The key to removing a broken bolt thread in any type of metal is to grind down the stud flush or flat (if it is broken slightly above the hole. Then use a center punch (appropriate size) to create an indentation in the broken thread so that drilling is easier. In my case the thread size is 1/4"x20, so I used a center punch and a very small high speed drill bit and drilled out about 1/8" (or a little more) hole. Then I used a 1/4" easy out which I hammered into the hole I made then turned out the broken thread.