Jun 12, 2023

Hard Facts About the Hardest Cutting Tools

Diamond and CBN cutting tools are powerful weapons, but when and how should they be used?

Two things everyone knows about diamonds: they’re very hard and they’re very expensive. And so it is with polycrystalline diamond (PCD) as well as cubic boron nitride (CBN) cutting tools. They’re hard enough to cut extremely difficult-to-machine materials—but at a cost that has kept them as a tool of last resort in most cases, used only when there is no better, cheaper, solution.

“CBN and PCD technologies are somewhat on the fringes of cutting tool application,” noted Iscar USA’s Tom Raun. “And so they tend to get overlooked a lot of the time.

There are two truths that manufacturers more in the mainstream of metalcutting should consider, however. First, while the upfront cost of hard cutting tools may seem a formidable barrier to their use, they can actually reap a great return on investment and reduce one’s cost per part when used in the right application. And second, both PCD and CBN tools are improving, making them more versatile and broadening the kinds of applications they are suitable for.

With the help of four experts (including Raun), we look at how both types are being used and how these tools are getting better and more useful.

“Both PCD and CBN materials are typically used for high production applications, meaning a large volume of parts need to be produced,” said Steve Howard, marketing and engineering manager for NTK Cutting Tools USA, Wixom, Mich. “Being the hardest cutting tool materials in the industry, they can offer the best tool life for various part materials. In the case of CBN, there are other cutting tool materials on the market that will perform the same machining operation at a higher cutting speed, but those cannot achieve CBN’s tool life. CBN can offer more parts and less insert indexing at lower surface footage [for] improved cycle time on a part.”

CBN grades are predominantly used for cutting hardened steel and powdered metals while PCDs are most applicable to non-ferrous material machining, Howard noted.

“However, parts manufacturers are seeing an evolution of material composition and new materials are being invented every day,” he said. “It is important to take the time to investigate the make-up and characteristics of the material to be machined to determine what cutting tool and parameters will work the best for machining it.”

The price of both PCD and CBN keep them off of the list of possible solutions for many shops until they find they have no other choice—and then some of them make the plunge without adequately researching the materials to be cut, according to Howard.

“With PCD and CBN options, the cost of the insert is the number-one deterrent for customers when selecting the cutting tool for their application,” he said. “One of the most difficult things to judge is [what] the exact tool life of these two cutting tool materials will be. Testing is the key to determine the maximum number of parts these inserts can cut before they wear out. Due to their high cost, you want to get every part you can out of them. Many users do not know if they’re getting maximum tool life, and may not be getting their money’s worth out of every index.”

Craig Bastian has seen the lightweighting trend in the automotive industry bring about greater use of PCD tools in his role as general manager of IT.TE.DI North America, a sister company to Ingersoll Cutting Tools Co. Both companies are based in Rockford, Illinois, and are part of the IMC Group.

“More and more automotive components over the past 10 or 15 years are moving from cast iron to aluminum, so PCD use is growing like crazy in this industry,” said Bastian.

“Just about every passenger car and light truck these days has an aluminum block and aluminum head. And in electric vehicles, even more components are aluminum. PCD is going to continue to grow over the next 20 to 25 years.”

The same lightweighting trend is also expanding CBN use, he said, offering the example of the move to CGI (compacted graphite iron) from cast iron in truck manufacturing.

“A lot of the big truck manufacturers that had still been using cast iron for engine blocks and heads are moving to CGI because they can maintain all of the strength of cast iron but lighten up the weight by reducing wall thickness,” he said. “CGI is a very difficult material to machine, so you need the high wear resistance of CBN.”

Another common automotive application for CBN is on hardened materials used for gear boxes or transmission components. Again, due to lightweighting requirements, these need to be made to ever tighter tolerances, according to Robert Keilmann, a turning project manager at Kennametal Inc., Latrobe, Pa.

The aerospace industry has also been expanding its use of CBN cutting tools, notably in engine manufacturing, Keilmann said. “If you look at the jet engine, you have the cold area in front, where the air intake is [made of] components usually made of titanium, due to its low weight. But the combustion area needs to have its very precisely made blades and disks and other components made of nickel-based, heat-resistant materials such as Inconel or Hastelloy.”

In the past, these heat-resistant materials were usually cut using tools with carbide inserts, he said. But CBN enables longer tool life and higher cutting speeds. “If you compare cutting Inconel 718, for example, with carbide and CBN, you’ll find that the speed you can run the CBN is ten times higher,” he said.

Another area ripe for growth in aerospace is the use of hard tools for machining carbon composites, noted Iscar’s Raun. He described one customer’s elaborate system to accurately drill holes in airframe composites for fasteners using carbide drills. The challenge was the shorter tool life of carbide compared to PCDs and CBNs, which was complicated by the steps taken to ensure drilling accuracy.

“They had a special bushing that was used to keep the drill in alignment,” he explained. “That meant a complex setup would need to take place every time they had to replace a drill. They would have to disassemble the whole bushing and the changeout took 10–15 minutes each time, at each of the multiple drilling stations.”

