机械工程英语第2版叶邦彦Unit 7

Unit 7 Metal Cuttingimportance of metal cuttingIntroduction of a typical cutting tool Chip FormationShear zoneCutting Tool MaterialsSurface FinishCutting Fluidsimportance of metal cuttingevery product we use in our daily life has undergone this process either directly or indirectly.In USA, more than $100 billions are spent annually on machining and related operations.A large majority (above 80%) of all the machine tools used in themanufacturing industry have undergone metal cutting.An estimate showed that about 10 to 15% of all the metal produced in USA was converted into chips.Introduction of a typical cutting toolFig 7.1 The general characteristics of a metal cutting toolA typical cutting tool in a simplified form is shown in Fig 7.1.The important features to be observed are follows：①Rake angle（前角）It is the angle between the face of the tool called the rake face and the normal to the machining direction.Higher the rake angle, better is the cutting and less are the cutting forces, increasing the rake angle reduces the metal backupavailable at the tool rake face. This reduces the strength of the tool tip as well as the heat dissipation through the tool.The maximum limit to the rake angle is generally of the order of l5°for high speed steel tools cutting mild steel. It is possible to have rake angles at zero or negative.②Clearance angle （后角）This is the angle between the machined surface and the underside of the tool called the flank face. The clearance angle is provided such that the tool will not rub the machined surface thus spoiling the surface and increasing the cutting forces. A very large clearance angle reduces the strength of the tool tip, and hence normally an angle of the order of 5-6°is used.The conditions which have an important influence on metal cutting ：work material（工件材料）cutting tool material（切削刀具材料）cutting tool geometry（切削刀具几何参数）cutting speed（切削速度）feed rate（进给量）depth of cut（切削深度）cutting fluid（切削液）Three cutting factors：cutting speed（切削速度）——vfeed rate（进给量）—————fdepth of cut（切削深度）——dChip FormationMetal cutting process is a very complex process. Fig 7.2 shows the basic material removal operation schematically. The metal in front of the tool rake face gets immediately compressed,first elastically and then plastically. Fig 7.2 The possible deformationsin metal cuttingThis zone is traditionally called shear zone（剪切区）in view of the fact that the material in the final form would be removed by shear from the parent metal. The actual separation of the metal starts as a yielding or fracture, depending upon the cutting conditions, starting from the cutting tool tip.Then the deformed metal (called chip ) flows over the tool (rake) face. If the friction between the tool rake face and the underside of the chip (deformed material) is considerable, then the chip gets further deformed, which is termed as secondary deformation. The chip after sliding over the tool rake face is lifted away from the tool, and the resultant curvature （曲率）of the chip is termed as chip curl.Chip formation in metal cutting could be broadly categorized intothree types (Fig.7.3).Discontinuous chipContinuous chipContinuous chip with BUE (Built up edge)Fig 7.3 Three main types of chipsa）Discontinuous chip b) Continuous chip c) Continuous chip with BUE积屑瘤Discontinuous ChipThe segmented chip separates into short pieces, which may or may not adhere to each other. Severe distortion of the metal occurs adjacent to the tool face, resulting in a crack that runs ahead of the tool. Eventually, the shear stress across the chip becomes equal to the shear strength of the material, resulting in fracture and separation.[1] With this type of chip, there is little relative movement of the chip along the tool face, Fig 7.3 (a).Continuous ChipThe continuous chip is characterized by a general flow of the separated metal along the tool face. [2] There may be some cracking of the chip, but in this case it usually does not extend far enough to cause fracture. This chip is formed at the higher cutting speeds when machining ductile materials. There is little tendency for the material to adhere to the tool. The continuous chip usually shows a good cutting ratio and tends to produce the optimum surface finish, but it may become an operating hazard, Fig 7.3 (b).Continuous with a Built-up EdgeThis chip shows the existence of a localized, highly deformed zone of material attached or “welded” on the tool face. Actually ,analysis of photomicrographs shows that this built-up edge is held in place by the static friction force until it becomes so large that the external forces acting on it cause it to dislodge , with some of it remaining on the machined surface and the rest passing off on the back side of the chip, Fig 7.3(c).Shear zonetwo schools of thought in the analysis of the metal removal process：①deformation zone is very thin and planar as shown in Fig 7.4 (a).②actual deformation zone is a thick one with a fan shape as shownin Fig 7.4 (b).Fig 7.4 Shear modelsCutting Tool Materialsimportant characteristics expected of a cutting tool material：Higher hardness—than that of the workpiece material being machined, so that it can penetrate into the work material.Hot hardness—which is the ability of the material to retain its hardness at elevated temperatures in view of the high temperatures existing in the cutting zone.Wear resistance—The cutting tool material should therefore have high abrasion resistance to improve the effective life of the tool.Toughness—it should have enough toughness to withstand the impact loads that come in the beginning of cut or to force fluctuations due to imperfections in the work material.Low friction—The coefficient [共同作用] of friction between the chip and tool should below. This would allow for lower wear rates and better chip flow.Thermal characteristics—Since a lot of heat is generated at the cutting zone, the tool material should have higher thermal conductivity to dissipate this heat in the shortest time, otherwise the tool temperature would become high, reducing its life.Surface FinishTwo factors of surface finish in a given machining operation：the ideal surface finish：which is a result of the geometry of the manufacturing process which can be determined by considering the geometry of the machining operation.the natural component：which is a result of a number of uncontrollable factors in machining, which is difficult to predict.Ideal Surface Finish in TurningThe actual turning tool used would have a nose radius in place of the sharp tool point, which modifies the surface geometry as shown in Fig 7.5 (a). If the feed rate is very small, as is normal in finish turning, the surface is produced purely by the nose radius alone as shown in Fig 7.5.For the case in Fig 7.5, the surface roughness value is to be ：Where: Ra is the surface roughness valueR is the nose Radius ； f is the feed rate ；2a 8f R 183RFig.7.5 Surface profile as produced by turning with a cutting tool having a nose radiusThe actual surface finish factors1.the cutting process parameters, speed, feed and depth of cut2.the geometry of the cutting tool3.application of cutting fluid4.work and tool material characteristics5.rigidity of the machine tool and the consequent vibrations.The major influence on surface finish is exerted by the feed rate and cutting speed.Cutting FluidsThe functions of cutting fluids (which are often erroneously called coolants) are：to cool the tool and workpieceto reduce the frictionto protect the work against rustingto improve the surface finishto prevent the formation of built-up edgeto wash away the chips from the cutting zone.Notes1. Severe distortion of the metal occurs adjacent to the tool face,resulting in a crack that runs ahead of the tool. Eventually, the shear stress across the chip becomes equal to the shearstrength of the material, resulting in fracture and separation.句意：金属的剧烈变形发生在刀具前刀面的附近，导致在运动的刀具前方金属层中产生裂缝。