Fundamentals of Machining PDF Chapter 21

M21 KALP2244 08 GE C21 page 636 Chapter 21 Fundamentals of Machining 21.1 Introduction 637 21.2 Mechanics of Cutting 639 21.3 Cutting Forces and Power 648 21.4 Temperatures in Cutting 652 21.5 Tool Life: Wear and Failure 654 21.6 Surface Finish and Integrity 661 21.7 Machinability 664 Examples...

M21 KALP2244 08 GE C21 page 636 Chapter 21 Fundamentals of Machining 21.1 Introduction 637 21.2 Mechanics of Cutting 639 21.3 Cutting Forces and Power 648 21.4 Temperatures in Cutting 652 21.5 Tool Life: Wear and Failure 654 21.6 Surface Finish and Integrity 661 21.7 Machinability 664 Examples: 21.1 Relative Energies in Cutting 651 21.2 Increasing Tool Life by Reducing the Cutting Speed 657 21.3 Effect of Cutting Speed on Material Removal 658 This chapter is an introduction to the fundamentals of machining processes and presents the basic concepts relevant to all machining operations. The chapter opens with a description of the mechanics of chip formation, including the model typically used for studying the basic cutting operations, which allows the calculation of force and power in machining. Temperature rise and its importance on the workpiece and cutting tool, and the mechanisms of tool wear are then discussed. The chapter concludes with a description of surface finish, integrity of the parts produced by machining, and the factors involved in the machinability of materials. 636  M21 KALP2244 08 GE C21 page 637 Introduction 637 21.1 Introduction Machining processes remove material from the surfaces of a workpiece by producing chips. Some of the more common cutting processes, illustrated in Fig. 21.1 (see also Fig. I.6e), are: Turning, in which the workpiece is rotated and a cutting tool removes a layer of material as the tool moves along its length, as shown in Fig. 21.1a. Cutting off, in which the tool moves radially inward and separates a piece (on the right in Fig. 21.1b) from the blank. Slab milling, in which a rotating cutting tool removes a layer of material from the surface of the workpiece (Fig. 21.1c). End milling, in which a rotating cutter travels to a certain depth in the workpiece, producing a cavity (Fig. 21.1d). In the turning process, illustrated in greater detail in Fig. 21.2, the cutting tool is set at a certain depth of cut (mm), and travels to the left with a certain cutting speed as the workpiece rotates. The feed, or feed rate, is Tool Tool (a) Straight turning (b) Cutting off Cutter End mill (c) Slab milling (d) End milling Figure 21.1: Some examples of common machining operations. Feed Depth of cut (mm/rev) (mm) Chip Tool Tool holder Figure 21.2: Schematic illustration of the turning operation, showing various features.  M21 KALP2244 08 GE C21 page 638 638 Chapter 21 Fundamentals of Machining the distance the tool travels per revolution of the workpiece (mm/rev); this movement of the cutting tool produces a chip, which moves up the face of the tool. In order to analyze this basic machining process in greater detail, a two-dimensional model of it is presented in Fig. 21.3a. In this idealized model, a cutting tool moves to the left along the workpiece at a constant velocity, V , and a depth of cut, to. Ahead of the tool, a chip is produced by plastic deformation, shearing the material continuously along the shear plane. This phenomenon can easily be demonstrated by slowly scraping the surface of a stick of butter lengthwise with a sharp knife, and observing how a chip is being produced. Chocolate shavings, used as decorations on cakes and pastries, are produced in a similar manner. In comparing Figs. 21.2 and 21.3, note that the feed in turning is equivalent to to , and the depth of cut in turning is equivalent to the width of cut (the dimension perpendicular to the page). These dimensional tc Rough surface Shiny surface Rake face Chip 2 1 Tool Shear plane Rake angle a V Flank face Relief or to f clearance angle Workpiece Shear angle (a) tc Rough surface Rake face Chip Tool 2 1 Primary Rake angle shear zone a Flank face V to Rough surface (b) Figure 21.3: Schematic illustration of a two-dimensional cutting process, also called orthogonal cutting: (a) Orthogonal cutting with a well-defined shear plane, also known as the M.E. Merchant model. Note that the tool shape, the depth of cut, to , and the cutting speed, V , are all independent variables. (b) Orthogonal cutting without a well-defined shear plane.  M21 KALP2244 08 GE C21 page 639 Mechanics of Cutting 639 relationships can be visualized by rotating Fig. 21.3 clockwise by 90◦. With this brief introduction as a background, the cutting process will now be described in greater detail. 21.2 Mechanics of Cutting The factors that influence the cutting operation are outlined in Table 21.1. In order to appreciate the con- tents of this table, consider the major independent variables in the basic cutting process: (a) tool material and coatings, if any; (b) tool shape, its surface finish and sharpness; (c) workpiece material and its processing history; (d) cutting speed, feed, and depth of cut; (e) cutting fluids, if any; (f) characteristics of the machine tool; and (g) the type of workholding device and fixturing. Dependent variables in machining are those that are influenced by changes made in the independent variables listed above. They include: (a) type of chip produced, (b) force and energy dissipated during cutting, (c) temperature rise in the workpiece, the tool, and the chip, (d) tool wear and failure, and (e) surface finish and surface integrity of the workpiece. The importance of establishing quantitative relationships among the independent and dependent vari- ables in machining can best be appreciated by considering some typical questions to be posed: Which of the independent variables should be changed first and to what extent (a) if the surface finish of the work- piece being machined becomes unacceptable, (b) if the cutting tool wears rapidly and becomes dull, (c) if the workpiece becomes very hot, and (d) if the tool begins to vibrate and chatter. In order to understand these phenomena and respond to the questions posed, consider that the me- chanics of chip formation have been studied extensively since the early 1940s. Several models, with varying degrees of complexity, have been proposed to describe the basic cutting process. More advanced machining models are being developed, especially computer simulation of the mechanics of the basic machining process. The simple model shown in Fig. 21.3a, and referred to as the M.E. Merchant model, developed in the early 1940s, is sufficient for the purposes of this introduction. This model is known as orthogonal cutting, because it is two-dimensional whereby the forces involved are perpendicular to each other. The cutting tool has a rake angle, α (positive as shown in the figure), and a relief or clearance angle. Microscopic examination of chips produced in actual operations reveal that they are produced by shear- ing (as modeled in Fig. 21.4a), a phenomenon similar to the movement of cards in a deck that is being deformed (see also Fig. 1.6). Shearing takes place within a shear zone (usually along a well-defined plane referred to as the shear plane) and at an angle φ (called the shear angle). Below the shear plane, the work- piece remains undeformed; above it, the chip (which is already formed) moves up the rake face of the tool. Table 21.1: Factors Influencing Machining Operations. Parameter Influence and interrelationship Cutting speed, Forces, power, temperature rise, tool life, type of chip, surface finish and integrity depth of cut, feed, cutting fluids Tool angles As above; influence on chip flow direction; resistance to tool wear and chipping Continuous chip Good surface finish; steady cutting forces; undesirable, especially in modern machine tools Built-up edge chip Poor surface finish and