Last year at Ontario's fall show I was asked how to remove the metal cap from a porcelain suspension. After some thought and considering the problems he had encountered, this is what I did.
Hand grinder with a zip disc works great. #1 was to cut 2/3 of the way around the top. Or you could go all the way. #2 was to cut completely around near the bottom of the cap just above the ridge. #3 was to make vertical cuts, 3 in total between #1 and #2. Using a screwdriver and prying metal against metal, this produced 2 pieces - #5. At this point or before, the top most part of the cap will have come loose and be off. All that is left is a narrow band around the porcelain which is opened with an angled cut that allows a screwdriver to pry, again metal against metal, in the cut and open the band. A little wiggling and it's off.