The idea came about to try to 3d print a synchro (3 phase rotary position sensor)
This technically counts as an 'air core' magnetic device, but that does not mean 
it can't work, I'd seen a motor 3d printed on twitter that gave me the notion to try it.
One of the design problems was a commutator and brushes for the armature, so I 
decided to go in all the way and design it with a brushless commutator. These types 
of commutators are used in VCR heads and some generators.
It did work, as expected being air core, the signals were somewhat weak. 
I would probably not use one of thse for position sensing, I later discovered that the 
analog signals from the hall sensors of a brushless motor are quite processable into an angle.
 They cycle several times per rotation, so there is a LOT of potential resolution, and
  are already assembled!