Axis as datum

Hey Dan,

In GD&T an axis datum is shown by a datum symbol on the cylindrical face that has that axis or if the view is side/crossection then the datum symbol can be attached to the DIA dimension.

in terms of solidworks auto dimension, picking a cylindrical/conical face as one of your datum will use its axis.

According to ASME, datums are features, not reference geometry.  It is recommended you use the cylinder feature as your selection for your datum rather than the axis.  However, let me ask, what is the reason you wish to use the axis?