When it comes to the number of flutes on an end mill, the right choice always depends on machine tool capabilities, material properties and part design. Shops that select the wrong number of flutes—or use a tool simply because they own it—may be disappointed to find that their part quality, tool life or both will suffer.
Toolmakers design end mills with flute counts and geometries that target specific jobs and machines. Heavy duty equipment with high horsepower but lower rpms cannot run at the high feed rates typically needed for high-flute-count end mills. Instead, these machines work best with lower flute counts that incorporate larger chip gullets or flute cavities—the chip evacuation spaces between cutting edges. With fewer flutes, these end mills work best for applications such as full slotting, which engages the full tool diameter and takes large radial stepovers.
High-speed machines with lower horsepower often have difficulty with the heavy roughing involved in full slotting using lower flute counts. Conversely, these high-speed machines excel with high-flute-count tools designed for fast feed rates and lighter radial stepovers. The more flutes, the lower the chip volume it can accommodate because the chip gullet size shrinks as the number of flutes increases. So, higher flute counts correlate with low-chip-volume applications.
Workpiece properties must also be considered. Freer-cutting materials such as aluminums, plastics, soft steels and non-ferrous materials need lower-flute-count tools that can handle high chip volumes, so shops can use end mills with two, three or four flutes and stepovers as high as 50 percent or more of the tool’s diameter.
On the opposite end of the scale, superalloys typically work best with higher flute counts because these materials are more difficult to cut. In fact, an attempt to cut challenging materials such as Inconel 718 with a four-flute end mill and full slotting will produce disappointing results.
The hard material will wear the tool prematurely, and the heat generated in heavy milling will work harden the material as it cuts. Heat transfer makes workpiece materials harder to machine because overly aggressive cutting changes the material’s molecular makeup, adding stresses that cause it to bend, flex or twist. Secondary processes are then needed to remove the stress and straighten the part. To avoid this, hard materials that push back need shallower depths of cut and lighter stepovers with six-, seven- or nine-flute end mills. Higher flute counts and lighter radial stepovers produce less cutting pressure, reducing the heat transference that causes work hardening.
Part complexity also plays a role in end mill flute selection. For simple 2D outside profiles, high-speed machining with large depths of cut, high feed rates and wide radial stepovers produces excellent results. With complex workpieces that include multi-level 3D features, however, shops must lower speeds and feeds to accommodate the required small, precise moves. In general, use fewer flutes for more complex parts and more flutes for simpler parts.
End mills can be characterized by application. Four-flute designs excel with steels and stainless steels, and are well suited for slot milling and ultra-heavy cuts with equally high radial stepovers and removal rates.
Traditionalists often consider five-flute end mills as finishing tools, but new versions of these tools offer versatility. Five-flute tools produce reasonably high MMR and tackle everything from slot milling to side mill finishing; they can also cope with fixturing limitations. This configuration is as close as any end mill can be to a universal design.