I haven't tried that. I did drill two 1.25" holes, 3/8" diameter in a hardened heavy angle bracket for my mill. Tough going, but I used a much slower speed then the company recommended, and plenty of oil - which, again, the company advises against. For your application I would imagine it would come down to which is harder - the drill bit, or the tap? Tough question - pardon the pun... How big is that tap? I am coming your way on Tuesday and could bring along the right size if you want to try out these bits. I think you are in Calgary, right? Maybe PM me?