I was stupid enough to break off a 3/16 whitworth tap in a large, part machined, block of aluminium. The tap has snapped off just above the start of the flutes so the usual aid of a walton extractor won't work. A little Googling suggested that sulphuric acid should shift the tap without damaging the aluminium. However 3 days fully immersed in concentrated H2SO4 has done nothing.
What options do I have before I give up and find a spark erosion man?
Thanks
Charles