This article throws light upon the types of material removal process. The types are:- 1. Miscellaneous Multipoint Machining Operations 2. Abrasive Machining and Finishing Operations. And also glance over the below given article to get an idea about the machining processes and operations.
Types of Material Removal Process # 1. Miscellaneous Multipoint Machining Operations:
Two other common machining operations, using multipoint tools, are broaching and cutting threads with taps and dies. In broaching, only one motion, i.e., the primary cutting motion, is provided by the machine, whereas the feed is obtained by placing the teeth progressively deeper. Figure 4.53 shows the principle of the broaching operation. Since there is no feed motion, the shape of the broach determines the shape of the machined part.
Broaching has maximum application in producing internal forms such as spin holes and noncircular holes. Depending on the situation, broaching is done either by pulling or pushing the broach through a hole drilled in the work piece.
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For a smooth operation, it is essential that at least two or three teeth be simultaneously engaged. The thumb rule sometimes followed to determine the tooth spacing (Fig. 4.54) is where s is the tooth spacing and l is the broached length in mm.
The cut per tooth ƒ is kept in the range 0.05-0.09 mm. The cutting speed in broaching normally lies in the range 0.1-0.5 m / sec.
The cutting force per tooth can be found out by using the basic mechanics of chip formation and the instantaneous broaching load is obtained as the product of the load per tooth and the number of teeth in contact. If Z is the maximum number of teeth in contact at a time, the load can be found out as –
Types of Material Removal Process # 2. Abrasive Machining and Finishing Operations:
In our discussion on the basic mechanics of the machining processes, we have emphasized that the material removal is accomplished by plastic deformation of the work material by the tool. This necessitates that the tool material be much harder than the work material, not only to maintain the form but also to avoid excessive wear. Thus, very hard materials are difficult to machine. Table 4.15 shows the hardness of some common materials in the Knoop scale.
Obviously, it is possible to make use of hard substances as tool materials. These include Al2O3, SiC, and B4C as they are naturally available and can also be produced synthetically without much problem. Diamond is also quite suitable but its higher cost restricts its application to special cases. However, it is not possible to produce the usual shapes of cutting tools with these materials and die only form in which these can be used is grains.
Since the grains of such materials have the capability to abrade the other materials, these grains are commonly known as abrasives and the machining process using such abrasives is called abrasive machining. Abrasives can be used either as powder or in definite geometric forms obtained by bonding these abrasives with some bonding material. The sharp edges of the abrasive grains act as cutting edges which are not only randomly distributed (in the cutting area) but also randomly oriented.
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The most common abrasive machining process is grinding where the abrasives are bonded to the shape of a wheel (known as the grinding wheel) which rotates at a high speed (Fig. 4.56). The other common finishing operations using abrasives are honing and lapping.
Grinding:
Though there are various types of grinding operations, to understand the basic mechanics we shall consider the most common grinding operation, namely, surface grinding. Figure 4.57a shows the basic arrangement of surface grinding, which has some similarity with the up milling operation except that the cutting points are irregularly shaped and randomly distributed (Fig. 4.57b).
The grains actually taking part in the material removal process are called the active grains. Gradually, the sharp edges of the active grains wear out and become blunt. This results in larger forces on the active grains during machining. When the cutting edge is too blunt and the force is sufficiently high, the grain may either get fractured or break away from the wheel.
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When a fracture takes place, new, sharp cutting edges are generated, and when the whole grain is removed, new grains (below the layer of the active grains) become exposed and active. This gives the grinding wheel self-sharpening characteristics. So, the bonding strength, which dictates the maximum force a grain can withstand, is an important characteristic. The strength of bonding is normally termed as the grade of the wheel. A wheel with a strong bond is called hard and vice versa.
Due to the nature of the process, very hot and small chips are produced which may readily get welded to either the grit (abrasive grain) or get welded back on to the work piece. Moreover, because of random grit orientation, a number of grits may have a very large negative rake angle (Fig. 4.57b) and may rub rather than cut. These two factors make the process of grinding quite inefficient in comparison with the other machining operations from the point of view of specific energy. Apart from this, since the material is removed in the form of exceedingly small chips, the size effect is very prominent.
Mechanics of Grinding:
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In our analysis of the grinding process, all grains are assumed to be identical. To explain the mechanics, we consider two different types of operations, namely – (i) plunge grinding, and (ii) surface grinding. Figure 4.58a shows a simple plunge grinding operation where a job of rectangular cross-section is being fed radially at a rate ƒ (mm/min).
