SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING LESSON THREE SURFACE QUALITY Surface quality is another aspect of machining accuracy. It consists of the surface finish and the physic mechanical state of the surface layer. It affects the proper functioning and service life of the parts Surface finish Machining accuracy is expressed quantitatively in machining error. Surface finish is expressed quantitatively in surface roughness. The surface roughness on a machined workpiece sur face is caused primarily by the geometrical factors(cutting tool geometry and cutting feed), the plastic deformation of the suf face material, as well as the vibration of the MFTW ystem during machining 「几何形状
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING LESSON THREE SURFACE QUALITY Surface quality is another aspect of machining accuracy. It consists of the surface finish and the physicmechanical state of the surface layer. It affects the proper functioning and service life of the parts. Surface Finish Machining accuracy is expressed quantitatively in machining error. Surface finish is expressed quantitatively in surface roughness. The surface roughness on a machined workpiece surface is caused primarily by the geometrical factors (cutting tool geometry and cutting feed), the plastic deformation of the surface material, as well as the vibration of the MFTW system during machining. 几何形状
SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING The sur face roughness of a workpiece after machining depends upon many technological factors, such as the workpiece material, tool material, tool geometry, cutting conditions(cutting peed and feed), rigidity of the MFTW system, cooling condition in machining process, and so on. The surface roughness is a kind of micro geometrical deviation or micro-irregularity of workpiece surfaces. It appears on all machined workplece sur faces no matter how smooth they look to the naked eye /In regard to the criteria for evaluating surface roughness, refer/to other textbooks, ISO and other standards and references.不规则性,不平度 criterion The surface roughness of machined parts influences their performance to a great extent. In order to ensure product quality. to improve its service life, and to reduce its production cost, the sur face roughness of machine parts has to be specified accurately in design and carefully controlled in manufacturing ISO: International Standardization Organization
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING The surface roughness of a workpiece after machining depends upon many technological factors, such as the workpiece material, tool material, tool geometry, cutting conditions (cutting speed and feed), rigidity of the MFTW system, cooling condition in machining process, and so on. The surface roughness is a kind of micro geometrical deviation or micro-irregularity of workpiece surfaces. It appears on all machined workpiece surfaces no matter how smooth they look to the naked eye. In regard to the criteria for evaluating surface roughness, refer to other textbooks, ISO, and other standards and references. The surface roughness of machined parts influences their performance to a great extent. In order to ensure product quality, to improve its service life, and to reduce its production cost, the surface roughness of machine parts has to be specified accurately in design and carefully controlled in manufacturing. 不规则性,不平度 criterion ISO:International Standardization Organization
SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING Physic-Mechanical State of Surface Layer intrusion In machining process, the metal nearest the‖硬挤性 workpiece surface deforms plastically. This is due to tool rake the intrusion of the cutting tool rake to the workpiece‖刀具前面 surface material, the friction between the tool relief刀具前角 and the machined surface, and the effect of the tool tool relief nose radius. This results in the surface layer on the刀具后面 workpiece in quite a different way from the internal具后角 material of the workpiece after machining. Until now, tool nose the evaluation of the physic-mechanical state of the T3se surface layer is still in the experimental investigation cold stage. Complete standards for the evaluation do not hardening exist. However, it is known that the variations of the‖冷作硬化 material on the surface layer from the original metalresidual are caused by cold hardening, metallographical stress change, and residual stress 金相组织变化 残余应力
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING Physic-Mechanical State of Surface Layer In machining process, the metal nearest the workpiece surface deforms plastically. This is due to the intrusion of the cutting tool rake to the workpiece surface material, the friction between the tool relief and the machined surface, and the effect of the tool nose radius. This results in the surface layer on the workpiece in quite a different way from the internal material of the workpiece after machining. Until now, the evaluation of the physic-mechanical state of the surface layer is still in the experimental investigation stage. Complete standards for the evaluation do not exist. However, it is known that the variations of the material on the surface layer from the original metal are caused by cold hardening, metallographical change, and residual stress. intrusion 硬挤性 tool rake 刀具前面 刀具前角 tool relief 刀具后面 刀具后角 tool nose 刀尖 cold hardening 冷作硬化 residual stress 金相组织变化 残余应力
SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING The cold hardening of the surface layer material is caused by plastic deformation of the material, resulting in the increase of its microhardness. The parameters which characterize the cold hardening are shown below The hardened depth h, which is the depth of the sur face layer, deformed plastically in the machining process The microhardness H. The degree of cold hardening N which is the ratio of the microhardness increases the surface layer to the microhardness of the original metal, i.e 6.3 minus N=[(HHo)Ha]×1009 where ho stands for the microhardness of the original metal
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING The cold hardening of the surface layer material is caused by plastic deformation of the material, resulting in the increase of its microhardness. The parameters which characterize the cold hardening are shown below: The hardened depth h, which is the depth of the surface layer, deformed plastically in the machining process. The microhardness H. The degree of cold hardening N, which is the ratio of the microhardness, increases the surface layer to the microhardness of the original metal, i.e., N = [(H-H0 )/H0 ] × 100% 6.3 where H0 stands for the microhardness of the original metal. minus
SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING A large part of the energy consumed in machining is transformed into heat. Which results in the increase of the temperature in the cutting or grinding area, especially on the workpiece sur face being machined. In general cutting processes, most of the heat generated is carried away by the chips, and the temperature of the workpiece surface is not very high. However, in the machining processes consuming a very large amount of energy for instance as in grinding, the temperature of the workpiece sur face may reach or exceed the critical point of phase change of workpiece material. This condition causes the metallographical variation of the surface material
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING A large part of the energy consumed in machining is transformed into heat. Which results in the increase of the temperature in the cutting or grinding area, especially on the workpiece surface being machined. In general cutting processes, most of the heat generated is carried away by the chips, and the temperature of the workpiece surface is not very high. However, in the machining processes consuming a very large amount of energy, for instance as in grinding, the temperature of the workpiece surface may reach or exceed the critical point of phase change of workpiece material. This condition causes the metallographical variation of the surface material
SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING The residual stress is generated in the surface layer after machining. The causes of the generation of the residual stress are (i) The material of the workpiece sur face expands while it is heated by the cutting or grinding heat, and it contracts when it is cooled The expansion and contraction are resisted by the internal material of the workpiece which results in the residual tensile stress in the surface layer after machining,(ii) The surface material plastically deforms under the extrusion and friction of the tool. while the 挤压 internal material close to the surface layer elastically deforms. After machining, the recovery of the elastic deformation of the interna material is restricted by the sur face material, which has deformed plastically. This results in the residual stress, usually compressive residual stress, in the surface layer;(iii The metallographical variation of the surface layer leads to its volume change, either expansion or contraction, which is restricted by the internal material and results in the residual stress(compressive or tensile)in the surface laver
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING The residual stress is generated in the surface layer after machining. The causes of the generation of the residual stress are: (i) The material of the workpiece surface expands while it is heated by the cutting or grinding heat, and it contracts when it is cooled. The expansion and contraction are resisted by the internal material of the workpiece which results in the residual tensile stress in the surface layer after machining; (ii) The surface material plastically deforms under the extrusion and friction of the tool, while the internal material close to the surface layer elastically deforms. After machining, the recovery of the elastic deformation of the internal material is restricted by the surface material, which has deformed plastically. This results in the residual stress, usually compressive residual stress, in the surface layer; (iii) The metallographical variation of the surface layer leads to its volume change, either expansion or contraction, which is restricted by the internal material and results in the residual stress (compressive or tensile) in the surface layer. 挤 压
SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING Influence of Surface Quality on the Performance of Machined parts A field failure of a machined part caused by its wear, fatigue or corrosion, etc, starts from the part sur face in most cases. The sur face quality of a machine part greatly influences its performance, such as the fitting between parts, wear resistance, corrosion resistance, and fatigue strength Influence on Fitting between Parts 强度间隙配合 The surface roughness influences directly the fitting condition between parts. For the clearance fit, the existence of the micro-irregularities on part surfaces leads to a rapid initial wear. Thus the clearance increases between the mating parts which cause the deterioration of the fitting condition
