ISO 21920-3:2021

Overview

ISO 21920-3:2021 - "Geometrical product specifications (GPS) - Surface texture: Profile - Part 3: Specification operators" - defines the complete specification operator for surface texture assessed by profile methods. It formalizes how a profile-based surface-texture specification is composed (including order of operations and default choices), so that drawings, specifications and verification activities are unambiguous and consistent.

This part of the ISO 21920 series replaces ISO 4288:1996 and provides default decision rules, setting classes (Sc1…Sc5) and practical rules to select filters, profile directions, profile positions and tolerance-acceptance rules when these elements are not explicitly stated on the drawing.

Key topics and technical requirements

• Complete specification operator: defines all operators required for an unambiguous profile surface-texture specification (order and contents).
• Default settings: a set of general defaults for unspecified elements to simplify drawing indications (see Tables and Annexes in the standard).
• Profile extraction and direction: default to mechanical profile extraction in the direction yielding maximum roughness height parameters (typically perpendicular to the dominant lay).
• Profile position rules:
  • Maximum tolerance acceptance rule: sample the area where critical values are expected.
  • 16% and median rules: use uniformly distributed traces across the surface.
• Filters:
  • Profile S-filter: Gaussian filter per ISO 16610-21 (default).
  • Profile L-filter (R‑parameters): Gaussian filter by default; for certain parameters (e.g., Rk, Rpk, Rvk and related R*-parameters) the default is the robust Gaussian regression filter (ISO 16610-31) to better remove large-scale components.
• F‑operator (form removal): association and removal of specified form using total least squares on the evaluation length.
• Defaults based on specification: defaults can be selected from the drawing indication, setting class, nesting indices or directly from tolerance limits for common R‑parameters and Pt.
• Verification considerations: the verification operator is the physical implementation of the specification operator and may affect measurement uncertainty if different.

Practical applications and users

• Intended for metrology and quality engineers, manufacturing engineers, drawing authors, and inspection laboratories who specify, measure or verify surface texture by profile methods.
• Useful for product designers and CAD/CAM engineers who must indicate surface texture on engineering drawings and data sheets.
• Relevant for developers of surface metrology instruments and software that implement profile filters, parameter calculations and default-setting logic.

Related standards

• ISO 21920-1 (indication of surface texture)
• ISO 21920-2 (terms, definitions, parameters)
• ISO 16610-21 (Gaussian linear profile filters)
• ISO 16610-31 (robust Gaussian regression filters)
• ISO 17450-2 (specification operators / GPS concepts)
• ISO 8015 and ISO 14253-1 (GPS fundamental rules and default decision rules)

ISO 21920-3:2021 organizes defaults and operators to improve clarity and reproducibility in profile-based surface texture specification and verification.