it kind of depends on what your drilling through, what kind of flint your using, the design of the bit,how hard your applying pressure.lots of variables here ect. but for a forshaft socket, a good hard type of flint made to be long as you need or bone.you dont want to drill out exact size. you will take a couple extra foreshafts made exactly alike and use that to spin into socket after you drill hole. youll get a great fit like that.Tony