0 INTRODUCTION Turning machining is the most basic, widest and most important process method in the mechanical manufacturing industry. It directly affects production efficiency, cost, energy consumption and environmental pollution. Due to the development of modern science and technology, a variety of high-strength, high-hardness engineering materials are increasingly being used. Traditional turning techniques are difficult or impossible to achieve for some high-strength, high-hardness materials, and modern Hard turning technology makes it possible and achieves significant benefits in production. 1 Hard turning and its characteristics Definition of hard turning Generally referred to as hard turning refers to the turning of hardened steel as the final processing or finishing process, which avoids the currently widely used grinding technology. Hardened steel usually refers to a martensitic microstructure after quenching, high hardness, high strength, almost no plastic workpiece material, hardness HRC> 55, its strength sb = 2100 ~ 2600MPa. Normally, the workpiece has been rough-finished before the heat treatment hardens, and only the finish is performed in the hardened state. Grinding is the most commonly used finishing technology for finishing, but its narrow processing range, large investment, low production efficiency, and easy to cause environmental pollution have plagued the economic and efficient processing of hardened steel. With the development of processing technology, hard turning instead of grinding has become possible and has achieved significant benefits in production. At present, the use of polycrystalline cubic boron nitride (PCBN) tools, ceramic tools or coated carbide tools in the lathe or turning machining center on the hardened steel (hardness HRC55 ~ 65) for cutting, the processing accuracy of up to 5 ~ 10μm, surface roughness root mean square value less than 20μm on average. The characteristics of hard turning High machining efficiency Hard turning has higher processing efficiency than grinding, which consumes 1/5 of the energy of ordinary grinding. Hard turning often uses large cutting depths and high workpiece speeds, and the metal removal rate is usually 3 to 4 times that of grinding. In the turning process, a plurality of surface processes (such as outer circle, interior hole, and slot) can be completed in a single setting. However, grinding requires multiple installations. Therefore, the auxiliary time is short and the position accuracy between surfaces is high. Hard turning is a clean process In most cases, hard turning requires no coolant. In fact, the use of coolant can adversely affect tool life and surface quality. Because, hard turning is to form a cutting by annealing and softening the material of the shearing part. If the cooling rate is too high, this effect caused by the cutting force will be reduced, thereby increasing the mechanical wear and shortening the tool life. At the same time, hard turning eliminates the need for cooling-related devices, lowers production costs, simplifies the production system, and forms chips that are clean and easy to recycle. Low equipment investment, suitable for flexible production requirements When the productivity is the same, the investment in the lathe is 1/3 to 1/20 of the grinding machine, and the auxiliary system cost is also low. For low-volume production, hard turning requires no special equipment, and high-volume machining of high-precision parts requires CNC machines with good rigidity, positioning accuracy, and high repeatability. The lathe itself is a flexible machining method with a wide processing range. The workpieces are fastened and the machining between 2 different workpieces can be easily converted using a modern CNC lathe equipped with a variety of tool discs or magazines. Hard turning is particularly suitable for this purpose. Processing. Therefore, hard turning can better adapt to flexible production requirements than grinding. Hard turning allows the part to achieve a good overall machining accuracy. Most of the heat produced during turning is carried away by the chips, without surface burns and cracks like grinding, with excellent machining surface quality, and with an accurate roundness of machining. , Can guarantee the high position precision between the processing surfaces. 2 Hard turning conditions Hard turning tool materials and their selection Carbide coated hard alloy tools are coated with one or more TiN layers with good wear resistance on tough tough carbide tools. TiCN, TiAlN and Al2O3, etc., coating thickness of 2 ~ 18μm, the coating usually plays the following two aspects: 1 has a much lower thermal conductivity than the tool matrix and the workpiece material, weakening the thermal effect of the tool matrix; 2 It can effectively improve the friction and sticking action of the cutting process and reduce the generation of cutting heat. Compared with cemented carbide tools, coated carbide tools have greatly improved in strength, hardness and wear resistance. For turning of HRC 45-55 workpieces, low-cost coated carbide tools enable high-speed turning. In recent years, some manufacturers have also improved the properties of coated tools by improving the coating material and ratio.
Such as the United States, Japan, some manufacturers use Swiss AlTiN coating materials and new coating patented technology blade, HV hardness up to 4500 ~ 4900, in the turning temperature up to 1500 ~ 1600 °C, the hardness is not reduced, no oxidation, the blade life is generally 4 times the coated blade, and the cost is only 50%, and the adhesion is good. It can process die steel with hardness HRC 47-52 at a speed of 498.56m/min.
