In daily engineering drawings, many users like to quote phrases such as “for unspecified tolerances, follow ISO2768-m” or “for unspecified tolerances, follow ISO2768-mK”. So what is the ISO2768 standard?
1. Introduction
In the ever-evolving field of precision manufacturing, achieving consistent quality and ensuring efficiency is paramount.
Tolerances—the allowable variation in a dimension—play a crucial role in maintaining the integrity of manufactured parts.
ISO 2768 is an international standard designed to simplify and streamline the specification of tolerances in technical drawings.
This blog explores ISO 2768 in detail, explaining its classifications, applications, and benefits in modern manufacturing.
Whether you’re a designer, engineer, or manufacturer, understanding ISO 2768 can significantly enhance your processes and outcomes.
2. What is ISO 2768?
It is an international standard that establishes general tolerances for linear, angular, and geometrical dimensions in technical drawings.
It eliminates the need to individually specify tolerances for every feature, simplifying the design process.
Primarily, ISO 2768 applies to parts made through machining, casting, and sheet metal fabrication.
For example, when a technical drawing specifies a dimension of 50 mm but does not indicate a tolerance,
ISO 2768 provides default tolerances based on the tolerance class (e.g., Fine or Medium).
This approach reduces ambiguity and ensures clarity in communication between designers and manufacturers, even across different countries and industries.
3. Key Classifications in ISO 2768
ISO 2768 is divided into two main categories that address different aspects of tolerances: general tolerances and geometrical tolerances.
Each category includes specific classifications to ensure clarity and precision in manufacturing and design.
General Tolerances
General tolerances in ISO 2768 apply to linear and angular dimensions that do not have individual tolerance specifications on the drawing.
They ensure that designers can avoid overloading drawings with unnecessary details while maintaining accuracy.
- Linear Dimensions:
Covers measurements like length, width, height, and thickness. For example, a dimension of 50 mm with a medium tolerance class (m) might allow a deviation of ±0.2 mm. - Angular Dimensions:
Addresses angular features such as chamfers, slopes, and inclinations.
Tolerances here depend on the angle size and the selected tolerance class, ensuring alignment without excess precision.
Geometrical Tolerances
This category covers the form and positional accuracy of features.
Geometrical tolerances help maintain functionality, especially in assemblies where misalignments could lead to performance issues.
Key elements include:
- Flatness: Ensures that a surface is even within specified limits.
- Straightness: Controls how much a line or edge can deviate from a straight path.
- Perpendicularity: Maintains the right-angle relationship between two features.
- Symmetry: Guarantees balanced and uniform features around a central axis.
Tolerance Classes
ISO 2768 introduces four tolerance classes to match the precision level to the application’s needs. These classes are:
- Fine (f): For applications requiring high precision, such as aerospace or medical devices.
- Medium (m): The most commonly used class, suitable for general-purpose applications.
- Coarse (c): Ideal for less critical dimensions or larger parts.
- Very Coarse (v): Used for parts with minimal complexity or large-scale components.
4. ISO 2768 Part 1: Linear and Angular Dimensions
ISO 2768 Part 1, titled “Unspecified Tolerances for Linear and Angular Dimensions,” is a critical component of the ISO 2768 standard suite.
It provides default tolerances for linear and angular dimensions that are not explicitly specified on technical drawings.
This part of the standard aims to simplify design documentation by reducing the need to specify individual tolerances for every dimension,
thereby streamlining the manufacturing process while ensuring that parts meet acceptable quality standards.
Scope and Application
ISO 2768 Part 1 applies to linear and angular dimensions in technical drawings where no specific tolerance is indicated.
It is intended for use in situations where general machining practices can achieve the necessary precision. The standard covers:
- Linear dimensions: Includes external and internal sizes, diameters, distances, chamfer heights, and radii.
- Angular dimensions: Covers angular measurements where specific tolerances are not indicated.
- Dimensions of machined and assembled parts: Applicable to both linear and angular dimensions produced during the machining of assembled components.