The active grains are assumed to be uniformly distributed. The uncut thickness per grit (t1) can be expressed as –
The uncut sections have approximately triangular cross-sections as shown in Fig. 4.58c. The ratio rg = b’ / t1 generally lies between 10 and 20. Since b’ can be written as equations (4.60) and (4.61) yield –
Thermal Aspects:
It is extremely difficult to develop a satisfactory analytical model for calculating the surface temperature during grinding. We shall therefore adopt an indirect approach for yielding the approximate results. It is not very illogical to assume that the grinding temperature depends directly on the energy spent per unit surface area ground. Thus (in surface grinding),
So, the temperature and also the defects caused by higher temperature can be reduced either by decreasing d, D, C, or N, or by increasing the table feed ƒ.
The temperature at the grain-chip interface θg during grinding reaches very high values and can easily go beyond 1500°C. Though this temperature may be above the melting temperature, the melting of the chips may or may not take place, depending on whether or not enough time is available. The time during which an individual grit remains in contact with the chip (in surface grinding) is –
where θ is a constant, v is the wheel surface speed, k is the thermal conductivity of the work material, and pc is the volume specific heat of the work material. Since Uc α t1max -0.4, we have θg α t1max0.1, we can conclude that, in surface grinding, θg α ƒ0.05. So, the grain-chip interface temperature may slightly increase with ƒ but the surface temperature decreases with ƒ.
The ground surface may get affected to a depth of about 0.2 mm by thermal and mechanical effects. As a consequence, large residual tensile stresses may develop (Fig. 4.61) which, if sufficiently high, may result in cracks. When the grinding temperature is sufficiently high, microstructural changes may also take place due to heating and rapid quenching; thus, in the case of a steel work piece, the surface layer may be heated so that it becomes austenite and, due to a quick quenching by the cutting fluid, may be transformed into martensite.
Grinding Wheel Characteristics:
The performance of a grinding wheel depends on the following important factors:
(i) Abrasive Type:
The abrasives generally used are aluminium oxide, silicon carbide, and diamond. Diamond is the hardest substance known and is used for very hard work materials such as glass, carbide, and ceramics. Aluminium oxide and silicon carbide are more commonly used for making the grinding wheels.
Silicon carbide is harder than aluminium oxide but dulls more rapidly. Generally, aluminium oxide abrasives are selected for the surface grinding of steels and bronzes, whereas silicon carbide is chosen for the surface grinding of cast iron, brass, aluminium, hard alloys, and carbides.
(ii) Grain Size:
The size of the grains is generally specified by the grit size. A 60 grit size, For example- is approximately 1/60 inch square. The larger the size of the grains, the more will be the material removal capacity, but the quality of the surface finish deteriorates. Thus, the grain size is determined primarily by the surface quality requirements.
(iii) Bonding Material:
The bond materials commonly used are vitrified clay, resinoid materials, silicates, rubber, shellac, and metals. The vitrified bond is strong and rigid. It is the most common type of bond used. The resin bonds are made from synthetic organic materials. Such bonds are strong and fairly flexible. The silicate bonds are essentially the silicates of soda (water glass). These bonds are not as strong as the vitrified bonds, and the grains are dislodged more rapidly. As a result, the operation is cooler.
Such bonds are used in grinding tools where the temperature rise should be as small as possible. The rubber bonds are used for making flexible wheels. A high speed operation is possible when the wheel is subjected to a side thrust. A fairly hard vulcanized rubber is used as the bonding material. The shellac bonds are used in making thin but strong wheels possessing some elasticity. Since a smooth finish on a hard surface can be achieved, the shellac bonded wheel is used in grinding parts such as cam shaft and mill roll.
(iv) Structure:
Since the grinding wheel is similar to a milling cutter with a very large number of teeth randomly oriented, it must have voids to allow space for the chips. If the voids are too small for the chips, the chips stay in the wheel, blocking the voids. This is known as loading of the wheel. Loading causes inefficient cutting. If the voids are too large, again the cutting action is inefficient since there will be too few cutting edges. In an open structure, the grains are not too densely packed, and in a wheel with a closed structure, the grains are tightly packed.
For grinding ductile work materials, larger chips are produced, and to reduce the tendency of loading, an open structure is preferred. In the case of hard and brittle work materials, a closed structure is selected. The structure depends on the required grade and also the nature of cut. For a rough cut, an open structure is more suitable.
(v) Grade:
The grade is determined by the strength of the bonding material. So, a hard wheel means strong bonding and the abrasive grains can withstand large forces without getting dislodged from the wheel. In the case of a soft wheel, the situation is just the opposite. When the work material is hard, the grains wear out easily and the sharpness of the cutting edges is quickly lost. This is known as glazing of the wheel. A glazed wheel cuts less and rubs more, making the process inefficient.
