OK, here is a method I used once succesfully to drill out a sheared subframe mounting bolt on a different machine. There was about 3/16 of the sheared bolt protruding from the aluminum main frame. I clamped two quarter inch alum plates together and drilled a 1/16 inch central hole and three locating holes. Unclamped and then drilled one of the central holes to the size of the bolt shaft. I used the locating holes to clamp the two plates together again. This made a jig for drilling out the bolt that centered the drill bit by placing the large hole side over it. I made a 1/16 pilot hole and went progressively higher until just the threads were left and I could good pick them out and then clean up with a tap. If I had screwed up I would have just gone to the next larger bolt size but I did not.