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Machining surface quality Mechanical Parts destruction, the general always start from the surface layer. Product performance, in particular its reliability and durability, to a large extent, depends on the surface layer of quality. Research machining surface quality objective is to master the mechanical processes of various technological factors on the surface quality of rules for the use of these laws to control the machining process, and ultimately improve the quality of the surface, improve product performance goals. Mechanical Parts destruction,9598
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the general always start from the surface layer. Product performance, in particular its reliability and durability, to a large extent, depends on the surface layer of quality. Research machining surface quality objective is to master the mechanical processes of various technological factors on the surface quality of rules for the use of these laws to control the machining process, and ultimately improve the quality of the surface, improve product performance goals. 1. machining surface quality of the machine using the performance impact (1) the quality of the surface wear resistance of one. Surface roughness right wear resistance of a just processed two pairs of contact between the surface, In the initial stage, only the rough surface of the peak contact, the actual contact area is much smaller than theoretical contact area, mutual access to the peaks of a huge stress units, the actual contact area of plastic deformation occurs. Elastic deformation and peak between the Department of shear failure, causing serious wear. General wear components can be pided into three stages, the initial wear stage, normal wear stage and dramatic stage wear. Surface roughness of the surface wear of parts of the significant influence. Generally speaking the smaller the value of surface roughness, wear better. However, the surface roughness was too small, not easily stored lubricants, surface contact between the high molecular bonding, but increased wear. Therefore, the contact surface roughness is one of the best, the value of work and parts of the work load increase, Wear initial increase and the best surface roughness values have also increased. 2. Coldwork hardening of the surface wear resistance of the surface of the cold-hardened so that friction surface layer metal microhardness increase , it will generally improve the wear resistance. It is not the higher the level of cold-hardening, the higher the wear resistance, This is because excessive cold-hardening metal will cause excessive tissue osteoporosis, and even the cracks and spalling of the metal surface. so wear resistance decreased. (2) the quality of the surface of metal fatigue by alternating load fatigue resulting from the damage occurred in parts often Table Chilled surface and the surface layer below, the surface quality of parts of fatigue significant influence. 1. Surface roughness on the impact of fatigue in alternating loads, The concave surface roughness Valley location easily lead to stress concentration, fatigue crack. Surface roughness values greater surface profile scar is, the smaller the radius of the end profile, anti-fatigue damage the end of the more capacity worse. 2. Residual stress, cold-hardening effect on the fatigue strength of the parts I stress fatigue strength of great influence. Surface layer of residual tensile stress fatigue cracks would expand and accelerate fatigue damage; and the surface layer of residual stress can prevent fatigue crack growth, delay fatigue damage have chilled surface residual stress generally associated with the rise, can prevent cracks and prevent the expansion of cracks has been to improve the fatigue strength favorable. (3) the quality of surface corrosion resistance of the corrosion resistance of components depends to a large extent on the surface roughness. Surface roughness values larger, Au Valley accumulation of corrosive substances will be. The more poor corrosion resistance. Surface layer of residual tensile stress will produce stress corrosion cracking, reducing the wear resistance of components, and the residual compressive stress is to prevent stress corrosion cracking. (4) the quality of the surface with the quality of surface roughness values will affect the size of the support surface with the quality. For clearance fit, roughness value of the General Assembly so that the wear increased, the gap increased, the requirements of the damage with nature. For the interference fit, the assembly process convex surface part of a check-Ping, the actual surplus was reduced too, reduced with the connecting pieces of strength. 2. effects of surface roughness factor (1) Cutting influence of surface roughness of a factor. Tool geometry of the cutter Reproducibility relative to the workpiece for Feed Movement, in the surface layer leaving a residue cutting area, its shape when the cutter geometry Reproducibility. Reduced feed rate, the main angle, and vice angle increases Corner Radius, may reduce residual height. In addition, the increase appropriate tool in the past to reduce the cutting of plastic deformation, reasonable choice lubricant and improve the quality of grinding tool to reduce the cutting of plastic deformation and inhibit tumor knife and stabbed the production scales, Surface roughness is reduced value of the effective measures. 2. Workpiece the nature of the material processed plastic materials, tools from the extrusion of metals produced plastic deformation, Moreover, chip tool and workpiece forced separation of tear, the surface roughness values increase. Workpiece material toughness better, the plastic deformation of metals bigger, the more rough surface. Processing of brittle materials, his chip was broken granular, as chip and the collapse of the broken surface left many Ma, roughen the surface. 