# Datum reference frames

Establishing repeatable geometric references from specified part features, precedence and constraint rules.

Reviewed: 2026-11-01

Tags: parts-and-design, specifications, gd-and-t, metrology

## Purpose and standard boundary

Establish the datum reference frame from the drawing before interpreting coordinates. The frame constrains the evaluation and can differ from the CAD or machine origin.

In the fixed-axis example, moving the inspection origin **+0.20 mm along X** changes the same physical point’s reported X coordinate from 40.00 to **39.80 mm**. The change belongs to the reference frame. This example excludes rotation, form and material-boundary effects.

Identify the governing standard and edition before interpreting a callout. ASME lists **Y14.5-2018 (R2024)** for dimensioning and tolerancing; its scope includes drawings, digital models and related documentation. Reaffirmation in 2024 does not rename the underlying edition to Y14.5-2024. The publisher's 2018 preview gives an actual issuance date of **February 11, 2019**. [ASME standard record](https://www.asme.org/codes-standards/find-codes-standards/y14-5-dimensiones-y-tolerancias/2018), [ASME preview, front matter](https://contentstaging12.asme.org/getmedia/88b96f5c-7fbb-4dfa-aa19-2ae5ddbf63c5/35976.pdf)

Under ISO GPS, **ISO 5459:2024**, edition 3, October 2024, addresses datums and datum systems. Its public scope distinguishes the specification operator from physical or mathematical verification methods, and directs detailed maximum/least material datum requirements to ISO 2692. Do not silently combine ISO and ASME defaults because symbols look similar. [ISO 5459 scope](https://www.iso.org/standard/87855.html)

## Three things to keep distinct

| Term | Meaning in this reference |
| --- | --- |
| Datum feature | The actual, imperfect feature selected on the manufactured part |
| Datum | The theoretically exact reference derived from that feature, such as a plane or axis |
| Simulator or counterpart | The ideal geometry, or its practical embodiment, used to establish the reference from the feature |

SME's GD&T teaching material distinguishes the nonideal physical datum feature, the ideal datum and the simulator. For example, a real mounting face can be contacted by a precision supporting surface to establish a datum plane. The face, supporting equipment and theoretical plane remain different objects. [SME Fundamentals of GD&T, slide 21](https://www.sme.org/globalassets/sme.org/engage/communities/direct-digital-manufacturing-group/fundofgdandt_part1.pdf)

Terminology changes with standards and editions. Hexagon notes that ASME 2018 uses **true geometric counterpart**, while its PC-DMIS documentation uses simulator as a convenient umbrella term. That documentation describes approximately equivalent concepts; it does not declare all ASME and ISO establishment rules interchangeable. [PC-DMIS 2021.1 datum terminology](https://docs.hexagonmi.com/pcdmis/2021.1/en/helpcenter/mergedProjects/core/geometric_tolerances/How_PC-DMIS_Solves_Datums.htm)

Assigning a datum identifier also does not manufacture a perfect surface. Where function needs a datum feature's form or orientation controlled, specify that requirement separately. Choose references that represent the intended functional relationship and can be accessed reliably in the planned verification setup.

## Sequence and degrees of freedom

A free rigid body can translate in three directions and rotate about three axes. Datum establishment constrains the motions relevant to the requirement, in the specified precedence. Later references must preserve constraints already established by earlier ones. Hexagon's implementation describes primary, secondary and tertiary fitting in that order, maintaining higher-precedence relationships throughout. A frame need not always constrain every motion; the requirement and feature geometry determine what is necessary. [PC-DMIS datum solution](https://docs.hexagonmi.com/pcdmis/2021.1/en/helpcenter/mergedProjects/core/geometric_tolerances/How_PC-DMIS_Solves_Datums.htm), [Creo 12 datum feature guidance](https://support.ptc.com/help/creo/creo_pma/r12/usascii/model-based_definition/datum_feature_dashboard.html)

For the **specific case of three mutually perpendicular planar references**, the familiar 3-2-1 model is useful. Let primary A define XY, secondary B define XZ and tertiary C define YZ. The following is an original constraint breakdown consistent with Creo’s documented orthogonal planar target arrangement. [Creo 12 planar references](https://support.ptc.com/help/creo/creo_pma/r12/usascii/model-based_definition/datums_from_targets.html)

| Stage | Newly constrained ideal motions | Motions remaining |
| --- | --- | --- |
| A | Translation Z; rotation about X and Y | Translation X/Y; rotation about Z |
| B, preserving A | Translation Y; rotation about Z | Translation X |
| C, preserving A and B | Translation X | None |

