The hottest turning technology and its application

2022-08-20
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Hard turning technology and its application

turning is the most basic, most extensive and most important process method in the machinery manufacturing industry. It directly affects the efficiency, cost, energy consumption and environmental protection of production. 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

definition of hard turning

generally speaking, hard turning refers to the turning of hard steel as the final processing or finishing process method, so as to 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 the hardness of hardened steel is 55HRC, its strength sb is about 2100 ~ 2600n/mm2. Generally, the rough machining process has been completed before the workpiece is hardened by heat treatment, 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 (55 ~ 65hrc) on lathes or turning centers, and the machining accuracy can reach 5 ~ 10 micror0; m. The root mean square value of surface roughness is less than 20 micr0 on average; m。

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 the machined surfaces is high

hard turning is a clean machining process

in most cases, hard turning does not require 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 ~ 1/2 of that of grinder, and the cost of its 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. Turning and 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

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

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: on the one hand, it has a much lower thermal conductivity than the tool matrix and workpiece material, The thermal effect of the tool matrix is weakened; On the other hand, it can effectively improve the friction and adhesion in the cutting process and reduce the generation of cutting heat. Generally, steel alloy tools with excellent alloy structure are used for hard coating. Compared with cemented carbide tools, they have a great improvement in strength, hardness and wear resistance. For turning workpieces with dry hardness between hrc45 and 55HRC, low-cost coated cemented carbide can realize high-speed turning. In recent years, some manufacturers have greatly improved the performance of coated tools by improving the coating materials and proportion. For example, the blades produced by some manufacturers in the United States and Japan using Swiss AlTiN coating materials and new coating patented technology have a hardness of 4500 ~ 4900hv. 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. The cost is only 30%, and the adhesion is good. It can process Die Steel with hardness of 47 ~ 52hrc at the speed of 498.56m/min

ceramic materials

ceramic cutting tools have the characteristics of high hardness (91 ~ 95hra), high strength (bending strength of 750 ~ 1000 MPa), good wear resistance, good chemical stability, good adhesion resistance, 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 a hardness of 62Hrc. Alumina based ceramics, silicon nitride based ceramics, cermets and whisker toughened ceramics are commonly used. In recent years, through a large number 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 cermet nx2525 developed by Mitsubishi metal company in Japan and the new cermet blade CT series and coated cermet blade series developed by ketman company in Sweden. The diameter of grain structure is as small as 1 m, The bending strength and wear resistance are much higher than ordinary cermet, which greatly broadens the application range of ceramic materials. The silicon nitride ceramic tool developed by Tsinghua University has also reached the international advanced level

cbn

cbn is second only to diamond in hardness and wear resistance, and 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 50HRC), 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

PCBN tools with high CBN content have high hardness, good wear resistance, high compressive strength and impact toughness. Their disadvantages are poor thermal stability and low chemical inertia. They are suitable for cutting heat-resistant alloys, cast iron and iron series sintered metals. The content of CBN particles in the composite PCBN tool is low. Using ceramics as binder, its hardness is low, but it makes up for the poor thermal stability and low chemical inertia of the former material, and is suitable for cutting hardened steel

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 necessary to conduct cost-benefit and processing quality analysis to determine which material is more economical. Figure 1 shows the VB wear of the cutting surface after machining gray cast iron with Al2O3, Si3N4 and CBN tools. As shown in the figure, the cutting performance of PCBN tool material is better than that of Al2O3. When dry cutting hardened steel with Si3N4, the cost of Al2O3 ceramic is lower than that of PCBN material. Ceramic tools have good thermochemical stability, but not as good as the toughness and hardness of PCBN tools. When cutting the workpiece with hardness below 6ohrc and small feed rate, ceramic tool is a better choice. PCBN tools are suitable for workpiece hardness higher than 60HRC, especially for automatic machining and high-precision machining. In addition, under the same flank wear, the residual stress on the workpiece surface after cutting by PCBN tool is also relatively stable than that of ceramic tool, as shown in Figure 2

Figure 1 Relationship between VB and cutting time when cutting gray cast iron

Figure 2 residual stress of cutting hardened steel with ceramics and PCBN tools

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 makes the metal in the front of the edge soften locally, which can effectively reduce the wear of PCBN tools. In addition, PCBN tools should also be used as much as possible in small cutting exploration. Due to the poor thermal conductivity of PCBN tools, the heat in the cutting area has no time to diffuse, and the shear area can also produce obvious metal softening effect and reduce the wear of cutting edges

determination of blade structure and geometry reference

reasonable determination of blade shape and geometric parameters is very important to give full play to the cutting performance of tools. In terms of 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 high strength should be selected as far as possible. For hard turning blades, the radius of the tool tip arc should also be as large as possible, and rough machining with circular and large radius blades. The radius of the tool tip during finish machining is about 0.8 ~ 1.2 m

hardened steel chips are red and soft forged bands, which are brittle, easy to break, and non cohesive. 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 larger than the main cutting force, so the tool should adopt a negative rake angle (go -5) and a large rake angle (ao=10 ~ 15). The main deflection angle depends on the rigidity of the machine tool, which is generally 45 ~ 60, To reduce workpiece and tool chatter

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 work is used to cut high hardness materials, 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 probe 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 rest value depends on the surface roughness value and productivity requirements. When the surface roughness is ra0.3 ~ 0.6 m, hard turning is more economical than grinding

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. However, 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 at the same time, 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

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