Module 2 · Lesson 2 of 6

Datums and datum reference frames: fixing the part in space

GD&T datums explained: datum features and datums, the 3-2-1 datum reference frame, why datum order matters, datum targets, and ISO 5459 vs ASME terms.

By The CheckedCalc editor Published

After this lesson you can:

  • Distinguish a datum feature on the part from the datum derived from it
  • Explain how primary, secondary and tertiary datums remove the six degrees of freedom
  • Show why the order of datum letters in a frame changes the result
  • Choose datum features from function, and recognise datum targets and common datums
  • Translate between ISO 5459 and ASME Y14.5 datum vocabulary
Estimated time
15 minutes
Before you start
Lesson 1: Why size tolerances are not enough
Contents
  1. Datum feature, datum and simulator are three different things
  2. How three datums remove six degrees of freedom
  3. Why the order of datum letters matters
  4. Datum feature symbols, targets and common datums
  5. ISO and ASME datum vocabulary side by side

A tolerance zone has a size and a shape, but it also has to be somewhere. “Perpendicular to what?” and “positioned from where?” are the questions datums answer. Get the datums right and inspection, machining and assembly all hold the part the same way. Get them wrong and a part can pass on the CMM, fail on the fixture and fit in neither.

The datum vocabulary here is that of ISO 5459:2024 and ASME Y14.5-2018 (R2024). Three datums lock a part in space, their order matters, and three special cases need care: datum targets, common datums and datums referenced at maximum material.

Datum feature, datum and simulator are three different things

Engineers say “datum A” for all three, and most misunderstandings about datums start there.

Datum feature, simulator and datum A block with an uneven underside rests on a surface plate. The real underside is the datum feature. The surface plate is the physical datum feature simulator. The perfect plane through the high points is the datum plane A, derived from the true geometric counterpart. Datum feature: the real, uneven underside of the part Datum feature simulator: surface plate (physical) Datum plane A: the perfect plane on the high points Unevenness exaggerated
Figure 1. The part touches the plate at its high points. The theoretical perfect plane that does the same job is, in ASME terms, the true geometric counterpart. The plate is its physical simulator, and the plane it establishes is the datum.

The datum feature is real and imperfect. The datum is ideal: a plane, axis, line or point. Between them sits the rule for getting from one to the other. ASME Y14.5-2018 calls the theoretical perfect boundary the true geometric counterpart. It calls the physical thing that realises it (a surface plate, a gauge pin, an expanding mandrel, a CMM fit) the datum feature simulator. ISO 5459:2024 describes the same step as fitting an ideal associated feature to the datum feature under stated association rules. The datum is then taken from the situation features of that associated feature: its plane, axis or point.

How three datums remove six degrees of freedom

A rigid part in space has six degrees of freedom: three translations and three rotations. A datum reference frame (ISO: datum system) is the set of three mutually perpendicular planes that removes them. It is built from the datums in the order they appear in the feature control frame.

The 3-2-1 principle on three datum planes A block sits in the corner formed by three perpendicular planes. It touches the primary plane at three points, the secondary plane at two points and the tertiary plane at one point. The primary removes translation in z and rotations about x and y, the secondary removes translation in y and rotation about z, and the tertiary removes translation in x. Primary A (floor): 3 points removes Tz, Rx, Ry Secondary B (back wall): 2 points removes Ty, Rz Tertiary C (side wall): 1 point removes Tx 3 + 2 + 1 = 6 xyz
Figure 2. The 3-2-1 principle. T is translation, R rotation. Contact points (blue) are the minimum for stable seating; real simulators touch the high points of real surfaces.

The primary datum takes the most, because it goes first. A plane seated on three high points stops movement normal to it and the two tilts. The secondary datum can then only remove what is left that it can reach: seated against a perpendicular wall, two points stop one translation and the rotation in the primary plane. The tertiary has a single degree of freedom left to stop.

A feature of size can also be a datum. A bore used as secondary after a face removes both translations across its axis in one go, leaving only rotation about that axis for the tertiary. Not every frame needs three letters; a perpendicularity tolerance to a face needs only A, and a coaxial feature may need only A and B.

Why the order of datum letters matters

Precedence is not alphabetical and not decorative. A | B | C means the part is seated on A first. It is then brought against B while staying on A, and against C last. If A and B are not exactly perpendicular, which they never are, swapping them tilts the whole frame.

