Picking the Right End Mill

Cutting tools are vital to any machining process. All of the engineering that goes into developing a machining center — the kinematics, the axes of movement, the structure of the machine bed, the temperature control systems, the spindle design — not to mention the technology that goes into the CAM system and control — exists to ensure the cutting tool meets the workpiece at the right position, angle and speed. In many ways, it is the vital point at which every program succeeds or fails.

But surely, many shop owners say, here we can skimp on the price.

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Of course, it’s not really about the price, but rather about getting the right cutting tool for the right job. Sometimes, that cutting tool will be the cheaper option, but putting careful thought into the cutting tools for a given job will often net a shop improved tool life, faster production speeds, or both. To discuss the considerations that go into cutting tool selection, I spoke with Kennametal global product manager Katie Myers to discuss the engineering that goes into solid carbide end mills.

Cutting Tool Basics

“The design of the cutting edge, coating and flute shape are vital when it comes to chip evacuation and finish,” Myers says. Generally speaking, roughing tools have lower flute counts to increase the space that enables chips to leave the cut, while finishing tools have higher flute counts and sharper edges to improve the finish. Cutting tool manufacturers engineer flute shapes to maximize chip evacuation and apply coatings to increase tool life.

Of course, most machinists work with ferrous metals, and the cutting tools designed for these materials tend to be generalists. “Most of the time, people are working with steel, stainless steel or cast iron, and they want a tool that can do it all.” Some cutting tools like Kennametal’s HARVI line are designed for such wide-ranging applications, with through-coolant capabilities and proprietary designs that reduce chatter and increase tool life. 

However, generalist cutting tools will not always enable shops to use the speeds and feeds necessary to produce parts profitably. “Most of the time, any cutting tool will work, but the cutting conditions and how you apply the tool will change,” Myers says. Shops working with non-ferrous materials that want to use aggressive machining strategies or improve chip management are sometimes best served by looking at milling tools designed for specific materials.

Matching Features to Materials

One common misconception is that a cutting tool that can handle steels can surely handle aluminum under similar cutting conditions. “People assume the softer material will cut even easier using the same end mill they use on steel,” she explains, “But if the edges aren’t sharp enough, it’s like cutting into a steak with a butter knife. It doesn’t matter how great the butter knife is, you’re going to tear it instead of shearing.”

The rough edges created by this tearing action can cause the chips to stick together, creating unmanageable swarf that gums up the cutting tool and spindle in addition to leaving a poor finish on the part. This is why cutting tools designed for aluminum often have sharp edges, so sharp that most cutting tool manufacturers do not coat solid end mills designed for aluminum cutting, as even the slight buildup in thickness from the coating can reduce the effectiveness of the cutting edge. Coatings are just not worth it for a lot of aluminum applications, as Myers explains. “Most coatings are designed to improve tool life, but the tool life for end mills used on aluminum tends to be pretty high already.” It is then preferable to focus on maximizing the sharpness of the flute and reducing the number of flutes to make chip evacuation much easier.

Other materials, such as titanium, create whole new problems by upending expectations. While under most circumstances cutting tools with fewer flutes are preferred for roughing and higher flute counts for finishing, titanium can turn this rule on its head. “Roughing titanium is very different,” Myers says. “We encourage people to use dynamic milling — taking a large axial depth of cut and a lower radial depth of cut. This requires increasing the speeds and feeds, and shops can use end mills with higher flute counts to increase the chip load and material-removal rate.” The higher flute count makes chip evacuation more difficult, but Kennametal combats this by including chipbreakers on these tools and developing flute geometries that improve chip evacuation.

The company has also developed coatings like the KCSM15A, which both improves tool life and makes the flutes smoother and more slippery. This helps the chips to slide much more easily, further improving chip evacuation. “[The coating] gives users the ability to increase the cutting parameters and still have higher tool life,” Myers says.

Additionally, Myers emphasizes that while coolant is often recommended for iron and aluminum, it is absolutely necessary for titanium. “Titanium is quite flammable, and without coolant it will likely combust.”

At the end of the day, however, Myers says it is how a machinist applies the tool that matters. “You can make a cutting tool work in a lot of different situations, but you need to approach each material differently, from machining strategy to toolpath to machining parameters,” she explains. “Using cutting tools designed for certain materials lets users machine more aggressively, but it’s up to the shop to consider which strategy serves them best.”