The role of superhard material tools in mechanical cold working

【Abstract】 In the process of machining, cutting tool is the basic and necessary tool. In addition to a good tool with high hardness, but also requires a high toughness. With the continuous maturity and improvement of machining technology, the requirements for machining tools have gradually become strict. In the face of increasingly complex and diverse machining, the superior performance of superhard material tools has also attracted the attention and research of relevant scientific researchers. Especially in the process of mechanical cold working, the super hard material knife has higher durability, excellent performance, and lower cost. All kinds of advantages make the application of super hard material cutting tools more widely.

【Key words】Machining; Cutting tool; Superhard material; Application

1 the basic concept and development of super hard material tool

From the current machining industry, the traditional superhard tool materials mainly include natural diamond, synthetic diamond and cubic boron nitride. Due to the scarcity of natural diamonds, it is difficult to meet the large-scale demand of the machining industry. Although synthetic diamond can be mass-produced, it is difficult to be widely used because of its high cost and high market price. At present, the mainstream materials used in superhard material tools in China are polycrystalline cubic boron nitride and its composites. Superhard cutting tools were used in the United States in the 1950 s. Scientists made a large number of boron nitride polycrystalline blocks by adding coagulants in experimental high temperature and high pressure conditions. In the following decades, scientists have developed diamond cemented carbide and boron nitride composite, basically solving the problem of the source of superhard tool materials. China's research on the related aspects of superhard tool materials began in the 1970 s. Although it started earlier, due to various external conditions at that time, both production technology and plant equipment conditions were relatively difficult, so the research work progressed slowly. It was not until the end of the twentieth century that the project research in this area began to be carried out in a real sense, and remarkable results have been achieved.

2 Types and performance analysis of super-hard material cutting tools

CVD diamond does not contain any metal catalysts, and its thermal stability and hardness are largely similar to natural diamond. CVD diamond has strong wear resistance and extremely high hardness, and is currently recognized as the most hard tool material in the world. In addition, CVD diamond also has a small coefficient of friction, high thermal conductivity, good cutting effect and many other advantages, in the cutting of metal and non-metallic materials have a very good effect. However, the thermal stability of CVD diamond is poor, and the hardness is more than, the toughness is insufficient, in the case of long-term cutting high temperature is prone to carbonization phenomenon, can not be used for hard alloy, steel alloy and other materials processing. Polycrystalline cubic boron nitride (PCBN) cutting tools, according to their different composition, can be divided into three types: polycrystalline cubic boron nitride composite, overall polycrystalline cubic boron nitride cutting tools and electroplating cubic boron nitride cutting tools. Among them, polycrystalline cubic boron nitride is a kind of superhard tool material widely used at present, with high wear resistance and thermal stability, suitable for most machining operations, and its material source is more extensive, manufacturing cost is lower than diamond, so it has become the mainstream choice of machining tool material. Artificial polycrystalline diamond is a kind of material formed by combining diamond single crystal materials with metal coagulant under high temperature and high pressure. In terms of hardness, it is between CVD diamond and natural diamond, but its toughness is higher than both, and it also has good compatibility and weldability. At present, artificial polycrystalline diamond is mostly used for the processing and cutting of precision small parts.

3 Application of superhard material cutting tools in mechanical cold working

Application of 3.1 in Turning

When turning steel materials, for the processing of hardened steel, superhard material tools can largely achieve the processing effect of turning instead of grinding. In the cutting process, because its cutting depth is much higher than that of grinding, it greatly improves the efficiency of grinding by car, thus reducing the cost of processing and manufacturing. It should be noted that when processing high-hardness mechanical parts, low-speed or medium-low-speed cutting methods should be adopted to prevent the super-hard material tool from deviating from the cutting track. When turning cast iron parts, for the exhaust valve seat of the engine cylinder head, you can choose copper and molybdenum high chromium alloy cast iron. The processing of this material generally has two processes: turning and milling. When the gray cast iron brake disc is processed with the tool made of PCBN material, the cutting speed is generally 700~2000 m/min.

