⊙ Datum target area
Datum target area · Datums y referencias · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
On the drawing, “⊙ A Ø25 on cast face” states a datum target area requirement. The locator is embodied by a flat-ended pin or pad of the exact size indicated in the symbol, which spreads the load over the whole hatched area. Compared with a point target, it tolerates rough or soft surfaces much better and avoids marking the part, at the cost of being somewhat less well defined if the face is warped. They are combined just like points in the 3-2-1 scheme: three areas for the primary datum, two or a line for the secondary, one for the tertiary. The size of the area is part of the requirement, just like its basic location, so changing the locator diameter changes the datum..
What controls
Defines the datum by contact over a dimensioned area of the surface, usually circular and sometimes rectangular. On the drawing the area is cross-hatched and its size is written in the upper half of the target symbol (or outside it, with a leader), and its location is fixed with basic dimensions. It is preferred over a point target when the surface is soft, rough or porous and a point locator would dig in or give erratic readings: raw castings, plastic, thin sheet, shot-blasted parts. Spreading the load over an area makes seating more stable and avoids marking the part. What it does NOT mean is that the whole face is the datum, or that the area has to be machined: it is only the effective contact surface.
How to verify
A flat-faced locator of the indicated diameter mounted at the target's basic coordinate, with the part seated without forcing. On a CMM without a fixture, several points spread within the area are probed (not just one in the center) and the datum is built from them, combining the areas of all the targets of the same datum. On as-cast surfaces it is advisable to increase the number of points within each area, because casting roughness makes sparse probing give widely scattered results. Probing outside the dimensioned area invalidates the construction.
Datums and notes
The locator is embodied by a flat-ended pin or pad of the exact size indicated in the symbol, which spreads the load over the whole hatched area. Compared with a point target, it tolerates rough or soft surfaces much better and avoids marking the part, at the cost of being somewhat less well defined if the face is warped. They are combined just like points in the 3-2-1 scheme: three areas for the primary datum, two or a line for the secondary, one for the tertiary. The size of the area is part of the requirement, just like its basic location, so changing the locator diameter changes the datum.
Common mistakes when interpreting it
- Resting on the whole face when the drawing dimensions a specific area, so the part seats differently and the measurements change.
- Using a locating pin of a different diameter from the one indicated: if smaller, the contact becomes a point; if larger, it imposes a seating the drawing does not call for.
- Probing a single point in the center of the area on the CMM instead of spreading several within the hatched zone.
How to get it in the workshop / if it comes out of tolerance
The diameter or size of the target indicates the actual contact area in inspection. Do not use the whole face if the drawing says target Ø20. In the fixture it is embodied by a flat-faced locator of exactly that diameter: a larger one imposes a seating the drawing does not call for and a smaller one behaves like a point. Keep those areas clean and free of dings, and avoid placing gates, ejector pin marks or parting lines there if you have a say in the raw part.
If the part rocks, check local flatness and dirt at the contact areas, and that the fixture locator is not worn on one edge (it then touches only along one rim and acts as a line). If the material is soft and the area is marked after machining, reduce clamping pressure or increase the area if design allows. And if castings arrive with too much variation in those areas, negotiate cleanup with the raw-part supplier, or machine locating pads in the first operation to serve as the reference for the rest of the sequence.
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