What Is ISO 2768？
ISO 2768 is a general tolerance standard that simplifies drawing specifications by providing permissible deviations for linear and angular dimensions without individual tolerance indications. It consists of two parts:
ISO 2768-1(Part 1): Linear and Angular Dimensions
Precision Levels
- ISO 2768-1 (Part 1): specifies general tolerances for linear and angular dimensions in four classes: fine (f), medium (m), coarse (c), and very coarse (v).
- The “medium” (m) class is most commonly used and provides permissible deviations ranging from ±0.1 mm for nominal lengths up to 3 mm to ±2.0 mm for nominal lengths over 2000 mm.
- For linear dimensions, the standard specifies permissible deviations and tolerances based on the size range. For instance, smaller components have tighter tolerances. This ensures components fit together seamlessly in assembly.
Linear Deviations
Permissible deviations for linear dimensions vary. They depend on whether the dimension, with its tolerance, is a length, diameter, or external radius. The standard provides a detailed chamfers table for reference.
For example, a dimension up to 30 mm in the fine category might have a tolerance of ±0.05 mm. In contrast, very coarse tolerances could allow deviations up to ±2.5 mm for dimensions over 400 mm.
Angular Deviations
Angular dimensions are crucial for ensuring parts assemble correctly. ISO 2768 specifies deviations in degrees and minutes.
A typical tolerance might be ±1 degree for coarse categories. For more precise assemblies, tolerances could tighten to ±30 minutes or even less.
Radii and Chamfers
The standard also addresses tolerances for external radii and chamfers. It ensures that edges meet specifications, affecting both aesthetics and functionality, within a certain tolerance.
Radii tolerances might range from ±0.2 mm in fine categories to ±2 mm in very coarse ones. Chamfer angles follow similar guidelines as other angular dimensions.
Table 1 – Linear Dimensions
| Permissible deviations in mm for ranges in nominal lengths | 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 |
Table 2 – External Radii and Chamfer Heights
Similarly, Table 2 indicates tolerance standards for external radii and chamfers.
| Permissible deviations in mm for ranges in nominal lengths | 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 |
Table 3 – Angular Dimensions
| Permissible deviations in mm for ranges in nominal lengths | f (fine) | m (medium) | c (coarse) | v (very coarse) |
|---|---|---|---|---|
| up to 10 | ±1° | ±1° | ±1°30′ | ±3° |
| over 10 up to 50 | ±0°30′ | ±0°30′ | ±1° | ±2° |
| over 50 up to 120 | ±0°20′ | ±0°20′ | ±0°30′ | ±1° |
| over 120 up to 400 | ±0°10′ | ±0°10′ | ±0°20′ | ±0°30′ |
| over 400 | ±0°5′ | ±0°5′ | ±0°10′ | ±0°20′ |
ISO 2768-2 (Part 2): Geometrical Tolerances for Features
Tolerance Ranges
ISO 2768-2 (Part 2) sets the stage for geometrical tolerances that are crucial in mechanical tolerance design. It defines three primary tolerance ranges: H, K, and L. These ranges apply to key geometrical features such as flatness, straightness, perpendicularity, symmetry, and run-out.
The H range signifies the most precise tolerance level, often applied in high-precision engineering where maximum accuracy is paramount. The K range represents a medium level of precision, with a suitable tolerance, for general manufacturing processes. Lastly, the L range offers the least strict tolerance, used in less critical applications where slight deviations are acceptable.
Table 4 – General Tolerances on Straightness and Flatness
| Ranges of nominal lengths in mm | 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 |
Parallelism and Perpendicularity
Parallelism within ISO 2768 Part 2 is unique. It’s defined as equal to either the numerical value of the size tolerance or the flatness/straightness tolerance, whichever is greater. This approach ensures that parts fit together perfectly without unnecessary stress or distortion.
Perpendicularity tolerances are also specified, providing a clear guide on how components should align with each other at right angles. This ensures the precision and quality of manufactured parts by eliminating ambiguities in engineering drawings.
Table 5 – General Tolerances on Perpendicularity
| Ranges of nominal lengths in mm | H | K | L |
|---|---|---|---|
| up to 10 | 0.2 | 0.4 | 0.6 |
| above 10 to 30 | 0.3 | 0.6 | 1.0 |
| above 30 to 100 | 0.4 | 0.8 | 1.5 |
| above 100 to 300 | 0.5 | 1.0 | 2.0 |
Table 6 – General Tolerances on Symmetry
| Ranges of nominal lengths in mm | H | K | L |
|---|---|---|---|
| up to 10 | 0.5 | 0.6 | 0.6 |
| above 10 to 30 | 0.5 | 0.6 | 1.0 |
| above 30 to 100 | 0.5 | 0.8 | 1.5 |
| above 100 to 300 | 0.5 | 1.0 | 2.0 |
Table 7 – General Tolerances on Circular Run-Out
| Ranges of nominal lengths in mm | H | K | L |
|---|---|---|---|
| 0.1 | 0.2 | 0.5 |
Benefits of Using ISO 2768-mK in CNC Machining
Implementing the ISO 2768-mK tolerance standard in CNC machining offers several key advantages:
Improved Interchangeability
ISO 2768-mK ensures that machined parts meet consistent dimensional and geometrical tolerances, allowing for improved interchangeability between components. This simplifies assembly and reduces the need for custom fitting or rework.