Take a plate on base A whose side face B leans 0.10 mm relative to A over its 20 mm height. With A primary, the frame is square to the base and B touches the plate only along its high edge. With B primary, the frame is square to the side face and tilted 0.10/20 = 0.005 rad relative to the base. A hole 60 mm up from the base would move 60 × 0.005 = 0.30 mm sideways between the two set-ups: far more than a typical position tolerance.

Choose the order from function. The face that seats in the assembly is usually primary, the feature that locates it sideways is secondary, and the one that stops rotation is tertiary.

Datum feature symbols, targets and common datums

A datum feature is identified by a capital letter in a square frame joined to a triangle on the feature. Where the triangle sits decides what the datum is:

Triangle placedDatum featureDatum
On the surface outline or an extension line, clear of any dimension lineThe surfaceA plane (or the surface itself for complex shapes)
In line with a size dimensionThe feature of sizeIts axis or median plane
On a leader to a group tagged with the same letterSeveral features togetherOne common datum (see below)

Datum targets. On castings, forgings, weldments and sheet metal, a whole rough surface would rock on the simulator. Datum targets specify contact only at chosen points, lines or small areas, labelled A1, A2, A3 and so on. The fixture then touches the part exactly where the drawing says, every time, and the machined datums that follow can be referenced to those first cuts.

Common datums. Two coaxial bearing journals, each with its own letter, can act together as one datum axis, referenced as A-B in the frame in both systems. Where several features carry the same letter, such as two coplanar pads, ISO references the result with the letter repeated and hyphenated, while ASME uses the single letter. ISO 5459:2024 treats common datums in full; ASME Y14.5-2018 renamed its multiple datum features to common datum features.

ISO and ASME datum vocabulary side by side

IdeaISO 5459:2024ASME Y14.5-2018
Real feature marked with a letterDatum featureDatum feature
Perfect geometry fitted to itAssociated feature, fitted by an association ruleTrue geometric counterpart
Physical or digital embodimentLeft to verification practiceDatum feature simulator
Ideal point, axis or plane obtainedDatum, from the situation featuresDatum
Ordered set of datumsDatum systemDatum reference frame
Several features acting as one datumCommon datumCommon datum features
Default for a datum feature of sizeFitted independent of size unless the maximum material requirement is shown (ISO 2692)Regardless of material boundary (RMB), from Rule #2
Datum feature referenced at maximum materialⓂ after the datum letterMaximum material boundary (MMB), Ⓜ after the letter
Part rocks on its datum featureHandled by the association rulesA single default solution that minimises separation from the counterpart

The words differ more than the results. The two rows that change inspection are the defaults for features of size and the handling of unstable (rocking) datum features, so check which standard the drawing cites.

Worked example: degrees of freedom on a flanged part

A flanged hub has three datum features. A is the mounting face of the flange. B is a Ø20 mm bore perpendicular to A. C is an 8 mm wide slot in the flange rim, centred 40 mm from the bore axis. Four M6 holes on a Ø60 mm pitch circle carry position Ø0.2 | A | B | C Ⓜ, with datum C referenced at maximum material boundary.

Flanged hub with datums A, B and C End view of a flange 100 mm in diameter with datum B labelling the central 20 mm bore. Datum C labels a slot at the top of the rim, and four M6 holes sit on a 60 mm pitch circle. A side view shows the mounting face labelled datum A. A feature control frame reads position diameter 0.2, A, B, C at maximum material boundary. BAC Ø0.2ABC Ⓜ 4x M6 8 Ø20 PCD Ø60 Side view Mounting face
Figure 3. Example part for the worked example: A seats, B centres, C clocks.
DatumFeatureRemovesLeft free
A (primary)Plane, the mounting faceTranslation along the axis, tilt about two axes (3)Two translations in the face, rotation about the axis
B (secondary)Axis of the Ø20 bore, perpendicular to ATwo translations in the face (2)Rotation about the axis
C (tertiary)Median plane of the 8 mm slotRotation about the axis (1)Nothing

Total 3+2+1=63 + 2 + 1 = 6: the holes’ tolerance zones are fully fixed. A coaxial groove toleranced to A | B alone would need no C, because rotation about the axis does not move it.

