A datum target is a specific point, line, or area on a part’s surface that serves as a reference for measurement and inspection. Rather than using an entire surface as a reference, datum targets designate precise locations where the part contacts a fixture or gauge. They are defined in the ASME Y14.5 standard and are essential for parts with irregular or unstable surfaces, such as castings, forgings, weldments, sheet metal, and plastic parts with curved geometry.
Why Datum Targets Exist
In geometric dimensioning and tolerancing (GD&T), a datum feature is normally an entire surface that establishes a reference plane. That works well when the surface is flat and machined. But many real parts don’t have clean, uniform surfaces. A rough casting has bumps and ridges. A sheet metal part can warp. A plastic housing might have complex curves. If you try to rest one of these parts against a flat reference surface, the contact will be unpredictable, and measurements taken from that reference will vary every time.
Datum targets solve this by replacing the full surface with a small number of defined contact locations. The designer chooses exactly where the part will touch the fixture, making setup repeatable regardless of surface irregularities. This is the key distinction: a datum feature uses a whole surface, while datum targets use specific, deliberately chosen spots on that surface.
The Three Types of Datum Targets
Datum targets come in three forms, each representing a different kind of physical contact between the part and its fixture or gauge.
- Point targets represent contact with a pointed pin or a pin with a spherical tip. On a drawing, they appear as an “X” at the contact location. These are the most common type and provide the simplest, most repeatable contact.
- Line targets represent contact along a line, typically between a cylindrical part and a matching cylinder on the fixture. On a drawing, they appear as a phantom line (or an “X” in edge view).
- Area targets represent contact with a flat-ended pin, either circular or rectangular. On a drawing, the contact zone is shown as a hatched area or a diameter on the part surface. Area targets spread the contact load and are useful when a point would damage softer materials or when a larger contact zone improves stability.
The 3-2-1 Placement Rule
Datum targets follow a principle called 3-2-1 locating, which controls a part’s position in all six degrees of freedom (three translations and three rotations).
Three points define the primary datum plane. These go on the largest surface of the part, spaced as far apart as practical for maximum stability. Think of a three-legged stool: three contact points always sit flat, even on an uneven floor. Two points define the secondary datum plane, perpendicular to the first. One point defines the tertiary datum plane, perpendicular to both. Together, these six points lock the part into a single, repeatable orientation.
This is why engineering drawings with datum targets typically show targets labeled A1, A2, A3 (three points on the primary datum), B1, B2 (two points on the secondary), and C1 (one point on the tertiary). The letter identifies which datum the target belongs to, and the number distinguishes individual targets within that datum.
Reading the Datum Target Symbol
On an engineering drawing, a datum target is called out with a circular symbol divided horizontally into two halves, connected to the part surface by a leader line.
The lower half contains the datum letter and the target number (for example, “A1” for the first target on datum A). The upper half is used only for area targets: it shows the size and shape of the contact area. For point and line targets, the upper half stays empty. If a target area is circular, the upper half displays its diameter.
How Datum Targets Become Physical Fixtures
Each datum target on a drawing translates directly into a physical feature on the inspection fixture or manufacturing jig. A point target becomes a pin with a spherical or pointed tip, positioned exactly where the drawing specifies. A line target becomes a cylindrical rest. An area target becomes a flat-ended pin, sized to match the area defined on the drawing.
This direct relationship between the drawing symbol and the fixture hardware is what makes datum targets so practical. The designer isn’t just specifying where to measure from in the abstract. They’re telling the toolmaker exactly where to place locating pins, what shape those pins should be, and in the case of area targets, how large the contact surface needs to be. Everyone involved, from machinist to quality inspector, sets up the part the same way.
Movable Datum Targets
Some parts require datum targets that can shift position depending on the assembly context. Movable datum targets were first introduced in the ASME Y14.8M-1989 standard for castings and forgings, and later incorporated into ASME Y14.5-2009. They allow a target to translate in a specified direction, which is useful when a part’s exact geometry varies within tolerance and the fixture needs some built-in adjustability to maintain consistent contact. On a drawing, the movable target symbol includes an arrow indicating the direction of permitted movement.
When Designers Choose Datum Targets
Datum targets are the standard approach for any part where using a full surface as a datum would produce inconsistent results. The most common situations include castings and forgings, where the as-cast or as-forged surfaces are rough and uneven. Weldments, which distort from heat, are another classic case. Sheet metal parts that flex under their own weight, and injection-molded plastic parts with free-form curves, round out the list.
Even parts with machined surfaces sometimes use datum targets when only a portion of a surface is functionally important, or when the part’s geometry makes full-surface contact impractical. The designer’s core question is always the same: will the part sit the same way every time it’s placed in a fixture? If the answer is no with a full surface datum, datum targets are the solution.