Originally Posted by S/W
Another method I have used is to shatter the tap with a center punch.
I have tried that method...it always left a broken tap in a completely f'ed up hole that could no longer be removed with any method other than our portable EDM.
I could see it working in thin plate where you only need to break a couple threads of the tap to get it to come out the other side. It has not worked for me in heavier materials.
To me, it is not even a viable option any more...YMMV.