Hi Vince - you're right about the "less material to remove" for the metric fine taps.
My favourite brand uses true serial taps - they cut sequentially more for each tap, and the last one is both a final and bottoming tap in one. For this brand, the standard metric taps are three to a set, and the metric fine ones two to a set. If a hole is tapped with only the first two taps for the three tap sets, or the first tap for the metric fine sets, even if tapped through-hole a normal screw won't fit - the last tap has to be used to get the hole to size. I nearly had a fit the first time I bought a metric fine set, as I happened to check the taps and there was one "missing"; the salesman had some explaining to do
My second-choice brand's taps aren't serial - even though there's three taps to a set for normal metric. On a thin workpiece with a through-hole, once the hole is tapped with the first tap, a screw will already screw in properly, and the other taps are only needed for blind or deeper holes.
For most material the second brand works well, but for more difficult stuff, the serial taps make life a lot easier. In fact, tapping something like 304 stainless with the serial taps feels the same as tapping mild steel with the non-serial ones - even though you have to make a pass or two more. The thread quality is also a lot better - materials that will show "tearing" in the threads with the non-serial taps are left with a very smooth thread finish for the serial ones.
So I guess it's not only how much material is removed per pass, but also depends on the material used and the quality of finish the taps will give that plays a role.
Kind regards, Arnold
