How do I simulate the datum target area diameter on the coordinate measuring machine?

Chinh Nguyen


The step-by-step instructions will vary depending upon the CMM software, but the basic requirements come from understanding datum target areas.

In cases where gage blocks or pins are unavailable, or the datum target area is a particularly unique shape, the target area can be simulated by probing the specified area with the stylus. Datum targets (points, lines, or areas) designate only a limited portion of the feature as acceptable for datum simulation, so the key is to produce a tangency from the points taken from this limited area.

Where multiple datum target areas are used to establish a datum (for example A1, A2, and A3), the datum plane must include a high point from each of the target areas.

Here’s an example from our Fundamentals of GD&T (’94) textbook.


Remember, where datum targets are specified there is a specific point,  line, or area from which points can be taken to simulate a datum plane. A common mistake by designers and inspectors is to think that any three points, or any 30mm diameter can be used to simulate the datum. Where targets are indicated, the drawing must specify how the target can be identified (dimensionally) so that part-to-gage repeatability issues do not impact part acceptance.


Michael Adcock
ASME GDTP Senior Level
Dimensional Engineering Mentor