integrity; if thin and stable, edge can protect tool surfaces Discontinuous chip Desirable for ease of chip disposal; fluctuating cutting forces; can affect surface finish and cause vibration and chatter Temperature rise Influences tool life, particularly crater wear and dimensional accuracy of workpiece; may cause thermal damage to workpiece surface Tool wear Influences surface finish and integrity, dimensional accuracy, temperature rise, forces and power Machinability Related to tool life, surface finish, forces and power, and type of chip produced  M21 KALP2244 08 GE C21 page 640 640 Chapter 21 Fundamentals of Machining Rake angle, a Chip (908 2 f 1 a) Tool Vc d (f 2 a) Vs a A (908 2 a) f C f Workpiece B V Shear A C f plane (f 2 a) O B (a) (b) Figure 21.4: (a) Schematic illustration of the basic mechanism of chip formation by shearing. (b) Velocity diagram showing angular relationships among the three speeds in the cutting zone. In this figure, the dimension d is highly exaggerated in order to show the mechanism involved in chip formation; this dimension has been found to be only on the order of 10−2 to 10−3 mm. Some materials, notably cast irons machined at low speeds, do not undergo shearing along a well- defined plane, but instead within a shear zone, as shown in Fig. 21.3b. The shape and size of this zone is important in the machining operation, as will be described in Section 21.2.1. Cutting Ratio. It can be seen from Fig. 21.3a that the chip thickness, tc , can be determined from the depth of cut, to , the rake angle, α, and the shear angle, φ. The ratio of to /tc is known as the cutting ratio, or chip-thickness ratio, r. It is related to the two angles by the following relationships: r cos α tan φ = (21.1) 1 − r sin α and to sin φ r= =. (21.2) tc cos (φ − α) Because the chip thickness is always greater than the depth of cut, the value of r is always less than unity. The reciprocal of r is known as the chip-compression ratio or chip-compression factor; it is a measure of how thick the chip has become as compared with the depth of cut. Thus, the chip-compression ratio always is greater than unity. As may be visualized by reviewing Fig. 21.3a, the depth of cut is also referred to as the undeformed chip thickness. The cutting ratio is an important and useful parameter for evaluating cutting conditions. Since the undeformed chip thickness, to , is easily specified as a machine setting, and is therefore known, the cutting ratio can be calculated by measuring the chip thickness, using a micrometer. With the rake angle also known for a particular cutting operation (since it is a function of the tool and workpiece geometries), Eq. (21.1) allows calculation of the shear angle. Although to is referred to as the depth of cut, note that in a machining process such as turning, shown in Fig. 21.2, this quantity is the feed or feed rate, expressed as the distance traveled per revolution of the workpiece. To visualize the situation, assume that the workpiece in Fig. 21.2 is a thin-walled tube, and that the width of cut is the same as the thickness of the tube. Then, by rotating Fig. 21.3 clockwise by 90◦ , the figure now becomes similar to the view in Fig. 21.2.  M21 KALP2244 08 GE C21 page 641 Mechanics of Cutting 641 Shear Strain. Referring to Fig. 21.4a, it can be seen that the shear strain, γ, that the material undergoes can be expressed as AB AO OB γ= = + , OC OC OC or γ = cot φ + tan (φ − α). (21.3) Note that large shear strains are associated with (a) low shear angles and (b) with low or negative rake angles. Shear strains of 5 or higher have been observed in actual cutting operations. The material removed from the workpiece undergoes greater deformation during cutting than in forming and shaping processes, as is also seen in Table 2.4. Furthermore, deformation in machining generally takes place within a very narrow zone; in other words, the dimension d = OC in Fig. 21.4a is very small. Thus, the rate at which shearing takes place in machining is high. The nature and size of the deformation zone is further described in Section 21.3. The shear angle has a major significance in the mechanics of machining operations, as it influences force and power requirements, chip thickness, and temperature rise in machining. One of the earliest analyses was based on the assumption that the shear angle adjusts in order to minimize the cutting force, or that the shear plane is a plane of maximum shear stress. This analysis yields the expression α β φ = 45◦ + − , (21.4) 2 2 where β is the friction angle, and is related to the coefficient of friction, μ, at the tool–chip interface by the expression μ = tan β. Among several other shear-angle relationships that have been developed, another approximate but useful formula is φ = 45◦ + α − β. (21.5) The coefficient of friction in metal cutting has been found to generally range from about 0.5 to 2 (see also Section 33.4), indicating that the chip undergoes considerable frictional resistance as it moves up the rake face of the tool. Experiments have shown that μ varies considerably along the tool–chip interface, because of large variations in contact pressure and temperature. Consequently, μ is also called the apparent mean coefficient of friction. Equation (21.4) indicates that (a) as the rake angle decreases or as the friction at the tool–chip interface increases, the shear angle decreases and the chip becomes thicker; (b) thicker chips indicate more energy dissipation, because the shear strain is higher, as can be noted from Eq. (21.2); and (c) because the work done during cutting is converted into heat, the temperature rise is also higher. Velocities in the Cutting Zone. Note in Fig. 21.3 that since the chip thickness is greater than the depth of cut, the velocity of the chip Vc has to be lower than the cutting speed V. Because mass continuity has to be maintained, V to = Vc tc or Vc = V r. Hence, V sin φ Vc =. (21.6) cos (φ − α) A velocity diagram also can be constructed, as shown in Fig. 21.4b. From trigonometric relationships, V Vs Vc = = (21.7) cos (φ − α) cos α sin φ where Vs is the velocity at which shearing takes place in the shear plane. Note also that to Vc r= =. (21.8) tc V  M21 KALP2244 08 GE C21 page 642 642 Chapter 21 Fundamentals of Machining These relationships will be utilized later in Section 21.3, describing power requirements in machining operations. 21.2.1 Types of Chips Produced in Metal Cutting The types of metal chips commonly observed in practice and their photomicrographs are shown in Fig. 21.5. The four main types are: Continuous Built-up edge Serrated or segmented Discontinuous. Note that a chip has two surfaces: 1. A surface that has been in contact with the rake face of the tool and has a shiny and burnished appearance, caused by sliding as the chip moves up the tool face. 2. A surface that is the original surface of the workpiece; it has a rough, jagged appearance (as can be seen on the chips in Figs. 21.3 and 21.5) caused by the shearing mechanism shown in Fig. 21.4a. Continuous Chips. Continuous chips are generally formed with ductile materials, machined at high cutting speeds and/or at high rake angles (Fig. 21.5a). Deformation of the material takes place along a narrow shear zone, called the primary shear zone. Continuous chips may develop a secondary shear zone (Fig. 21.5b) because of high friction at the tool–chip interface; this zone becomes wider as friction increases. Deformation in continuous chips also may take place along a wide primary shear zone with curved boundaries (see Fig. 21.3b), unlike that shown in Fig. 21.5a. Note that the lower boundary of the deformation zone in Fig. 21.3b projects below the machined surface, subjecting it to distortion, as depicted by the distorted vertical lines within the machined subsurface. This situation generally occurs in machining soft metals, at low