I am struggling to add a mate to the center of a hole. How do I select the centerline?
"Alt A" should show the the axis, and then you will be able to select them.
If the "Alt A" combination isn't part of your keyboard customization, the View > Temporary Axes should work.
You could also add a permanent Reference Axis to the hole by selecting Insert > Reference Geometry > Axis and then the inner surface of the hole.
And depending on the method used to create the hole, you may also be able to use a portion of the hole feature's sketch.
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