Research on the characteristics and application of

Research on the characteristics and application of hard turning

preface

turning is the most basic, most extensive and most important process method in the machinery manufacturing industry, which directly affects production efficiency, cost, energy consumption and environmental pollution. Due to the development of modern science and technology, all kinds of high-strength and high hardness engineering materials are more and more used. The traditional turning technology is not competent or can not realize the processing of some high-strength and high hardness materials at all, but the modern hard turning technology makes it possible and obtains obvious benefits in production

1 hard turning and its characteristics

1) definition of hard turning

generally speaking, hard turning refers to the turning of hardened steel as the final processing or finishing process method, which can avoid the currently widely used grinding technology. Hardened steel usually refers to workpiece materials with martensitic structure, high hardness, high strength and almost no plasticity after quenching. When hardness hrc>55, its strength sb=2100 ~ 2600mpa. Generally, the workpiece has been rough machined before heat treatment and hardening, and only the finish machining is carried out in the hardened state. Fine grinding is the most commonly used processing technology for finishing, but its narrow processing range, large investment, low production efficiency, easy to cause environmental pollution, has been plagued by the economic and effective processing of hardened steel. With the development of machining technology, hard turning instead of grinding has become possible, and obvious benefits have been achieved in production. At present, polycrystalline cubic boron nitride (PCBN) tools, ceramic tools or coated carbide tools are used to cut hardened steel (hardness HRC55 ~ 65) on lathes or turning centers. Wang Chuanfu, the king of battery, is very depressed. The machining accuracy can reach 5 ~ 10 m, and the root mean square value of surface roughness is less than 20 m on average

2) characteristics of hard turning

· high machining efficiency

hard turning has higher machining efficiency than grinding, and its energy consumption is 1/5 of that of ordinary grinding. Hard turning often adopts large cutting depth and high workpiece speed, and its metal removal rate is usually 3 ~ 4 times that of grinding. When turning, one clamping can complete a variety of surface processing (such as the outer circle of the car, the inner hole of the car, the slot of the car, etc.), while grinding requires multiple installations. Therefore, its auxiliary time is short and the position accuracy between surfaces is high

· hard turning is a clean machining process

in most cases, hard turning does not need coolant. In fact, the use of coolant will adversely affect the tool life and surface quality. Because hard turning is formed by annealing and softening the material in the shear part. If the cooling rate is too high, this effect caused by cutting force will be reduced, so as to increase mechanical wear and shorten tool life. At the same time, hard turning can eliminate the devices related to coolant, reduce production costs, simplify the production system, and the formed chips are clean and easy to recycle

· less equipment investment, suitable for flexible production requirements

at the same productivity, the investment of lathe is 1/3 ~ 1/20 of that of grinder, and the cost of auxiliary system is also low. For small batch production, hard turning does not need special equipment, while large batch processing of high-precision parts requires CNC machine tools with good rigidity, high positioning accuracy and repeated positioning accuracy

lathe itself is a flexible processing method with a wide range of processing. Workpiece clamping is fast. Using modern CNC lathes equipped with a variety of tool turntables or tool libraries, it is easy to realize the processing conversion between two different workpieces, and hard turning is particularly suitable for this kind of processing. Therefore, compared with grinding, hard turning can better meet the requirements of flexible production

· hard turning can make parts obtain good overall machining accuracy

glass magnesium plate JC688 ⑴ 998 most of the heat produced in hard turning is taken away by chips, which will not produce surface burns and cracks like grinding. It has excellent machining surface quality, accurate machining roundness, and can ensure high position accuracy between machining surfaces

2 conditions for hard turning

1) tool materials for hard turning and their selection

· coated cemented carbide tools are coated with one or more layers of tin, TiCN, TiAlN and Al2O3 with good wear resistance on cemented carbide tools with good toughness. The thickness of the coating is 2 ~ 18 M. the coating usually plays the following two roles: ① it has a much lower thermal conductivity than the tool matrix and workpiece material, The thermal effect of the tool matrix is weakened; ② It can effectively improve the friction and adhesion in the cutting process and reduce the generation of cutting heat. Compared with cemented carbide tools, coated cemented carbide tools have greatly improved in strength, hardness and wear resistance. For turning workpieces with hardness ranging from hrc45 to 55, low-cost coated carbide tools can realize high-speed turning. In recent years, some manufacturers have greatly improved the properties of coated tools by improving the coating materials and proportion. For example, some manufacturers in the United States and Japan use Swiss AlTiN coating materials and new coating patented technology to produce blades. The HV hardness is as high as 4500 ~ 4900. When the turning temperature is as high as 1500 ~ 1600 ℃, the hardness is still not reduced and does not oxidize. The service life of the blades is 4 times that of the general coated blades, while the cost is only 50%, and the adhesion is good. It can process Die Steel with hardness hrc47 ~ 52 at the speed of 498.56m/min

