Not tried the pyramid type but when you think of things like scrapers and scribers they work on ferrous materials and have negative rake. May not work so well if the cut per tooth were deep but at the sort of feeds recommended, they are really only scratching a small amount of metal off per tooth. Ferrous may be a couple of tenths of a thou per tooth chip load. I dare say if you took an HSS tool blank, lent it over to give good negative rake and dragged it over a bit of steel it would scratch a groove.

Main advantages are they are stronger and can do the job in 1/3rd the time of the traditional half ground style as you get 3 cuts per rev not one.

I was very impressed by Huub’s demonstration of thread-milling

Me to, I just wanted this for milling threads on wood. I didn’t expect it to work at aluminum and steel but since the cutter only was 30 cents, I gave it a try. I was also surprised that the Dremel milled the tread in steel pretty decent.

For chamfering I use 3 flute carbide (CNC router) or HSS 5% cobalt (manual mill) bits. They do a decent job.

For engraving I tried some self ground 1 flute HSS bits but the cheap Chinese 1 flute carbide bits do a better job. Even in steel they last pretty long.

For PCB isolation routing milling I use 20° 1 flute, 0.1 mm tip, cheap Chinese carbide bits. The perform excellent and last very long. The trace they cut is actual 0.2 mm but that is even better.

You need a minimum cutting dept (per tooth = chip load) otherwise the tool starts rubbing, gets hot and dulls pretty fast. At the recommended speed, small bits require a high RPM. A 3 flute bit needs triple the feed rate to maintain the same chip load. It also generates triple the cutting forces. If the machine can’t handle the feed rate or cutting forces, reducing the flutes is a way to solve/reduce these problems.

Single flute cutters are better in evacuation of chips. When milling soft materials like wood or plastic, I increase the chip load and reduce the speed (RPM) to prevent heating up the chips/stock (plastics melts, wood burns). Then I use a single flute cutter for better chip evacuation.

On my more rigid manual mill, in general I use 2 and 4 flute bits. On my not so rigid CNC router, I use most of the time 2 flute bits to reduce the cutting forces.