金属切削外文翻译.doc

METAL CUTTING The importance of machining processes can be emphasised by the fact that every product we use in our daily life has undergone this process either directly or indirectly. (1) In USA, more than $100 billions are spent annually on machining and related operations. (2) A large majority (above 80%) of all the machine tools used in the manufacturing industry have undergone metal cutting.(3) An estimate showed that about 10 to 15% of all the metal produced in USA was converted into chips.These facts show the importance of metal cutting in general manufacturing. It is therefore important to understand the metal cutting process in order to make the best use of it. A number of attempts have been made in understanding the metal cutting process and using this knowledge to help improve manufacturing operations which involved metal cutting. A typical cutting tool in simplified form is shown in Fig.7.1. The important features to be observed are follows. 1. Rake angle. 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 backup available at the tool rake face.This reduces the strength of the tool tip as well as the heat dissipation through the tool. Thus, there is a maximum limit to the rake angle and this is generally of the order of 15for high speed steel tools cutting mild steel. It is possible to have rake angles at zero or negative. 2. Clearance angle. 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 56is used. The conditions which have an important influence on metal cutting are work material, cutting tool material, cutting tool geometry, cutting speed, feed rate, depth of cut and cutting fluid used. The cutting speed, v, is the speed with which the cutting tool moves through the work material. This is generally expressed in metres per second (ms-1). Feed rate, f, may be defined as the small relative movement per cycle (per revolution or per stroke) of the cutting tool in a direction usually normal to the cutting speed direction. Depth of cut, d, is the normal distance between the unmachined surface and the machined surface.Chip Formation Metal 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. This zone is traditionally called shear zone in view of 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 of the chip is termed as chip curl. Plastic deformation can be caused by yielding, in which case strained layers of material would get displaced over other layers along the slip-planes which coincide with the direction of maximum shear stress. A chip is variable both in size and shape in actual manufacturing practice. Study of chips is one of the most important things in metal cutting. As would be seen later, the mechanics of metal cutting are greatly dependent on the shape and size of the chips produced. Chip formation in metal cutting could be broadly categorised into three types: (Fig.7.3) (1) Discontinuous chip (2) Continuous chip(3) Continuous chip with BUE (Built up edge) Discontinuous Chip. The segmented chip separates into short pieces, which may or may not adhere to each other. Severe distortion of the metal occurs adjacent to the 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. With this type of chip, there is little relative movement of the chip along the tool face, Fig.7.3a. Continuous chip. The continuous chip is characterized by a general flow of the separated metal along the tool face. 