Doesn't the metal need to be annealed periodically when you are
trying to stretch it? I have broken a few ring shanks on the
inexpensive ring stretcher beacuse the metal was too hard. On a
mandrel, if it is too work hardened it will not stretch at all or
only in very tiny increments. I f you are trying to stretch it
with a stone still in it, and can't heat it, what would you do?