Datum Target — Points, Lines & Areas

Points, lines and areas on rough surfaces that collectively establish a datum.

Definition

A datum target is a specific point, line, or small area on a datum feature, called out individually when the feature's actual surface is too rough or irregular to reference in full. The datum simulator contacts those specific targets — and only those targets — and the datum is established through them. Rough castings, forgings, and weldments are the classic cases: a wavy as-cast surface touches a surface plate at unpredictable high spots, so the drawing makes the contact points deliberate instead of accidental.

Why targets exist

A machined face references a surface plate the way it was made — everywhere, repeatably. A rough casting does not: its high spots are wherever the mold happened to leave them, so two copies of the same part establish different datums if the whole surface is referenced. Targets fix the datum in space: the drawing says the simulator contacts A1, A2 and A3 — no more, no less — so the datum plane is the same on every copy. The functional contact the part will actually make in service is often point-like too, which makes targets the honest way to reference sheet metal and large thin castings.

How targets are called out

Each target is marked with the target symbol — a circle divided by a horizontal line — carrying the datum letter plus a number: A1, A2, A3 for datum A's targets. A point target is the plain symbol. A line target adds a leader showing the line's length and orientation. An area target encloses the area in phantom boundary lines, with the symbol and a size. Whatever the type, the letter says which datum the target helps establish and the number tells the inspector which target it is.

Establishing the datum from targets

Targets establish a datum exactly the way surfaces do, through the same 3-2-1 logic: three point targets on the primary datum feature establish the primary plane; two more (or a line target) establish the secondary; one completes the frame. The simulator contacts each target — fixed points, adjustable tooling points, or rest buttons on a fixture — and the plane through the three primary contacts is the datum plane. For thin sheet metal, area targets spread the contact so the part doesn't deform at the points.

When to use them

• Rough castings and forgings whose surface cannot be referenced in full
• Sheet-metal parts where the real functional contact is point-like and full-surface contact would deform the part
• Large weldments where referencing the whole surface would over-constrain the datum
• Parts that will be located in a fixture by specific pads or tooling points anyway — targets document the fixture contact

When not to

A machined surface that can be referenced in full should be referenced in full. Putting targets on a flat, repeatable face adds nothing but fixture complexity, and it throws away the datum quality the machined surface already provides. Targets are the tool for irregular surfaces, not a decoration on good ones.