Diamond and cubic boron nitride tools are powerful weapons, but when and how should they be used?

Everyone knows two things about diamonds: they are very hard and very expensive. The same is true for polycrystalline diamond (PCD) and cubic boron nitride (CBN) tools. They are sufficient to cut extremely difficult materials-but their cost makes them the last tool in most cases, only used when there is no better and cheaper solution.

"CBN and PCD technologies are on the edge of tool applications to some extent," points out Tom Raun of Iscar USA. "So they tend to be ignored many times.

However, there are two facts that mainstream metal cutting manufacturers should consider. First, although the upfront cost of hard cutting tools seems to be a huge obstacle to their use, when they are used in the right application, they can actually achieve a huge return on investment and reduce the cost of each part. Second, PCD and CBN tools are improving, making them more versatile and broadening the types of applications they are suitable for.

With the help of four experts (including Raun), we studied how these two types are used and how these tools can become better and more useful.

"PCD and CBN materials are often used in high-volume applications, which means a large number of parts need to be produced," said Steve Howard, US marketing and engineering manager for NTK Cutting Tools in Wixom, Michigan. "Become the hardest material in the cutting tool industry, they can provide the best tool life for a variety of part materials. As far as CBN is concerned, there are other tool materials on the market that can perform the same machining operations at higher cutting speeds. , But can’t reach the tool life of CBN. CBN can provide more parts and fewer blade indexing at lower surface footage [thus] improving part cycle time."

Howard pointed out that CBN grades are mainly used for cutting hardened steel and powdered metals, while PCD is most suitable for processing non-ferrous materials.

"However, component manufacturers are seeing the evolution of material composition and are inventing new materials every day," he said. "It is important to spend time studying the composition and characteristics of the material to be processed to determine which tools and parameters are most suitable for processing."

According to Howard, the prices of PCD and CBN prevented many factories from choosing possible solutions until they found that they had no choice, and then some of them plummeted without fully studying the material to be cut.

"For the PCD and CBN options, the cost of the insert is the biggest deterrent when customers choose cutting tools for their applications," he said. "One of the hardest things to judge is the exact tool life of these two cutting tool materials. Testing is the key to determining the maximum number of parts that these blades can cut before they wear out. Because of their high cost, you want to learn from them Get all the possible parts of it. Many users don’t know if they get the longest tool life and may not get value from each index."

During his tenure as general manager of IT.TE.DI North America (sister company of Ingersoll Cut Tools Co.), Craig Bastian witnessed the lightweight trend in the automotive industry leading to more use of PCD tools. Both companies are located in Rockford, Illinois and are part of the IMC Group.

"In the past 10 or 15 years, more and more auto parts are switching from cast iron to aluminum, so the use of PCD in this industry is growing wildly," Bastian said.

"Almost every passenger car and light truck now has an aluminum block and aluminum head. In electric vehicles, there are even more aluminum parts. PCD will continue to grow in the next 20 to 25 years."

He said that the same lightweight trend is also expanding the use of CBN, providing an example of the shift from cast iron to CGI (dense graphite) in truck manufacturing.

"Many large truck manufacturers that still use cast iron to make engine blocks and cylinder heads are turning to CGI because they can maintain all the strength of cast iron, but reduce weight by reducing wall thickness," he said. "CGI is a very difficult material, so you need the high wear resistance of CBN."

Another common automotive application of CBN is hardened materials for gearboxes or transmission components. According to Robert Keilmann, turning project manager at Kennametal Inc. (Latrobe, Pa), similarly, these parts need to have tighter tolerances due to lightweight requirements.

Keilmann said the aerospace industry has also been expanding its use of CBN cutting tools, especially in the field of engine manufacturing. "If you look at a jet engine, you will find that there is a cold area in front, the air intake is made of parts usually made of titanium [made] because of its light weight. But the combustion area requires very precise manufacturing of the blades. And discs and other parts made of nickel-based heat-resistant materials (such as Inconel or Hastelloy)."

He said that in the past, these heat-resistant materials were usually cut using tools with carbide blades. But CBN can achieve longer tool life and higher cutting speed. "For example, if you compare the cutting of Inconel 718 with cemented carbide and CBN, you will find that the cutting speed of CBN is ten times higher," he said.

Iscar's Raun pointed out that another mature growth area in the aerospace industry is the use of hard tools to process carbon composite materials. He described a complex system that a customer used a cemented carbide drill bit for fasteners to accurately drill holes in the fuselage composite material. The challenge is that compared with PCD and CBN, the tool life of cemented carbide is shorter, which is complicated by the steps taken to ensure drilling accuracy.

"They have a special bushing to keep the drill bit aligned," he explained. "This means that every time a drill bit must be changed, a complex setup is required. They will have to disassemble the entire casing, and at each of the multiple drilling stations, each change takes 10 to 15 minutes."

