Modernising metrology

The British Standards Institution has recently published the much-anticipated BS EN ISO 21920 series of international standards on surface properties in technical product documentation. Here the BSI outlines some of the changes

Surface texture plays an important role in determining how an object will interact with its environment. It can have an impact on everything from friction and corrosion to heat transfer and wear. It’s an aspect of mechanical-component development that has been measured and verified for more than 80 years across every field of engineering in order to improve the performance of manufactured products.

The handful of international standards that are related to profile specifications, measurements and analysis were developed with the aim of guiding metrologists and designers to comply with the agreed best practices with regard to profiling surface texture. However, many of these industry specifications were last revised during the late 1990s and have since been regularly adopted without any changes.

Because some of the previous standards no longer corresponded to the current state of the art, it wasn’t always possibly to provide a complete description of the surface finish on a manufactured component. For example, new production processes, such as additive manufacturing, pose a new set of measurement challenges when compared to the traditional ablative manufacturing processes.

Given these changes in production processes, as well as recent advancements in the technology that governs engineering techniques and the publication of more recent standards regarding geometrical product specifications, it became apparent that it was time to refresh the standardisation for surface texture.

The result is the recently published international BS EN ISO 2190 suite of surface-texture standards, developed by the global committee ISO/TC 213. This new standard series will support the work of mechanical engineers, design engineers and engineering designers who operate in the aerospace, defence, automotive, rail, nuclear and medical-devices sectors. It will also be useful to engineering or manufacturing organisations that produce engineering drawings – whether they use 2D manual drawings or 3D CAD-generated models – in their product design and development processes.

The new standard series consists of three individual parts:

■ BS EN ISO 21920-1:2022 Geometrical product specifications (GPS) – Surface texture: Profile – Indication of surface texture provides a modern approach to the indication of surface texture requirements on mechanical engineering drawings (and CAD models), specifying the rules for using graphical symbols to indicate surface texture in technical product documentation. It also adds to the International Standard Organization’s geometrical product specifications toolbox of requirements (enhancing the full suite of ISO/TC 213 standards).

■ BS EN ISO 21920-2:2022 Geometrical product specifications (GPS) – Surface texture: Profile – Terms, definitions and surface texture parameters develops the terminology used in describing surface textures, providing updated terms, definitions and parameters for the surface texture requirements used on engineering drawings and CAD models. It replaces several standards – BS EN ISO 4287:1998, BS EN ISO 12085:1997, BS EN ISO 13565-2:1998 and ISO 13565-3:2000 – which have been the main surface texture standards for industry for many years.

■ BS EN ISO 21920-3:2022 Geometrical product specifications (GPS) – Surface texture: Profile – Specification operators provides specifications on the complete specification operator for surface texture by profile methods.

Taken together, the new guidelines within these standards will be used by manufacturing and production engineers, as well as metrologists and quality-control teams, who measure and verify that the specifications for the surface texture of components have been met.

It’s hoped that the introduction of this new standard series will result in the modernisation of metrology practices. Importantly, the series will replace the long-standing industry front-runner – BS EN ISO 1302 – as the definitive specification for texture indication. As a result, metrologists will likely need to revise their processes, instrument manufacturers will need to update their analysis software and designers will need to adapt their specifications in order to adhere to the new industry guidance.

One of the main changes introduced in the new series is that parameters are now defined on the evaluation length. In practice, this means that they are no longer calculated for five sampling lengths and then averaged. Instead, there will be only one Ra (or Rq) value calculated on the profile. However, Rp, Rv and Rz will still be averaged in order to reduce the influence of outliers. Moreover, the term ‘sampling length’ has been changed to ‘section length’ in order to avoid confusion with the sampling of points on a discrete profile.

Among the other key changes are the fact that the maximum tolerance acceptance rule is now the default tolerance acceptance rule and the inclusion of definitions of new parameters, in particular parameters based on the watershed transformation. Additionally, as with all BS 8888 technical drawing standards, the BS EN ISO 21920 series is aimed at users of the former national standard, BS 308. Because many organisations are still using the withdrawn BS 308 for their drawings, this new international standard will encourage a move away from traditional, outdated practices.