⊙ A Datum target
Datum target · Datums y referencias · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
On the drawing, “⊙ A (20 mm circular target area)” states a datum target requirement. Targets do not replace the datum: they simulate it. The letter is still A, B or C and the order of precedence works the same way; the only difference is that contact is limited to the marked areas..
What controls
Defines a point, a line or a localized area of a surface as the datum contact, instead of using the whole face. It is used when the surface is too irregular to seat repeatably — castings, forgings, sheet metal, molded plastic — so that the part always sits in the same places. The symbol is a divided circle: the size or shape of the contact area on top, and the datum letter with its number (A1, A2, A3) below. Target locations are always dimensioned with basic dimensions.
How to verify
They are simulated with locating fixtures that physically reproduce each target: a spherical tip for a point target, a knife-edge rest for a line target and a flat pad of the stated diameter for an area target. On a CMM, probing is done at exactly those locations (defined by basic dimensions) and the datum is constructed from those points only, not from the whole surface. The usual layout is 3-2-1: three targets define the primary plane, two the secondary and one the tertiary.
Datums and notes
Targets do not replace the datum: they simulate it. The letter is still A, B or C and the order of precedence works the same way; the only difference is that contact is limited to the marked areas.
Workshop guideline values
- General
- Contact areas of ⌀6–⌀15 mm on ordinary castings or forgings.
- Accuracy
- ⌀3–⌀6 mm with ground rests when workholding repeatability is critical.
- High precision
- Point targets with spherical rests and 0.01–0.02 mm repeatability on inspection fixtures.
Guidance only: the drawing's value always wins.
Common mistakes when interpreting it
- Seating the part on the whole face in addition to the targets: if the fixture touches outside the indicated areas, the datum is no longer the one the drawing calls for.
- Dimensioning target locations with toleranced dimensions: they always use basic dimensions, like any theoretically exact location.
- Placing targets on areas that are machined later, so the reference disappears partway through the sequence.
How to get it in the workshop / if it comes out of tolerance
On parts with a lot of form variation, circular or line targets are used as the datum instead of the whole face.
If the part rocks in the fixture or every measurement gives a different result, check that it seats on all three targets at once and that none of them lands on a parting line, a gate or a casting flash. If a target sits on an area that is machined later, you lose the reference partway through the process: targets must be on rough surfaces that survive every operation. A worn or sunken rest in the fixture shifts the entire datum without showing up in any dimension.
Related symbols
Other symbols in Datums and references
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