3. Cutting consumption (2) grinding surface roughness effects of factors like cutting surface roughness at the formation process , the grinding surface roughness also formed by the surface geometry factors and plastic deformation of metals to decide. Grinding surface roughness effects of the main factors : Wheel size Wheel hardness grinding wheel dressing grinding radial velocity feed rate and the number of workpiece Mitsuma circumference feed speed and Feed volume axial cooling lubricant three affected surface layer physical and mechanical properties of the factors in machining, Due to the workpiece cutting force and cutting heat, the surface layer of metal physical properties have changed, The main change is the surface layer of metal microhardness changes, the changes in microstructure and residual stress from arising. As grinding as the plastic deformation and thermal cutting edge than the more serious cutting, thus grinding the surface layer above the physical mechanical properties of three changes to be very high. (1) surface layer coldwork sclerosis. Coldwork sclerosis and Assessment machining parameters for the process of cutting force role in the plasticity Deformation and lattice distortion, distortion, the grain between the shear slip, elongated grains and fibrosis, or even broken, All these will add to the metal surface layer hardness and strength increase, a phenomenon known as the cold-hardening (or enhanced). Surface layer metal reinforcing findings will increase resistance to deformation of the metal, reducing the metal plastic. metal of the physical nature will also change. Coldwork sclerosis by the metal-at the high-energy state of instability, only one possible, Metal instability it is necessary to the relatively stable state of transformation, a phenomenon known as dilution. Weakening the role of the size depends on the level of temperature, duration of temperature and the length of the degree of strengthening. Because the metal machining process and the same time be hot, and in After processing the metal surface depends on the final nature of strengthening and weakening of the combined results. Evaluation coldwork sclerosis three indicators that the metal surface microhardness HV, hardening layer depth h and N sclerosis. 2. Cold-working hardening of the main factors blunt the cutting edge radius increased, the surface of the metal compression increased, plastic deformation increased, resulting in enhanced Chilled. Tool flank wear increased flank with the processing of surface friction intensified, plastic deformation increased, resulting in enhanced Chilled. Cutting speed increases, tool and workpiece role shorten the time so that the plastic deformation expand the depth decreases, Chilled depth decreases. Cutting speed increases, cutting heat in the surface layer of shortening the time for music, Chilled level will increase. Feed rate increases, cutting force also increased, the surface of the plastic deformation of metal intensified, Chilled greater role. Workpiece material greater plasticity, the more serious Chilled phenomenon. (2) surface layer material microstructure changes so that when the heat was cutting surface temperature over the phase transition temperature. The metal surface microstructure there will be changes. 1. Grinding mill was burned when the workpiece surface temperature phase transition temperature above, metal surface microstructure occurred changes so that the surface of metal strength and hardness reduction, and associated with the residual stress generated. even microscopic cracks, a phenomenon known as grinding burn. In the grinding hardened steel, the following three possible burn : Tempering grinding burn if the temperature does not exceed hardened steel phase transition temperature, but over the martensitic transformation temperature, workpiece surface of the metal will be tempered martensite into lower hardness tempering Organization (sorbite's body or giving), Tempering such as burns burns. If grinding burn quenching temperature exceeds the phase transition temperature, coupled with the cooling liquid chill, Second occurred metal surface hardening and surface metals secondary hardening martensitic, its hardness than the original tempered martensite, in its class, cooling slowly a hardness than the original tempered martensite low temper (or giving sorbite's CARICOM), Quenching such as burns burns. If grinding burn annealing temperature exceeds the phase transition temperature, and the grinding coolant region without access, will have a metal surface annealing, surface hardness of sharp decline, such as annealing burns burns. 2. Improved means of grinding burn grinding heat caused grinding burn is the root of it to improve by grinding burn two ways : First, as far as possible to reduce the grinding heat generated; Second is to improve the cooling conditions as possible, to produce less heat into the workpiece. Correct choice for a reasonable choice Cutting Wheel dosage to improve cooling conditions (3) surface layer of residual stress one. Have the original residual stress a. by cutting the surface layer of metal with plastic deformation, The metal surface hematocrit increased only because of the plastic deformation on the surface of metal, while the metal surface of the hematocrit increases, volume expansion, we will inevitably be linked with it by the metal layer prevents, So on the surface of the metal layer of residual stress and the metal layer have residual tensile stress. B. machining, cutting area there will be a lot of cutting heat generated c. different microstructures with different density, also have different hematocrit If the surface layer of metal produced changes in the microstructure, Hematocrit metal surface changes will inevitably be linked with the base metal obstacles, which will have a residual stress. 2. Parts major work surface machining processes ultimate choice of the main components work surface processes processing methods final choice critical, because ultimately the work processes left by the surface residual stress will have a direct impact on the use of machine parts performance. Choose parts final major work surface machining processes, shall consider the main parts of the surface of specific working conditions and possible failure modes. AC load, machine parts on the surface of the local micro-cracks because of the role of stress so that the original crack expanded, finally led to parts fracture. Improve resistance from parts of fatigue damage point of view, The final surface processes should choose the surface residual compressive stress of processing methods.