ISO 21204:2020

Overview

ISO 21204:2020 - Geometrical product specifications (GPS) - Transition specification provides a standardized way to specify extended edge transition features between two adjacent integral features on technical drawings and in CAD. It defines operators, specification modifiers, and drawing indications (including the proportions and dimensions of graphical symbols) for edges where the nominal geometry of the transition matters. The standard is intentionally limited to transitions between two planes and between a cylinder and a plane nominally perpendicular to it. Corners (transitions between three or more features) are excluded.

Key topics and technical requirements

• Definition of edge transition features and the concept of an extended edge transition feature (includes portions of adjacent features).
• Specification operators and modifiers to express toleranced edge transitions; rules for applying them to any line in a defined direction of the extended feature.
• Graphical symbols and drawing indications: symbol usage, dimensions and proportions (see Annex C for relations and dimensions).
• Profile types covered: circular (fixed/variable/consistent), chamfer (fixed/variable/consistent), elliptical, and CAD-defined profiles.
• Boundary specifications: maximum material and least material boundary options, combinations with profile specifications.
• Supplemental indication rules: extent of toleranced feature, tolerance zone offsets, indication of adjacent reference sections and multiple toleranced features.
• CAD integration: rules for CAD profile specifications and CAD-based tolerance boundaries.
• Informative materials: Annex A presents a preliminary algorithm for identifying toleranced features and adjacent reference sections (subject to change), Annex B examples, Annex D relation to the GPS matrix model.

Practical applications

• Ensuring functional edge shapes for assembly without interference, mating parts, sealing surfaces, and safety-critical edges.
• Providing clear drawing and CAD-based specifications where the nominal transition geometry (not a sharp edge) affects function.
• Streamlining communication between design, manufacturing, and inspection by standardizing how edge transitions are documented.

Who should use ISO 21204:2020

• Design and mechanical engineers specifying part geometry
• CAD modelers and drafting teams producing 2D/3D documentation
• Quality, inspection and metrology engineers interpreting tolerances
• Manufacturing engineers responsible for tooling and process design
• Standards writers and technical documentation specialists

Related standards (for further context)

• ISO 1101 - Geometrical tolerancing (form, orientation, location, run-out)
• ISO 13715 - Indication of edges of undefined shape
• ISO 14638, ISO 8015, ISO 14253-1 - GPS framework and decision rules
• ISO GPS matrix model references in Annex D of ISO 21204

Keywords: ISO 21204:2020, transition specification, edge transition feature, GPS, geometrical product specifications, CAD profile specification, drawing indications, graphical symbols, edge tolerancing.