Datum Feature — Feature vs. Datum

The actual surface labeled A/B/C on the part — and the perfect theoretical datum derived from it.

Definition

A datum feature is an actual, physical feature of the part — a face, a bore, a shoulder — labeled with a datum letter (A, B, C) and selected to serve as the origin for measurements. The datum itself is something else entirely: the perfect theoretical plane, axis, or point derived from that feature. Every symbol page in this library that references datums — position, profile with datums, orientation, runout — is really saying: measure from the datum, which is established by the datum feature.

The feature vs. the datum

No real surface is perfectly flat, and no real bore is perfectly round. The datum feature is what you can touch; the datum is the perfect reference the feature simulates against — a surface plate, a chuck, a V-block, a mandrel. The drawing's datum triangle attaches to the feature; the measurements attach to the datum. Blurring the two is the most common conceptual error in GD&T, and it is exactly what causes mislabeled drawings: a callout referencing datum A means the derived datum plane, not the wavy surface that produced it.

Selecting datum features

• Choose the surfaces the part actually assembles against — mounting faces, mating bores, bearing shoulders — not convenient flats on the drawing
• Prefer the largest, most stable surface as the primary datum: three widely spaced contact points make the most repeatable plane
• Make sure the feature can be simulated in inspection — a surface plate, a mandrel, or a CMM fixture must be able to reproduce the datum
• Prefer machined features over cast or forged surfaces when the datum must be repeatable
• Keep the datum chain short: every datum is itself located from the previous one, so each addition stacks error

Precedence: primary, secondary, tertiary

Datums are established in the order they appear in the feature control frame. The primary datum is established first — a plane, simulated by three points of contact. The secondary datum is then established relative to the primary — typically two points, a line or an axis. The tertiary datum completes the reference frame with a single point. This is the 3-2-1 rule, and it is why the order in the frame matters: A B C is a precedence order, not a menu. Each datum removes degrees of freedom, and the frame lists them in the order the part is physically constrained.

When the surface can't be a datum feature

A datum feature must be repeatable. When the actual surface is too rough or irregular to establish a stable reference — rough castings, forgings, weldments — the drawing calls out specific points, lines, or areas instead: datum targets. The targets are contacted by the simulator and collectively establish the datum. See the Datum Target page for that mechanism; the distinction to keep here is that the datum feature is the whole physical feature, and the datum target is the alternative when the feature can't be referenced in full.

Measurement

Simulation — the classic method: the part is placed against a physical simulator (surface plate for a plane datum, mandrel for a bore axis, V-block for a cylinder) and everything is measured from the simulator, never from the imperfect surface directly.

CMM — the datum feature is measured, then the perfect datum (plane, axis, point) is fitted to the measured data exactly as the simulator would establish it, and the controlled features are evaluated relative to that derived datum.