1、 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 (a
2、bove 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 import
3、ant 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
4、 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 re
5、duces 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
6、 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 for
7、ces. 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
8、 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
9、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 fron
10、t 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 fr
11、acture, 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,
12、 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 ot
13、her 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 grea
14、tly 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 piec
15、es, 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 th
16、is 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 en
17、ough 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 oper
18、ating 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 b
19、ecomes 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 schoo
20、l 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 impossib
21、le 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 makin
22、g 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 vie
23、w 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
24、 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 machin
25、ed, 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 sever
26、e 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 be
27、ginning 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
28、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
29、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 woul
30、d 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 geome
31、try 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 t
32、he 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
