Superhard Cutting Tools: Types And Characteristics

Superhard tool material is a tool material that is harder than ceramic material.

Including: single crystal diamond, polycrystalline diamond (PCD), polycrystalline cubic boron nitride (PCBN) and CVD diamond.

Superhard cutting tools are mainly made of diamond and cubic boron nitride, in which the artificial diamond composite (PCD) cutting tools and cubic boron nitride (PCBN) cutting tools are dominant.

Many machining concepts, such as green machining, turning instead of grinding, milling instead of grinding, hard machining, high-speed cutting and dry cutting, have been developed due to the application of superhard tools, so superhard tools have become an indispensable and important means in machining.

With the progress of science and technology, the rapid development of manufacturing industry, the rapid development of CNC machining technology and the universal use of CNC machine tools, the production and application of superhard tools are becoming more and more extensive.

PCD and PCBN cutting tools have been widely used in various industries of machining, greatly promoting the rapid development of cutting and advanced manufacturing technology.

The manufacturing process of superhard tools is as follows:

(1) To make sintered body, the mixed powder of diamond or CBN and binder and cemented carbide substrate are filled into the sealing box, and sintered under high temperature and high pressure to combine with the substrate at the same time.

As shown in Fig. 2-33.

manufacturing process of sintered body

Fig. 2-33 manufacturing process of sintered body

(2) In the manufacturing process from the sintered body to the finished tool, the raw material of the sintered body is first cut off, then welded on the cemented carbide matrix to form a blade, or welded on the steel tool support, and then the cutting edge is ground and finished.

As shown in fig. 2-34.

manufacturing process of sintered body tool

Fig. 2-34 manufacturing process of sintered body tool

(1) PCBN tool

Cubic boron nitride CBN was first synthesized by General Electric (GE) under high temperature and high pressure in the 1950s.

Its hardness is second only to diamond and much higher than other materials.

Therefore, it is collectively referred to as superhard materials with diamond.

CBN has high hardness, chemical inertness and thermal stability at high temperature, so it is widely used in grinding as an abrasive.

Because CBN has the characteristics superior to other tool materials, people tried to apply it to cutting processing from the beginning.

However, the single crystal CBN has small particles, which makes it difficult to make tools, and the CBN sintering property is very poor, which makes it difficult to make large CBN sintered bodies.

Until the 1970s, the former Soviet Union, China, the United States, Britain and other countries successively developed polycrystalline cubic boron nitride PCBN sintered bodies as cutting tools.

The structure of CBN sintered body is shown in Fig. 2-35.

Since then, PCBN has been applied in various fields of machining with its superior cutting performance, especially in the machining of high hardness materials and difficult to machine materials.

After more than 30 years of development and application, PCBN tool materials for machining different materials have emerged.

The PCBN tool is shown in Fig. 2-36.

microstructure of CBN sinter

Fig. 2-35 microstructure of CBN sinter

PCBN tool (piece)

Fig. 2-36 PCBN tool (piece)

1. Cutting performance of PCBN tool

(1) It has high hardness and wear resistance.

CBN crystal structure is similar to diamond, with the same chemical bond type and similar lattice constant, so it has the same hardness and strength as diamond.

The microhardness of CBN powder is 8000 ~ 9000hv, and the hardness of sintered PCBN is generally 3000 ~ 5000HV.

When cutting wear-resistant materials, its wear resistance is 50 times that of cemented carbide tools, 30 times that of coated cemented carbide tools, and 25 times that of ceramic tools.

The use of PCBN tools not only has a high metal removal rate, but also can obtain good machining accuracy and machined surface quality, that is, to maintain the integrity of the machined surface, and can change the traditional process method that divides machining into cutting rough machining with tools before quenching and grinding finish machining with grinding wheels after quenching, so that rough machining before quenching and finish machining after quenching can be carried out on the same equipment.

In this way, the investment in grinder equipment can be reduced, the number of processes can be reduced, and the productivity and processing quality can be improved.

Therefore, the use of PCBN tools is very suitable for small batch and small and medium-sized enterprises.

In addition, because PCBN tool has high hardness and wear resistance, it can process high-precision parts for a long time at high cutting speed, with small size dispersion, and can greatly reduce the number of tool changes and the time spent on tool wear compensation shutdown.

Therefore, it is an ideal tool for automatic lines and automatic processing equipment.

(2) It has high thermal stability.

The heat resistance of CBN can reach 1400 ~ 1500 ℃, and the hardness of PCBN at 800 ℃ is higher than that of ceramics and cemented carbide at room temperature.

Therefore, PCBN tool can be used to cut high-temperature alloy and hardened steel at a speed 3~5 times higher than that of cemented carbide tool.

(3) It has excellent chemical stability.

The chemical inertia of CBN is very large. CBN does not react with ferrous materials at 1200 ~ 1300 ℃, and reacts with carbon at 2000 ℃;

In neutral and reducing gases, it is stable to acid and alkali;

Its adhesion and diffusion to various materials are much smaller than that of cemented carbide, so PCBN cutters are especially suitable for processing steel materials.

(4) It has good thermal conductivity.

Among all kinds of tool materials, the thermal conductivity of CBN is second only to diamond, with a thermal conductivity of 1300W/ (m · K), which is 3.2 times that of red copper and 20 times that of cemented carbide.

