▱ A Datum feature symbol
Datum feature symbol · Datums y referencias · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
The ▱ A label identifies the physical face that acts as the primary datum. The |A|B|C| frames give the order in which the reference frame is built to measure the other tolerances.
What controls
Identifies the actual surface, axis or physical point on the part that serves as the origin for measuring the other geometric tolerances. What gets labeled is always a real, tangible feature (a face, a diameter, a slot); the datum itself is the perfect geometric element — plane, axis or median plane — derived from it by seating it or by fitting it in software. That is why the symbol does not by itself impose any tolerance on that face: if the datum has to be flat or round, it must be controlled separately with its own form tolerance.
How to verify
In the shop the datum is simulated physically: face A seated on a surface plate or base plate, the datum diameter mounted on an expanding arbor or between centers, and everything else is measured from there. On a CMM, the datum feature is probed with well-distributed points (at least three on a plane, four to six on a cylinder, and never all in the same area) and the software constructs the reference plane or axis from them; the machine coordinate system cannot be used as a datum. The typical mistake is probing too few points too close together: the constructed plane ends up tilted and carries that error into every subsequent measurement.
Datums and notes
It is the feature that gives the datum its name (A, B, C...). With the symbol attached to the surface, the datum is that face; placed in line with the dimension line of a feature of size, the datum becomes its axis or median plane, not the surface. This is the distinction that most changes the outcome of an inspection.
Workshop guideline values
- General
- Datum face with 0.05–0.2 mm flatness; sufficient for ordinary workholding and assembly.
- Accuracy
- 0.01–0.03 mm when tight positions are measured from that datum.
- High precision
- 0.002–0.008 mm on datums for tooling, gauges or inspection beds.
Guidance only: the drawing's value always wins.
Common mistakes when interpreting it
- Believing that labeling a face as a datum requires it to be flat or round: the symbol only names it as a reference; its form must be toleranced separately.
- Choosing as the datum the face that is easiest to measure instead of the one that actually seats in the assembly: the datum must reproduce how the part is assembled, not how it is easy to hold on the CMM table.
- Probing the datum only in a small area and extending the result to the whole face: the reference ends up tilted and every subsequent measurement inherits that error.
How to get it in the workshop / if it comes out of tolerance
The A-B-C order matters: A is primary (normally a plane), B secondary and C tertiary. In assembly, A is usually the first face that seats against the reference.
If measurements are inconsistent from one check to the next, suspect the datum itself before the dimension: burrs, chips or paint on the seating face lift the part and shift the whole reference frame. Always deburr and clean the datum face, and machine it early in the sequence so it also serves as the workholding reference for subsequent operations. If the datum is a rough cast or forged surface, it will not seat repeatably: in that case the drawing should use datum targets instead of the full face.
Alternative symbol:△ A
Related symbols
Other symbols in Datums and references
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