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Bottoming (or plug) taps are used when threads need to reach as close to the bottom of a hole as possible. However, such taps generally still have a few threads of lead-in taper. On occasion, I've needed to get every last thread possible out of a hole (e.g. in thin material), so I took a cheap tap and ground the end off square. After using a "normal" bottoming tap, the modified tap will cut a thread right to the bottom.
 Normal (upper) and modified (lower) taps |
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 Cross-section of threads produced |
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Quick video:
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