· ceramic materials

ceramic tools have the characteristics of high hardness (hardness hra91 ~ 95), high strength (bending strength 750 ~ 1000MPa), good wear resistance, good chemical stability, good anti Hertz performance, low friction coefficient and low price. When in normal use, the durability is extremely high, and the speed can be increased by 2 ~ 5 times than that of cemented carbide. It is especially suitable for processing high hardness materials, finishing and high-speed processing. It can process all kinds of hardened steel and hardened cast iron with hardness of HRC62. Alumina based ceramics, silicon nitride based ceramics, cermets and whisker toughened ceramics are commonly used. In recent years, through a lot of research, improvement and adoption of new manufacturing processes, the bending strength and toughness of ceramic materials have been greatly improved, such as the new type of cermet nx2525 developed by Mitsubishi metal company in Japan and the new CT series and coated cermet blade series developed by Sandvik company in Sweden. The diameter of grain structure is smaller than 1 m, and the bending strength and wear resistance are much higher than ordinary cermet, The application scope of ceramic materials has been greatly expanded. The silicon nitride ceramic tool developed by Tsinghua University has also reached the international advanced level

· CBN

the hardness and wear resistance of CBN are second only to diamond, and it has excellent high-temperature hardness. Compared with ceramic tools, its heat resistance and chemical stability are slightly poor, but its impact strength and crushing resistance are better. It is widely used for cutting hardened steel (above HRC50), pearlitic gray cast iron, chilled cast iron and superalloys. Compared with cemented carbide tools, its cutting speed can even be increased by an order of magnitude

in the application field of cutting gray cast iron and hardened steel, ceramic tools and CBN tools can be selected at the same time, so it is very necessary to conduct cost-benefit and processing quality analysis to determine which material is more economical. By analyzing the workpiece with cutting hardness below HRC60 and small feed rate, ceramic tool is a better choice. PCBN tools are suitable for workpiece hardness higher than HRC60, especially for automatic machining and high-precision machining. In addition, under the same flank wear, the residual stress on the workpiece surface after cutting with PCBN tool is also relatively stable than that with ceramic tool

dry cutting hardened steel with PCBN tools should also follow the following principles: choose a large cutting depth as far as possible when the rigidity of the machine tool allows, so that the heat generated in the cutting area softens the metal in the front edge area locally, which can effectively reduce the wear of PCBN tools. In addition, when using PCBN tools in small cutting depth, it should also be considered to use PCBN tools. The poor thermal conductivity makes it impossible for the heat in the cutting area to diffuse, and the shear area can also produce obvious metal softening effect, Reduce the wear of cutting edge

2) determination of blade structure and geometric parameters

reasonable determination of blade shape and geometric parameters is crucial to give full play to the cutting performance of the tool. According to the tool strength, the tip strength of various blade shapes from high to low is: round, 100 ° diamond, square, 80 ° diamond, triangle, 55 ° diamond, 35 ° diamond. After the blade material is selected, the blade shape with the highest strength should be selected. The hard turning blade should also choose the largest tip arc radius, rough machining with circular and large radius blades, and the tip radius during finish machining is 0.8 ~ 1.2 m

hardened steel chips are red and soft forged bands, which are brittle, easy to break and do not bulge. Generally, there is no chip buildup on the cutting surface, and the machined surface quality is high. However, the cutting force of hardened steel is relatively large, especially the radial cutting force is greater than the main cutting force, so the negative rake angle should be used for the tool（ γ 0 ≥ -5 °) and larger rear angle（ α 0=10 ~ 15 °), the main deflection angle depends on the rigidity of the machine tool, which is generally 45 ~ 60 ° to reduce the chatter of the workpiece and tool

3) selection of cutting parameters

the higher the hardness of the workpiece material, the smaller the cutting speed should be. The suitable cutting speed for hard turning finishing is 80 ~ 200m/min, and the common range is 10 ~ 150m/min. Large cutting depth or strong intermittent cutting of high hardness materials should be adopted, and the cutting speed should be maintained at 80 ~ 100M/min. Generally, the cutting depth is 0.1 ~ 0.3mm. When the machining surface roughness is required to be high, a small cutting depth can be selected, but it should not be too small, and it should be appropriate. The feed rate can usually be 0.05 ~ 0.25mm/r, and the specific value depends on the surface roughness value and productivity requirements. When the surface roughness is ra0.3 ~ 0.6 m, hard turning is much more economical than grinding

4) requirements for process system

in addition to selecting reasonable tools, hard turning has no special requirements for lathes or turning centers. If the rigidity of lathes or turning centers is sufficient and the required accuracy and surface roughness can be obtained when processing soft workpieces, they can be used for the processing of hardened steel. In order to ensure the stability and continuity of turning operation, the common method is to use rigid clamping device and medium rake tool. It is generally believed that hard turning requires a lathe with high rigidity, that is, the key to hard turning is that the machine tool has enough rigidity, and the cutter, workpiece and clamping device are compact and have the same rigidity. If the positioning, support and rotation of the workpiece can be kept quite stable under the action of cutting force, the existing equipment can be used for hard turning

3 application of hard turning technology

after 10 years of development, popularization and application, hard turning technology has achieved great economic and social benefits. The following illustrates the promotion and application of hard turning technology in production with examples from roll processing and other industries

1) roll processing industry

more than a dozen large roll enterprises in China have used hard turning technology to cut and process all kinds of rolls such as chilled cast iron and hardened steel, such as rough turning, rough turning and fine turning, and have achieved good benefits. The average processing efficiency is increased by 2 ~ 6 times, and significant benefits of saving 50 ~ 80% of processing hours and power are achieved. as