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Normally I would agree that a tap won't work. The channels are too wide to keep the blank engaged and turning properly. The mysterious 'tap' is cut from scratch, or a bolt that has been modified...

How about a 'tap' with your desired pitch held in a lathe. The gear blank is held onto the tool post, but allowed to spin. The idea is that as the blank is fed into the homemade tap, the tap itself...