Surface roughness sampling length is part of a defined measurement setup, not a universal number that can be chosen from Ra alone. ISO 21920-3:2021 provides default setting classes that connect a specified Ra upper limit with filter nesting indices and evaluation length. This reference lists five common classes and their associated values. Before measuring, read the drawing for the parameter, tolerance form, standard edition and any explicitly specified settings; those instructions take precedence over a default lookup.

Surface Roughness Sampling Lengths

What the settings mean

The setting class identifies a bundle of default measurement conditions. The table relates an Ra upper-limit band to the L-filter nesting index λc and the evaluation length le. For section-length parameters such as Rz, it also lists a section length lsc and the number of sections. The sampling distance and nominal stylus-tip radius are additional instrument-related limits in the guide. These terms are linked, but they are not synonyms: the cutoff filters the profile, while the evaluation length defines how much filtered profile is assessed.

The classes step through λc values of 0.08, 0.25, 0.8, 2.5 and 8 mm, with evaluation lengths of 0.4, 1.25, 4, 12.5 and 40 mm. The roughness-related upper-limit ranges shown here progress from very small Ra values through values above 10 µm. The selected table focuses on the upper tolerance limit path; bilateral tolerance or lower-limit specifications can lead to different defaults in the standard. It should not be used to derive a setting for a different profile parameter by substituting its numeric value into the Ra bands.

How to use the table

First, identify the specified parameter and the drawing’s edition of the surface-texture standard. If the drawing gives a single-sided upper Ra limit and invokes ISO 21920-3:2021 default settings, locate the corresponding band and record its setting class. For example, an Ra upper limit of 0.8 µm falls in Sc3’s band, so the default λc is 0.8 mm and le is 4 mm. The instrument must still use the required filter, stylus characteristics, sampling distance and evaluation operator. The table does not decide whether the surface passes; acceptance depends on the stated tolerance and the evaluated result.

For a section-length parameter, the default setting uses five sections. An evaluation length of 4 mm corresponds to five 0.8 mm sections in Sc3. This is not a direction to measure any profile over five arbitrary pieces: the parameter definition and standard specify how sections are evaluated. Setup must also account for the instrument’s traverse and any pre-travel or post-travel needed by its filter implementation.

Standards and drawing checks

Older drawings may cite ISO 4287 or ISO 4288, while newer drawings may specify the ISO 21920 series or an explicit filter and evaluation condition. Do not silently convert a legacy note to a new class. Record the standard, parameter, filter, tolerance limits and measurement direction so the inspection result can be reproduced. A stylus instrument’s selected tip radius and sampling spacing matter especially when the surface contains narrow valleys, steep slopes or fine periodic features.

Common mistakes and limitations

A common error is treating five sections as five independent acceptance results or confusing the individual section length with the total evaluation length. Another is selecting a filter solely from a familiar Ra value while ignoring the drawing’s standard or bilateral tolerance. Neither practice is justified by this reference. The table contains default settings, not universal settings for every profile, material, instrument or functional surface.

Surface texture parameters describe different aspects of a profile. Ra is an arithmetic mean and can conceal a deep isolated valley or a peak that matters to sealing, wear or coating. Use the required parameters and inspection plan for the function. If the drawing is ambiguous, resolve the intended standard and operator with the responsible design or quality authority before collecting acceptance data.

Sources

Default setting values are taken from Mitutoyo’s ISO 21920-3:2021 quick guide. The official ISO 21920-3 edition page identifies the standard and its publication status. The standard itself governs when specified by the drawing.