Within the Assembly module, you use the Datum toolset to provide additional reference geometry (vertices, edges, and surfaces) that is not provided by the assembly. You use the reference geometry to help you define position constraints and to position part instances. For example, you can use a datum plane when creating a parallel face or face-to-face constraint if the desired surface does not exist. Similarly, you can use a datum axis when creating a parallel edge or edge-to-edge constraint if the desired edge does not exist. A datum is a parent feature of any constraint in which it was selected. Datums do not modify the geometry of a part instance; as a result, you can create datums that refer to both independent and dependent part instances.
Datum geometry that you create in the Part module is transferred along with the rest of the part's geometry when you create a part instance in the Assembly module. In addition, when you translate and rotate a part instance in the Assembly module, a datum created in the Part module is translated and rotated along with the instance. In contrast, a datum created in the Assembly module follows only the reference points that were used to create the datum. As a result if you translate and rotate the part instance, the behavior of the datum may not reflect your design intent. If you know that a datum should be associated with a part, you should create the datum in the Part module.
Figure 13–21 illustrates a model in which a deformable curved shell will be compressed between two rigid surfaces.
The shell is positioned easily by applying an edge-to-edge position constraint between a selected edge of the lower rigid surface (the fixed part instance) and a datum axis associated with the shell (the movable part instance). The datum axis was created with the deformable part in the Part module and moves along with the movable part instance when the position constraint is applied. In contrast, Figure 13–22 illustrates an edge-to-edge position constraint applied between three movable part instances and a fixed datum axis that provides reference geometry. In this example the datum axis was created along the X-axis of the assembly and is not associated with any part instance. Applying three edge-to-edge position constraints, one to each of the three part instances shown, would result in alignment of the three instances along the datum axis.A datum is a feature of the assembly and is regenerated along with the rest of the assembly. You can make datum geometry invisible while still retaining it in the assembly by selecting View
Assembly Display Options from the main menu bar. For more information, see “Controlling datum display,” Section 50.6.
The triad indicating the origin and the orientation of the global coordinate system is a datum coordinate system created by the Assembly module. You can suppress or delete it, but you cannot modify it.
