Tolerance classes iaw. ISO 22081 and DIN ISO 2768: Optimized use of general tolerances

This article deals with the topic of general tolerances and their tasks in the design process.

Tolerances are an essential aspect for manufacturing mechanically moving parts, as they ensure the correct interaction of components. General tolerances and their tolerance classes are used to simplify drawings.

The standard DIN ISO 2768 specifies tolerances. Many design engineers use general tolerances as a simplified variation of tolerances to facilitate the design process.

What are tolerances in design?

Tolerances in design are generally defined as the deviation of a dimension from its ideal value. This deviation is divided into tolerance classes that determine the effects of the deviation. DIN ISO 2768 defines which tolerances are appropriate for which applications.

The most important terms are:

• Nominal dimension
• Upper dimension (+/-), also called maximum dimension
• Lower dimension (+/-), also called minimum dimension
• Limit dimension
• Adjustment: Is the dimensional relationship (type and manner) of two components. For example, how a bearing and a shaft fit together; how much “clearance” there is between the two components.

The tolerance with the upper and lower dimensions is also referred to as the tolerance limit.

For machined parts, DIN ISO 2768 applies, which describes the following features:

• for lengths, broken edges and diameters
• for angles
• for concentricity and axial run-out
• for shape and position
• for symmetry
• for perpendicularity
• for flatness and straightness

Going forward, DIN ISO 2768 will be replaced by the new ISO 22081.

Tolerance according to ISO 22081

As a design engineer, thanks to ISO 22081 you can simplify product specifications and proof of conformity optimally by describing all non-function-relevant features clearly and completely, taking into account the special features of production technology and material physics. Careful application of general tolerances can help to simplify design and to ensure that the results meet requirements.

Design engineers using ISO 22081 must make optimized use of general tolerance classes and general geometric specifications. Instead of specifying tolerance values, ISO 22081 only determines the specified feature:

• Feature for position
• Feature for surface profile
• Feature for line profile
• Feature for total runout and axial runout
• Feature for straightness
• Feature for symmetry
• Feature for perpendicularity
• Feature for position and shape
• Feature for flatness
• Feature for distance
• Feature for parallelism
• Feature for coaxiality
• Feature for concentricity
• Feature for slope and angle
• Feature for roundness

The user must then decide for himself which tolerance value (constant or variable) is best suited for implementing the feature and must also determine the mandatory complete datum system with all tolerated nominal geometric elements.

NOTE: Complete and clear product documentation and product conformity is a mandatory prerequisite to meet increasing global cooperation and outsourcing. As a design engineer, it is important to know the tolerance classes and ISO 22081 in order to use them optimally for design.

What do you need to consider when using DIN ISO 22081?

If you, as the designer, wish to make optimized use tolerance classes and DIN ISO 22081, you should consider the following exclusion criteria to avoid ambiguities. We have listed the most important exceptions here:

• DIN ISO 22081 does not apply to all integral geometric elements derived from integral size measurement elements that can be used with an individual geometric specification. These are defined either directly in the technical product documentation or indirectly by CAD attributes in the digital data set for the product definition.
• DIN ISO 22081 does not apply to linear dimensioned elements or to partial regions of linear dimensioned elements that are subject to a general dimension specification or that have an individual dimension specification that is defined either directly in the technical product documentation or indirectly by CAD attributes in the digital data set for the product definition.
• It is important to know that ISO 22081 does not apply to the reference elements specified in the reference field of the tolerance indicator of the general geometric specification.

Tolerances according to DIN ISO 22081

The International Standardization Organization (ISO) has developed DIN ISO 22081 for tolerance classes in design and mechanical engineering. DIN ISO 22081 includes four basic tolerances: fit tolerance, geometry tolerance, dimensional tolerance, and position and alignment tolerance.

The fit tolerance defines the dimensions and fits that must be observed when installing components. It also determines how close the gap can be between the parts.

The geometry tolerance defines the shape and size of certain components and ensures that they have the required accuracy.

The dimensional tolerance concerns the dimensions and tolerances of a component and is used to minimize the effects of undesirable deviations when manufacturing components.

The position and alignment tolerance governs the direction and position in which a component must be installed to achieve a certain accuracy.

What you must observe in the DIN ISO 2768 standard

New international standards for general tolerances were published at the end of 2022. DIN ISO 22081 will soon replace the old standard DIN ISO 2768.

The old standard DIN ISO 2768 is still valid (as of March 2023) and can therefore continue to be used.

For new designs, use of the new standard DIN ISO 22081 is recommended.

As a Japanese manufacturer, MISUMI manufactures its products according to the Japanese standard JIS B0401, which is identical to the standard ISO 22081.

Selecting the appropriate tolerance class and what must be observed

General tolerances are divided into classes.

The design engineering process must consider the following factors:

• required fit accuracy
• Costs

IMPORTANT: The tolerance class also defines the expense of manufacturing or procuring the component. The lower the tolerance, the higher the costs

General tolerances are divided into:

• f (f) fine – e. g. precision engineering
• m (m) medium – e. g. mechanical engineering (typical machining accuracy)
• c (c) coarse – e. g. castings
• v (v) very coarse - hardly used nowadays because modern manufacturing methods allow for higher precision

How are tolerances and tolerance classes called out and shown in engineering drawings?

The title block of an engineering drawing defines the tolerance for the entire drawing with an abbreviation. The exact specification is described in the standards DIN 406-10 and DIN 406-11.

For example:

• DIN ISO 2768-m (for medium)
• General tolerance DIN ISO 2768-c (for coarse)

CAD programs automatically display the tolerance settings, which therefore do not have to be inserted manually.

What tolerances still need to be observed?

DIN ISO 2768 or ISO 22081 applies to machined parts.

Other standards apply to other manufacturing processes, for example:

• DIN 16742 for injection molded parts
• DIN 12020 for manufacturing aluminum profiles
• ISO 20457 for plastic parts

Quality Control for Tolerances and Components - How to Deliver Accurate Results

As manufacturers, we know how important it is that the components we design and produce meet the highest quality standards. Measuring tolerances is an important part of our quality control.

By using different tolerances, your component can be manufactured such that it meets the design requirements.

As a Japanese manufacturer, MISUMI manufactures its products according to JIS B0401, which is identical to ISO 22081.

MISUMI uses the particularly precise tolerances h5/g6 for many components. For more information please use our tolerance tables iaw. JIS B04011, 2 (1998).

Tolerance tables for dimensions iaw. DIN ISO 2768-1

DIN ISO 2768-1 specifies the dimensional tolerances.

Tolerance tables for shape and position iaw. DIN ISO 2768-2

DIN ISO 2768-2 defines the shape and position tolerances.

In addition to general tolerances, you can restrict the tolerances of shafts and shanks further in the option tables here.