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In the not-too-distant past, before carbide became so pervasive, a high-speed-steel tool with a 30-degree helix angle could have been called an "across the board" solution. Today, specialized tools serve specific needs, and carbide has machined what was in the past thought to be unmachinable—titanium included.
Tool-geometry considerations can be boiled down to helix and rake angle of the tooth, designed specifically to cut into a specific material as efficiently as possible. And for titanium, this evolution has resulted in a unique geometry to handle some major challenges. Known for its high strength to weight ratio, titanium on a metallurgical level really can't be said to be "stronger" than steel. Most titanium falls in the 20 to 30 Rockwell range. (For comparison Inconel 718 is about 45 to 50 Rockwell).
Titanium's memory makes it a bear in the machining center. With a low elastic modulus, it "remembers" its shape, and when a cutting tool applies pressure, a small amount of that material springs back—hence its "gummy" characteristics, the worry about deflection during machining and the need to make relatively light, fast cuts.
Nevertheless, recent tooling advancements hope to push the depth-of-cut limits in titanium. With the medical and aerospace markets soaring, demand for tooling that can give that productivity edge, however slight, has hit the footlights.
THE GEOMETRY ADVANTAGE
"The evolution of [tool geometries for titanium] has involved higher helix angles—50 and 60 degrees—along with positive rake angles," says Stephen Jean, milling products manager for Emuge Corp., West Boylston, Mass.
That high helix angle results in good chip evacuation for titanium. A straight, zero-degree flute would take significant bites out of the metal simultaneously, producing among other things a chip traffic jam in the work area. For softer, gummy materials like titanium, the long, thin chips (analogous to stainless) "get a ride" up that flute, which propels the chip, and the heat it contains, away from the work zone. Such an angle means it takes less rotation of the end mill to "eject" the chips away from the cutting zone. "The higher the helix angle," Jean explains, "the more benefit is created to chip removal and evacuation."
The positive rake angle makes the cutting edge sharp, cutting through gummy titanium like a knife through butter, to use a rough analogy. Contrast this with hardened steel, which requires a negative rake, closer to perpendicular to the actual cut; this makes the cutting edge less sharp but much stronger, able to bludgeon the brittle material with great force and produce the characteristically small chips.
To push titanium (and similar materials) a bit further, tooling designers have introduced staggered tooth geometries. Here, no longer are flutes set off consistently from each other—for instance, by 90 degrees in a four-flute cutter. Instead, one flute may be set at 87 degrees, another at 93 degrees and so on. This geometry, done primarily with flat-ended end mills, "makes for smooth cutting and eliminates the vibration that comes from repetitive motion," Jean explains.
Consider a slotting operation using a four-flute cutter, with those flutes offset 90 degrees from one another. Looking from overhead, one tooth on the east side wall would be just coming out of the cut, while the tooth on the west side wall would be just entering the cut. Meanwhile, the tooth to the north would be the only one fully engaged. This pattern would repeat multiple times a second, and it is this repetition, Jean explains, that produces harmonics that result in vibration and chatter.
With a staggered geometry, on the other hand, the "north" tooth would never be fully engaged by itself without other teeth engaged to some degree, "balancing" the motion and reducing the effect of the repetitive motion. Balancing those cutting forces helps reduce the vibration, avoid chatter and assist in pushing depths-of-cuts slightly further.
SUBSTRATE AND COATING
For tooling substrates, there has been the traditional trade-offs between hardness and durability. The harder a material is the more brittle it is. For more durability, you must reduce hardness. Regarding titanium, "you can trade off some of the hardness that you don't need for cutting material like titanium, and have the result be more durability that allows a higher rake-angle edge to hold up better."
Fortunately, the new micro-grain and sub-micro-grain geometries available today makes that trade-off between hardness and durability not quite so stark. A micro-grain can boast high durability and relatively high hardness—which makes for a more flexible tool-geometry design, including high rake angles.
Also adding to durability has been coating advances. With end mills TiAlN coating has remained the most popular, and new coating combinations continually appear on the horizon, Jean says, adding that chromium nitride—CrN—exhibits real potential for titanium applications.
GAINING A COMPETITIVE EDGE
The combination of geometry advancements, micro-grains in the substrate and coatings, according to Jean, has pushed the titanium milling from between 150 and 200 surface feet per minute to about 400 sfm—still slow compared to hardened steel, which typically sees 900 sfm or more. But in percentage terms, that represents significant productivity gains.
Jean groups titanium with materials like stainless and Inconels that he calls "the great equalizers. Our company, for instance, recently introduced a tool that just eats through steel. But for titanium, which has a relatively low overall cutting speed, the actual gain is relatively low. You unfortunately can't change the physical characteristics of the material. It's harder to stand out."
More work has swayed away from commercially pure (CP) titanium and toward materials like Ti-6Al-4V, which add a dose of vanadium to make life easier for the machinist—but not that much easier. Within the machining center, spindle, toolholder and workholding, technology has taken rigidity to new levels, and titanium has seen the benefit.
But not as much benefit as many would like. Indeed, make any major productivity improvement in milling titanium represents a big accomplishment. Baby-step increases in production can give any metal manufacturer a major competitive edge.
Editor's Note: Photo courtesy of Emuge Corp., www.emuge.com.
author: By Tim Heston