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.7.3b. Continuous with a built-up edge. This 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.7.3c.Shear Zone There are basically two schools of thought in the analysis of the metal removal process. One school of thought is that the deformation zone is very thin and planar as shown in Fig.7.4a. The other school thinks that the actual deformation zone is a thick one with a fan shape as shown in Fig.7.4b. Though the first model (Fig.7.4a) is convenient from the point of analysis, physically it is impossible to exist. This is because for the transition from undeformed material to deform to take place along a thin plane, the acceleration across the plane has to be infinity.Similarly the stress gradient across the shear plane has to be very large to be practical. In the second model (Fig.7.4b) by making the shear zone over a region, the transitions in velocities and shear stresses could be realistically accounted for. The angle made by the shear plane with the cutting speed vector, is a very important parameter in metal cutting. Higher the shear angle better is the cutting performance. From a view of the Fig.7.4a, it can be observed that a higher rake angles give rise to higher shear angles. Cutting Tool Materials Various cutting tool materials have been used in the industry for different applications. A number of developments have occurred in the current century. A large variety of cutting tool materials has been developed to cater to the variety of materials used in these programmes. Before we discuss the properties of these materials, let us look at the important characteristics expected of a cutting tool material. 1. Higher hardness than that of the workpiece material being machined, so that it can penetrate into the work material. 2. 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. 3. Wear resistanceThe chip-tool and chip-work interfaces are exposed to such severe conditions that adhesive and abrasion wear is very common. The cutting tool material should therefore have high abrasion resistance to improve the effective life of the tool. 4. ToughnessEven though the tool is hard, it should have enough toughness to withstand the impact loads that come in the beginning of cut or force fluctuations due to imperfections in the work material. This requirement is going to be more useful for the interrupted cutting, e.g. milling. 5. Low frictionThe coefficient of friction between the chip and tool should be low. This would allow for lower wear rates and better chip flow. 6. Thermal characteristicsSince 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. All these characteristics may not be found in a single tool material. Improved tool materials have been giving a better cutting performance.Surface Finish Machining operations are utilized in view of the better surface finish that could be achieved by it compared to other manufacturing operations.Thus it is important to know what would be the effective surface finish that can be achieved in a machining operation. The surface finish in a given machining operation is a result of two factors: (1) 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, and (2) the natural component, which is a result of a number of uncontrollable factors in machining, which is difficult to predict.Ideal Surface Finish in Turning The 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.7.5a. 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 Ra=8f2/(18R3)Where: Ra is the surface roughness value R is the nose radius f is the feed rate The above are essentially geometric factors and the values represent an ideal situation. The actual surface finish obtained depends to a great extent upon a number of factors such as: (1) the cutting process parameter, speed, feed and depth of cut (2) the geometry of the cutting tool (3) application of cutting fluid (4) work and tool material characteristics (5) rigidity of the machine tool and the consequent vibrations. The major influence on surface finish is exerted by the feed rate and cutting speed. As the feed decreases, from the above equations, we can see that the roughness index decreases. Similarly as the cutting speed increases, we have better surface finish. Thus