Profile standards are set to change - What you need to know

The upcoming revision of profile parameter standards is set to change the way metrologists, technicians and engineers perform and analyze measurements. Here is an update of the current work at ISO.

Surface metrology is vastly dominated by profile measurements and profile parameters specifications. Many international standards related to profile specifications, measurements or analysis have been developed with the aim of guiding metrologists and designers:

ISO 1302 explains how to specify surface texture on drawings

ISO 4287 describes the main profile parameters (Ra, Wa and so on)

ISO 4288 explains how to apply parameters and select cut-offs

ISO 16610-21 describes the Gaussian profile filter

ISO 3274 describes the basic requirements for stylus profilometers

ISO 12179 explains how to calibrate a stylus profilometer

ISO 5436 describes the material measures used to calibrate instruments

ISO 12085 describes the motifs method and parameters (also called "French motifs"

ISO 13565 describes a robust filter and functional parameters used in the automotive industry

Most of these standards were published or last revised in the late 1990’s and have since been regularly confirmed without modification. However, the recent publication of ISO 25178 established a new universal basis for surface texture by initially targeting areal surface texture. It makes sense today to apply the same concepts to profile surface texture and therefore revise or replace existing standards.

The ISO TC213/WG16 has already started work on this subject and has been discussing the draft of a new three-part standard for eighteen months now. This draft should become an official work item in 2016 (with an ISO number) and could be published within the three next years.

A new standard in three parts

This project is organized into three parts, exactly as the first three parts of ISO 25178 which correspond to the first three columns of the GPS matrix (see ISO 14638):

1. Indication of profile surface texture on drawings

2. Terms, definitions and surface texture parameters

3. Specification operators

Example of roughness specification on a drawing.

Note the line segment above the triangle on the left of the radical sign, which identifies the specification as a profile surface texture specification.

The new profile standard is labelled ISO 21920 and is in three parts.

Part 1 of this new profile standard will basically incorporate ISO 1302 with some additions coming from ISO 1101 and other specification documents.

Part 2 will incorporate all parameters of existing profile standards and add new ones, either old parameters that have been dropped or ones adapted from ISO 25178; for example, Pvv will provide the void volume of the valleys on the primary profile (adapted from Svv).

Part 3 will give default specification values that can be omitted on a drawing, such as units, nesting index, filter type, etc.

Changes to profile lengths

One of the main achievements of this draft is that parameters are now defined on the evaluation length. This means they are no longer calculated several times and then averaged. Instead, there will be only one Ra (or Rq) value calculated on the profile. The only exceptions are for Rp, Rv and Rz which will still be averaged to reduce the influence of outliers. Moreover the name "sampling length" is changed to "section length" to avoid confusion with the sampling of points on a discrete profile (see ISO 14406).

Current averaging method (see ISO 4288). The above profile is divided into five sampling lengths (L1 to L5) on which five estimated values of a parameter are calculated and averaged. Some parameters are calculated on the evaluation length (Le) such as Rt. This will no longer be the case with the new standard.

Calculating parameters on the evaluation length is not new. ASME B46.1, the American standard for surface texture already specifies profile parameters without averaging the parameters on a number of sampling lengths.

Mountains® software already allows users to choose the profile length used to calculate parameters. (Go to File menu > Preferences (or F7) and Metrology > Filtering.)

The default setting is five sampling lengths but any number of sampling lengths can be defined. Alternatively, parameters can be calculated on all sampling lengths available on the profile or on the evaluation length.

Replacement of the 16% rule

Another important change concerns the 16% rule defined in ISO 4288, which is quite complex and not very well understood by users. The 16% rule will not be the default rule anymore. It may be replaced, if necessary, by specifying multiple measurements and setting a tolerance on a statistical parameter. Otherwise tolerances and specifications will be verified with respect to a single measurement.

So far, there are no plans to write documents describing instruments as they are already well specified in ISO 25178-60x for their metrological characteristics and ISO 25178-700 for their calibration. Furthermore, ISO 5436-1 is already incorporated into ISO 25178-70 which provides material measures for the calibration of surface texture instruments, areal or profile.

Of course it is too early to speculate on the final versions of these documents as they can be amended by experts at the different stages of voting which will take place before final publication. But one thing is sure: metrology practices will be changed and modernized. Metrologists will likely need to revise their procedures, designers to adapt their specifications and instrument manufacturers to update their analysis software.

What will change in Mountains®

Mountains® software (including MountainsMap® Profile) will be updated accordingly as soon as details of the new standard are agreed. The update will be made available at the FDIS final draft stage allowing Mountains® users to implement the new parameters even before official publication of the standard.

Current standards will, of course, continue to remain available and users will be able to set preferences defining which standards should be applied by default.

This text was first published in the Surface Newsletter, Fall 2015.