One way to accomplish this, that I have done, is this.
I start to create the axis, and when it asks for references, I then start creating a 2 successive datum planes on the fly (within the axis command). These two datum planes will intersect each other creating a center point, that will update as the model changes without a relationship. This of course will depend on your specific geometry, and might not lend itself to all cases.
In my example I selected opposite edges of a rectangle extrusion for each datum plane (DTM1 & DTM2) to create an intersection that is centered along the extrusion path. Axis A_1 is automatically placed at the intersection as I was in the command already. Note I resized the datum planes (DTM1 & DTM2) to the feature for clarity.
Please contact me if you would like more information.
Doug Pogatetz
Mechanical Design Engineer
Northrop Grumman Corporation
