Overview
ISO 12780-2:2011 - Geometrical product specifications (GPS) - Straightness - Part 2: Specification operators defines the complete specification operator for straightness of integral features (line-type features). It covers only complete straightness profiles and prescribes how straightness specifications must define the transmission band, probing system, and associated measurement parameters so that a straightness requirement is unambiguous and reproducible.
Key topics and requirements
- Complete specification operator: A full, ordered set of unambiguous specification operations (as defined in ISO 17450-2) that together define straightness for a feature of type line.
- Transmission band / filtering:
- Uses a phase‑correct longwave‑pass filter (per ISO 11562) that transmits infinite wavelength and progressively attenuates shorter undulations around the cut‑off.
- Typical cut‑off lengths given: 8 mm, 2.5 mm, 0.8 mm, 0.25 mm, 0.08 mm.
- Associated maximum sample point spacing and maximum stylus tip radii (to avoid profile distortion) are specified. For example:
- λ = 8 mm → max spacing 1.14 mm, max stylus radius 5.0 mm
- λ = 2.5 mm → max spacing 0.357 mm, max stylus radius 1.5 mm
- λ = 0.8 mm → max spacing 0.114 mm, max stylus radius 0.5 mm
- λ = 0.25 mm → max spacing 0.0357 mm, max stylus radius 0.15 mm
- λ = 0.08 mm → max spacing 0.0114 mm, max stylus radius 0.05 mm
- Minimum sampling: At least seven sample points per cut‑off are recommended (Nyquist-related practical minimum).
- Probing system:
- Contacting probing with a spherical stylus tip is part of the operator.
- Probing force specified as 0 N (to exclude elastic deformation effects from the specification operator).
- Ambiguity warning: ISO 12780‑2 does not assume defaults for filter cut‑off, probe tip radius or method of association - these must be explicitly stated in the specification or the requirement is ambiguous (ISO 17450‑2).
- Measurement integrity: Addresses harmonic content, aliasing, Nyquist sampling criteria and the need for band limitation (probe, analogue/digital filters) to avoid aliasing errors.
- Compliance: Proving conformance or non‑conformance follows decision rules in ISO 14253‑1.
Applications and users
ISO 12780-2 is used where formal, reproducible straightness specifications are required:
- Quality engineers and metrologists defining straightness tolerances for parts and assemblies.
- Manufacturing engineers and inspection planners specifying measurement setup (filters, stylus radius, sampling).
- Calibration and test laboratories developing measurement procedures for line features.
- Procurement and design teams who must unambiguously specify straightness in drawings and technical specifications.
Benefits include consistent measurement results, reduced ambiguity in supplier inspections, and improved interoperability between CAD/tolerance specifications and metrology systems.
Related standards
Keywords: ISO 12780-2:2011, straightness specification operators, geometrical product specifications (GPS), transmission band, longwave-pass filter, stylus tip radius, Nyquist sampling, ISO 17450-2, ISO 14253-1.