Enhanced Quality Control
By adhering to the specified tolerances in ISO 2768-mK, manufacturers can maintain tighter control over the quality of machined parts. This leads to reduced defects, improved reliability, and enhanced overall product quality.
Reduced Production Costs
Standardizing tolerances through ISO 2768-mK can streamline the manufacturing process, reducing setup times, material waste, and the need for rework. This translates to lower overall production costs and improved efficiency.
Simplified Communication
ISO 2768-mK provides a common language for designers and manufacturers to communicate tolerance requirements. This minimizes misunderstandings and ensures that parts are produced to the intended specifications, reducing costly errors.
Increased Efficiency
By eliminating the need to specify individual tolerances for every dimension and feature, ISO 2768-mK simplifies the design and manufacturing process. This allows for faster turnaround times and improved overall efficiency in CNC machining operations.In summary, adopting the ISO 2768-mK tolerance standard in CNC machining offers significant advantages in terms of interchangeability, quality control, cost reduction, communication, and efficiency.
Common Challenges When Implementing ISO 2768-mK
When implementing the ISO 2768-mK tolerance standard, manufacturers may face several challenges. Here are some of the most common ones:
Measurement Uncertainty
Accurately measuring parts to ensure they meet the specified tolerances can be challenging, especially for smaller features. Measurement uncertainty can arise from factors like environmental conditions, equipment calibration, and operator skill.
Communication Between Designers and Manufacturers
Effective communication between designers who specify the tolerances and manufacturers who produce the parts is crucial. Misunderstandings about the intended tolerances can lead to costly rework or scrap.
Maintaining Consistency Across Suppliers
When sourcing parts from multiple suppliers, ensuring that all parts meet the same tolerance requirements can be difficult. Variations in manufacturing processes and measurement methods between suppliers can result in inconsistent quality.
Selecting the Appropriate Tolerance Class
Choosing the correct tolerance class (f, m, c, or v for linear dimensions; H, K, or L for geometrical tolerances) for each feature is important.
Specifying tolerances that are too tight can increase manufacturing costs, while tolerances that are too loose may result in functional issues.
Documenting and Maintaining Records
Proper documentation of tolerance requirements and maintaining records of measurements and inspections is essential for demonstrating compliance with ISO 2768-mK. Inadequate record-keeping can make it difficult to identify and address quality issues.
Training and Educating Employees
Ensuring that all employees involved in the design, manufacturing, and inspection processes understand the requirements of ISO 2768-mK is crucial. Lack of training can lead to errors and inconsistencies.To overcome these challenges, manufacturers should:
- Implement robust measurement systems and train employees in proper measurement techniques
- Foster clear communication between designers and manufacturers through the use of standardized terminology and documentation
- Establish consistent quality control procedures across all suppliers
- Carefully review tolerance requirements for each feature and adjust as needed
- Maintain detailed records of all measurements and inspections
- Provide comprehensive training to all employees on the requirements of ISO 2768-mK
Final Remarks
ISO 2768 plays a key role in ensuring precision and consistency in CNC machining by defining critical tolerance standards. Understanding and applying both parts of this standard can enhance the quality, interchangeability, and overall success of your components. Integrating ISO 2768 into your manufacturing processes not only improves product quality but also streamlines operations and builds client trust.
Take the next step with Witcool Machinery—we apply ISO 2768 standards to ensure top-quality CNC machined parts. Let us help elevate your projects to international standards. Contact us today to see how we can assist you in achieving precision and excellence!
FAQs
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.
Why ISO 2768 is Important?
Every feature on a component always has a geometrical shape and size. Deviating from theoretically exact dimensions often impair the function of the part. This is why it is important to complete the tolerancing on technical drawings.
What is ISO 2768-mK Meaning?
The tolerance class “m” for Part 1 means medium. The “K” class, on the other hand, is part of ISO 2768-2. Therefore, ISO 2768-mK means that such a component must meet the “medium” tolerance range for Part 1 and the tolerance class “K” for Part 2.
How does ISO 2768 impact the quality of machined parts?
By adhering to ISO 2768, manufacturers can maintain consistent quality and precision in their components, reducing errors and ensuring that parts meet international standards for fit and functionality.
How does ISO 2768 differ from other international standards?
Unlike many specific standards, ISO 2768 provides general geometrical tolerances, making it broadly applicable across various manufacturing processes, not just CNC machining.
Why should I integrate ISO 2768 into my design process?
Integrating ISO 2768 into your design ensures that your components are built to international tolerance standards, improving the accuracy, quality, and reliability of your parts while meeting global expectations.
