In the past I have made a good number of small taps in sizes around 8-12BA (although usually with other pitches) using the "watchmaker's" triangular three sided form, which are then sharpened by rubbing on a fine arkansas stone after hardening and tempering. These taps work quite well, even on steel when used with care. In those days I regarded 10BA as a small tap.
I've tried making some even smaller sizes (14BA and 16BA) and have enountered a new problem. If you don't sharpen them after tempering they are blunt and don't cut and end up stripping the thread. But they are so small that when trying to sharpen them on the stone, it is very easy to accidentally round off the thread form. I've managed to make some which work, but not nearly as well as the 12BA taps which I made previously.
I know I could buy them (e.g. Arc Euro trade sell 14BA and 16BA) but they don't do the even smaller sizes -- is there some trick known to other makers of tiny taps?
Alan