4.1 Common Datum Features and Datum Reference Frames

The Datum Reference Frame Concept

A datum reference frame (DRF) is the complete, three-dimensional coordinate system established on a part from which all geometric tolerances are measured. ASME Y14.5-2009 builds a DRF from three mutually perpendicular datum planes — primary, secondary, and tertiary — that together arrest the six degrees of freedom (three translation, three rotation) a rigid body has in space. Every feature control frame that references datums measures its tolerance zone relative to this frame, not to the part's imperfect surfaces.

The DRF is not a single datum. It is the system assembled from the ordered set of datums in the feature control frame. The order matters: the primary datum is contacted first and arrests the most degrees of freedom; the secondary is contacted next and arrests additional freedom but is constrained by the primary; the tertiary completes the lock-up. Changing the order changes the DRF and therefore the inspection result.

Building a DRF from Primary, Secondary, Tertiary

A typical rectangular plate uses three mutually perpendicular faces:

1. Primary datum A — the bottom face, contacted on a surface plate. Arrests 1 translation (Z) + 2 rotations (about X, about Y) = 3 DOF.
2. Secondary datum B — a side face, contacted against an angle plate perpendicular to A. Arrests 1 translation (Y) + 1 rotation (about Z) = 2 DOF.
3. Tertiary datum C — an adjacent side face, contacted against a stop perpendicular to A and B. Arrests the final 1 translation (X) = 1 DOF.

Total: 6 DOF locked. The DRF is fully defined and repeatable.

Common Datum Features — One Feature, Two Datums

A common datum feature is a single physical feature that is used as two datums in the DRF — typically the primary and secondary at once. The classic example is a cylindrical bore used as the primary datum. The bore's axis becomes one datum (an axis), and a point on that axis (often defined by a datum target or the end face) becomes the second. Together, the axis plus the point establish a DRF even before a tertiary is added: the axis arrests two translations and two rotations (4 DOF), and the point on the axis arrests one more translation along the axis (1 DOF) = 5 DOF, leaving only rotation about the axis for the tertiary to handle.

Y14.5-2009 notation for a common datum feature in a feature control frame hyphenates the two datum letters in a single compartment, e.g. A-B when two features together form a single datum axis, or uses a single letter when one feature supplies two datums. The "common datum" concept is distinct from the term "compound datum" used loosely in industry — the standard's term is common datum feature.

Worked Example: Shaft Located by a Common Bore Datum

Consider a gear blank with a central bore (datum A) and an end face (datum B). A four-bolt flange pattern is toleranced |POS|Ø0.25|A-B| where A is the bore axis and B is the face. The DRF is:

• Datum A (bore axis) — primary, arrests 4 DOF (X-Y translation, rotation about X and Y).
• Datum B (face) — secondary, arrests 1 DOF (Z translation) and orients the DRF along the axis.

The bolt pattern's position tolerance zone is located relative to the bore axis and the face — exactly how the gear sits on its shaft in assembly. A DRF built only from the outside diameter would not reflect function and would accept parts that bind on the bore.

Multiple DRFs in One Drawing

A single drawing may carry multiple DRFs when different features on the part are located by different functional references. Y14.5-2009 allows each feature control frame to carry its own datum reference set; the DRF is local to that feature, not global to the drawing.

Use multiple DRFs when:

• A part has two functional interfaces that are independently located in assembly (e.g. two mating bores that bolt to different housings).
• A feature is most repeatable from a local reference (e.g. a hole pattern relative to its own pocket, not the part's overall edges).
• The manufacturing sequence differs from the inspection sequence and a separate DRF simplifies gaging.

Avoid multiple DRFs when one DRF can do the job — extra DRFs add inspection setup cost and stack-up complexity. The Senior-level judgment is: use the minimum set of DRFs that preserves functional intent.

How the DRF Locates Features

Once the DRF is established, every tolerated feature is located by basic dimensions from the DRF origin. The tolerance zone is centered on the true position (or true profile) defined by those basics, and the zone's size and shape come from the feature control frame. The datums themselves do not carry a tolerance — they are the reference — but the relationship between the DRF and each controlled feature does.

Simultaneous Requirements (Preview)

When two or more feature control frames share the same datum references, in the same order, with the same modifiers, Y14.5-2009 treats them as a simultaneous requirement by default: the features are inspected in a single setup relative to one DRF, and their zones must all fit at once. This couples the features — one cannot "use up" the DRF independently of the other. To decouple them, the drafter adds the SIM REQT notation to a frame or changes a datum reference. Simultaneous requirements are covered in depth in Chapter 10; the key here is that the DRF is the anchor, and shared DRFs imply shared inspection.