The ratio of thermal conductivity of cubic boron nitride to ceramic is 37.1 (1300:35), and the ratio of thermal diffusivity is 65.5 (74.7:1.14).

With the increase of temperature, the thermal conductivity of PCBN increases, while that of alumina decreases, as shown in fig. 2-37.

The high thermal conductivity of PCBN tool can reduce the temperature at the tool tip, and the reduction of tool wear is conducive to the improvement of machining accuracy.

Under the same cutting conditions, the cutting temperature of PCBN tool is lower than that of cemented carbide tool.

thermal conductivity of PCBN material

Fig. 2-37 thermal conductivity of PCBN material

(5) Low friction coefficient.

The friction coefficient CBN with different materials is 0.1 ~ 0.3, and that of cemented carbide is 0.4 ~ 0.6.

With the increase of cutting speed, the friction coefficient decreases.

This characteristic leads to the reduction of tool chip friction, cutting deformation, cutting force and surface quality.

2. The performance comparison between PCBN tool material and other tool materials is shown in table 2-39.

Table 2-39 performance comparison of various tool materials

Material properties

Material type

high speed steel

Cemented carbide

ceramics

cubic boron nitride

diamond

Density (g/em)

8.0~8.8

8.0~8.8

3.6~4.7

3.44~3.49

3.47~3.56

hardness

HRC63-70

HRA89-94

HRA91~95

HV8000~9000

HV10000

Bending strength /mpa

2500~4000

900~2450

450-800

300

210-490

Compressive strength /mpa

2500~4000

3500~5900

3000-5000

800~1000

2000

Impact toughness / (kj/m2)

100~600

25-60

5~12

 

 

Modulus of elasticity /gpa

200~230

420~630

350-420

720

900

Thermal conductivity / [w/ (MK)]

16.7~25.1

20.93~83.74

20.93

79.54

146.5

Coefficient of thermal expansion( × 10/℃)

9~12

5~7

6.3~9

2.1~231400~

0.9~1.18

Heat resistance / ℃

600~650

800~1000

>1200

1500

700~800

(2) PCD Tool

1. Development of PCD tools

As a kind of superhard tool material, diamond has been used in cutting for hundreds of years.

In the development of cutting tools, from the end of the 19th century to the middle of the 20th century, high-speed steel is the main representative of cutting tool materials;

In 1927, Germany first developed cemented carbide tool materials and widely used them;

In the 1950s, Sweden and the United States synthesized synthetic diamond respectively, and the cutting tools have entered a period represented by superhard materials.

In the 1970s, polycrystalline diamond (PCD) was synthesized by high-pressure synthesis technology, which solved the problem of rare and expensive natural diamond, and expanded the application range of diamond tools to aviation, aerospace, automotive, electronics, stone and other fields.

The PCD tool is shown in fig. 2-38.

PCD Tool (piece)

Fig. 2-38 PCD Tool (piece)

Diamond tools have the characteristics of high hardness, high compressive strength, good thermal conductivity and wear resistance.

They can obtain high machining accuracy and efficiency in high-speed cutting.

The above characteristics of diamond tools are determined by the state of diamond crystal.

In diamond crystal, the four valence electrons of carbon atoms form bonds according to tetrahedral structure, and each carbon atom forms covalent bonds with four adjacent atoms to form diamond structure.

This structure has strong binding force and directivity, which makes diamond have extremely high hardness.

Because the structure of polycrystalline diamond (PCD) is fine grain diamond sintered body with different orientations, its hardness and wear resistance are still lower than that of single crystal diamond, although the binder is added.

However, because PCD sinter is isotropic, it is not easy to crack along a single cleavage plane.

2.PCD tool cutting performance

(1) The hardness of PCD can reach 8000hv, which is 80 ~ 120 times that of cemented carbide.

(2) The thermal conductivity of PCD is 700W/mk, which is 1.5 ~ 9 times that of cemented carbide, and even higher than that of PCBN and copper.

Therefore, the heat transfer of PCD tools is rapid.

(3) The friction coefficient of PCD is generally only 0.1 ~ 0.3 (the friction coefficient of cemented carbide is 0.4 ~ 1), so PCD tools can significantly reduce the cutting force.

(4) The coefficient of thermal expansion of PCD is only 0.9 × 10-6~1.18 × 10-6 ℃, only equivalent to 1/5 of cemented carbide, so PCD tools have small thermal deformation and high machining precision.

(5) The affinity between PCD tools and non-ferrous and non-metallic materials is very small, and the chips are not easy to bond on the tool tip to form chip nodules in the machining process.

3.PCD tool application

The industrial developed countries have carried out the research on PCD cutting tools earlier, and its application has been relatively mature.

Since the first synthesis of synthetic diamond in 1953, a large number of achievements have been made in the research on the cutting performance of PCD tools.

The application scope and usage of PCD tools have expanded rapidly.

At present, the world’s famous manufacturers of synthetic diamond composite chips mainly include E-6 company, GE company of the United States, Sumitomo Electric Co., Ltd. of Japan, etc.

The application scope of PCD tools has expanded from initial turning to drilling and milling.

At present, the processing range of PCD tools has expanded from traditional metal cutting to stone processing, wood processing, metal matrix composites, glass, engineering ceramics and other materials.

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