speeds, and low rake angles. It usually results in a poor surface finish and surface residual stresses, which may be detrimental to the properties of a machined part in its service life. Although they generally produce a good surface finish, continuous chips are not necessarily desirable as they tend to become tangled around the toolholder, the fixturing, and the workpiece. They also interfere with chip-disposal systems, described in Section 23.3.7. This situation can be alleviated using chip breakers (see below), as well as by changing processing parameters, such as cutting speed, feed, and depth of cut, or by using appropriate cutting fluids. Built-up Edge Chips. A built-up edge (BUE) consists of layers of material from the workpiece that gradually are deposited on the tool tip, hence the term built-up (Fig. 21.5c). As it grows larger, a BUE becomes unstable, and eventually breaks apart. A portion of the BUE material is carried away by the tool side or rake face of the chip; the rest is deposited randomly on the workpiece surface. Note that, in effect, a built-up edge changes the geometry of the cutting edge and dulls it, as can be seen in Fig. 21.6a. The cycle of BUE formation and destruction is repeated continuously during the cutting operation. Built-up edge is a major factor that adversely affects surface finish, as can be seen in Figs. 21.5c and 21.6b and c. On the other hand, a thin, stable BUE is generally regarded as desirable, because it reduces tool wear by protecting its rake face. Cold-worked metals have a lower tendency to form BUE than those in their annealed condition. Because of work hardening and deposition of successive layers of material, the BUE hardness is significantly higher than that of the workpiece (Fig. 21.6a). The tendency for BUE formation can be reduced by one or more of the following means: Increase the cutting speed Decrease the depth of cut  M21 KALP2244 08 GE C21 page 643 Mechanics of Cutting 643 Secondary shear zones Tool Chip Chip Tool Primary shear Workpiece Primary BUE zone shear zone (a) (b) (c) Low shear strain High shear strain (d) (e) Figure 21.5: Basic types of chips produced in orthogonal metal cutting, their schematic representation, and photomicrographs of the cutting zone: (a) continuous chip, with narrow, straight, and primary shear zone; (b) continuous chip, with secondary shear zone at the chip–tool interface; (c) built-up edge; (d) segmented or nonhomogeneous chip; and (e) discontinuous chip. Source: After M.C. Shaw, P.K. Wright, and S. Kalpakjian. Increase the rake angle Use a sharper tool Use a cutting tool that has lower chemical affinity for the workpiece material Use an effective cutting fluid. Serrated Chips. Serrated chips, also called segmented or nonhomogeneous chips (Fig. 21.5d), are semicontinu- ous chips with large zones of low shear strain and small zones of high shear strain (called shear localization). These chips have a sawtooth-like appearance (not be confused with the illustration in Fig. 21.4a, in which the dimension d is highly exaggerated). Metals that have low thermal conductivity and strength that decreases sharply with temperature (called thermal softening) exhibit this behavior, and is most notably observed with titanium.  M21 KALP2244 08 GE C21 page 644 644 Chapter 21 Fundamentals of Machining Chip 316 Built-up edge Hardness (HK) 474 306 661 372 588 329 289 (b) 565 492 325 331 588 286 289 371 418 656 604 432 684 383 386 656 589 306 466 704567 578 261 281 361 289 587 704512639 565 327 281 704 410 308 734770655 341 297 409 544 503 377 231 229 317 201 266 251 Workpiece 230 (a) (c) Figure 21.6: (a) Hardness distribution in a built-up edge in 3115 steel. Note that some regions within the built-up edge are as much as three times harder than the bulk metal being machined. (b) Surface fin- ish produced in turning 5130 steel with a built-up edge. (c) Surface finish on 1018 steel in face milling. Magnifications: 15×. Source: (b) and (c) Courtesy of TechSolve, Inc. Discontinuous Chips. Discontinuous chips consist of segments, either firmly or loosely attached to each other (Fig. 21.5e). Discontinuous chips generally develop under the following conditions: Brittle workpiece materials, because they do not have the capacity to undergo the high shear strains encountered in machining Workpiece materials containing hard inclusions and impurities, or have structures such as the graphite flakes in gray cast iron (see Fig. 4.11a) Very low or very high cutting speed, V Large depth of cut, d Tools with low rake angle, α Lack of an effective cutting fluid (Section 22.12) Low stiffness of the toolholder or the machine tool, thus allowing vibration and chatter to occur (Section 25.4). Another factor in the formation of discontinuous chips is the magnitude of the compressive stresses on the shear plane. The maximum shear strain at fracture increases with increasing compressive stress. Because of the discontinuous nature of chip formation, cutting forces continually vary during machin- ing. Consequently, the stiffness or rigidity of the cutting-tool holder, the workholding devices, and the machine tool and its condition (see Chapters 23 through 25) are significant factors in machining with ser- rated or discontinuous chips. If not sufficiently rigid, the machine tool may begin to vibrate and chatter, as  M21 KALP2244 08 GE C21 page 645 Mechanics of Cutting 645 described in detail in Section 25.4. This condition, in turn, adversely affects the surface finish and dimen- sional accuracy of the machined part, and it may cause premature wear or damage to the cutting tool. Even the components of the machine tool may be damaged if the amplitude of the vibration is excessive. Chip Curl. In all cutting operations performed on metals and nonmetallic materials, chips develop a cur- vature (chip curl) as they leave the workpiece surface (Fig. 21.5). Among the factors affecting chip curl are: The distribution of stresses in the primary and secondary shear zones Thermal effects in the cutting zone Work-hardening characteristics of the workpiece material The geometry of the cutting tool Process parameters Cutting fluids. The first four items above are complex phenomena and beyond the scope of this text. As for the effects of process parameters: as the depth of cut decreases, the radius of curvature of the chip generally decreases (i.e., the chip becomes more curly). Also, cutting fluids can make chips become more curly, thus reducing the tool–chip contact area (see Fig. 21.7a) and concentrating the heat closer to the tip of the tool (Section 21.4). As a result, tool wear increases. Chip Breakers. As stated above, continuous and long chips are undesirable in machining operations be- cause they tend to become severely entangled, interfere with the machining operation, and can also become a potential safety hazard. The usual procedure employed to avoid such a situation is to break the chip intermittently with special features on cutting tools, called chip-breakers, as shown in Fig. 21.7. The basic principles of a chip breaker on a tool’s rake face is to bend and break the chip periodically. Cut- ting tools and inserts (see Fig. 22.2) now have built-in chip-breaker features of various designs (Fig. 21.7). Chips also can be broken by changing the tool geometry to control chip flow, as in the turning operations shown in Fig. 21.8. Experience indicates that the ideal chip size to be broken is in the shape of either the letter C or the number 9, and fits within a 25-mm square space. Controlled Contact on Tools. Cutting tools can be designed such that the tool–chip contact length is de- liberately reduced by recessing the rake face of the tool some distance away from its tip. The reduction in contact length then affects the chip-formation mechanics; primarily, it reduces the cutting forces and, thus, the energy and temperature in machining. Determining an optimum length is important, as too small a contact length would concentrate the heat at the tool tip, increasing tool wear. Machining Nonmetallic Materials. The mechanics of cutting metals are generally applicable to polymers as well as metals. A variety of chips are encountered in cutting thermoplastics (Section 7.3), depending on the type of polymer and process parameters, such as depth of cut, tool geometry, and cutting speed. Because they are brittle, thermosetting plastics (Section 7.4) and ceramics (Chapter 8) generally produce discontinuous chips. The characteristics of other machined materials are described in Section 21.7.3. 