You can position a datum axis by selecting a straight edge on the model to define as the datum axis.
To create a datum axis along a straight edge:
From the main menu bar, select Tools
Datum.
The Create Datum dialog box appears. The dialog box outlines the types of datum geometry you can create.
Abaqus/CAE displays prompts in the prompt area to guide you through the procedure.
Tip:
You can also create a datum axis using the 
tool, located in the module toolbox. For a diagram of the datum tools in the toolbox, see “Using the Datum toolset,” Section 62.2.
From the list of types at the top of the dialog box, choose Axis.
The Method list indicates the methods you can use to create a datum axis.
From the Method list, select Straight edge.
From the part or assembly in the current viewport, select a straight edge.
The datum axis appears. You cannot modify a datum axis created with this method; you must delete the old axis and create a new one.