The solution Iscar offered was twofold: first, by using PCD rather than carbide, tool life was greatly increased, so that the number of times drilling needed to be interrupted was greatly reduced. And secondly, Raun said, the Iscar SUMOCHAM system of using tools—drills, in this case—with replaceable heads allowed the customer to quickly change only drill heads, rather than perform the time-consuming process of replacing the entire drill. Without disturbing the bushing, “they have room to go in there and quickly change just the head. It literally takes seconds rather than 15 minutes per drill,” he said. The resultant savings were “astronomical,” according to Raun.

PCDs and CBNs start out with the advantage of longer tool life than their most common rival, carbide. In the right application, tool life is enough longer that it makes the higher up-front costs of the former irrelevant when considering the total cost of production.

But for some suppliers, that up-front cost is nonetheless a barrier. All of the makers of these tools continue to find ways to improve their products in order to make them better solutions in an ever-widening sphere of applications.

For example, Iscar’s changeable head technology can allow PCD to be used in composites and other areas. According to Raun, the company is also improving its method of braising PCD or CBN onto a carbide insert. “Usually, braising happens on straight edges, the edge of the carbide against the straight edge of the tiny, flat wafer of PCD or CBN,” he said. “That join can be a point of weakness.”

Iscar has increased the surface area being braised by changing the shape of the two edges (see photo on page 61).

“It’s almost like a spline or an ‘S’ shape, with a mating pattern on the substrate,” he explained. “This creates a much stronger brazed connection, with better rigidity, and that allows us to be more aggressive with our depths of cuts and our feed rates.” And while Iscar is changing the shape of the PCD and CBN where it joins the carbide, it has also started putting chip formers on the cutting edge to enable better chip control, he said.

A notable development at Ingersoll is how the PCD cutting edge is formed, according to Bastian. Traditionally, that’s done with either wire EDM or grinding, both of which have a limitation.

“Polycrystalline diamonds, as the name would suggest, are hundreds of thousands of tiny diamond particles compacted together under heavy heat and pressure to make up that PCD wafer, onto which a cutting edge is machined,” he said. “But if you look at the cutting edge under a microscope, you’ll see little peaks and valleys, like a mountain range on what should be a flat cutting edge.” The jagged surface is because the EDM and grinding processes both unavoidably pull some of those tiny diamond particles out of the compacted material.

“Those peaks and valleys lend themselves to more quickly creating cutting edge failure,” said Bastian. “So, these days we’re moving more towards preparing the cutting edge by means of laser.” The laser cuts through the diamond particles rather than pulling them out, he explained. “So now we’ve got a much smoother edge to begin with, which lends itself to much longer tool life.”

Bastian noted that the company has been using the laser method for about seven years, and while some competitors are also doing it this way, the high cost of the necessary laser equipment is a barrier for some of their smaller rivals. “We’re lucky. We belong to the IMC Group and we can afford machines like that.”

NTK Cutting Tools improves its products by working with its customers to create an optimal cutting edge for each application, according to Howard.

“Insert edge preparation is a critical component to machining success,” he said. “A broad application range for these inserts can lead to adjustments to edge treatment, based on the part material and cutting conditions. Analyzing the application and identifying the appropriate edge preparation can play a major role in maximizing insert tool life. As we say, one size does not fit all.”

Paramount to the customer’s—and NTK’s—success is the latter’s emphasis on working closely with the customer to understand their requirements, processes and part materials, Howard explained.

“We partner with customers to develop a solution that maximizes productivity and in turn we gain a greater understanding of how PCD and CBN work. Real-life conditions are the greatest help for tooling manufacturers to learn and perfect the optimal combination of the grade of PCD or CBN with edge preparations for each material. Gathering data helps us to offer customers the best solutions.”

Edge preparation as well as coating technology is what has enabled Kennametal to meet the continually tightening tolerances of its customers, according to Keilmann.

“The precision requirements of the tools have been driven by the needs of the applications, such as tighter tolerances on automotive gearboxes to improve fuel efficiency,” he said. And the tools have come a long way.

“Twenty, thirty years ago we couldn’t have prepared a cutting edge in a way that we can do today. The capability of the grinding machines we now use to process our inserts enable higher accuracy and tighter tolerances than even a decade ago.”

The cutting edge prep improvements are abetted by new coating technology and other innovations, he said, using a recently launched double-sided CBN for turning hard metals as an example.

“The KBH-10B and KBH-20B grades feature a completely new PVD TiN/TiAlN/TiN coating for added wear resistance. But along with the coating, we implemented a new type of honing process.”

On the double-sided tool, one edge features a “trumpet” style hone for heavier and interrupted cuts, while the other has a light hone for continuous turning. The hones enable “lower cutting forces and at the same time improved surface quality and tool life,” Keilmann said. “It was a nice combination of developing a new edge shape and a new coating, then implementing them in the double-sided inserts.”

For Keilmann, even though some people have a static idea of what PCD and CBN tools can do, tool development is anything but static as the tools keep getting better. “What I always say is, ‘we don’t reinvent the carbide, we do not reinvent the CBN. The only thing we invent is the design of the tools and how they can be applied to make the processes better, and to work with a greater range of materials.’”

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Michael C. Anderson