while making a choice of cutting process parameters for finish, it is desirable to have high cutting speed and small feed rates.Cutting Fluids The functions of cutting fluids (which are often erroneously called coolants) are: To cool the tool and workpiece To reduce the friction To protect the work against rusting To improve the surface finish To prevent the formation of built-up edge To wash away the chips from the cutting zone However, the prime function of a cutting fluid in a metal cutting operation is to control the total heat. This can be done by dissipating the heat generated as well as reducing it. The mechanisms by which a cutting fluid performs these functions are: cooling action and lubricating action. Cooling action. Originally it was assumed that cutting fluid improves the cutting performance by its cooling properties alone. That is why the name coolant was given to it.Since most of the tool wear mechanisms are thermally activated, cooling the chip tool interface helps in retaining the original properties of the tool and hence prolongs its life. However, a reduction in the temperature of the workpiece may under certain conditions increase the shear flow stress of the workpiece, thereby decreasing tool life. It has been shown through a number of investigations that cooling in fact is one of the major factors in improving the cutting performance. Lubricating action. The best improvement in cutting performance can be achieved by the lubricating action since this reduces the heat generated, thus reducing the energy input to the metal cutting operation. However, if the cutting fluid is to be effective, it must reach the chip tool interface. But it is not easy to visualize how it is accomplished in the case of a continuous turning with a single point turning tool, specially when the chip-tool contact pressure is as high as 70 MPa. Merchant thought that minute asperities existed at the chip-tool interface and the fluid was drawn into the interface by the capillary action of the interlocking network of these surface asperities. 金属切削 机加工过程的重要性可通过日常生活使用的每件产品都直接或间接经历这一过程的事实来强调。 (1)在美国，每年花在机加工及其相关作业上的费用都多于千亿美元。 (2) 用于制造业的全部机床中的大多数(多于80%)都经历过金属切削。 (3) 有估计显示美国生产的所有金属中约10到15%转变成了切屑。 这些事实说明了金属切削在常规制造中的重要性。因此了解金属切削过程以充分利用它是重要的。在了解金属切削过程并运用这些知识帮助改善与金属切削有关的制造作业方面已经做了许多努力。 典型切削刀具的简化形式如图7.1所示。要注意的重要特征如下。 1.前角：它是被称为前倾面的刀具面与垂直机加工方向的夹角。前角越大，则切削越好且切削力越小，增加前角可以减少刀具前倾面上产生的金属阻塞。但这会和减少通过刀具散发的热量一样减少刀尖强度。因此前角有一最大限制，用高速钢刀具切削低碳钢通常为15。前角取零度或负值也是可能的。 2. 后角：这是机加工面与被称为后侧面的刀具底面夹角。后角使刀具不产生会损坏机加工面的摩擦和增加切削力。很大的后角会削弱刀尖的强度，因此一般采用56的后角。 对金属切削有重要影响的条件有工件材料、刀具材料、刀具几何形状、切削速度、进给率、切削深度和所用的切削液。 切削速度v指切削刀具经过工件材料的移动速度。通常用米每秒 (ms-1)表示。 进给率f可定义为每循环(每转或每行程)切削刀具在通常为垂直于切削速度方向的次要相对运动。 切削深度d是未加工面与已加工面之间的垂直距离。 切屑的形成 金属切削过程是一个很复杂的过程。图7.2用图的形式显示了基本材料去除作业。在刀具前倾面前的金属直接受到压缩，首先弹性变形然后塑性变形。考虑到最终形状中的材料是通过剪切从母体金属去除的，此区域传统上称为剪切区。金属的实际分离始于屈服或断裂(视切削条件而定)，从切削刀尖开始。然后变形金属(称为切屑)流过刀具(前倾)面。如果刀具前倾面与切屑(变形金属)底面之间的摩擦相当大，那么切屑进一步变形，这也叫做二次变形。滑过刀具前倾面的切屑被提升离开刀具，切屑弯曲的结果被称为切屑卷。 屈服能导致塑性变形，在这种情况下材料变形层沿着与最大剪应力方向一致的滑移面被其它层所取代。 在实际加工过程中切屑的尺寸和形状都是变化的。对切屑的研究是金属切削最重要的事情之一。如同后面将要看到的那样，金属切削力学极大地依赖于所产生切屑的形状和尺寸。 金属切削中的切屑形成可以宽泛地分成三个类型(图7.3)： (1)间断切屑 (2)连续切屑 (3)带切屑瘤的连续切屑 间断切屑：分段的切屑分散成小碎片，既可能相互附着也可能不相互附着。在靠近切削面处发生金属的剧烈变形，导致在运动刀具前方金属层产生裂缝。最后，横过切屑的剪切应力与材料的剪切强度相等，造成断裂和分离。生成这类切屑时，切屑沿刀具面几乎没有相对运动，见图7.3a。 连续切屑：连续的切屑一般具有分离金属沿刀具面流动的特征。切屑可能有一些破裂，但在这种情况下切屑通常不会延长到足以引起断裂。这种切屑形成于用较高切削速度机加工有延展性的材料时。材料几乎没有粘附刀具的倾向。连续切屑通常具有良好的切削率和趋向于产生最适宜的表面光洁度，但可能成为操作的危险之源，见图7.3b。 带切屑瘤的连续切屑：这种切屑显示了粘合或“焊接”在刀具面上材料局部高度变形区的存在。实际上，对显微照片的分析显示这种切屑瘤受到静摩擦力抑制直至它变得大到作用在它上面的外力使其移动，一些留在机加工表面上而另一些延伸到切屑的背面，见图7.3c。剪切区 在对金属去除过程的分析中主要存在两种思想学派。一种思想学派认为变形区如图7.4a所示那样非常薄而平坦。另一学派则认为真实变形区象图7.4b所示那样为一厚的带有扇形的区域。 虽然第一种模型(图7.4a)从分析的角度看是方便的，但实际上是不可能存在的。这是由于未变形的材料沿着剪切面发生变形，而且越过剪切面的加速度无穷大。同样在实际运用中越过剪切面的应力梯度必须很大才行。在第二种模型(图7.4b)中让剪力区分布于一个范围，速度和剪应力的转变能说明得更符合实际。 由剪切面和切削速度矢量形成的角度在金属切削中是一个十分重要的参数。剪切角越大，切削作业越好。从图7.4a观察，