ISO 17450-3:2016

Overview

ISO 17450-3:2016 - part of the ISO Geometrical Product Specification (GPS) family - defines default rules and definitions for toleranced features used in dimensional, geometrical and surface texture specifications. The standard establishes how extracted features (integral or derived) of a workpiece are defined and interpreted in GPS specifications, providing default geometrical features such as integral features, median features and opposing point pairs that underpin verification and tolerancing.

Key Topics and Requirements

• Scope and intent: Gives default definitions for extracted features that become the toleranced feature in GPS specifications. It is a fundamental GPS standard and follows general GPS tenets (ISO 8015) and decision rules (ISO 14253‑1).
• Extracted integral features: Defines integral features (points, lines, surfaces) and states that, by default, the toleranced feature is the complete extracted integral feature. Elementary toleranced features can be portions, lines or point sets of the integral feature.
• Extracted integral line and point:
  • An extracted integral line is formed by intersection of the real (non‑ideal) feature with an intersection feature; if the intersection is not fully constrained, multiple extracted lines form the complete toleranced feature.
  • An extracted integral point results from the intersection of a non‑ideal integral feature with a locked intersection straight line.
• Opposing point pair and two‑point size: An opposing point pair (two points established simultaneously) defines local size; the centre of such a pair can be used to derive median surfaces or centres.
• Median features: The document addresses derived median point/line/surface definitions (used to characterize size, location or orientation of features of size).
• Default association rules: Unless otherwise specified, associations to establish geometrical features use the total least squares (Gaussian) objective without material constraint. Final associations for datums and characteristics depend on the specification.
• Notes on practical effects: The standard highlights how uneven point distribution or removed portions (e.g., keyways) can bias association results (e.g., axis location shifts).

Applications and Who Uses It

ISO 17450-3 is used by:

• Design and mechanical engineers for consistent GD&T and tolerancing definitions.
• Metrology and quality control teams for measurement plans and verification of features of size and form.
• CAD/CAM and inspection software developers to implement default extraction and association algorithms (e.g., median extraction, opposing point pair logic).
• Standards writers and technical committees aligning GPS matrix models and related standards.

Practical applications include specification of holes, shafts, slots, wedges and other features of size where extracted/median features and datum associations are required for inspection and acceptance.

Related Standards

• ISO 17450‑1 (Model for geometrical specification and verification)
• ISO 17450 (other parts), ISO 17450‑4
• ISO 22432, ISO 25378 (GPS terminology/characteristics)
• ISO 14253‑1, ISO 8015, ISO 14638
• ISO 14405 (size), ISO 1101 (geometrical tolerancing), ISO 1302 (surface texture)