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING Influence of Surface Quality on the Performance of Machined Parts A field failure of a machined part caused by its wear, fatigue or corrosion, etc., starts from the part surface in most cases. The surface quality of a machine part greatly influences its performance, such as the fitting between parts, wear resistance, corrosion resistance, and fatigue strength. Influence on Fitting between Parts The surface roughness influences directly the fitting condition between parts. For the clearance fit, the existence of the micro-irregularities on part surfaces leads to a rapid initial wear. Thus the clearance increases between the mating parts, which cause the deterioration of the fitting condition. 强度 间隙配合
SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING As to the interference fit, the strength of fit interference fit depends upon the surface roughness of the part过盈配合 When one part is press-fitted into another, the ridges of the micro-irregularities are extruded: ridge it uk This reduces the actual interference and thus the strength of fit. The actual interference can be calculated by the following equation e=(D-D)-1.2[(H max/s +(Hna6.4 where e-actual interference (Hma s(Hma maximum height of irregularities of shaft and hole, respectively, D-diameter of shaft D, -diameter of hole
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING As to the interference fit, the strength of fit depends upon the surface roughness of the parts. When one part is press-fitted into another, the ridges of the micro-irregularities are extruded: This reduces the actual interference and thus the strength of fit. The actual interference can be calculated by the following equation: e = (Ds - Dh ) - l.2[(Hmax) s + (Hmax) h ] 6.4 where e—actual interference, (Hmax) s , (Hmax) h—maximum height of irregularities of shaft and hole, respectively, Ds—diameter of shaft, Dh—diameter of hole. interference fit 过盈配合 ridge 波峰
SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING The surface roughness and surface hardening influence greatly the wear resistance of part sur faces. When dry friction exists between two fitting sur faces, only the ridges of the irregularities on both sur faces are in contact with each other at the initial stage. The pressure between the two mating parts is concentrated on these small areas For example, the actual contacting area for turned or milled surfaces is only 15-20% of the total area, and for the finely ground surfaces, 30-50%. Due to the high unit pressure, the irregularities on the part surfaces deform elastically and plastically and portions are sheared because of the relative movement of the surfaces. These/fallen particles are scattered, which intensify the weaf between the contacting sur faces shear剪切,剪断
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING The surface roughness and surface hardening influence greatly the wear resistance of part surfaces. When dry friction exists between two fitting surfaces, only the ridges of the irregularities on both surfaces are in contact with each other at the initial stage. The pressure between the two mating parts is concentrated on these small areas. For example, the actual contacting area for turned or milled surfaces is only 15-20% of the total area, and for the finely ground surfaces, 30-50%. Due to the high unit pressure, the irregularities on the part surfaces deform elastically and plastically and portions are sheared because of the relative movement of the surfaces. These fallen particles are scattered, which intensify the wear between the contacting surfaces. shear 剪切,剪断
SCIENCE AND TECHNOLOGY ENGLIAH FOR MECHANICAL ENGINEERING The situation of the wet friction is more complicated. At the initial stage of the wet friction the lubricant films are pierced through by the ridges of the roughness to form a dry friction between two fitting surfaces. The intensive initial wear changes the proper fit of the mating parts. However, as the wear gradually increases. the unit pressure between the surfaces reduces, and the wear slows down. When a certain point is reached. the wear is intensified again.亲合力亲合性 lubricant filn润滑油膜 A reduction of the sur face roughness can increase the wear resistance of part surface. However, the relationship between them is not linear. It has been proved by experiments that an optimal value for the surface roughness exists under a given condition with which the minimum wear can be expected. If the fitting surfaces are too smoo oth, they will be in close contact with each they other, which leads to a larger affinity between the molecules of the surfaces. This will result in an intensive wear of the surfaces
SCIENCE AND TECHNOLOGY ENGLISH FOR MECHANICAL ENGINEERING The situation of the wet friction is more complicated. At the initial stage of the wet friction the lubricant films are pierced through by the ridges of the roughness to form a dry friction between two fitting surfaces. The intensive initial wear changes the proper fit of the mating parts. However, as the wear gradually increases, the unit pressure between the surfaces reduces, and the wear slows down. When a certain point is reached, the wear is intensified again. A reduction of the surface roughness can increase the wear resistance of part surface. However, the relationship between them is not linear. It has been proved by experiments that an optimal value for the surface roughness exists under a given condition with which the minimum wear can be expected. If the fitting surfaces are too smooth, they will be in close contact with each other, which leads to a larger affinity between the molecules of the surfaces. This will result in an intensive wear of the surfaces. 亲合力,亲合性 lubricant film 润滑油膜