Surface roughness cutoff length

A profile cutoff is a horizontal wavelength, usually in millimetres. Ra and Rz are height parameters, usually in micrometres. The profile, filter and evaluation settings together determine the result. NIST profile-filter explanation

Filtering is part of the measurement definition

Illustrative filter calculation: for a sinusoidal profile component with amplitude 0.20 µm exactly at a filter’s stated 50% transmission wavelength, the corresponding transmitted amplitude is 0.20 × 0.50 = 0.10 µm in that filter response. This is not a prediction of the Ra of a real multiscale surface. It illustrates why “the cutoff removes everything above this wavelength” is an inadequate description. Transmission definition

In legacy profile terminology, the long-wavelength cutoff λc separates roughness from longer-scale waviness. The short-wavelength cutoff λs limits the other end of the roughness bandwidth. Form removal is another operation. KEYENCE’s JIS profile guide distinguishes the S-filter, which removes short-scale components, from the L-filter, which removes long-scale components, and from the F-operator for nominal form. Selecting λc alone does not completely define the processing chain. KEYENCE profile filters

A filter is not an ideal switch that preserves every shorter wavelength unchanged and deletes every longer one. Evident’s surface-roughness guide describes cutoff wavelength through a 50% amplitude-transmission point. Its footer carries document code N8600858-022023; its profile terminology is tied to the legacy JIS references listed in that guide. Preserve filter type as well as its scale when comparing results. Evident guide, profile glossary and footer

Distinguish the lengths

ZEISS Academy’s ISO 21920 poster distinguishes le, section length lsc, section count nsc, and pre/post travel. Thus a 4 mm evaluation does not imply exactly 4 mm of available traverse. The extra travel depends on acquisition and processing; obtain it from the configured instrument/measurement procedure. ZEISS ISO 21920 surface-parameter poster

Legacy ISO 4287/4288 context

ISO 4287 supplied legacy profile definitions and parameters; ISO 4288 supplied assessment rules. Mitutoyo’s technical page explicitly labels its selection tables JIS B 0633:2001 / ISO 4288:1996. It lists separate tables for nonperiodic profiles using Ra-related or Rz-related criteria, and for periodic profiles using spacing RSm. These are different selection routes. A table headed by measured/estimated roughness must not be presented as a universal setting for every profile or drawing. Mitutoyo legacy assessment tables

For example, that page’s nonperiodic Ra table assigns lr = 0.8 mm and ln = 4 mm for 0.1 < Ra ≤ 2 µm. The arithmetic is 5 × 0.8 = 4 mm. This is a bounded legacy-table example, not a recommendation to set every machined surface to 0.8 mm. The page also provides an iterative selection procedure when sampling length is not specified. Mitutoyo ISO 4288-based lengths and selection

What changes with ISO 21920

ISO lists ISO 21920-3:2021, first edition published December 2021, as the specification-operator standard replacing withdrawn ISO 4288:1996 and its corrigendum. A replacement in the standards catalogue does not by itself rewrite an existing drawing or agreed inspection plan: determine the requirement and edition governing the part before selecting the instrument’s standards mode. ISO 21920-3 publication and lifecycle

Mahr explains that ISO 21920 introduces nesting indices Nic and Nis, the evaluation-length notation le, and removes the periodic/nonperiodic distinction for choosing the relevant nesting index. Its calibration guidance also shows why this is more than renaming: evaluation and filtering can differ for particular parameter families, including core-roughness parameters. Do not claim equivalent results merely because a legacy cutoff and a modern nesting index have the same numerical value. Mahr ISO 21920 calibration changes

ZEISS’s poster shows drawing-tolerance-based defaults for specified parameters and setting classes Sc1–Sc5. As a bounded Sc3 example, it lists Nic = 0.8 mm, le = 4 mm, Nis = 2.5 µm, maximum point spacing 0.5 µm, and maximum nominal stylus-tip radius 2 µm. For section-based parameters it lists lsc = 0.8 mm and nsc = 5. Those values belong to the stated setting-class context; a specified override or a different parameter/tolerance case needs its applicable operator. The poster carries document code EN_60_050_0038I and print mark CZ-VIII/2023; it does not explicitly label a revision date. ZEISS setting-class table

Select and report a reproducible setup

Read the complete surface requirement: parameter, tolerance type/value, standard/edition, specified filter or nesting indices, evaluation length, and any acquisition or direction requirements. Confirm that the instrument can acquire the needed usable profile on the actual feature. A short land does not justify quietly selecting a shorter cutoff: that changes the assessed bandwidth and can undermine comparison with the requirement. NIST explains that a defined bandwidth is important when comparing results from different instruments. NIST filtering and comparison

When legacy and modern results disagree, retain the raw profile where available and compare the complete operators before attributing the difference to manufacturing. Mahr describes issuing surface-standard calibration results under both legacy and ISO 21920 frameworks to make the differences visible. Such side-by-side reporting preserves the basis of comparison; it does not establish a universal conversion between the results. Mahr dual-framework calibration reporting

A usable inspection record includes the location and direction, acquisition method, instrument/probe, tip and point-spacing settings, form removal, filter types/indices, section/evaluation lengths, standard mode, parameter result, and acceptance rule. Keep units explicit: 0.8 mm = 800 µm, not 0.8 µm. A roughness number without its operator can be insufficient for either repeat measurement or acceptance review.

Related reference

Inspection and quality documents