How Many Flutes Does My End Mill Need?

An end mill with a higher flute count isn't a better tool unless it's the best match for the job at hand. To offer ideal performance, end mill flute counts need to match three characteristics of each job, including the capabilities of the machine tool, the properties of the material and the design of the part. When you select your tools, evaluate these criteria on a case-by-case basis.

First, look at the capabilities of your machine. High flute counts typically mean high feed rates, which heavy-duty, high-horsepower equipment with lower RPMs can't maintain. Instead, for these machines, select end mills with lower flute counts that incorporate larger chip gullets or flute cavities, the chip evacuation spaces between cutting edges.

These end mills work best for applications such as full slotting, which engages the full tool diameter and takes large radial stepovers.  Although it's possible to use a tool with a high flute count for slot milling, it's more efficient to approach the task with three or four flutes and make use of the larger spacing among them for good chip evacuation.

Conversely, machine tools with lower horsepower and higher RPMs typically struggle to complete the kinds of heavy roughing involved in full slotting with lower flute-count end mills. Instead, these machines' forte lies in tasks that require fast feed rates and lighter radial stepovers and jobs with lower chip volumes that can evacuate efficiently through the smaller chip gullet of a higher flute count end mill.

Workpiece material forms the second criterion in end mill selection. The more freely a material cuts, the more chip volume it creates, so aluminums, plastics, soft steels and non-ferrous materials need low flute count tools and stepovers as high as 50% of tool diameter.

At the other end of the scale, super alloys typically work best with higher flute counts because these materials are more difficult to cut. In fact, attempting 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 create premature tool wear, and the heat generated from this type of heavy milling will work harden the material, changing its molecular structure and making it even more difficult to cut.

Heat transfer from overly aggressive cutting adds stresses that cause the material to bend, flex or twist, which then requires secondary processes to remove the stress and straighten out the part. To avoid that scenario, hard materials that push back need shallower depths of cut and lighter stepovers with six or seven-flute tools. Higher flute counts produce the higher cutting pressures necessary to machine these materials without heat transfer.

Along with machine characteristics and material properties, part design also becomes a consideration in end mill selection. High-speed machining with large depths of cut, high feed rates and wide radial stepovers produces excellent results with lower flute counts on simple 2D outside profiles. Complex multi-level 3D features, however, need lower speeds and feed rates for the small, precise moves with higher flute-count tools to produce these highly dimensional contours.

By the numbers, shops can generalize about flute counts in simple terms. Four-flute designs suit slot milling and ultra-heavy cuts with high radial stepovers and large metal removal rates. New designs of five-flute end mills have come as close as any tools to offering a universal option, with the ability to produce reasonably high MRR and take on anything from slot milling to side mill finishing. Six, seven and nine-flute end mills suit machines with fast lookaheads, high feed rates, high RPMs and lower horsepower create less byproduct heat in parts and do a great job with difficult-to-machine materials.

In the final analysis, the best end mill suits the job at hand, and that means choosing the flute count that's right for the combination of machine, material and workpiece characteristics. When our customers face tool selection challenges, the Seco Tools Tec Team always can help determine which end mill will offer the best machining performance possible.