′ | ±0º30′ |
| over 400 | ±0º5′ | ±0º5′ | ±0º10′ | ±0º20′ |
#2 General Tolerances ISO 2768-2
ISO 2768-2 stands for the geometrical tolerances for features without individual tolerance indications. It indicates the general geometrical tolerances range of flatness & straightness, cylindricity, and circularity. This standard includes 3 classes of tolerance – H, K, and L:
Table 4 – General Tolerances on Straightness and Flatness
| Ranges of nominal lengths in mm | Tolerance Class | ||
|---|---|---|---|
| H | K | L | |
| up to 10 | 0.02 | 0.05 | 0.1 |
| above 10 to 30 | 0.05 | 0.1 | 0.2 |
| above 30 to 100 | 0.1 | 0.2 | 0.4 |
| above 100 to 300 | 0.2 | 0.4 | 0.8 |
| above 300 to 1000 | 0.3 | 0.6 | 1.2 |
| above 1000 to 3000 | 0.4 | 0.8 | 1.6 |
Table 5 – General Tolerances on Perpendicularity
| Ranges of nominal lengths in mm | Tolerance Class | ||
|---|---|---|---|
| H | K | L | |
| up to 100 | 0.2 | 0.4 | 0.6 |
| above 100 to 300 | 0.3 | 0.6 | 1.0 |
| above 300 to 1000 | 0.4 | 0.8 | 1.5 |
| above 1000 to 3000 | 0.5 | 1.0 | 2.0 |
Table 6 – General Tolerances on Symmetry
| Ranges of nominal lengths in mm | Tolerance Class | ||
|---|---|---|---|
| H | K | L | |
| up to 100 | 0.5 | 0.6 | 0.6 |
| above 100 to 300 | 0.5 | 0.6 | 1.0 |
| above 300 to 1000 | 0.5 | 0.8 | 1.5 |
| above 1000 to 3000 | 0.5 | 1.0 | 2.0 |
Table 7 – General Tolerances on Circular Run-Out
| Ranges of nominal lengths in mm | Tolerance Class | ||
|---|---|---|---|
| H | K | L | |
| 0.1 | 0.2 | 0.5 | |
This general tolerance allows the manufacturer to choose the appropriate tolerance level that suits their needs best. For example, if the part is expected to be used in a project with high-level tolerance requirements, it would be wise to choose a small tolerance range. On the contrary, a larger tolerance range would be more cost-effective if the part is produced in high volumes for lower-level tolerance applications.Â
In the End
The level of tolerance that a part needs will depend on how it will be used and what the needs of the application are. When figuring out the right level of tolerance for a part or product, factors like materials, proceses, and costs should also be taken into consideration. For example, if you want to build a precise metal part with tight tolerance for your project, then CNC machining will be a good choice. CNC machines are highly precise and can produce parts with very tight tolerances, typically CNC machining tolerance can ranging from ±0.001″ to ±0.0001″.
A brief PDF version of ISO 2768: General Din ISO-2768 mk.pdf :
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