Humm...

Of the three tap types, there is a tapered tap for the initial thread cutting, an intermediate type, this is used to progress the thread.

Finally, a "Bottoming" tap, this is used to obtain the full thread depth when cutting a thread that does not go the whole of the way trough the piece. (Your hole to be tapped has a bottom to it AKA (pocket).

So you would put a taper on it and then some slots/flutes:



:)