A datum target is a point, line, or limited zone on a target with which machines and instruments for processing, measuring, and inspection come into contact in order to establish a datum.
The Need for Datum Targets
If a plane defined as a datum is warped or otherwise shaped inappropriately, using that entire surface as a datum feature for indication can cause a significant error during processing or inspection.
Measuring under such conditions can result in deteriorated repeatability and reproducibility.
Using a concaved bottom as a datum plane may also cause problems in placing it directly on the surface plate.
In such cases, you can indicate a datum target to define the minimum required part as a datum.
Specifying a Datum Target
A datum target is described using a circular frame with a horizontal line drawn through the middle (datum target frame). In the bottom half of the datum target frame, you write the letter/symbol indicating the same datum as that of the entire feature, along with the number of the datum target.