Tolerances for Linear Dimensions
The table below outlines the ISO 2768 tolerance limits for linear dimensions across different nominal size ranges：
| 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 |
How to read the table: For a part with a nominal dimension range of 50 mm, under the Fine (f) tolerance class, the acceptable deviation would be ±0.15 mm.
Tolerances for External Radius and Chamfer Heights
The table below shows the ISO 2768 standard tolerances for external radii and chamfer heights.
These tolerances define permissible deviations for curved surfaces and chamfered edges.
| EXTERNAL RADIUS 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 | ±0.1 | ±1.0 | ±2.0 | ±2.0 |
How to read the table: For an external radius of 4 mm, the applicable nominal dimension range is ‘over 3 to 6 mm.’
If you select the Fine (f) tolerance class, the acceptable deviation would be ±0.5 mm.
Tolerances for Angular Dimensions
The table below details the ISO 2768 tolerances for angular dimensions, expressed in degrees and minutes. These tolerances apply to the shorter leg of an angle.
| 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 | ±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°15′ | ±0°30′ |
| over 400 | ±0°5′ | ±0°5′ | ±0°10′ | ±0°20′ |
How to read the table: For an angular measurement with a nominal dimension range of 30 mm, under the Fine (f) tolerance class, the acceptable deviation would be ±0°30′.
5. ISO 2768 Part 2: Geometrical Tolerances for Features
ISO 2768 T2 refers to the part of ISO 2768 that governs geometrical tolerances, focusing specifically on form, orientation, location, and runout tolerances for features.
These tolerances are critical for ensuring proper functionality, assembly precision, and overall quality of manufactured components.
Scope and Application
ISO 2768 T2 applies to:
- Geometrical tolerances that are not explicitly mentioned on technical drawings.
- Components where precision in geometry is crucial for assembly or operation.
- General-purpose manufacturing, with predefined tolerance levels to balance quality and cost.
Geometrical Tolerances Defined in T2
ISO 2768 T2 specifies tolerances for the following features:
1. Form Tolerances:
- Flatness: Ensures a surface lies within a defined plane.
- Straightness: Controls the straightness of an edge or axis.
- Roundness: Maintains circular consistency.
- Cylindricity: Ensures cylindrical surfaces remain consistent.
2. Orientation Tolerances:
- Parallelism: Maintains parallel relationships between surfaces or axes.
- Perpendicularity: Ensures surfaces or features are at 90° angles.
- Angularity: Specifies a precise angle between surfaces.
3. Location Tolerances:
- Position: Defines the allowable deviation from the intended position.
- Concentricity: Ensures the center of one feature aligns with another.
- Symmetry: Controls symmetry for balanced designs.
4. Runout Tolerances:
- Circular Runout: Limits the deviation of a feature during rotation.
- Total Runout: Controls the overall deviation of a surface in motion.
6. The Importance of ISO 2768 in Manufacturing
ISO 2768 provides multiple advantages for manufacturers:
- Standardization: Ensures parts from different suppliers meet consistent quality standards.
- Clear Communication: Reduces misinterpretation in technical drawings, minimizing errors.
- Global Compatibility: Facilitates collaboration across international supply chains.
For example, a multinational company can use ISO 2768 to ensure that parts sourced from different regions fit together seamlessly, reducing delays and rework.
7. How ISO 2768 Works
ISO 2768 provides a standardized approach to tolerances in manufacturing, simplifying design, communication, and production processes.
It works by defining general tolerances for dimensions and geometrical features when specific tolerances are not explicitly stated on technical drawings.
Here’s a detailed explanation of how ISO 2768 functions:
Step-by-Step Explanation
1. Incorporation into Design
- General Tolerances: Instead of specifying tolerances for every dimension, engineers use ISO 2768 to apply default tolerances.
For example, a shaft length listed as 100 mm would automatically include a tolerance range defined by ISO 2768, such as ±0.2 mm for medium (m) class. - Geometrical Tolerances: Features like flatness or perpendicularity are governed by ISO 2768 Part 2, ensuring consistency in form and alignment.
2. Communication in Technical Drawings
- The technical drawing includes a note like “ISO 2768-mK,” where:
- m indicates the medium tolerance class for linear and angular dimensions (Part 1).
- K refers to the geometrical tolerances for features (Part 2).