Ceramic material ceramic tool has high hardness (hardness HRA91～95), high strength (bending strength 750～1000MPa), good wear resistance, good chemical stability, good anti-Hegel performance, low friction coefficient and low price etc. Features. When used normally, the durability is extremely high, and the speed can be increased by 2 to 5 times than that of cemented carbide. It is especially suitable for high hardness materials processing, finishing and high speed machining. It can process various hardness steels and hardened cast irons with hardness of HRC62. Commonly used alumina-based ceramics, silicon nitride-based ceramics, metal ceramics and whisker toughened ceramics. In recent years, through a great deal of research, improvement, and adoption of new manufacturing processes, the flexural strength and toughness of ceramic materials have been greatly improved, such as the new type of metal ceramic NX2525 developed by Mitsubishi Metals of Japan and developed by Sandvik, Sweden. The new CT series of metal ceramic inserts and the coated ceramic ceramic inserts series have a grain size as small as 1 μm or less, bending strength and wear resistance are much higher than ordinary cermets, greatly broadening the application of ceramic materials. The silicon nitride ceramic material cutter successfully developed by Tsinghua University has also reached the international advanced level. The hardness and wear resistance of CBN CBN is second only to diamond, and it has excellent high-temperature hardness. Compared with ceramic tools, its heat resistance and chemical stability are slightly inferior, but impact strength and crush resistance are better. It is widely used in the cutting of hardened steel (HRC50 and above), pearlite gray cast iron, chilled cast iron and high-temperature alloys. Compared with cemented carbide tools, the cutting speed can even be increased by an order of magnitude. The PCBN tool with high CBN content has high hardness, good wear resistance, high compressive strength and good impact toughness. Its disadvantages are poor thermal stability and low chemical inertness. It is suitable for cutting of heat-resistant alloys, cast iron and iron-based sintered metals. . The composite PCBN tool has a lower content of CBN particles, and uses ceramics as a nucleating agent, which has a low hardness, but makes up for the shortcomings of poor thermal stability and low chemical inertia of the former material, and is suitable for cutting and processing of hardened steel. In cutting grey and hardened steel applications, ceramic tools and CBN tools are available for simultaneous selection. Therefore, it is necessary to perform cost-effective analysis and processing quality analysis to determine which material is more economical. By analyzing workpieces with cutting hardness below HRC60 and small feeds, ceramic tools are a better choice. PCBN tools are suitable for workpiece hardness higher than HRC60, especially for automated processing and high-precision machining. In addition, under the same flank wear condition, the residual stress on the workpiece surface after the PCBN cutting tool is also relatively stable than the ceramic tool. The use of PCBN tools for dry cutting of hardened steels should also follow the following principles: Select a large depth of cut as much as possible with the rigidity of the machine, so that the heat generated in the cutting zone will locally soften the metal in the blade area, which can effectively reduce the wear of PCBN tools. PCBN tools should also be considered when the cutting depth is small. The thermal conductivity is so poor that the heat in the cutting zone is too late to spread, and the shear zone can also produce a significant metal softening effect and reduce the wear of the cutting edge. The proper configuration of the blade geometry and geometric parameters to determine the shape of the blade and the geometric parameters are critical to the full play of the cutting performance of the tool. According to the cutter strength, the blade tip strengths of various blade shapes are: round, 100° diamond, square, 80° diamond, triangle, 55° diamond, and 35° diamond. After the blade material is selected, the blade shape with the highest possible strength should be selected. Hard turning inserts should also choose the largest radius of the tool tip arc, rough machining with round and large radius blades, and the radius of the tool tip when finishing is 0.8-1.2 μm. Hardened steel chips are red and soft forging belts, brittle, easy to break, no burglary, generally no built-up edge on the cutting surface, the surface quality of processing is high, but the hardened steel cutting force is relatively large, especially The radial cutting force is even larger than the main cutting force, so the tool should adopt negative rake angle (g0≥-5°) and large relief angle (a0=10～15°). The main deviation angle depends on the rigidity of the machine tool. Take 45 ~ 60 °, to reduce workpiece and tool chatter. The choice of cutting parameters The higher the hardness of the workpiece material, the lower its cutting speed should be. The suitable cutting speed for hard turning finishing is 80～200m/min, the common range is 10～150m/min, the high hardness material is cut with large depth of cut or strongly interrupted, the cutting speed should be maintained at 80～100m/min. In general, the depth of cut is 0.1 to 0.3 mm. The roughness of the machined surface can