Creating Datums 

This task shows you how to specify a datum element.

 
  • This scenario illustrates the creation of a datum element by selecting geometry, but you can also select any Part Design or Generative Shape Design feature in the specification tree. In this case, the created annotation will not be attached to the selected feature, but to its geometrical elements at the highest level.
  • When you select a non-canonical geometry a datum support, the created datum will be invalid because there is no enough information to define datum plane orientation. Two possibilities to avoid this problem:
    • Edit the invalid datum and define the plane orientation in the Plane Direction field.
    • Create the datum using the Tolerancing Advisor which recognizes automatically the selected geometry as non-canonical and asks you to define the plane orientation in the Plane Direction field. See Creating Datum on Non-Canonical Surfaces.
  • See also Datum Principles for more information.
Open the Annotations_Part_04.CATPart document.

  1. Activate the Front View.1 annotation plane.

  2. Click the Datum Element icon:

  1. Select the attachment surface of the datum feature.

    The Datum Feature dialog box displays with D as default identifier.
     

    The Tools Palette is displayed according to the type of face selected depending on the canonicity. In this scenario the Tools Palette is not used. For more information, refer to Propagating Geometry Selection for Feature Creation.

  2. Click OK to create the datum if the identifier corresponds to your choice.

    The datum feature is created in a specific annotation plane.
    The "Datum" entity (identified as Simple Datum.xxx) is added to the specification tree. 

 
The datum is only a 3D annotation without any semantic link to the geometrical tolerancing.
 

  1. Select the datum and drag it anywhere. You can notice that it remains in the annotation plane.
 

  1. Release the datum.
To edit a datum, double-click the datum, enter the new label in the Datum Definition dialog box that is displayed, and click OK. The modification is simultaneously taken into account.
  Two datum elements must not have the same label. A datum label must be unique to ensure that tolerance specifications are consistent.