Characteristics and cutting applications of polycrystalline cubic boron nitride cutting tools

1. Features of polycrystalline cubic boron nitride (PCBN) cutter

PCBN tool is made of CBN particles as raw material, sintered at high temperature and high pressure with or without binder, and its structural forms include integral sintering and cemented carbide substrate composite sheet. Since PCBN is mainly made of CBN, the PCBN tool has various excellent properties of CBN. Its performance indicators are shown in Table 1. Its main features are:

Table 1 Mechanical properties of various tool materials

(1) High strength and abrasion resistance

The crystal structure of polycrystalline cubic boron nitride is similar to that of diamond, and it has hardness and compressive strength close to that of diamond. When using PCBN tools for cutting, its wear resistance is also much higher than that of hard It can be used for machining hard cast iron and steel or other alloy materials with high strength, high hardness and high heat sensitivity.

(2) Excellent chemical stability

CBN has high chemical stability and will not react with iron-based materials when the temperature reaches 1200-1300 °C, and at a high temperature of 1000 °C CBN will not be oxidized and has strong oxidation resistance, so PCBN tools have less chemical wear during cutting than other tool materials, and can be used to cut a variety of materials.

(3) Good thermal conductivity and low friction coefficient

The thermal conductivity of CBN is much higher than that of high-speed steel, ceramics and cemented carbide, and the thermal conductivity of PCBN tools will increase with the increase of temperature. This makes the temperature of the tool tip not too high during cutting, which is beneficial to improve tool durability and prolong tool life. When CBN interacts with other materials, its friction coefficient is about 0.1-0.3, which is much smaller than the friction coefficient (0.4-0.6) between cemented carbide and other materials, which reduces cutting force and cutting temperature, reducing cutting The sticking phenomenon during processing can effectively avoid the generation of chip accumulation, thereby improving the surface processing quality of the workpiece.

2. Application of polycrystalline cubic boron nitride (PCBN) tool

PCBN tools are mainly used for machining difficult-to-machine materials and high-hardness materials (>HRC35), such as tool steel, high-speed steel, hardened steel, austenitic soft iron, etc.; wear-resistant materials Such as gray cast iron, sintered iron, etc.; difficult-to-machine materials such as surface coating materials, surface hardening alloys, ductile iron, etc., and PCBN tools can replace the traditional grinding process, the machining accuracy can be as high as IT5-IT6, and the surface roughness can be as high as IT5-IT6. It can reach Ra0.8~0.2μm, which can achieve the precision of grinding.

2.1 Machining grey cast iron

High-speed cutting High-speed cutting of gray cast iron material is one of the typical applications of PCBN tools. When PCBN tools are high-speed cutting gray cast iron, the tool life can reach 20-30 times that of ceramic tools. For different grades of PCBN tools, the higher the CBN content, the higher the hardness, wear resistance and thermal conductivity of the PCBN tool. When the ferrite content in the gray cast iron material is less than 10%, it is more suitable for the cutting of PCBN tools. At this time, the life of PCBN tools is longer, because when there is too much ferrite, it will interact with the boron element in CBN. The chemical reaction occurs at high temperature during cutting, which increases the chemical wear of the PCBN tool and reduces the service life of the tool. When machining gray cast iron, one aspect that needs extra attention is aging, that is, the time after the workpiece is cast until the PCBN tool is used for cutting. This is because a large amount of oxygen and nitrogen will be generated when gray cast iron is cast. It is harmful to the durability of the PCBN tool, so it takes a period of time for oxygen and nitrogen to volatilize to ensure the normal service life of the tool. The general aging days are 10 days.

2.2 Machining hardened steel

Hardened steel is a typical hard-to-machine and wear-resistant material. Grinding has been used to complete the finishing of hardened steel parts. When using PCBN tools to process hardened steel, the processing efficiency is faster than grinding, and it is more suitable for cutting Machining parts of different shapes is more flexible than grinding. Because PCBN tools often use dry cutting, it can also avoid the pollution of cutting waste fluid and save energy. The quality of the machined surface is comparable to that of grinding, or even higher than that of grinding. In the process of high-speed cutting of hardened steel, the high heat generated by the PCBN tool continuously softens the cutting area of the workpiece material, so as to carry out cutting processing. Compared with the carbide tool, the efficiency is greatly improved, and the surface roughness of the workpiece can be improved. Precision.

2.3 Difficult Materials

Relatively speaking, the application and advantages of PCBN tools in machining ductile or vermicular graphite iron are relatively small. In the process of machining ductile or vermicular graphite iron, PCBN tools are prone to severe chemical wear and reduce tool life. Therefore, in order to ensure the dimensional accuracy and geometric tolerance of the parts, when the PCBN tool has to be used, it is usually coated to protect the PCBN tool and increase its durability. In addition to the above typical applications, PCBN tools can also be used to cut nickel-based superalloys, case hardened alloys, cemented carbides, ceramics, powder metallurgy and other materials. It has good advantages in processing efficiency, workpiece dimensional accuracy, surface roughness and tool durability, and is widely used.

3. Epilogue

PCBN With high tool processing efficiency, long tool life and low processing cost, it has been widely used in the field of ferrous metal processing such as gray cast iron and hardened steel. High-speed cutting of difficult-to-machine materials can be realized by turning grinding, which greatly improves the processing efficiency. The application of PCBN tool solves the processing problem of ferrous metal hard cutting, which is of great significance for promoting high-speed cutting technology.