be selected with high cutting depth, but it should not be too small. The feed rate can usually be selected from 0.05 to 0.25 mm/r, depending on the surface roughness value and productivity requirements. When the surface roughness is Ra 0.3 to 0.6 μm, hard turning is much more economical than grinding. The requirements for the process system In addition to the selection of a reasonable tool, hard turning does not require special requirements for lathes or turning centers. If the lathe or turning center is sufficiently rigid and the required precision and surface roughness are obtained when machining soft workpieces, ie Can be used for hardened steel processing. In order to ensure a smooth and continuous turning operation, a common method is to use a rigid clamping device and a medium front angle tool. It is generally believed that hard turning requires a highly rigid lathe, that is, the key to hard turning is that the machine tool has enough rigidity, and the tool, workpiece, and clamping device are compact and have the same rigidity. If the workpiece is positioned, supported and rotated under the action of the cutting force, it can remain fairly stable and existing equipment can be used for hard turning. 3 Application of Hard Turning Technology After 10 years of development and application of hard turning technology, hard turning technology has gained tremendous economic and social benefits. The following examples illustrate the popularization and application of hard turning technology in the production industry. In the domestic roll processing industry, more than a dozen large-scale roll mills in China have used hard-bending technology to carry out cutting, processing, and roughing of chilled cast iron, hardened steel, and other types of rolls, all of which have achieved good results. On average, the processing efficiency is increased by 2 to 6 times, and a significant benefit of saving processing time and electricity by 50% to 80% is achieved. For example, at the Wuhan Iron and Steel Roll Company, the cutting speed is increased three times for chilled cast iron rolls with a hardness of HS 60 to 80, and the cutting speed is increased by a factor of three. One roll for each vehicle saves electricity and labor costs by more than 400 yuan, saving tooling costs. Nearly 100 yuan, has achieved great economic benefits. Such as Weifang Institute of Mechanical and Electrical Experimental Center, using FD22 metal ceramic tool turning HRC58 ~ 63 86CrMoV7 hardened steel roll (v = 60m/min, f = 0.2mm / r, cutting depth ap = 0.8mm), single-blade continuous cutting roll The path is up to 15000m (VCmax=0.2mm), which satisfies the requirements for grinding with precision turning cars. Industrial pump processing industry Currently, 70% to 80% of domestic retting pump production plants have adopted hard turning technology. Reel pump is widely used in mines, electric power and other industries. It is an urgently needed product at home and abroad. Its sheathing and guard plates are Cr15Mo3 hard iron castings with hardness HRC6367. In the past, it was difficult to turn it because of various tools. Therefore, it was necessary to adopt a process of roughening after annealing, softening, and then quenching. After the hard processing technology was adopted, the problem of primary hardening processing was solved smoothly, and two processes of annealing and quenching were eliminated, saving a lot of man-hours and electricity. The automotive processing industry often encounters machining problems in crankshafts, camshafts, drive shafts, tool measuring tools, and equipment maintenance in mass production industries such as automobiles and tractors. For example, a locomotive and rolling stock factory in China needs to process the inner ring of the bearing during equipment maintenance. The hardness of the bearing inner ring (material GCr15) HRC60, inner ring diameter is 285mm, grinding process is adopted, and the grinding allowance is uneven, requiring 2h Only hard turning can be used, and an inner ring can be machined in just 45 minutes. 4 Conclusion After many years of research and exploration, China's hard turning technology has made great progress, but hard turning technology is not widely used in production. The main reasons are: (1) The production companies and operators do not know enough about the effects of hard turning. It is generally believed that hard materials can only be ground. (2) The tool cost is considered too high. The initial tooling cost for hard turning is more expensive than ordinary cemented carbide (such as CBN is ten times more expensive than ordinary cemented carbide), but the cost of allocating to each part is lower than that of grinding, and the effect is better than ordinary hard. The quality of the alloy is much better. (3) The study of the hard turning machining mechanism is not enough; (4) The specifications of the hard turning machining are insufficient to guide the production practice. Therefore, in addition to the in-depth study of the hard turning mechanism, it is necessary to strengthen the training of hard turning machining knowledge, successful experience demonstrations, and strict operating specifications, so that this highly efficient and clean processing method is more applied to production. At present, if the combination of hard turning and fine grinding, the cost of processing a general part will be 40% to 60% lower than the cost of roughing and finishing.