Position tolerance

Original bounded coordinate calculation

Take a straight hole axis parallel to nominal Z in a fixed datum frame. Its nominal XY location is (40.00, 25.00) mm and its actual location is (40.06, 24.92) mm throughout the controlled length. Assume no tilt, form error, datum mobility or material modifier.

At offsets of +0.06 mm in X and −0.08 mm in Y, the required zone diameter is 2 × √(0.06² + 0.08²) = 0.20 mm. The ideal axis fits a 0.25 mm diameter zone. Independent ±0.10 mm limits on X and Y allow the corner at (0.10, 0.10) mm, which needs a diameter of 2 × √(0.10² + 0.10²) ≈ 0.28284 mm.

This compares geometry for a fixed-frame, parallel-axis example. A production decision also depends on measurement uncertainty, the applicable decision rule and independent size requirements.

Meaning and governing framework

Position tolerance limits departure of a controlled feature from its specified position. Read the feature identity, theoretically exact location, zone shape and size, datum references and modifiers together. For an axial feature, the zone can constrain orientation as well as location over the controlled extent. A coordinate reported at one section does not necessarily evaluate the whole requirement.

KEYENCE's introductory position illustration places a circle's center within a circular zone of stated diameter. It is useful for understanding a simple section, but should not be read as a universal three-dimensional inspection procedure. KEYENCE position illustration

Identify the governing edition. ASME lists Y14.5-2018 (R2024) for dimensioning and tolerancing. ISO lists ISO 1101:2017, edition 4, February 2017, confirmed in 2022, for geometrical-specification symbols and interpretation. Reaffirmation or confirmation does not create a new technical edition. Do not silently transfer defaults between these frameworks. ASME record, ISO 1101 record

Zone shape, datum constraints and extent

A diameter symbol defines a diametrical zone, not independent plus/minus coordinate limits. PC-DMIS 2023.1 also documents axial planar zones requiring specified orientation. Use the actual indication rather than assuming every position zone is cylindrical. Hexagon position zone shapes

Datum references establish the applicable relationship to the part; preserve their order and modifiers. A machine coordinate system or convenient best fit is not automatically the required datum reference frame. Hexagon allows zero or more datum references: a valid position application need not have an A/B/C frame. Hexagon position introduction

Preserve the controlled extent. A tilted bore can have a satisfactory center at one height and a larger departure elsewhere. If a projected zone is specified, its extent also belongs in the evaluation definition. Position does not replace independent size acceptance or every other surface requirement.

What feature is evaluated?

The acquired surface, a fitted cylinder and the feature used by the tolerance calculation are not automatically identical. Hexagon's Deriving the Toleranced Feature documentation describes distinct constructions for different feature types and for ASME versus ISO interpretations. It also distinguishes features with surface data from features supplied without it. PC-DMIS 2023.1 feature derivation

A program containing a feature named “hole axis” therefore needs more context: measured coverage, construction method, governing standard and any material condition. Inspect enough of the actual geometry to support the intended evaluation. A single fitted circle cannot reveal all variation along a long hole.

KEYENCE describes establishing a datum and measuring the target with a CMM. Treat its example as an arrangement to understand, rather than a mandated instrument, sampling density or uncertainty. Instrument selection must suit the feature, tolerance and access. KEYENCE measuring position

Material modifiers and allowance

For ISO GPS, ISO 2692:2021, edition 4, June 2021, confirmed in 2026, addresses maximum/least material and reciprocity requirements. Its public scope limits these to linear features of size of cylindrical or two parallel opposite planes type. A software feature list does not broaden that standard's scope. ISO 2692

PC-DMIS documents MMC/LMC position options and calculates additional tolerance from the relevant envelope-size departure. A local diameter or average fit should not silently replace the required size characteristic. The example above deliberately includes no such allowance. Hexagon position material modifiers

Keep the modifier following the tolerance value separate from one following a datum reference. In Hexagon's ASME-oriented controls, the former concerns feature material condition; the latter concerns RMB/MMB/LMB datum boundaries. A specified boundary size is not simply the datum feature's nominal or MMC size. Preserve the full requirement instead of adding an unexplained “datum bonus” to a feature's tolerance. PC-DMIS feature and datum controls

Patterns and composite position

Hexagon's named implementation evaluates multiple considered features together under their datum constraints. Its composite-position description distinguishes the upper segment from lower segments, whose zone patterns retain internal nominal relationships while allowing specified translation relative to the frame. For that command, lower datum references must be a permitted subset in the same order, with the same modifiers. PC-DMIS 2023.1 patterns and composite position

Do not independently relocate each hole to obtain a favorable pattern result or treat stacked single-segment controls as automatically equivalent to a composite control. Interpret the actual indication under the governing edition, including any simultaneous requirements. The simple coordinate formula above supplies no composite or pattern-level acceptance result.

Handoff and source scope

A useful inspection definition preserves the revision and governing edition, nominal geometry, complete tolerance indication, datum realization, feature construction, sampling coverage and software release/settings. Report the geometric result separately from size, any additional allowance and the acceptance decision. Retain measured data where needed to explain disagreement.

This article uses public standard records and release-specific PC-DMIS 2023.1 documentation; its Position page prints March 8, 2023. No technical issue date is inferred for KEYENCE's live tutorials from copyright years. The numerical examples and proposed handoff guidance are original explanations, not universal production tolerances.

Datum reference frames · Maximum material condition · Dimensions, tolerances and specifications