Datum shift at C. The Ⓜ after C means the slot is simulated by a fixed key, not by an expanding one. Assuming the slot has no geometrical tolerance of its own that enlarges the boundary, the key is 8.00 mm wide, the slot’s maximum material size. If the slot is made 8.05 mm wide, the part can rotate on the key by half the clearance at the slot, 40 mm from the axis:

θ=(8.05−8.00)/240=0.02540=0.000625 rad\theta = \frac{(8.05 - 8.00)/2}{40} = \frac{0.025}{40} = 0.000625\ \text{rad}

A hole on the 30 mm radius pitch circle moves sideways by

δ=r θ=30×0.000625=0.019 mm\delta = r\,\theta = 30 \times 0.000625 = 0.019\ \text{mm}

That ±0.019 mm is extra freedom the inspector may use, on top of the Ø0.2 mm zone, because the functional key in the mating part allows it. Referenced RMB instead, the simulator would grow to fill the slot and no shift would be allowed.

Maximum material boundaries and the bonus tolerance they bring are the subject of lesson 4, which is released on 6 October 2026. You can see datums A and B used on a complete part drawing in Module 1, lesson 6.

On the job

Pick datum features by asking how the part is held in its assembly, not how it is held on the machine. The face that bolts down, the bore that locates on a spigot and the slot or dowel that stops rotation are the usual answers. Then check the datum features are big enough and stable enough to seat repeatably: a 3 mm wide land makes a poor primary datum. If the part is cast or forged, give the first machining set-up datum targets, and reference the finished datums to the faces it produces. Talk to the inspection team before release; a CMM programmer who has to guess your datum order will guess the convenient one.

Check your understanding

5 questions. Choose an answer, then check it. Nothing is saved.

  1. Question 1 of 5How many degrees of freedom does a primary datum plane remove from a rigid part?
  2. Question 2 of 5A frame references A | B | C, where A is a flange face, B a bore perpendicular to A, and C a slot in the flange rim. What does datum C remove?
  3. Question 3 of 5In ASME Y14.5-2018 terms, what is a granite surface plate used to establish datum A?
  4. Question 4 of 5On an ASME drawing, a bore is referenced as secondary datum B with no modifier. How is the datum axis established?
  5. Question 5 of 5A clocking slot, MMC width 8.00 mm, is referenced at maximum material boundary. The actual slot is 8.05 mm wide and its centre is 40 mm from the part axis. How far can a hole on a 30 mm radius move sideways because of datum shift?

Key terms

Datum feature
A real, imperfect surface or feature of size on the part, identified by a datum feature symbol and a letter.
Datum
The theoretically exact point, axis, line or plane derived from a datum feature, from which tolerance zones are located or oriented.
Datum reference frame
Three mutually perpendicular planes established by the datums cited in a frame, in order of precedence (ISO: datum system).
True geometric counterpart
ASME term for the theoretically perfect boundary, such as a perfect plane or cylinder, used to establish a datum from a datum feature.
Datum feature simulator
ASME term for the physical or digital embodiment of the counterpart: a surface plate, gauge pin, chuck, fixture or CMM fit.
Datum target
A point, line or area on a datum feature where contact is specified, used where a whole surface would be unstable, such as on castings.
Regardless of material boundary (RMB)
The ASME default for a datum feature of size: the simulator expands or contracts until it makes full contact, so the datum is centred on the actual feature.

Standards referenced

StandardEditionTitleUsed here for
ISO 54592024GPS, Geometrical tolerancing, Datums and datum systemsDatum features, associated features, common datums and datum systems
ASME Y14.52018 (R2024)Dimensioning and TolerancingDatum reference frames, true geometric counterparts, datum feature simulators, RMB and MMB
ISO 11012017GPS, Geometrical tolerancing, Tolerances of form, orientation, location and run-outDatum letters in the tolerance indicator
ISO 26922021GPS, Geometrical tolerancing, Maximum material requirement (MMR), least material requirement (LMR) and reciprocity requirement (RPR)Maximum material requirement applied to a datum reference

Editions checked on 29 Sep 2026. Standards are copyright documents; this lesson explains and paraphrases them. Buy or borrow the current edition before relying on a value.