Found this on a thread on google. Just might be the way to go. My only reservation is the spindle should be fairly rigid to keep chatter from occuring. Take very light passes, use coolant, make sure the stone is trued and balanced.
"Grinding the cores is probably your only option, if for no other reason they would be rather difficult to hold while machining, though not impossible.
If it were for me to do, I would do them on a surface grinder, because setting them up and holding them is no problem, nor is removing precisely the amount you care to remove.
You can accomplish the same thing on a mill, but you should have a magnetic chuck to hold them. It could be placed on an angle plate and a wheel mounted in the spindle. Dress the wheel using the quill, dropping the wheel past a diamond held in your mill vise. I think you get the idea here. The wheel would have to be dressed narrow to pass the edges of the cores. You would have to spend a little time lining the cores up, and it would be so
much easier if you could do this in a flat plane rather than vertical.
(Like on a surface grinder). You could make the same type setup on a mill if you have a right angle head. Lots of ways to get there, but the surface grinder is really the easiest and best one.
If you do this on a mill, mask off all the slides until you are completely finished with the job. Allowing abrasive to land on your ways is really not a good idea. In cleaning up afterwards, be diligent in removing even the smallest specks. Remember you are working with aluminum oxide abrasive.
DO NOT USE A SILICON CARBIDE WHEEL ON THE FERRITE. Don't know if this helps, but it is the best I can offer."
If you have set up questions about this let me know. Good luck.