Sweep the article, take you to see through and understand PCBN

In the last article, the author introduced the concept of hard cutting and its development direction. In this article, the author will combine the performance requirements of hard cutting for tool materials and introduce the most suitable tool for hard cutting: PCBN tool

1. The requirements of hard cutting for tool materials

The development of hard cutting technology is largely due to the emergence and development of superhard tool materials。

The prerequisite for the formation of the hard cutting principle is to accumulate enough cutting heat to form a "metal softening effect" on the workpiece material. Therefore, whether it is dry cutting or extremely high cutting parameters, it is the result of conventional tool materials. unbearable.

Therefore, hard cutting has clear and special requirements for the tool material used:

1. High hardness and wear resistance

2. High temperature resistance and good chemical stability

3. High resistance to mechanical stress and impact

In short, the tool material must have sufficient high temperature wear resistance!

To meet the above requirements, the tools suitable for hard cutting mainly include PCBN tools, diamond tools, ceramic tools, ultra-fine carbide tools and coated carbide tools.

Although I don’t want to say it, it still comes to the inevitable “but” part:

However, due to the inability of diamond tools to process ferroalloy materials, ceramic tools with sufficient hardness but insufficient toughness, and the wear resistance of cemented carbide and its coated tools can only be considered entry-level, therefore, in practical hard cutting applications, the most widely used A broad tool material is PCBN, or polycrystalline cubic boron nitride.

2. Development history of PCBN tools

In 1957, inspired by the synthetic method of synthetic diamond, the General Electric Company (GE) of the United States successfully developed a new type of tool material, Cubic Boron Nitride (CBN), which has a hardness second only to diamond.

Although the properties of CBN materials are relatively stable, the synthesized CBN single crystals are relatively small in size, easy to dissociate, and have obvious internal anisotropy, which limits their practical application.

To solve this problem, PCBN materials came into being. The Soviet Union was the first country to successfully develop a PCBN tool. In 1972, the Soviet Union showed its PCBN sintered body turning tool to the public for the first time, and it could meet the needs of industrial production and conduct mass production.

3. PCBN material cutting performance

PCBN is made of many small CBN single crystals and binders sintered in a high temperature and high pressure environment.

The main properties of PCBN materials are as follows:

1. Extremely high hardness and wear resistance

The hardness of the PCBN sintered body reaches 3000 to 5000HV. When machining wear-resistant workpieces, the wear resistance of PCBN tools is much higher than that of carbide tools and ceramic tools. Its wear resistance is 50 times that of carbide tools and 30 times that of ceramic tools.

 2. Good thermal stability

PCBN has high heat resistance, and the crystal begins to soften when the temperature reaches 1370°C or more, and PCBN has excellent red hardness. At 800°C, it can still maintain the hardness of 94HRA, which is equivalent to the normal temperature hardness of ordinary ceramic tools. In the environment of 1300 ℃, the PCBN tool can still ensure sufficient hardness and strength, and the cutting performance is stable.

3. Good chemical stability

PCBN grains have good chemical inertness, stable chemical properties under high temperature conditions, no oxidation reaction at 1000°C, no chemical reaction with iron at 1200-1300°C, and good stability under acid and alkali conditions. sex.

 4. Good thermal conductivity

Although the thermal conductivity of PCBN is slightly lower than that of diamond, it is much higher than that of ceramic tools and carbide tools.

During the cutting process, as the cutting temperature increases, the thermal conductivity of the PCBN tool material will also increase, so that the cutting heat of the tool can be dissipated more quickly, avoiding excessive temperature at the tip of the tool, which is conducive to slowing tool wear and prolonging the cutting tool. life, to ensure the processing quality.

 5. Lower friction coefficient

The friction coefficient between PCBN and other materials is small, about 0.1 to 0.3, which is much smaller than that of cemented carbide (about 0.4 to 0.6), and with the increase of cutting speed, the friction coefficient slightly There is a decrease. The lower friction coefficient is beneficial to reduce the cutting force, reduce the cutting temperature and prevent the bonding between the tool material and the material to be processed, thereby improving the surface processing quality of the workpiece.

4. PCBN tool use

It is precisely because of the above advantages of PCBN that PCBN tools are very suitable for high-speed hard dry cutting of some difficult-to-machine materials.

Materials suitable for processing PCBN tools include: hardened steel materials with high hardness, wear-resistant cast iron, heat-resistant iron-based alloys and nickel-based alloys. The higher the hardness of the material to be processed, the better it can reflect the superiority of the PCBN tool.

In recent years, the industry using PCBN tools has become more and more extensive, and the usage has also increased year by year.

According to the statistics of General Electric Company of the United States, PCBN tools are mainly used in the finishing of hardened steel materials in the automobile industry, heavy machinery industry and bearing and gear industry.

It is mainly used for turning processing, accounting for about 60%, followed by boring processing, accounting for 32.5%, and PCBN tools used for milling processing are used less, accounting for only 7.5%.

Just like there is no one-size-fits-all truth, PCBN knives are not universal!

1. Too soft materials are not suitable

Compared with the use of ordinary tool materials, the advantage of PCBN tools is that they can process workpiece materials with high hardness. If the hardness of the material is too low, PCBN tools cannot exert their high hardness and high chemical stability, and they cannot reflect the cost-effectiveness advantage.

2. Avoid chemical wear

When using PCBN tools to cut certain materials (such as austenitic stainless steel), abnormal chemical wear and diffusion wear of the tool are prone to occur, resulting in the loss of binder elements in the tool material, the reduction of tool hardness and the reduction of tool life.

3. High requirements for equipment

Due to the high brittleness of the PCBN tool, there is a high requirement for the overall rigidity of the machine tool to avoid the tool breakage caused by the vibration of the machine tool.

This also explains in disguise why hard cutting is mainly used in continuous machining conditions such as turning and boring, while it is less used in interrupted machining milling, and why "hard cutting" is also called "hard turning" .

4. Edge passivation treatment

PCBN tools are brittle and cannot withstand excessive cutting force, and when the cutting edge is sharpened, there will always be tiny serrations on the cutting edge. The particles on the blade fall off, resulting in micro chipping, which in severe cases will lead to tool chipping, thus greatly reducing the service life of the tool.

Therefore, in order to ensure that the PCBN tool can be cut normally, the edge of the PCBN tool is usually processed into a certain shape, so as to achieve the purpose of increasing the strength of the blade.

The state of the cutting edge after edge treatment mainly includes: rounding, chamfering and compound edge. As shown below

The first two articles introduced the principles of hard cutting and applicable tool materials, which laid the foundation for subsequent exploration.

As the application of hard cutting in the field of high-hardness material finishing is more and more extensive, the research on cutting technology and tool application is also increasing. Advantages and Disadvantages", "Chamfering, the life of PCBN tools! " and "There is no best, only better, the matching of working conditions with tools and parameters" and other articles, I hope you will pay attention to reading.