21.2.2 Oblique Cutting The majority of machining operations involve tool shapes that are three dimensional, whereby the cutting action is oblique. The basic difference between oblique and orthogonal cutting can be seen in Fig. 21.9a and c. In orthogonal cutting, the chip slides directly up the face of the tool and it becomes a spiral, whereas in oblique cutting, the chip becomes helical and leaves the workpiece surface at an angle i, called the inclination angle (Fig. 21.9b). Note the lateral direction of chip movement in oblique cutting is similar to the action of a snowplow blade, whereby the snow is thrown sideways as the plow travels straight forward.  M21 KALP2244 08 GE C21 page 646 646 Chapter 21 Fundamentals of Machining (a) (b) Chip breaker Without chip breaker Chip Rake face of tool With chip breaker Clamp Tool Chip breaker Tool Workpiece (c) (d) Rake face Radius Positive rake 08 rake (e) Figure 21.7: (a) Machining aluminum using an insert without a chip breaker; note the long chips that can in- terfere with the tool and present a safety hazard. (b) Machining aluminum with a chip breaker. (c) Schematic illustration of the action of a chip breaker; note that the chip breaker decreases the radius of curvature of the chip and eventually breaks it. (d) Chip breaker clamped on the rake face of a cutting tool. (e) Grooves in cutting tools acting as chip breakers; the majority of cutting tools are now inserts with built-in chip-breaker features. Source: (a) and (b) Courtesy of Kennametal, Inc. Note in Fig. 21.9a that the chip moves up the rake face of the tool at an angle αc (called the chip flow angle), measured in the plane of the tool face. Angle αi is the normal rake angle, and is a basic geometric feature of the tool. It is the angle between line oz normal to the workpiece surface and line oa on the tool face.  M21 KALP2244 08 GE C21 page 647 Mechanics of Cutting 647 Shank (a) (b) (c) (d) Figure 21.8: Chips produced in turning: (a) tightly curled chip; (b) chip hits workpiece and breaks; (c) continuous chip moving radially away from workpiece; and (d) chip hits tool shank and breaks off. Source: After G. Boothroyd. z Top view a Tool at ac Chip a Tool y i o i 5 08 Chip o i 5 158 i Workpiece i 5 308 x Workpiece Chip (a) (b) (c) Figure 21.9: (a) Schematic illustration of cutting with an oblique tool; note the direction of chip move- ment. (b) Top view, showing the inclination angle, i. (c) Types of chips produced with tools at increasing inclination angles. In oblique cutting, the workpiece material approaches the cutting tool at a velocity V and leaves the surface (as a chip) with a velocity Vc. The effective rake angle, αe , is calculated in the plane of these two velocities. Assuming that the chip flow angle, αc , is equal to the inclination angle (an assumption that has been verified experimentally), the effective rake angle, αe , is   αe = sin−1 sin2 i + cos2 i sin αn. (21.9) Since both i and αn can be measured directly, the effective rake angle can now be calculated. Note that as i increases, the effective rake angle increases, the chip becomes thinner and longer and, as a consequence, the cutting force decreases. The influence of the inclination angle on chip shape is shown in Fig. 21.9c. A typical single-point turning tool, used on a lathe, is shown in Fig. 21.10a; note the various angles involved, each of which has to be selected properly for efficient cutting. Although these angles have tradi- tionally been produced by grinding (Chapter 26), the majority of cutting tools are now widely available as inserts, as shown in Fig. 21.10b and described in detail in Chapter 22. Various three-dimensional cutting tools, including those for drilling, tapping, milling, planing, shaping, broaching, sawing, and filing, are described in greater detail in Chapters 23 and 24. Shaving and Skiving. Thin layers of material can be removed from straight or curved surfaces by a process similar to the use of a plane in shaving wood. Shaving is used particularly for improving the surface finish and dimensional accuracy of sheared sheet metals and punched holes, as shown in Fig. 16.9.  M21 KALP2244 08 GE C21 page 648 648 Chapter 21 Fundamentals of Machining k an Sh is Ax Side-rake Face angle, 1 (SR) Toolholder Cutting edge Back-rake angle, 1 (BR) Clamp screw Axis Nose radius Clamp Flank Insert End-cutting- edge angle Side-relief angle Seat or shim (ECEA) Side-cutting-edge angle (SCEA) Clearance or end-relief angle Axis (a) (b) Figure 21.10: (a) Schematic illustration of a right-hand cutting tool. The various angles on these tools and their effects on machining are described in Section 23.2. Although these tools traditionally have been pro- duced from solid-tool steel bars, they have been replaced largely with (b) inserts, typically made of carbides and other materials; they are available in a wide variety of shapes and sizes. A common application of shaving is in finishing gears, using a cutter that has the shape of the gear tooth (see Section 24.7). Parts that are long or have complicated shapes are shaved by skiving, using a specially shaped cutting tool that moves tangentially across the length of the workpiece shaved. 21.3 Cutting Forces and Power Studying the cutting forces and power involved in machining operations is important for the following reasons: Data on cutting forces is essential so that 1. Machine tools can be designed to minimize distortion of their components, maintain the desired dimensional accuracy of the machined part, and help select appropriate toolholders and work- holding devices. 2. The workpiece, the workholding devices, and the fixtures are capable of withstanding these forces without excessive distortion. Power requirements must be known to enable the selection of a machine tool with sufficient capacity or to select process parameters that can be achieved by the machine selected. The forces acting in orthogonal cutting are shown in Fig. 21.11a. The cutting force, Fc , acts in the direction of the cutting speed, V , and supplies the energy required for cutting. The ratio of the cutting force to the cross-sectional area being cut (i.e., the product of width of cut and depth of cut) is referred to as the specific cutting force. The thrust force, Ft , acts in a direction normal to the cutting force. These two forces produce the resul- tant force, R, as can be seen from the force circle diagram shown in Fig. 21.11b. Note that the resultant force can be resolved into two components on the tool face: a friction force, F , along the tool–chip interface, and a normal force, N , perpendicular to it.  