When tapping threads there are often times when the fastener "seizes" or is otherwise difficult to turn in the new threads. To eliminate this problem, run a screw/bolt of the same thread size into the newly-threaded piece all the way through the threads. Lubricate the threads with grease or heavy oil before doing this. Afterward, the threads will be smooth and will hold firmly.
What you are doing is a crude approximation of "roll-forming," a highly desirable way of forming threads, in which the metal is actually deformed by the roll-forming cutters. This makes for exceptionally smooth-faced threads that are stronger than those cut with regular threading devices.
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