In CNC machining, international tolerance standards are crucial for ensuring that parts meet specific design requirements and are interchangeable between different manufacturers and regions. The most commonly used general tolerance international standard is...ISO 2768。
ISO 2768 is divided into two parts:
- ISO 2768-1:Specify general tolerances for linear and angular dimensions.
- ISO 2768-2:Specify general geometric tolerances (shape and position).
These standards are typically applied to engineering drawings.未Specific tolerances are indicated. They provide a default tolerance grade based on the nominal size of the feature and the selected tolerance grade.
The following is a breakdown of the main tables in ISO 2768:
CNC International Tolerance Standard Table (ISO 2768)
ISO 2768-1: General tolerances for linear and angular dimensions
ISO 2768-1 classifies tolerances into four grades:
- f (fine)Fine level
- m (medium)Intermediate level
- c (coarse)Roughness level
- v (very coarse)Very rough
Table 1: Permissible Deviations of Linear Dimensions (mm)
| Nominal size range (mm) | f (fine) | m (Medium) | c (rough) | v (very rough) |
| 0.5 | ± 0.05 | ± 0.1 | ± 0.2 | ± 0.3 |
| 3 | ± 0.05 | ± 0.1 | ± 0.3 | ± 0.5 |
| 6 | ± 0.1 | ± 0.2 | ± 0.5 | ± 1.0 |
| thirty | ± 0.15 | ± 0.3 | ± 0.8 | ± 1.5 |
| 120 | ± 0.2 | ± 0.5 | ± 1.2 | ± 2.5 |
| 400 | ± 0.3 | ± 0.8 | ± 2.0 | ± 4.0 |
| 1000 | ± 0.5 | ± 1.2 | ± 3.0 | ± 6.0 |
| 2000 | – | ± 2.0 | ± 4.0 | ± 8.0 |
- Note: For nominal dimensions less than 0.5 mm, the deviation should be indicated next to the relevant nominal dimension.
Table 2: Permissible Deviations (mm) for External Fillet Radius and Chamfer Height
| Nominal size range (mm) | f (fine) | m (Medium) | c (rough) | v (very rough) |
| 0.5 | ± 0.2 | ± 0.4 | ± 0.4 | ± 0.4 |
| 3 | ± 0.5 | ± 1.0 | ± 1.0 | ± 1.0 |
| x>6 | ± 1.0 | ± 2.0 | ± 2.0 | ± 2.0 |
- Note: For nominal dimensions less than 0.5 mm, the deviation should be indicated next to the relevant nominal dimension.
Table 3: Permissible Deviations of Angular Dimensions (degrees, minutes)
| Length range of the shorter side of the angle (mm) | f (fine) | m (Medium) | c (rough) | v (very rough) |
| x≤10 | ± 1∘ | ± 1∘ | ± 1∘3 0′ | ± 3∘ |
| 10 | ± 0∘3 0′ | ± 0∘3 0′ | ± 1∘ | ± 2∘ |
| 50 | ± 0∘2 0′ | ± 0∘2 0′ | ± 0∘3 0′ | ± 1∘ |
| 120 | ± 0∘1 0′ | ± 0∘1 0′ | ± 0∘1 5′ | ± 0∘3 0′ |
| x>400 | ± 0∘5′ | ± 0∘5′ | ± 0∘1 0′ | ± 0∘2 0′ |
ISO 2768-2: General geometric tolerances for form and position
ISO 2768-2 classifies geometric tolerances into three categories:
- H (High Precision): High precision
- K (medium precision): Medium accuracy
- L (low precision)Low precision
Table 4: General tolerances for straightness and flatness (mm)
| Nominal length range (mm) | 哈 | 钾 | 左 |
| x≤10 | 0.02 | 0.05 | 0.1 |
| 10 | 0.05 | 0.1 | 0.2 |
| thirty | 0.1 | 0.2 | 0.4 |
| 100 | 0.2 | 0.4 | 0.8 |
| 300 | 0.3 | 0.6 | 1.2 |
| 1000 | 0.4 | 0.8 | 1.6 |
Table 5: General tolerances for perpendicularity (mm)
| Nominal length range of the shorter side (mm) | 哈 | 钾 | 左 |
| x≤100 | 0.2 | 0.4 | 0.6 |
| 100 | 0.3 | 0.6 | 1.0 |
| 300 | 0.4 | 0.8 | 1.5 |
| 1000 | 0.5 | 1.0 | 2.0 |
Table 6: General tolerances for symmetry (mm)
| Nominal length range (mm) | 哈 | 钾 | 左 |
| x≤100 | 0.5 | 0.6 | 0.6 |
| 100 | 0.5 | 1.0 | 1.0 |
| 300 | 0.5 | 1.5 | 1.5 |
| 1000 | 0.5 | 2.0 | 2.0 |
Table 7: General Tolerances for Circular Runout (mm)
| Nominal diameter range (mm) | 哈 | 钾 | 左 |
| x≤10 | 0.05 | 0.1 | 0.2 |
| 10 | 0.1 | 0.2 | 0.4 |
| thirty | 0.15 | 0.3 | 0.6 |
| 100 | 0.2 | 0.4 | 0.8 |
| 300 | 0.25 | 0.5 | 1.0 |
| 1000 | 0.3 | 0.6 | 1.2 |
Important Note:
- Default tolerance: The ISO 2768 series of standards aims to simplify engineering drawings by providing default tolerances for dimensions where tolerances are not explicitly specified. If tolerances are explicitly specified on the drawing, those specified tolerances shall prevail.
- Application scope: These tolerances primarily apply to parts manufactured through material removal processes (such as CNC machining). For other manufacturing processes, special evaluations may be required.
- GD&T (Geometric Dimensions and Tolerances): For parts requiring more precise control, GD&T standards such as ASME Y14.5 or ISO 1101 are typically used, as they provide more detailed symbols and rules to define the geometric properties of the parts. ISO 2768 is usually used as a supplement to features not specified in GD&T.
- Practical applications: In actual production, choosing the appropriate tolerance grade requires balancing functional requirements, manufacturing costs, and processing capabilities. Too tight a tolerance will increase manufacturing costs, while too loose a tolerance may affect product performance.
- Revision: International standards are revised periodically. In practical applications, please be sure to refer to the latest version of ISO 2768 or other relevant standards.
This table provides the most commonly used tolerance data in ISO 2768. In actual CNC machining, the ISO 2768 standard and the corresponding tolerance grade are usually specified in the drawing title block or technical requirements, such as "ISO 2768-m" or "ISO 2768-mk".
