ISO 4351:2023

Overview

ISO 4351:2023 - Geometrical product specifications (GPS) - Association defines the terminology and fundamental concepts for feature association in the ISO GPS system. It describes how ideal (mathematical) features are fitted to measured or extracted input features using an optimisation-based association criterion, and covers associated topics such as objective functions, association constraints and material offset. ISO 4351:2023 is a general GPS standard (part of the ISO GPS matrix) and complements other GPS standards that define defaults and syntax.

Key topics and technical requirements

• Association (definition): fitting one or more ideal features to one or more input features according to an association criterion.
• Input feature / Associated feature / Restricted associated feature: precise terminology for measured or derived geometry, the ideal fitted model, and the subset of the associated feature where local deviations exist.
• Association criterion: an optimisation setup made of an objective function (S-function or L-function) plus optional association constraints and optional material offset. The chosen criterion determines the association result.
  • S-function: objective based on the size of an associated feature (e.g., maximum inscribed / minimum circumscribed concepts).
  • L-function: objective based on the set of signed local geometrical deviations between input and associated feature.
• Association constraints: restrictions applied during optimisation such as orientation, location, material, and size constraints. Constraints control degrees of freedom or enforce material-boundary relationships.
• Material offset: optional post-optimization adjustment relative to the material boundary of the input feature.
• Multiple-feature association: associations can be done independently, simultaneously (with internal constraints), or in specific order using external constraints.
• Normative links: specifies relationships to ISO 17450-1 and ISO 17450-4 and references GPS fundamentals (e.g., ISO 8015, ISO 14253-1).

Practical applications and users

ISO 4351:2023 is directly applicable wherever ideal geometric models must be established from measured data for downstream specification, verification or manufacturing control:

• Defining datums and reference features for GD&T (geometrical tolerancing)
• Establishing associated toleranced features, dimensioning characteristics, intersection/orientation/collection planes, and direction features
• CMM and inspection software algorithms that compute best-fit features and report tolerances
• CAD/CAM model validation, measurement planning, and quality assurance

Primary users:

• Metrologists and quality engineers
• Design engineers and GD&T specialists
• CMM programmers and inspection software developers
• Standards writers and conformity assessors

Related standards

• ISO 17450-1 (GPS general concepts - model)
• ISO 17450-4 (GPS - geometrical characteristics for quantifying deviations)
• ISO 8015 and ISO 14253-1 (decision rules and GPS fundamentals)

Keywords: ISO 4351:2023, GPS association, feature association, association criterion, L-function, S-function, material offset, datum, metrology, GD&T, CMM, inspection software.