M21 KALP2244 08 GE C21 page 649 Cutting Forces and Power 649 a a Tool Chip V Chip R F Ft Fs b Tool Fc Fc f N V Fs a b2a Fn Ft Fn R f F Workpiece R b Workpiece N (a) (b) Figure 21.11: (a) Forces acting in the cutting zone during two-dimensional cutting. Note that the resultant force, R, must be colinear to balance the forces. (b) Force circle to determine various forces acting in the cutting zone. It can also be shown that F = R sin β (21.10) and N = R cos β. (21.11) Note that the resultant force is balanced by an equal and opposite force along the shear plane, and is resolved into a shear force, Fs , and a normal force, Fn. These forces can be expressed as Fs = Fc cos φ − Ft sin φ (21.12) and Fn = Fc sin φ + Ft cos φ. (21.13) Because the area of the shear plane can be calculated by knowing the shear angle and the depth of cut, the shear and normal stresses in the shear plane can thus be determined. The ratio of F to N is the coefficient of friction at the tool–chip interface, μ, and the angle β is the friction angle (as in Fig. 21.11). The magnitude of μ can be determined as F Ft + Fc tan α μ= =. (21.14) N Fc − Ft tan α Although the magnitude of forces in actual cutting operations is generally on the order of a few hundred newtons, the local stresses in the cutting zone and the pressure on the cutting tool are very high because the contact areas are very small. For example, the tool–chip contact length (see Fig. 21.3) is typically on the order of 1 mm. Consequently, the tool tip is subjected to very high stresses, which lead to wear as well as chipping and fracture of the tool. Thrust Force. The thrust force in cutting is important because the toolholder, the work-holding devices, and the machine tool itself must be sufficiently stiff to support that force with minimal deflections. For example, if the thrust force is too high or if the machine tool is not sufficiently stiff, the tool will deflect away from the workpiece. This movement will, in turn, reduce the depth of cut, resulting in poor dimensional accuracy in the machined part.  M21 KALP2244 08 GE C21 page 650 650 Chapter 21 Fundamentals of Machining The effect of rake angle and friction angle on the magnitude and direction of thrust force can be determined by noting, from Fig. 21.11b, that Ft = R sin (β − α) , (21.15) or Ft = Fc tan (β − α). (21.16) The magnitude of the cutting force, Fc , is always positive, as shown in Fig. 21.11, because it is this force that supplies the work required in cutting. However, the sign of the thrust force, Ft , can be either positive or negative, depending on the values of β and α. Note that when β > α, the sign of Ft is positive (downward), and when β < α, the sign is negative (upward). It is therefore possible to have an upward thrust force under the conditions of (a) high rake angles, (b) low friction at the tool–chip interface, or (c) both. A negative thrust force can have important implications in the design of machine tools and workholders and in the stability of the cutting process. Power. It can be seen from Fig. 21.11 that the power input in cutting is Power = Fc V. (21.17) The power is dissipated mainly in the shear zone (due to the energy required to shear the material) and on the rake face of the tool (due to tool–chip interface friction). From Figs. 21.4b and 21.11, the power dissipated in the shear plane is Power for shearing = Fs Vs. (21.18) Denoting the width of cut as w, the specific energy for shearing, us , is given by Fs V s us =. (21.19) wto V Similarly, the power dissipated in friction is Power for friction = F Vc , (21.20) and the specific energy for friction, uf , is F Vc Fr uf = =. (21.21) wto V wto The total specific energy, ut , is thus ut = us + uf. (21.22) Because numerous factors are involved, reliable prediction of cutting forces and power still is based largely on experimental data, such as those given in Table 21.2. The wide range of values seen in the table can be attributed to differences in strength within each material group, and to other factors, such as friction, use of cutting fluids, the wide range in process parameters, and the sharpness of the tool tip. Dull tools require higher power and result in higher forces because the tip rubs against the machined surface and makes the deformation zone ahead of the tool larger. Measuring Cutting Forces and Power. Cutting forces can be measured using a force transducer (typically with quartz piezoelectric sensors), a dynamometer, or a load cell (with resistance-wire strain gages placed on octagonal rings) mounted on the cutting-tool holder. It is also possible to calculate the cutting force from the power consumption during cutting, using Eq. (21.4). It should be recognized that Eq. (21.4) represents the power in the machining process itself, and the machine tool will need additional power in order to overcome friction. Thus, to determine the cutting  M21 KALP2244 08 GE C21 page 651 Cutting Forces and Power 651 Table 21.2: Approximate Range of Energy Requirements in Cutting Operations at the Drive Motor of the Machine Tool, Corrected for 80% Efficiency (for dull tools, multiply by 1.25). Specific energy Material W-s/mm3 Aluminum alloys 0.4–1 Cast irons 1.1–5.4 Copper alloys 1.4–3.2 High-temperature alloys 3.2–8 Magnesium alloys 0.3–0.6 Nickel alloys 4.8–6.7 Refractory alloys 3–9 Stainless steels 2–5 Steels 2–9 Titanium alloys 2–5 force from the measured machine power consumption, the mechanical efficiency of the machine tool must be known. The specific energy in cutting, such as that shown in Table 21.2, also can be used to estimate cutting forces. Example 21.1 Relative Energies in Cutting Given: In an orthogonal cutting operation, to = 0.1 mm, V = 2 m/s, α = 10◦ , and the width of cut is 5 mm. It is observed that tc = 0.20 mm, Fc = 500 N, and Ft = 200 N. Find: Calculate the percentage of the total energy that goes into overcoming friction at the tool–chip interface. Solution: The percentage of the energy can be expressed as Friction energy F Vc Fr = = , Total energy Fc V Fc where to 0.1 r= = = 0.50, tc 0.20 F = R sin β, Fc = R cos (β − α) , and   R= Ft2 + Fc2 = 2002 + 5002 = 538 N. Thus, 500 = 538 cos (β − 10◦ ) , so β = 32◦ and F = 538 sin 32◦ = 285 N. Hence, (285)(0.5) Percentage = = 0.28, or 28%. 500  M21 KALP2244 08 GE C21 page 652 652 Chapter 21 Fundamentals of Machining 21.4 Temperatures in Cutting As in all metalworking processes involving plastic deformation (Chapters 13 through 16), the energy dis- sipated in cutting is converted into heat which, in turn, raises the temperature in the cutting zone and the workpiece surface. Temperature rise is a major factor in machining because of its various adverse effects: Excessive temperature lowers the strength, hardness, stiffness, and wear resistance of the cutting tool; tools may also soften and undergo plastic deformation, thus the altering tool shape. Heat causes uneven dimensional changes in the part being machined, thus making it difficult to control its dimensional accuracy and tolerances. An excessive temperature rise can induce thermal damage and metallurgical changes (Chapter 4) in the machined surface, adversely affecting properties. The main sources of heat in machining are: (a) work done in shearing in the primary shear zone, (b) energy dissipated as friction at the tool–chip interface, and (c) heat generated as the tool rubs against the machined surface, especially with dull or worn tools. Much effort has been expended in establishing relationships among temperature and various material and process variables in cutting. It can be shown that, in orthogonal cutting, the mean temperature, Tmean , in K is  0.000665σf 3 V to Tmean = , (21.23) ρc K where σf is the flow stress (see Section 14.2), in MPa, ρc is the volumetric specific heat in kJ/m3 ·K, and K is the thermal diffusivity (ratio of thermal conductivity to volumetric specific heat) in m2 /s. Because the material parameters in this equation also depend on temperature, it is important to use appropriate values that are applicable to the predicted temperature range. It can be seen from Eq. (21.23) that the mean cutting temperature increases with workpiece strength, cutting speed, and depth of cut, and decreases with increasing specific heat and thermal conductivity of the workpiece material. A simple expression for the mean temperature in turning on a lathe is given by Tmean ∝ V a f b , (21.24) where V is the cutting speed and f is the feed of the tool, as shown