I had the same problem, I went to Timpsons and they said they will do it but if the blade jumps out of the cutting tool and ruins it its my problem. It was fine and they didn't charge me he said its because i supplied the blank
I assume they mean soft metal could dull there cutter or even melt on the cutting surface!Just go to the back street people if they are still about they have older cutter machines that will do a better job.