I had the same problem tapping a 3/8" x 10TPI x 1" long thread in phosphor bronze. The first broken tap I put down to heavy handedness - the second broke when being extra careful and I think it was impossible with the lead-in on the tap. So many teeth
cutting created more torque than the tap could withstand however carefully done. The tap flutes are deeper at the front causing further weakness. I salvaged a stub tap from the remains of the broken one which utilized the strongest part of the tap and
also had a much higher specific tooth load as fewer teeth were cutting. Going carefully after that did the job but, as there is little lead-in on the tap, the lead-in has to be created by tapering the hole which works fine.
Nigel
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