in Fig. 21.2. Approximate values of the exponents a and b are a = 0.2 and b = 0.125 for carbide tools and a = 0.5 and b = 0.375 for high-speed steel tools. Temperature Distribution. Because the sources of heat generation in machining are concentrated in the primary shear zone and at the tool–chip interface, it is to be expected that there will be severe temperature gradients within the cutting zone. A typical temperature distribution is shown in Fig. 21.12; note the presence of severe gradients, and that the maximum temperature is about halfway up the tool–chip interface. The temperatures typically developed in a turning operation on 52100 steel are shown in Fig. 21.13. The temperature distribution along the flank surface of the tool is shown in Fig. 21.13a for V = 60, 90, and 170 m/min as a function of the distance from the tip of the tool. The distributions at the tool–chip interface for the same three cutting speeds are shown in Fig. 21.13b as a function of the fraction of the contact length. Thus, zero on the abscissa represents the tool tip, and 1.0 represents the end of the tool–chip contact length. Note from Eq. (21.23) that the temperature increases with cutting speed and that the highest tem- perature is almost 1100◦ C. The presence of such high temperatures in machining can be verified simply by observing the dark-bluish color of the chips (caused by oxidation) typically produced at high cutting speeds. Chips can indeed become red hot, and thus create a safety hazard. From Eq. (21.24) and the values for the exponent a, it can be seen that the cutting speed, V , greatly influences temperature. The explanation is that, as speed increases, the time for heat dissipation decreases,  M21 KALP2244 08 GE C21 page 653 Temperatures in Cutting 653 Chip 600 600 00 5 Temperature (8C) 450 650 400 70 0 360 Tool 65 38 0 0 600 500 130 80 30 Workpiece Figure 21.12: Typical temperature distribution in the cutting zone. Note the severe temperature gradients within the tool and the chip, and that the workpiece is relatively cool. Local temperature at tool–chip interface (8C) 1100 Work material: AISI 52100 Annealed: 188 HB Flank surface temperature (8C) Tool material: K3H carbide 2.8 m/s 700 2.8 m/s 900 V5 1. 5 600 700 1. 0 1. 5 Feed: 0.14 mm/rev 1.0 500 500 300 400 0 0.5 1.0 1.5 0 0.2 0.4 0.6 0.8 1.0 mm Fraction of tool–chip contact length measured in the direction of chip flow (a) (b) Figure 21.13: Temperatures developed in turning 52100 steel: (a) flank temperature distribution and (b) tool–chip interface temperature distribution. Source: After B.T. Chao and K.J. Trigger. and hence the temperature rises, eventually becoming almost an adiabatic process. The effect of speed can be simulated easily by rubbing hands together faster and faster. As can be seen in Fig. 21.14, the chip carries away most of the heat generated. In a typical machining operation, it has been estimated that 90% of the energy is removed by the chip, with the remainder taken by the tool and the workpiece. Note also that, as the cutting speed increases, a larger proportion of the total  M21 KALP2244 08 GE C21 page 654 654 Chapter 21 Fundamentals of Machining l Too iece p ork Energy (%) W Chip Cutting speed Figure 21.14: Proportion of the heat generated in cutting transferred to the tool, workpiece, and chip as a function of the cutting speed. Note that the chip removes most of the heat. heat generated is carried away by the chip, and less heat is transferred elsewhere. This is one reason for the continued trend of increasing machining speeds (see high-speed machining, Section 25.5). The other main benefit of higher cutting speeds is associated with the favorable economics in reducing machining time (see Section 25.8). Techniques for Measuring Temperature. Temperatures and their distribution in the cutting zone may be determined using thermocouples, embedded in the tool or the workpiece. The mean temperature can be determined using the thermal emf (electromotive force) at the tool–chip interface, which acts as a hot junc- tion between two different materials (tool and chip). A third method is monitoring the infrared radiation from the cutting zone, using sensors; however, this technique indicates only surface temperatures, and its accuracy depends on the emissivity of the surfaces, which can be difficult to determine accurately. 21.5 Tool Life: Wear and Failure It can be noted from the previous sections that cutting tools are subjected to (a) high localized stresses at the tip of the tool, (b) high temperatures, especially along the rake face, (c) sliding of the chip at relatively high speeds along the rake face, and (d) sliding of the tool along the newly machined workpiece surface. These conditions induce tool wear, a major consideration in all machining operations (as are mold and die wear in casting and metalworking processes). Tool wear, in turn, adversely affects tool life, the quality of the machined surface, its dimensional accuracy, and, consequently, the economics of machining operations. Wear is a gradual process (see Section 33.5), much like the wear of the tip of an ordinary pencil. The rate of tool wear (that is, volume worn per unit time) depends on the workpiece material, tool material and its coatings, tool geometry, process parameters, cutting fluids, and characteristics of the machine tool. Tool wear and the resulting changes in tool geometry (Fig. 21.15) are generally classified as: flank wear, crater wear, nose wear, notching, plastic deformation, chipping, and gross fracture. 21.5.1 Flank Wear Flank wear occurs on the relief (flank) face of the tool, as shown in Fig. 21.15a, b, and e. It generally is attributed to (a) rubbing of the tool along the machined surface, thereby causing adhesive or abrasive wear and (b) high temperatures, adversely affecting tool-material properties. In a classic study by F.W. Taylor on the machining of steels conducted in the early 1890s, the following approximate relationship for tool life, known as the Taylor tool life equation, was established: V T n = C. (21.25) where V is the cutting speed, T is the time (in minutes) that it takes to develop a certain flank wear land (shown as V B in Fig. 21.15a), n is an exponent that depends on tool and workpiece materials and cutting  M21 KALP2244 08 GE C21 page 655 Tool Life: Wear and Failure 655 Rake Flank wear Depth-of-cut line face Crater Crater Rake face wear wear depth Tool R Nose VBmax VB (KT) radius Flank face Flank Flank New wear face tool Depth-of-cut line (a) Rake face Rake face Flank wear Crater wear Flank face Flank face (b) (c) Thermal BUE cracking Flank face Rake face (d) (e) Figure 21.15: (a) Features of tool wear in a turning operation. The VB indicates average flank wear. (b)–(e) Examples of wear in cutting tools: (b) flank wear, (c) crater wear, (d) thermal cracking, and (e) flank wear and built-up edge. Source: (a) Terms and definitions reproduced with the permission of the International Organization for Standardization, ISO, copyright remains with ISO. (b)–(e) Courtesy of Kennametal Inc. conditions, and C is a constant. Each combination of workpiece and tool materials and each cutting condi- tion have their own n and C values, both of which are determined experimentally, often based on surface finish requirements. Moreover, the Taylor equation is often applied even when flank wear is not the domi- nant wear mode (see Fig. 21.15), or if a different criterion (such as the machining power required) is used to define C and n. Generally, n depends on the tool material, as shown in Table 21.3, and C on the workpiece material. Note that the magnitude of C is the cutting speed at T = 1 min. To appreciate the importance of the exponent n, Eq. (21.25) can be rewritten as  1/n C T = , (21.26) V where it can be seen that for a constant value of C, the smaller the value of n, the lower is the tool