Changes in roughness measurements with stylus profilometers: the new series of standards ISO 21920

The evaluation of the surface quality by means of stylus profile measurements has long been firmly established in quality assurance in the manufacturing industry and is still the most common method of measuring roughness. The roughness parameters and the procedures for their measurement and evaluation are standardized to allow interchangeability in construction.
The previous standards have been revised by the responsible ISO committees and brought together in the new three-part series of standards ISO 21920.

Part 1 of ISO 21920 deals with the drawing entries and replaces the previous ISO 1302; the possibilities for drawing indications have been significantly expanded and thus take today's production requirements and potentials into account.
Part 2 defines the roughness parameters, which replaces the previous standards ISO 4287, ISO 13565-2 and ISO 13565-3. A major change is that parameters are now primarily calculated directly for the entire evaluation length and no longer individually for individual section lengths first.
The parameters for characterizing the contact areas of stratified surfaces are determined from roughness profiles, which are obtained by numerical filtering to eliminate form and longwave components so that pronounced dales are assigned more clearly to the roughness. The filter method previously specified for this in ISO 13565-1 has therefore been replaced by a more robust filter that is more suitable for plateau-like areas with retention volumes. This robust filter is defined in ISO 16610-31.
The ISO 21920-3 standard is dedicated to the specification operators, which primarily include the required measurement lengths, permissible stylus tip radii and filters to be used, whereby these aspects are largely regulated in accordance with the previous ISO 4288. The most important change in ISO 21920-3 is a stronger orientation towards the drawing entries.

A general goal of the revision was to largely adapt the procedures for profile-based measurement methods to the level of standardization that has now been reached for areal measurements, for which the new multi-part standards series ISO 25178 is being developed for several years. The ISO 25178 series focuses in particular on optical surface measurement methods, which are becoming more and more widespread in industrial measurement technology. In addition, shortcomings in the previous standards that were recognized in the meantime have been eliminated by defining parameters more precisely in the ISO 21920-2 standard and defining algorithms for their determination. Ambiguities and potentially different interpretations could thus be eliminated.

On the roughness standards manufactured by grinding and ultra-precision diamond turned superfine roughness standards that are regularly calibrated at PTB, the differences are in most cases only a small fraction of the specified measurement uncertainty when changing from the old to the new standards. Due to the differences in the filtering, deviations in the contact area or material proportion parameters for the stratified surfaces can occur that are in some cases in the order of the stated uncertainty.
Since the beginning of 2022, the roughness values in the PTB calibration certificates for roughness standards are specified according to the new standards (ISO 21920 and 16610-31) in addition to the results based on the previous standards (such as ISO 4287, 4288 and 13565). The evaluation for the calibrations within PTB is carried out with the roughness analysis software RPTB, which is continuously being further developed at PTB. This software also serves as reference software and is available worldwide via web interface, see www.ptb.de/rptb. In the course of the revision of the standards, RPTB was also revised accordingly. The software offers the possibility to analyse according to both the old and the new standards.
In order to ensure that the DAkkS laboratories accredited for roughness can work according to the new standards ISO 21920 and ISO 16610-31 as soon as possible, an interlaboratory comparison is currently running to validate the respective software of all participating DAkkS laboratories using synthetic reference data sets. This comparison focuses on checking the numerical implementation of the evaluation algorithms.
Furthermore, a re-evaluation according to the new standards is made available to the DAkkS laboratories for all the profile data of the currently valid calibrations, so that these standards can also be used for traceability in the laboratories according to the new series of standards without having to submit them to PTB for re-measurement in the meantime. A transitional phase lasting several years is assumed before the new standards are generally implemented.

International editions:

• ISO 21920-1 Geometrical product specifications (GPS) — Surface texture: Profile — Part 1: Indication of surface texture
• ISO 21920-2 Geometrical product specifications (GPS) — Surface texture: Profile — Part 2: Terms, definitions and surface texture parameters
• ISO 21920-3 Geometrical product specifications (GPS) — Surface texture: Profile — Part 3: Specification operators
• ISO 16610-31 Geometrical product specifications (GPS) — Filtration — Part 31: Robust profile filters: Gaussian regression filters

Corresponding German editions:

• DIN EN ISO 21920-1 Geometrische Produktspezifikation (GPS) — Oberflächenbeschaffenheit: Profile — Teil 1: Angabe der Oberflächenbeschaffenheit
• DIN EN ISO 21920-2 Geometrische Produktspezifikation (GPS) — Oberflächenbeschaffenheit: Profile — Teil 2: Begriffe und Parameter für die Oberflächenbeschaffenheit
• DIN EN ISO 21920-3 Geometrische Produktspezifikation (GPS) — Oberflächenbeschaffenheit: Profile — Teil 3: Spezifikationsoperatoren