To avoid this problem, a soft wheel should be used so that the grains which lose the sharpness get easily dislodged as the machining force on the individual grains increases. Thus, the layers of new grains are exposed, maintaining the sharpness of the wheel. When the work material is soft, a hard wheel should be used since the problem of glazing will be absent and a longer wheel life will be achieved.
So, for a work material, there exists an optimal grade—too hard a wheel causes glazing, whereas too soft a wheel wears out very fast. Figure 4.62 shows the nature of growth of power requirement as the grinding operation continues for too hard, optimum, and too soft wheels.
Wheel Specification and Selection:
Generally, an alphanumeric system is used for a complete specification of a grinding wheel.
For getting the optimum results, a grinding wheel must be properly selected. The important guidelines that should be observed when choosing a wheel; it should however be noted that such a selection also depends on experience.
Wheel Life:
The study of grinding wheel life is a much more complex problem than that of an ordinary cutting tool. The usable period depends not only on the wheel wear but also on the wheel loading and glazing. However, if the wheel is properly selected, the loading and glazing may not pose a serious problem, and the wheel wear may be the predominant factor in determining the wheel life. The abrasive grains also have a finite life. An abrasive grain may lose volume in two ways, namely – (i) by a gradual wear of its sharp edges (known as attrition wear), and (ii) by a fracture of a portion.
A grinding wheel wears more rapidly than the tools used in the other conventional machining operations. The commonly-used parameter for evaluating the performance of a wheel is the grinding ratio (Gr) given by the relation –
If the wheel wear is plotted against the volume of work material removed, the general characteristic would be as shown in Fig 4.64.
We see here that the wear curve is quite similar to that of the other cutting tools. Also, there is an initial breakdown period followed by a region of uniform wear rate. Finally, the wheel again starts breaking down rapidly. The middle zone (see Fig. 4.64) is the real usable period which determines the life of a wheel.
Once the period of uniform wear rate ends, the wheel should be reconditioned (as when regrinding a tool) before using it again. The reconditioning of a wheel is done by a process commonly known as wheel dressing. This requires holding a hard tipped tool against the rotating wheel and giving the dresser some feed motion to cover the whole width of the wheel. By this process, the dull, used layer is crushed, exposing the fresh surface. Thus, all the problems accumulated by glazing and loading are removed.
Types of Grinding Operations:
The common grinding operations are – (i) surface grinding with horizontal spindle, (ii) surface grinding with vertical spindle, (iii) external cylindrical grinding, (iv) internal cylindrical grinding, (v) centre less grinding, and (vi) form grinding. The basic principles of these grinding operations are illustrated in Fig. 4.65.
Unlike in cylindrical grinding, in centre less grinding the cylindrical work piece is supported on a rest. The feed is provided to the long work piece by keeping the regulating wheel slightly tilted from the vertical position. If ɸ is the angle of tilt and vr is the surface speed of the regulating wheel, the feed velocity then is vr sin ɸ.
2. Finishing Operations:
The other important operations using abrasives are:
(i) Honing, and
(ii) Lapping.
However, here the material removal rate is generally very small as compared with that in grinding, and these operations are used only for finishing purposes. Apart from these two operations, there are various finishing operations, e.g., super finishing and buffing, where abrasives are used.
The honing operation is used for finishing the inside surface of a hole. Here, abrasives in the form of sticks are mounted on a mandrel which is then given a reciprocating movement (along the hole axis) superimposed on a uniform rotary motion (Fig. 4.66). The grit size normally varies from 80 mesh to 600 mesh. Because of the nature of the path of the abrasive grits on the surface of the work, a random cross-marked surface finish (desirable for lubrication) is obtained.
Depending on the work material, the honing speed may vary from 15 m / min to 60 m / min, and the honing pressure lies in the range 1-3 N / mm2. In special cases, material up to 0.5 mm may be removed by honing. The tolerance and finish achieved in this operation are of the order of 0.0025 mm and 0.25 μm, respectively.
Lapping is another operation for improving the accuracy and finish. It is accomplished by abrasives in the range 120-1200 mesh. A lap is generally made of a material softer than the work material. In this process, straight, narrow grooves are cut at 90° on the lap surface and this surface is charged by sprinkling the abrasive powder. The work piece is then held against the lap and moved in unrepeated paths.
A suitable cutting fluid is applied for lapping. In hand lapping, the work is moved over the lap along a path in the form of 8. Machines are also available for lapping. The material removal is seldom more than 0.0025 mm and the lapping pressure is generally kept in the range 0.01-0.2 N / mm2, depending on the hardness of the work material.