3.3 Datum Targets

What Datum Targets Are and When to Use Them

A datum target is a designated point, line, or small area on a part that is used to establish a datum from a feature that is too rough, too large, or too irregular to use as a full-contact datum feature. Instead of the datum feature simulator contacting the whole surface, it contacts the part only at the specified targets, and the datum is derived from those target locations.

Datum targets are essential when:

• The surface is an uneven or irregular casting or forging where full-surface contact would rest on unpredictable high spots and give a non-repeatable DRF.
• The surface is large; contacting the whole surface is impractical or would amplify form error.
• The function of the part depends on a specific contact region — for example, the three pads on a casting that actually bear against a mating housing.
• Repeatability is critical and the drafter wants to force every inspector to engage the same three points.

Datum Target Symbols and Types

ASME Y14.5-2009 defines three datum target types:

Each datum target is labeled with a datum target symbol: a circle divided horizontally, with the datum reference letter in the lower half and the target number in the upper half (e.g., A1, A2, A3 for three targets on primary datum A).

Locating Datum Targets with Basic Dimensions

The location of each datum target is defined by basic dimensions (enclosed in a rectangular frame, theoretically exact). Because the targets, not the whole surface, define the datum, the basic dimensions locate the targets relative to each other and to the datum reference frame. There is no tolerance on the target location itself — it is basic by definition — and the tolerances apply to the features inspected against the resulting DRF.

Establishing Primary, Secondary, and Tertiary Datums with Targets

The 3-2-1 concept maps directly onto datum targets:

• Primary datum — typically three datum targets (often target areas) define a plane. Three points, not in a line, fix the plane and lock one translation plus two rotations.
• Secondary datum — typically two datum targets define a line on a second, perpendicular datum. Two points fix a second translation and the remaining rotation.
• Tertiary datum — typically one datum target fixes the final translation.

The exact count depends on the geometry — a cylindrical datum established by targets may use a different arrangement — but the principle holds: targets replace full-surface contact and are counted to match the DOF each datum must remove.

Inspection Setup with Datum Targets

In inspection, datum targets translate directly into the fixture: a primary target area becomes a fixed, flat-faced pin of the specified diameter at the basic location; a datum target line becomes a knife-edge or narrow bar; a datum target point becomes a spherical-tip pin. The part is placed on the fixture so that it rests on the designated targets, and only those targets, before any measurement is taken. This guarantees that every inspector — and the manufacturing process — engage the same contact points, which is the whole point of using targets.

Movable Datum Targets (Preview)

Y14.5-2009 introduced movable datum targets. A movable target can translate along its datum axis (the axis normal to the datum plane it helps define) to accommodate part taper or other irregularity, while still establishing the datum. The symbol is the standard datum target symbol with an added arrow indicating the direction of allowed movement. Movable targets appear most often on cast and forged parts where draft and taper make fixed contact points impractical. They are covered in detail in Chapter 4 (Datum Referencing II), where datum shift and the interaction between material condition and target motion are developed.

Worked Example: Cast Pump Housing

A cast pump housing has a rough, as-cast bottom flange that, in service, bolts to a machined baseplate through three bosses. The three bosses are the actual load-bearing contact regions; the rest of the flange is cosmetic and may not even touch the baseplate.

Functional datum scheme using datum targets.

• Primary datum A — three datum target areas (A1, A2, A3) of Ø15 mm, located by basic dimensions on the three bosses. The target areas ensure the datum plane is the plane of the three bosses that actually bear, not some average of the rough flange.
• Secondary datum B — two datum target points (B1, B2) on a machined edge, fixing clocking and one translation in the plane of A.
• Tertiary datum C — one datum target point (C1) on a second machined edge, fixing the final translation.

Trap 1 — full-surface contact. If an inspector rests the whole rough flange on a surface plate, the contact points are whatever three high spots happen to be tallest. Two inspectors will get two different DRFs and two different results. The datum targets force the same three bosses every time.

Trap 2 — wrong target type. Using a datum target point on a rough cast boss is fragile — the point lands on whatever single asperity is highest. A target area of specified diameter averages the local surface and matches the real contact patch of the bolted joint. Choosing the right target type is part of the design, not an afterthought.

Trap 3 — tolerancing target location. The target locations are basic by definition. A drafter who adds a ± tolerance to a target location has misunderstood the concept: the targets are the datum, so they cannot be toleranced against themselves.