There are two solutions provided by Iscar: First, by using PCD instead of cemented carbide, the tool life is greatly increased, thereby greatly reducing the number of interrupted drilling. Second, Raun said, the Iscar SUMOCHAM system uses tools (drills in this case) and replaceable heads, allowing customers to quickly replace the drill bit instead of performing the time-consuming process of replacing the entire drill bit. Without disturbing the casing, “they have room to get in there and change their heads quickly. Literally, each drill takes only a few seconds instead of 15 minutes,” he said. According to Raun, the resulting savings are "astronomical."

The advantage of PCD and CBN is that the tool life is longer than their most common competitor carbide. In the right application, the tool life is long enough that the higher upfront cost of the former becomes irrelevant when considering the total production cost.

But for some suppliers, the upfront cost is still an obstacle. All manufacturers of these tools are constantly looking for ways to improve their products in order to make them better solutions in the ever-expanding fields of application.

For example, Iscar's variable head technology allows PCD to be used in composites and other fields. According to Raun, the company is also improving its method of brazing PCD or CBN to carbide blades. "Usually, brazing occurs on the straight edge, and the edge of the carbide rests on the straight edge of a tiny flat wafer of PCD or CBN," he said. "This connection may be a weakness."

Iscar increased the surface area to be stewed 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 matching pattern on the substrate," he explained. "This creates a stronger brazed connection with better rigidity, which allows us to be more aggressive in terms of depth of cut and feed rate." He said, although Iscar is changing the PCD and CBN that connect cemented carbide Shape, but it also began to place chip formers on the cutting edge to achieve better chip control.

According to Bastian, a notable development of Ingersoll is the way the PCD cutting edge is formed. Traditionally, this is done by wire EDM or grinding, both of which have limitations.

"Polycrystalline diamond, as the name suggests, is hundreds of thousands of tiny diamond particles compacted together under high temperature and high pressure to form a PCD wafer on which a cutting edge is processed," he said. "But if you look at the cutting edge under a microscope, you will see small peaks and small valleys, just like mountains. The cutting edge should be flat." The jagged surface is because EDM and the grinding process inevitably start from compacting the material. Pull out some tiny diamond particles.

"These peaks and valleys help to produce cutting-edge failures faster," Bastian said. "So, now we are more inclined to use lasers to prepare the tips." He explained that the laser cuts diamond particles instead of pulling them out. "So now we have a smoother edge, which helps to extend tool life."

Bastian pointed out that the company has been using laser methods for about seven years, and although some competitors are doing so, the high cost of the necessary laser equipment is an obstacle to some of its smaller competitors. "We are lucky. We belong to the IMC Group and we can afford such a machine."

According to Howard, NTK Cutting Tools improves its products by working with customers to create the best cutting edge for each application.

"Insert edge preparation is a key component of successful machining," he said. "The wide range of applications of these inserts can adjust the edge treatment according to the part material and cutting conditions. Analyzing the application and determining the appropriate edge preparation plays an important role in maximizing the life of the insert tool. As we said, one size does not Not for everyone."

Howard explained that for the customer and NTK's success, the most important thing is the latter's emphasis on working closely with customers to understand their requirements, processes and parts materials.

"We work with customers to develop solutions that maximize productivity, and we have a deeper understanding of the working principles of PCD and CBN. The reality is that moldmakers learn and perfect the PCD or CBN grades and the edge of each material. The greatest help of the best combination of preparations. Collecting data helps us provide the best solutions for our customers."

Keilmann said that edge preparation and coating technology enable Kennametal to meet customers' ever-tightening tolerance requirements.

"The accuracy requirements of tools are driven by application requirements, such as the tighter tolerances of automotive gearboxes to improve fuel efficiency," he said. These tools have made considerable progress.

"Twenty or thirty years ago, we could not be ready for the cutting edge the way we are today. Compared with ten years ago, the grinders we use to process blades now have higher precision and tighter tolerances."

He said that taking the recently introduced double-sided CBN for turning hard metals as an example, new coating technologies and other innovations have promoted improvements in cutting-edge preparation.

"KBH-10B and KBH-20B grades use a new PVD TiN/TiAlN/TiN coating to improve wear resistance. But in addition to the coating, we also implemented a new honing process."

On the double-sided tool, one edge has a "trumpet" type sharpener for heavier interrupted cutting, and the other end has a light sharpener for continuous turning. Honing can "reduce cutting forces while improving surface quality and tool life," Keilmann said. "This is the perfect combination of developing new edge shapes and new coatings, and then implementing them in double-sided blades."

For Keilmann, although some people have a static concept of the functions of PCD and CBN tools, as tools continue to improve, tool development is by no means static. "I always say,'We don't reinvent cemented carbide, we don't reinvent CBN. The only thing we invent is the design of tools and how to apply them to improve processes and use a wider range of materials.'"