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Bottoming taps have only 1 to 2 undeveloped/chamfered/beveled threads at the tip and should be used only when full amount of threads specified can't be achieved by taper or plug taps.
The bottom tap should only chase the threads originally cut by the plug or taper tap. The bottom tap is only forming the last few threads. Bronze oxide surface treatment. Bright finish polished. The shortest chamfer for threading close to the bottom of blind holes. Usually used after Plug or Taper style taps are used.