Application of 3.2 in Milling

When milling aluminum alloy with a tool made of PCD material, the speed can reach 300~4000 m/min; when milling gray cast iron with a tool made of PCBN, the speed is generally in the range of 1000~2000 m/min. At present, the cutting speed of PCD face milling cutter for processing aluminum alloy cylinder head in China has reached 4021 m/min, and its feed speed is 5670 mm/min, which is twice the efficiency of foreign equipment introduced in China in the early 1990 s. The milling speed of CBN face milling cutter for finishing gray cast iron cylinder has also reached 2000 m/min, and its efficiency is 10 times that of carbide face milling cutter. In glass deep processing, PCD spiral inner row liquid surface milling cutter for milling glass is widely used.

Application of 3.3 in Broaching

In broaching processing, the composite involute tooth jumping superhard material broach is generally used to process the workpiece hole. Its advantages mainly include two points: one is the tooth structure of the spline blade with side clearance and the reasonable tooth jumping arrangement, which ensures the manufacturing quality of the broach, and its manufacturing process is not complicated, and the cost is not much different from that of the ordinary composite involute broach. The second is that this processing method ensures the coaxiality between the various surfaces of the inner hole of the workpiece. In the subsequent processing process, the small diameter circular surface can be uniformly selected as the positioning reference, thus simplifying the production of the subsequent inspection mandrel and the positioning mandrel, and at the same time ensuring the position accuracy of each processed surface.

Application of 3.4 in reaming machining

In the reaming of hard casting holes or hardened steel, PCBN electroplated reamers are widely used. This kind of reamer with 9CrSi or 42HRC 45# steel as the matrix, with cutting and front and rear guide parts. The matrix design is very reasonable, and the manufacturing precision is very high, the cutting part is 0.04mm larger than the front part, the workpiece hole depth is less than the length of the cutting zone, the length of the cutting zone is slightly smaller than the length of the rear guide. For example, a factory processing a hardened steel workpiece with a diameter of about 12mm has a hardness of 45HRC, a bottom size of about 12mm, a cylindricity of 0.005mm and a surface roughness of 0.2 μm. At this time, a group of 5 electroplated CBN reamers are generally used for processing. Some engine manufacturers use PCBN or PCD electroplated reamers to hone the main bearing holes of the cylinder block instead of honing. This practice improves the work efficiency several times and the quality of the processed products is stable.

Application of 3.5 in Drilling

The phenomenon of processing engineering ceramics, glass and stone with PCD drill bits is very common. Many glass shops and glass processing companies use PCD drills to drill holes in glass and mirrors. At present, the diameter of common diamond drill bits in China is generally between 3 and 130mm. Glass drill bit has resin diamond, also have inlaid diamond. Processing stone and engineering ceramics with PCD drill bits has the advantages of high efficiency and good quality. Diamond electrolytic drilling can be used to form cylindrical holes (blind holes and through holes), special holes and conical holes on various difficult-to-machine conductive materials. Conclusion: At this stage, the requirements of the mechanical processing department for mechanical processing are becoming more and more complex and diverse. In addition to ensuring the processing quality, higher requirements are also put forward in the processing flatness, smoothness and wear resistance. Traditional metal or metal alloy cutting tools can no longer meet the demanding mechanical processing requirements. The use of superhard material tools can solve this processing problem very well, and the unique advantages of superhard materials themselves have made remarkable achievements in different fields of machining work. In order to realize the large-scale use of superhard material tools and improve the quality and efficiency of machining, the majority of mechanical processing personnel need to continuously strengthen their professional skills learning, in-depth understanding and analysis of the problems and deficiencies of superhard material tools in machining, so as to take corresponding optimization and improvement measures, and provide basic guarantee for the further development of mechanical processing.

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