ISO 12780-2:2011

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

• ISO 12780‑1 (vocabulary and parameters of straightness)
• ISO 17450‑2 (GPS specification operators and basic tenets)
• ISO 11562 (phase‑correct filters for profile measurement)
• ISO 14253‑1 (decision rules for proving conformance)
• ISO 3274 / ISO 4287 / ISO 4288 (related surface texture and stylus instrument 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.