This link should help.
MOTOR Magazine Article | MOTOR Information Systems
I would exhaust all other efforts before cutting the head off the bolt.
My experience with drilling out a bolt has not been good largely depending on the hardness and diameter of the bolt. It has been at least a 50/50 chance the first few drill bits will break off making the problem much harder to fix. I have had success with heating the surrounding area (with flame wrench) and cooling the bolt if the bolt was fairly large and when I had an excellent fit on the bolt head coupled with a breaker bar and a big hammer hitting it in both directions. If the bolt head is well rounded I have ground the flats and driven a smaller 6 point socket or box wrench. The very best solution for me has been to keep soaking it with penetrating oil several times a day and let it sit overnight and then use the breaker bar and big hammer method.