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Form and Positional Tolerances (GD&T)

Definition

geometric tolerancing per ISO-GPS standards, verified via CT measurement for form, orientation, location and run-out

Synonyms:
GD&TGPSGeometric Dimensioning and TolerancingForm and LocationForm Tolerances

What are Form and Positional Tolerances?

Form and positional tolerances (GD&T — Geometric Dimensioning and Tolerancing) describe the permitted deviations of a real part from its ideal geometric form. The ISO-GPS family of standards (Geometrical Product Specifications) defines internationally harmonised symbols and evaluation procedures for form, orientation, location and run-out deviations. CT allows all GPS characteristics to be tested in a single measurement — including on hidden surfaces.

Scientific background

From the CT volume dataset, surface points are extracted and fitted to reference geometries (plane, cylinder, cone) (Least-Squares, Chebyshev or Min-Max per ISO 22432). Datum features are determined per ISO 5459 from the measurement point set. GPS tolerance zones (tolerance zone, envelope condition, maximum material condition MMC) are evaluated according to the drawing annotation.

Relevant key metrics

  • Measurement uncertainty per VDI/VDE 2630 Part 2.1 must be less than 1/3 of the tolerance range.
  • Roughness of the CT surface influences form measurements; filtering per ISO 16610 is required.
  • Datum alignment (best-fit vs. datum feature alignment) has significant influence on the result.

Standards and thresholds

  • Standards: ISO 1101:2017 (Geometrical tolerancing — tolerances of form, orientation, location and run-out), ISO 14405-1:2016 (linear-size tolerancing), ISO 5459:2011 (datums), ISO 22432:2011 (reference geometries), VDI/VDE 2630 Part 2.1:2015.
  • Typical thresholds: Conformity decision per ISO 14253-1; with MMC/LMC entries, bonus tolerance must be accounted for analytically.
  • Validity: Always prove measurement system capability per VDA 5 or MSA R&R before series release.

Application in industrial practice

  • GPS-conforming evaluation of all drawing requirements for EMPB/PPAP without multiple clamping.
  • Verification of conicity, flatness, roundness and cylindricity on rotational parts.
  • Complete form analysis on injection-moulded and die-cast parts after first sampling.

Sources and reference date

  • ISO 1101:2017.
  • ISO 14405-1:2016.
  • ISO 5459:2011.
  • ISO 14253-1:2018.
  • VDI/VDE 2630 Part 2.1:2015-06.
  • Reference date: March 2026.

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