Awl --
I've taken to form tapping aluminum (cnc rigid tapping mode), and have noticed a wide variety of geometries in these taps. The name-brand form taps I have (Nachi, Emuge) almost look like cut or hand taps, as they have between 2 and 5 sharp-edged flutes. I have others which have no flutes whatsoever (they look almost like a screw!), and others with a slight recess instead of a true groove,, and one that looks almost oval.
Any insights into the preferred style of form tap? Does this style vary with context, metal, etc.?? Suggestions on best bang fer the form tap buck?