ISO 21920-1:2021

Overview

ISO 21920-1:2021 - "Geometrical product specifications (GPS) - Surface texture: Profile - Part 1: Indication of surface texture" defines the rules for indicating profile-based surface texture on technical product documentation. It standardizes the use of graphical symbols, specification elements and decision rules used when specifying surface finish on engineering drawings and documentation. This part of the ISO 21920 series replaces ISO 1302:2002 and aligns with the GPS framework (see ISO 14638, ISO 8015, ISO 14253‑1). Note: population (batch) requirements are outside its scope (see ISO 18391).

Key topics and requirements

• Graphical symbols for profile surface texture and their proportions (normative annexes).
• Specification elements: mandatory and optional items that must appear to unambiguously define a profile specification.
• Tolerance acceptance rules: default and alternative rules (including the maximum tolerance acceptance rule as the default, the 16% rule, and median acceptance rule).
• Parameter types: references to profile parameters (R-, P-, W-parameters) and how to indicate them on documentation.
• Filters and extraction: designation of profile S‑filters, L‑filters, nesting indexes and methods of profile extraction.
• Evaluation and section lengths: requirements for evaluation length, section length and number of sections for measurement.
• Additional elements: F‑operator association, manufacturing process note, surface lay/direction, profile direction, setting class (Scn).
• Placement rules: positioning and orientation of the graphical symbol on technical product documentation.
• Inspection guidance: annexes that cover inspection procedures (e.g., for the 16% rule) and criteria for default rules.

Applications and who uses it

ISO 21920-1 is used by:

• Design and CAD engineers to place unambiguous surface texture requirements on drawings.
• Quality, metrology and inspection teams to interpret profile specifications and set up measurement procedures.
• Manufacturing engineers and process planners to translate texture specs into process controls.
• Procurement and contract authors for clear acceptance criteria in specifications and contracts.
• Standards committees and documentation authors aligning surface texture requirements with the GPS matrix.

Practical uses include specifying surface finish for mating parts, functional surfaces (sealing, bearing, lubrication), appearance requirements, and setting acceptance/inspection criteria across supply chains.

Related standards

• ISO 21920 series (other parts on measurement and parameters)
• ISO 1302:2002 (superseded)
• ISO 14638, ISO 8015, ISO 14253‑1 (GPS framework and decision rules)
• ISO 18391 (population/batch specifications)

Keywords: ISO 21920-1, surface texture, profile, GPS, graphical symbols, surface finish specification, tolerance acceptance rule, R-parameters, S-filter, L-filter, technical product documentation.