Introduction to the "new" ISO 21920-1: precision in surface texture

Key changes compared with ISO 1302

The introduction of ISO 21920 brings several significant changes compared with the previous standard ISO 1302. Here are the most important ones at a glance:

New standard number and symbols: the new standard is designated ISO 21920-1 and supersedes ISO 1302. A surface texture specification per ISO 21920-1 can be identified unambiguously by the new symbol.

New symbols of ISO 21920-1 vs. the historical ones from ISO 1302

Any manufacturing process permitted

ISO 21920-1:2022

Default Tmax

ISO 1302 historical

Default 16% rule

Material shall be removed

ISO 21920-1:2022

Default Tmax

ISO 1302 historical

Default 16% rule

Material removal not permitted

ISO 21920-1:2022

Default Tmax

ISO 1302 historical

Default 16% rule

New symbols and specification elements

ISO 21920 introduces a series of new symbols and specification elements that allow surface texture to be specified and interpreted more precisely. Here are some of the most important ones:

Tmax symbol

This symbol indicates that the Tmax rule applies to a given parameter. Tmax is the default in ISO 21920-1.

T16% symbol

This symbol specifies the 16% tolerance acceptance rule, under which 16% of the measured values may exceed the upper and lower tolerance limits.

Tmed symbol

This symbol designates the median tolerance acceptance rule, under which the median of all measured values must stay within the tolerance limits. At least three measured values are required to form the median. The symbol OR(n) specifies additional requirements, such as a higher number of measurements for applying the median acceptance rule.

Application and examples

To illustrate how the new standard is applied, here are a few examples:

Tmax rule: a part is measured and 10% of the measured values exceed the upper tolerance limit. Under the Tmax rule this would not be acceptable, since no measured value may exceed the limit.

T16% rule: a part is measured and 10% of the measured values exceed the upper tolerance limit. Under the T16% rule this would be acceptable, since the exceedance lies within the 16% limit.

Tmed rule: when measuring a surface texture parameter, the median of all measured values lies within the specified tolerance, although some individual values lie above and others below it. This satisfies the median tolerance acceptance rule, and the part is considered conforming.

Conclusion

ISO 21920 is a significant step forward in the specification and measurement of surface texture. With new symbols and rules and a more precise definition of tolerances, the standard meets the requirements of modern manufacturing processes and provides uniform, understandable documentation of technical products.

We hope this information helps you understand the changes in ISO 21920 and apply them in practice. For further questions or detailed explanations, our ISO GPS consulting is at your disposal.

Further relevant standards for surface texture specifications

ISO 21920-2:2021 Surface texture: Profile — Part 2: Terms, definitions and surface texture parameters

ISO 21920-3:2021 Surface texture: Profile — Part 3: Specification operators