- This shorthand eliminates the need to detail tolerances for each dimension individually, saving time and reducing errors.
3. Application in Manufacturing
- During production, manufacturers follow the ISO 2768 tolerance class specified on the drawings.
- Tolerance guidelines ensure that deviations within the limits do not impact part functionality or fit.
- Consistency is maintained across batches, even with different suppliers.
4. Inspection and Quality Control
- Measurement Tools: Inspection teams use calipers, micrometers, and CMM machines to verify that dimensions and geometrical features meet ISO 2768 tolerances.
- Tolerance Stacking: Evaluates how dimensional deviations might accumulate and affect assembly. Proper application of ISO 2768 minimizes issues caused by excessive stacking.
Example:
A drawing specifies a hole diameter of 20 mm under ISO 2768-f. For a fine tolerance class, the permissible deviation might be ±0.1 mm.
During inspection, a measured diameter of 20.08 mm would conform to the standard, but 20.12 mm would not.
Advantages of How ISO 2768 Functions
- Clarity in Communication
- Reduces ambiguity by providing a clear, universal guideline for tolerances.
- Promotes better collaboration between designers, manufacturers, and suppliers.
- Efficiency in Production
- Streamlines the manufacturing process by eliminating the need for detailed tolerance specifications.
- Encourages the use of cost-effective and consistent practices.
- Quality Assurance
- Ensures parts meet design intent without requiring overly tight tolerances, which can increase costs unnecessarily.
- Facilitates robust quality control processes with well-defined standards.
Common Missteps and How to Avoid Them
- Ignoring ISO 2768 Classes: Ensure the appropriate tolerance class (fine, medium, coarse, very coarse) is selected based on the application’s precision requirements.
- Over-Specification: Avoid assigning tighter tolerances than necessary, as this can increase manufacturing costs.
- Tolerance Stacking Mismanagement: Be mindful of accumulated tolerances when designing assemblies to prevent misalignment or fitment issues.
8. How to Choose the Right Tolerance
Choosing the correct tolerance is vital to achieving a balance between functionality, fit, cost, and manufacturability.
Tolerances that are too tight can increase manufacturing complexity and costs, while overly loose tolerances may compromise part performance and assembly.
The goal is to select a tolerance level that ensures proper part functionality without unnecessary expense.
Key Considerations in Tolerance Selection
- Functionality
- Determine the operational requirements of the part, such as load-bearing capacity, movement, or sealing performance.
- Identify whether the part must align with other components and the precision required for proper assembly.
- Manufacturing Process
- Understand the capabilities of the chosen manufacturing process. For example:
- CNC machining generally supports tighter tolerances than 3D printing.
- Sheet metal fabrication may have limitations for fine tolerances.
- Material Choice
- Certain materials, like plastics, may require looser tolerances due to thermal expansion or flexibility, while metals can typically hold tighter tolerances.
- Cost vs. Precision
- Tight tolerances usually increase production costs due to additional machining time and quality control.
- Opt for looser tolerances when high precision isn’t critical.
- Standards
- Refer to standardized tolerance classes such as ISO 2768 or ISO 286 to ensure consistency and compatibility in global manufacturing.
