Overview
ISO 3443-3:1987 - "Tolerances for building - Part 3: Procedures for selecting target size and predicting fit" provides standardized procedures to relate joint clearances and target sizes within dimensional coordination (including modular coordination). It gives a practical basis for predicting the probability of fit between manufactured components and in‑situ work, and for selecting target sizes (work sizes) so that joints meet required clearance limits.
Key topics and requirements
- Scope and purpose: Relates component sizes, in‑situ space sizes, accuracy and tolerances to predict fit and select target sizes for standard and non‑standard components.
- Core concepts: Uses the concepts of extension (space target size minus coordinating size) and deduction (component target size less than coordinating size) to maintain dimensional coordination.
- Statistical basis: Assumes size deviations follow normal distributions; uses mean (systematic deviation) and standard deviation from measurement surveys or tolerance specifications.
- Risk and probability of fit: Requires selection of acceptable risks of misfit (too large or too small joint clearances). Procedures reference multipliers (Q and q) to set target limits for chosen probabilities.
- Procedure content: Provides calculations for lower and upper component target limits, maximum and minimum space extension, and deduction values. Worked examples appear in Annex B; inherent deviations are covered in Annex A.
- Assumptions & limitations: Assumes components can be adjusted/placed to achieve required clearances or that constructed spaces can be measured prior to insertion. Best applied where variability data for component types or construction forms are available.
Practical applications
- Component manufacturers: Determine optimum target sizes for standard components to maximise the probability of fit across applications.
- Building designers and engineers: Evaluate applicability of standard components, set target sizes for on‑site construction, and design assemblies with predictable joint behavior.
- Contractors and façade specialists: Use procedures to predict joint clearances, choose suitable jointing techniques, and assess acceptance risk on erection.
- Quality and production engineers: Translate measurement surveys and tolerance specifications into design targets and acceptance criteria.
Related standards
- ISO 3443-1: Basic principles for evaluation and specification
- ISO 3443-2: Statistical basis for predicting fit
- ISO 3443-4: Methods for predicting deviations of assemblies and allocation of tolerances
- ISO 1791, ISO 1803-1, ISO 1803-2: Vocabulary and derived terms related to modular coordination and tolerances
Keywords: ISO 3443-3:1987, tolerances for building, target size, predicting fit, dimensional coordination, modular coordination, joint clearance, extension and deduction, probability of fit, component manufacturers, building designers.