This is an explanatory planar example, not a universal rule that every primary feature removes three freedoms. A cylindrical primary reference leaves axial translation and rotation available. Nor does “3-2-1” authorize selecting any three, two and one probe points on imperfect surfaces. The specified features, establishment rules and measurement strategy still govern. [PC-DMIS planar and cylindrical constraints](https://docs.hexagonmi.com/pcdmis/2021.1/en/helpcenter/mergedProjects/core/geometric_tolerances/How_PC-DMIS_Solves_Datums.htm)

Datum precedence belongs to each applicable tolerance reference. Do not replace it with alphabetical sorting or a single shop alignment used indiscriminately for every characteristic. Changing which imperfect feature is primary can change the established orientation and therefore the evaluation.

## Material boundaries and other modifiers

In the **ASME-oriented controls documented for PC-DMIS 2021.1**, a datum reference without a material modifier is RMB, regardless of material boundary. MMB and LMB denote maximum and least material boundaries. The software explicitly cautions that a specified boundary size is not simply the feature's nominal size or its maximum/minimum material size. Read the full callout and applicable rules before calculating a boundary. [PC-DMIS feature-control-frame datum controls](https://docs.hexagonmi.com/pcdmis/2021.1/en/helpcenter/mergedProjects/core/geometric_tolerances/Feature_Control_Frame_Tab.htm)

Keep a modifier attached to a **datum reference** distinct from one attached to the **tolerance value**. They occupy different positions and affect different parts of the requirement. Do not automatically add a datum-reference modifier to a feature's geometric tolerance or replace boundary analysis with a generic “bonus tolerance” calculation.

The same implementation documents translation modifiers and customized frames as advanced, separately enabled options for specified ASME tolerances. These are explicit controls, not permission for an inspector to release constraints to improve a result. ISO material requirements must instead be interpreted under the applicable ISO system. [PC-DMIS advanced options](https://docs.hexagonmi.com/pcdmis/2021.1/en/helpcenter/mergedProjects/core/geometric_tolerances/Feature_Control_Frame_Tab.htm), [ISO 5459 scope](https://www.iso.org/standard/87855.html)

## Targets and measurement implementation

Datum targets identify a more specific establishment arrangement than an unspecified whole-surface contact. As a bounded software example, Creo 12 describes a three-point target plane as a primary reference, a two-point target plane as secondary to a primary plane, and particular one-point arrangements for the remaining constraint. These implementation rules should be read with the drawing's selected standard, not used to invent target locations absent from the design. [Creo 12 datums from targets](https://support.ptc.com/help/creo/creo_pma/r12/usascii/model-based_definition/datums_from_targets.html)

For inspection, preserve the actual referenced features, precedence, modifiers, targets and associated requirements in the measurement plan. Identify the software release and evaluation settings. Inspect cleanliness, contact accessibility and part restraint, and use a sampling strategy suited to the actual feature. A setup that distorts a thin part or samples too little of an imperfect face can give an unrepresentative result even when its coordinate labels are correct.

## Original bounded coordinate example

Assume the ideal planar A/B/C frame above is established and a point has coordinates `(40.00, 25.00, 10.00) mm`. An incorrectly translated inspection origin, displaced **+0.20 mm along X** with unchanged axis directions, reports that same point as `(39.80, 25.00, 10.00) mm`:

`reported X = physical X − origin displacement = 40.00 − 0.20 = 39.80 mm`.

Nothing about the part changed. The 0.20 mm discrepancy comes from the reference. This illustrates why frame establishment must precede dimensional interpretation. It is not a position-tolerance evaluation, an uncertainty budget or an acceptance criterion; rotation errors, feature form and material-boundary effects are excluded by assumption.

## Source versions

This article uses the ASME 2018/R2024 record and publisher preview, ISO 5459:2024's public scope, SME's February 3, 2023 teaching presentation (footer **02/03/2023**, with the date spelled out on slide 4), and release-specific PC-DMIS 2021.1 and Creo 12 documentation. Software documentation demonstrates named implementations rather than certifying equivalence between standards or all software releases.

[Dimensions, tolerances and specifications](index.md)