Guidance for Selecting Tolerance Standards
| Application | Description | Recommended tolerance standard | Reason for tolerance choice |
|---|---|---|---|
| Precision machined parts | High-precision components are used in aerospace, automotive, or medical devices where exact fit is critical. | ISO 2768 Fine and ISO 286 Grade 6 (IT6) or higher | Ensures minimal variation for linear and angular dimensions (ISO 2768) and tight control for cylindrical fits (ISO 286). |
| Interchangeable mechanical parts | Parts are designed to be easily replaced or exchanged, like gears, bearings, and fasteners in assemblies. | ISO 2768 Fine and ISO 286 Grade 7 (IT7) or higher | Allows for precise linear/angular fits (ISO 2768) and standardized fits for shafts and holes (ISO 286). |
| General mechanical assemblies | Components in general machinery that require good fits but not ultra-high precision, like housings or brackets. | ISO 2768 Medium | Provides a balance between precision and manufacturability for linear and angular dimensions. |
| Large fabricated structures | Parts used in construction or heavy machinery where exact fits are less critical, such as beams or plates. | ISO 2768 Medium | Tolerances accommodate larger dimensions and processes like welding or fabrication. |
| Plastic components | Molded or machined plastic parts for consumer products or electronics, where some dimensional variability is acceptable. | ISO 2768 Medium and ISO 286 Grade 8 (IT8) or higher | Tolerances consider material flexibility (ISO 2768) and accommodate standard fits (ISO 286) for plastics. |
| Shafts and holes for rotating components | Components like shafts and holes in rotating machinery require specific fits to ensure proper function. | ISO 2768 Fine and ISO 286 Grades 6 or 7 (IT6, IT7) | Ensures precise linear/angular dimensions (ISO 2768) and tight fits for rotational balance (ISO 286). |
| Sheet metal parts | Parts made from sheet metal for enclosures, panels, and brackets where tight fits are not critical. | ISO 2768 Medium | Tolerances are suitable for processes like bending and forming, accommodating inherent variabilities. |
| Electrical enclosures and casings | Enclosures for electrical components that must fit together but do not require tight tolerances. | ISO 2768 Medium | Provides sufficient accuracy for assembly while reducing costs for non-precision parts. |
| Consumer product components | Parts in consumer electronics or appliances where aesthetic finish and function are prioritized over tight tolerances. | ISO 2768 Medium and ISO 286 Grade 8 (IT8) | Balances manufacturing efficiency with adequate fit and function, using standard tolerances for general fits. |
9. ISO vs. ASME Tolerance Standards
ISO and ASME standards serve as critical frameworks for defining tolerances, ensuring consistency, and facilitating efficient global manufacturing practices.
While both aim to achieve precision and clarity in engineering drawings, their application and regional prevalence differ significantly.
- ISO Standards: Primarily used in Europe, the UK, Turkey, and parts of Asia, focusing on general tolerances (e.g., ISO 2768) and specific fit systems (e.g., ISO 286).
These standards simplify dimensional tolerances and ensure uniformity across industries. - ASME Standards: Dominant in the United States, these standards (e.g., ASME Y14.5 and ASME B4.1) emphasize geometric dimensioning and tolerancing (GD&T)
with detailed guidelines for defining form, orientation, and positional tolerances.
Comparison of ISO and ASME Tolerance Standards
| ISO Standard | Equivalent ASME Standard | Application | Key Difference |
|---|---|---|---|
| ISO 2768 for Angular Dimensions | ASME B4.2 | Angular dimension tolerances | Similar angular tolerance ranges, but ASME B4.2 may offer more detailed instructions for specific applications. |
| ISO 1101 (Geometric Tolerancing) | ASME Y14.5 (GD&T) | Geometric tolerancing of shapes and features | Both provide frameworks for GD&T, but ASME Y14.5 is more detailed and widely used in the US. |
| ISO 286 (Grade 6, 7, 8) | ASME B4.1 (Grade 6, 7, 8) | Tolerances for cylindrical fits and distances between parallel surfaces | Both standards define similar tolerance grades for fits, but ASME includes additional guidance specific to US practices. |
| ISO 2768 (Fine, Medium) | ASME Y14.5 | General tolerances for linear and angular dimensions | ISO 2768 provides general tolerances, while ASME Y14.5 offers detailed geometric dimensioning guidelines (GD&T). |
Example of Equivalency
- General Dimensional Tolerances:
- ISO 2768-m aligns with ASME B4.1 for medium precision.
- Geometric Tolerances:
- ISO 1101 covers similar principles as ASME Y14.5, but ASME provides more detailed guidelines for complex assemblies.
10. Conclusion
ISO 2768 is a foundational tool for precision manufacturing, simplifying the specification of tolerances and enhancing efficiency.
By promoting standardization and clarity, it reduces costs, minimizes errors, and ensures high-quality results.
Adopting ISO 2768 in your design and manufacturing processes can lead to smoother operations, better collaboration, and superior products.
If you’re looking for professional manufacturing services that comply with ISO 2768, contact our experts today and take your projects to the next level.