life. Cutting speed is the most important variable associated with tool life, followed by depth of cut and feed, f. For turning, Eq. (21.25) can be modified as V T n dx f y = C, (21.27)  M21 KALP2244 08 GE C21 page 656 656 Chapter 21 Fundamentals of Machining Table 21.3: Ranges of n Values for the Taylor Equation [Eq. (21.25)] for Various Tool Materials. High-speed steels 0.08–0.2 Cast alloys 0.1–0.15 Carbides 0.2–0.5 Coated carbides 0.4–0.6 Ceramics 0.5–0.7 where d is the depth of cut and f is the feed in mm/rev, as shown in Fig. 21.2. The exponents x and y must be determined experimentally for each cutting condition. Taking n = 0.15, x = 0.15, and y = 0.6 as typical values encountered in machining practice, it can be seen that cutting speed, feed rate, and depth of cut are of decreasing importance. Equation (21.27) can be rewritten as T = C 1/n V −1/n d−x/n f −y/n , (21.28) or, using typical values for the exponents, as T ≈ C 7 V −7 d−1 f −4. (21.29) For a constant tool life, the following observations can be made from Eq. (21.29): If the feed or the depth of cut is increased, the cutting speed must be decreased, and vice versa. Depending on the magnitude of the exponents, a reduction in speed can result in an increase in the volume of the material removed, because of the increased feed or depth of cut. Tool-life Curves. Tool-life curves are plots of experimental data, obtained from cutting tests for various materials and under different cutting conditions, such as cutting speed, feed, depth of cut, tool material and geometry, and cutting fluids. Note in Fig. 21.16, for example, that (a) tool life decreases rapidly as the cutting speed increases, (b) the condition of the workpiece material has a strong influence on tool life, and (c) there is a large difference in tool life for different microstructures of the workpiece material. Heat treatment of the workpiece is important, due largely to increasing workpiece hardness; for ex- ample, ferrite has a hardness of about 100 HB, pearlite 200 HB, and martensite 300 to 500 HB. Impurities 120 5 Hardness 1 Ferrite Pearlite Tool life (min) (HB) 80 2 3 1 As cast 265 20% 80% 4 2 As cast 215 40 60 40 3 As cast 207 60 40 4 Annealed 183 97 3 5 Annealed 170 100 — 0 50 100 150 200 250 m/min Figure 21.16: Effect of workpiece hardness and microstructure on tool life in turning ductile cast iron. Note the rapid decrease in tool life (approaching zero) as the cutting speed increases. Tool materials have been developed that resist high temperatures, such as carbides, ceramics, and cubic boron nitride, as described in Chapter 22.  M21 KALP2244 08 GE C21 page 657 Tool Life: Wear and Failure 657 300 100 Cast-cob Tool life (min) High-speed Ce arbide 20 ram C ic 10 a lt alloy steel 5 n 1 50 300 3000 Cutting speed (m/min) Figure 21.17: Tool-life curves for a variety of cutting-tool materials. The negative reciprocal of the slope of these curves is the exponent n in the Taylor tool-life equation [Eq. (21.25)], and C is the cutting speed at T = 1 min, ranging from about 60 to 3000 m/min in this figure. and hard constituents in the material or on the workpiece surface, such as rust, scale, and slag, also are important factors, because their abrasive action (see Section 33.5) reduces tool life. The exponent n can be determined from tool-life curves (Fig. 21.17). Note that the smaller the value of n, the faster the tool life decreases with increasing cutting speed. Although tool-life curves are somewhat linear over a limited range of cutting speeds, they rarely are linear over a wide range. Moreover, n can indeed become negative at low cutting speeds, meaning that tool-life curves actually can reach a maximum and then curve downward. Caution should therefore be exercised in using tool-life equations beyond the range of cutting speeds to which they are applicable. Because temperature has a major influence on the physical and mechanical properties of materials (see Chapters 2 and 3), it is to be expected that temperature also strongly influences wear. Thus, as temperature increases, wear increases. Example 21.2 Increasing Tool Life by Reducing the Cutting Speed Given: Assume that for a given tool and workpiece combination, n = 0.5 and C = 400. Find: Calculate the percentage increase in tool life when the cutting speed is reduced by 50%, using the Taylor equation [Eq. (21.25)] for tool life. Solution: Since n = 0.5, the Taylor equation can be rewritten as V T 0.5 = 400. Denote V1 as the initial speed and V2 as the reduced speed; thus, V2 = 0.5V1. Because C is a constant at 400,   0.5V1 T2 = V1 T1. Simplifying this equation, T2 1 = = 4. T1 0.25 Thus the change in tool life is   T2 − T1 T2 = − 1 = 4 − 1 = 3, T1 T1 or that tool life is increased by 300%. Note that a reduction in cutting speed has resulted in a major increase in tool life. Note also that, for this problem, the magnitude of C is not relevant.  M21 KALP2244 08 GE C21 page 658 658 Chapter 21 Fundamentals of Machining Table 21.4: Allowable Average Wear Land (see V B in Fig. 21.15a) for Cutting Tools in Various Machining Operations. Allowable wear land (mm) Operation High-speed steel tools Carbide tools Turning 1.5 0.4 Face milling 1.5 0.4 End milling 0.3 0.3 Drilling 0.4 0.4 Reaming 0.15 0.15 Note: Allowable wear for ceramic tools is about 50% higher. Allowable notch wear (see Section 21.5.3), V Bmax , is about twice that for V B. Allowable Wear Land. Cutting tools need to be resharpened or replaced when (a) the surface finish of the machined workpiece begins to deteriorate, (b) cutting forces increase significantly, or (c) the temperature rises significantly. The allowable wear land, indicated as V B in Fig. 21.15a, is given in Table 21.4 for various machining conditions. For improved dimensional accuracy and surface finish, the allowable wear land may be smaller than the values given in the table. The recommended cutting speed for a high-speed steel tool (see Section 22.2) is generally the one that yields a tool life of 60 to 120 min, and for a carbide tool (Section 22.4), it is 30 to 60 min. Optimum Cutting Speed. Recall that as cutting speed increases, tool life decreases rapidly. On the other hand, if the cutting speed is low, tool life is longer, but the rate at which material is removed is also low. Thus, there is an optimum cutting speed, based on economic or production considerations, where the tool life is long and production speeds are reasonably high. This topic is described in greater detail in Section 25.8. Example 21.3 Effect of Cutting Speed on Material Removal The effect of cutting speed on the volume of metal removed between tool changes or resharpenings can be appreciated by analyzing Fig. 21.16. Assume that a material is being machined, in the as-cast condition, with a hardness of 265 HB. Note that when the cutting speed is 60 m/min, tool life is about 40 min. Therefore, the tool travels a distance of 60 m/min × 40 min = 2400 m before it has to be replaced. However, when the cutting speed is increased to 120 m/min, tool life is reduced to about 5 min, and thus the tool travels 120 m/min × 5 min = 600 m before it has to be replaced. Since the volume of material removed is directly proportional to the distance the tool has traveled, it can be seen that by decreasing the cutting speed, more material is removed between tool changes. Note, however, that the lower the cutting speed, the longer is the time required to machine a part, which has a significant economic impact on the operation (see Section 25.8). 