ISO 12780-1:2011

Overview

ISO 12780-1:2011 - Geometrical product specifications (GPS) - Straightness - Part 1: Vocabulary and parameters of straightness defines the terminology, concepts and quantitative parameters used to describe straightness of individual integral features. The standard covers complete straightness profiles only and establishes how to refer deviations from straightness to unambiguous reference lines (e.g., minimum zone, least squares). It is a GPS vocabulary standard intended to support consistent specification and verification of straightness in design, manufacturing and metrology.

Key topics

• Scope and purpose
  • Defines terms and concepts related to straightness of individual integral features; excludes derived axes of cylinders (see ISO 12180 series).
• Profiles and extraction
  • Distinguishes an extracted line (intersection of the real surface and the straightness plane) and a straightness profile (extracted line after filtering).
• Reference lines
  • Formalizes reference-line concepts used for evaluation:
    • Minimum zone reference lines (MZ) - two parallel lines with least separation enclosing the profile.
    • Least squares reference line (LS) - line minimizing the sum of squared deviations.
    • Mean minimum zone line - arithmetic mean of MZ lines.
• Filter function and cut-off
  • Notes that extraction and optional profile filtering influence measurements; the currently defined filter is the phase-correct mean line filter (see ISO 11562). Cut-off wavelengths and transmission bands must be specified.
• Parameters and modifiers
  • Defines principal straightness parameters:
    • Peak-to-valley straightness deviation (STRt) - for all reference lines (only parameter defined for MZ).
    • Peak-to-reference (STRp) and reference-to-valley (STRv) - relative to least squares reference.
    • Root-mean-square straightness deviation (GQ) - rms deviation relative to LS.
  • Specification modifiers such as GT, GP, GV, GQ indicate the parameter and reference element used.

Applications

ISO 12780-1 is used by:

• Quality engineers and metrology laboratories for specifying and verifying straightness tolerances on parts.
• Design and manufacturing engineers specifying form-tolerances on precision components.
• Inspection software and coordinate measuring equipment (CMM) vendors to implement measurement and reporting conventions. Practical applications include straightness control of machined edges, tool guidance surfaces, linear slides and precision components where form of a line affects assembly, function or motion.

Related standards

• ISO 12780-2:2011 - Straightness - Part 2: Specification operators (defines extraction and verification operators).
• ISO 11562 - Surface texture filters (phase-correct mean line filter).
• ISO 12180‑1/2, ISO 12181‑1 - Related GPS standards for cylinder and roundness features.
• General GPS framework: ISO 17450, ISO 14660, ISO 8015.

Keywords: ISO 12780-1, straightness, geometrical product specifications, GPS, reference line, minimum zone, least squares, straightness profile, peak-to-valley, RMS straightness, cut-off wavelength.