When it came to the number of flutes in end mills, the rule of thumb was to use two flutes for aluminum and four for steel. This was due to the size of the flutes and their ability to efficiently evacuate chips. Since the more flutes you have the smaller the flute size, it’s hard to get big chips out of the way when you add flutes to the end mill. Endmills, however, have come a long way and can now accommodate more flutes, up to 12 in some configurations. For non-ferrous machining, the standard is now three flutes and ferrous/HRSA machining can go as high as seven.
It’s also been said, “More flutes, more feed”. Be careful. As mentioned above, flute size decreases as you add them to the end mill, and this restricts how chips are evacuated during the milling process. Simply cranking up feed rates when going to a six-flute design when milling titanium may not work. It may even make sense to reduce the number of flutes for your tool.
Flute technology is not where it used to be, but that still doesn’t mean more is better.