ISO2768 is an international standard by the International Organization for Standardization (ISO) and it simplifies mechanical tolerance specifications in engineering drawings. ISO 2768 provides general tolerances for linear, angular, and geometrical dimensions, reducing complexity and ensuring consistency across industries. This guide will explore ISO 2768’s details and includes key tolerance tables for practical use.
What is ISO 2768?
ISO 2768 defines general tolerances for parts produced via material removal processes, like CNC machining or sheet metal fabrication. It eliminates the need to specify tolerances for every dimension, enhancing clarity between designers, manufacturers, and inspectors. The standard comprises two parts:
- ISO 2768-1: General tolerances for linear and angular dimensions without individual tolerance indications.
- ISO 2768-2: Geometrical tolerances for features without individual tolerance indications.
ISO 2768 ensures acceptable dimensional and geometrical variations, minimizing costs and errors. It’s widely used in aerospace, automotive, electronics, and medical device industries.
ISO 2768-1: Tolerances for Linear and Angular Dimensions
ISO 2768-1 simplifies tolerances for linear and angular dimensions without specific notations, covering:
- External and internal sizes
- Step sizes
- Diameters and radii
- Distances
- External radii and chamfer heights
- Angular dimensions, including right angles (90°) and uniform polygon angles (unless referenced to ISO 2768-2)
Tolerance Classes
ISO 2768-1 offers four precision levels:
- f (fine): Tightest tolerances for high-precision applications.
- m (medium): Balances precision and cost for general use.
- c (coarse): For less critical dimensions.
- v (very coarse): For large acceptable variations.
Below are the tolerance tables for linear and angular dimensions per ISO 2768-1 (in millimeters for linear, degrees/minutes for angular). For nominal sizes below 0.5 mm, tolerances must be specified individually.
Table 1: Permissible Deviations for Linear Dimensions
| Nominal Size Range (mm) | f (fine) | m (medium) | c (coarse) | v (very coarse) |
| 0.5 to 3 | ±0.05 | ±0.1 | ±0.2 | / |
| Over 3 to 6 | ±0.05 | ±0.1 | ±0.3 | ±0.5 |
| Over 6 to 30 | ±0.1 | ±0.2 | ±0.5 | ±1.0 |
| Over 30 to 120 | ±0.15 | ±0.3 | ±0.8 | ±1.5 |
| Over 120 to 400 | ±0.2 | ±0.5 | ±1.2 | ±2.5 |
| Over 400 to 1000 | ±0.3 | ±0.8 | ±2.0 | ±3.0 |
| Over 1000 to 2000 | ±0.5 | ±1.2 | ±3.0 | ±6.0 |
| Over 2000 to 4000 | / | ±2.0 | ±4.0 | ±8.0 |
Table 2: External Radii and Chamfer Heights
| Nominal Size Range (mm) | f (fine) | m (medium) | c (coarse) | v (very coarse) |
| 0.5 to 3 | ±0.2 | ±0.2 | ±0.4 | ±0.4 |
| Over 3 to 6 | ±0.5 | ±0.5 | ±1.0 | ±1.0 |
| Over 6 | ±1.0 | ±1.0 | ±2.0 | ±2.0 |
Table 3: Angular Dimensions
| Nominal Size Range (mm) | f (fine) | m (medium) | c (coarse) | v (very coarse) |
| up to 10 | ±1° | ±1° | ±1°30′ | ±3° |
| Over 10 to 50 | ±0°30′ | ±0°30′ | ±1° | ±2° |
| Over 50 to 120 | ±0°20′ | ±0°20′ | ±0°30′ | ±1° |
| Over 120 to 400 | ±0°10′ | ±0°10′ | ±0°20′ | ±0°30′ |
| Over 400 | ±0°5′ | ±0°5′ | ±0°10′ | ±0°20′ |
ISO 2768-2: Geometrical Tolerances
ISO 2768-2 defines geometrical tolerances for features without individual indications, addressing:
- Straightness and Flatness: Limits deviation from a straight line or plane.
- Perpendicularity: Ensures correct angular relationships to a datum.
- Symmetry: Ensures alignment around a central plane or axis.
- Circularity and Cylindricity: Controls roundness or cylindrical uniformity.
- Runout: Circular features positional variation during rotation.
Tolerance Classes
Three tolerance classes:
- H: Tightest for high-precision.
- K: Moderate for general use.
- L: Loosest for less critical applications.
Table 4: 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: 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 |
Note: Circularity tolerance equals the diameter tolerance but does not exceed concentricity limits.
Table 6: 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 | |
What is ISO 2768-mK?
ISO 2768-mK combines “medium” (ISO 2768-1) and “K” (ISO 2768-2). It’s common for sheet metal fabrication, indicating:
- Linear/angular dimensions follow “m”.
- Geometrical features follow “K”.
Conclusion
ISO 2768 streamlines manufacturing with standardized tolerances, ensuring quality and efficiency. Its tables above provide practical guidance for linear, angular, and geometrical tolerances. Use ISO 2768-1 and -2 to simplify designs and meet global standards.