21.5.2 Crater Wear Crater wear occurs on the rake face of the tool, as shown in Fig. 21.15a and c, and Fig. 21.18, which also illustrates various types of tool wear and failures. It can be seen that crater wear alters the tool–chip contact geometry. The most significant factors that influence crater wear are (a) the temperature at the tool– chip interface and (b) the chemical affinity of the tool and workpiece materials. Additionally, the factors influencing flank wear also may affect crater wear. Crater wear is generally attributed to a diffusion mechanism: the movement of atoms across the tool– chip interface. Because diffusion rate increases with increasing temperature, crater wear also increases as temperature increases. Note in Fig. 21.19, for example, how rapidly crater wear increases with tem- perature within a narrow range. Applying protective coatings to tools is an effective means of slowing  M21 KALP2244 08 GE C21 page 659 Tool Life: Wear and Failure 659 Thermal cracks in interrupted cutting 1 Flank wear (wear land) 2 Crater wear 2 Chamfer 5 3 Primary groove or 2 5 depth-of-cut line 2 6 4 4 3 1 4 Secondary groove 3 (oxidation wear) 1 4 5 Outer-metal chip notch 1 6 Inner chip notch Carbide High-speed steel Ceramic (a) 1 Flank wear Chamfer 2 2 2 Crater wear 5 3 Failure face 4 3 4 Primary groove or depth-of-cut line 3 1 1 5 Outer-metal chip notch 6 6 Plastic flow around failure face High-speed steel tool, thermal Ceramic tool, chipping, softening, and plastic flow and fracture (b) Figure 21.18: (a) Schematic illustrations of types of wear observed on various cutting tools. (b) Schematic illustrations of catastrophic tool failures. A wide range of parameters influence these wear and failure patterns. Source: Courtesy of V.C. Venkatesh. Crater-wear rate eed steel 0.30 mm3/min bide ide arb 0.15 car -sp 1c gh C5 Hi C 0 500 700 900 1100 Average tool–chip interface temperature (8C) Figure 21.19: Relationship between crater-wear rate and average tool–chip interface temperature: (1) high- speed steel, (2) C1 carbide, and (3) C5 carbide (see Table 22.5). Note how rapidly crater-wear rate increases with an incremental increase in temperature. Source: After B.T. Chao and K.J. Trigger. the diffusion process, and thus reducing crater wear. Typical tool coatings are titanium nitride, titanium carbide, diamondlike carbon, titanium carbonitride, and aluminum oxide, and are described in greater detail in Section 22.6. In comparing Figs. 21.12 and 21.15a, it can be seen that the location of the maximum depth of crater wear, KT , coincides with the location of the maximum temperature at the tool–chip interface. An actual cross section of this interface, for steel machined at high speeds, is shown in Fig. 21.20. Note that the wear pattern on the tool face coincides with its discoloration pattern, an indication of the presence of high temperatures.  M21 KALP2244 08 GE C21 page 660 660 Chapter 21 Fundamentals of Machining Rake face Crater wear Chip Flank face Figure 21.20: Interface of a cutting tool (right) and chip (left) in machining plain-carbon steel. The discol- oration of the tool indicates the presence of high temperatures. Compare this figure with the temperature profiles shown in Fig. 21.12. Source: After P.K. Wright. 21.5.3 Other Types of Wear, Chipping, and Fracture Nose wear (Fig. 21.15a) is the rounding of a sharp tool due to mechanical and thermal effects. It dulls the tool, affects the type of chip formation, and causes rubbing of the tool over the workpiece, raising temper- ature and inducing residual stresses on the machined surface. A related phenomenon is edge rounding, as shown in Fig. 21.15a. An increase in temperature is particularly important for high-speed steel tools, as can be appreciated from Fig. 22.1. Tools may also undergo plastic deformation, because of temperature rises in the cutting zone where temperatures can easily reach 1000◦ C in machining steels, and can even be higher depending on the strength of the material machined. Notches or grooves that develop on cutting tools, as shown in Figs. 21.15a and 21.18, have been at- tributed to the fact that the region where they occur is the boundary where the chip is no longer in contact with the tool. Known as the depth-of-cut line (DOC), see Fig. 21.15a, this boundary oscillates, because of inherent variations in the cutting operation. If sufficiently deep, the groove can lead to gross chipping of the tool tip because of (a) its now reduced cross section and (b) the notch sensitivity of the tool material. Scale and oxide layers on a workpiece surface also contribute to notch wear, because these layers are hard and abrasive. Thus, light cuts should be avoided on such workpieces. In Fig. 21.3 for example, the depth of cut, to , should be greater than the thickness of the scale on the workpiece. In addition to being subjected to wear, cutting tools may also undergo chipping, where a small frag- ment from the cutting edge of the tool breaks away. This phenomenon, which typically occurs in brittle tool materials such as ceramics, is similar to chipping of the tip of a pencil if it is too sharp. The chipped fragments from the cutting tool may be very small (called microchipping or macrochipping, depending on its size), or they may be relatively large, in which case they are variously called gross chipping, gross fracture, and catastrophic failure (Fig. 21.18). Chipping also may occur in a region of the tool where there is a preexisting small crack or a defect during its production. Unlike wear, which is a gradual process, chipping is a sudden loss of the tool mate- rial, thus changing the tool’s shape. As can be expected, chipping has a major detrimental effect on surface finish, surface integrity, and the dimensional accuracy of the workpiece being machined.  M21 KALP2244 08 GE C21 page 661 Surface Finish and Integrity 661 Two main causes of chipping are: Mechanical shock, such as impact due to interrupted cutting, as in turning a splined shaft on a lathe. Thermal fatigue, due to cyclic temperature variations within the tool in interrupted cutting. Thermal cracks usually perpendicular to the cutting edge of the tool, as shown on the rake face of the carbide tool in Figs. 21.15d and 21.18a. Major variations in the composition or structure of the workpiece material also may cause chipping, due to differences in their thermal properties. Chipping can be reduced by selecting tool materials with high impact and thermal-shock resistance, as described in Chapter 22. High positive-rake angles can contribute to chipping because of the small included angle of the tool tip, as can be visualized from Fig. 21.3. Also, it is possible for the crater-wear region to progress slowly toward the tool tip, thus weakening the tip because of reduced volume of material. 21.5.4 Tool-condition Monitoring With rapid advances in computer-controlled machine tools and automated manufacturing, the reliable and repeatable performance of cutting tools is a major consideration. As described in Chapters 23 through 25, modern machine tools operate with little direct supervision by an operator. Moreover, they are typically enclosed, making it virtually impossible to closely monitor the machining operation and the condition of the cutting tool. It is thus essential to indirectly and continuously monitor the condition of the cutting tool. In machine tools, tool-condition monitoring systems are now integrated into computer numerical control and programmable logic controllers. Techniques for tool-condition monitoring typically fall into two general categories: direct and indirect. The direct method for observing the condition of a cutting tool involves optical measurements of wear, such as periodic observation of changes in the tool profile. This is a common technique, and is done us- ing a toolmakers’ microscope. However, this method requires that the cutting operation be stopped for tool observation. Another direct method involves programming the tool to contact a sensor (touch probe) after each machining cycle; this approach allows the measurement of wear and/or the detection of broken tools. Indirect methods involve correlating the tool condition with parameters such as cutting forces, power, temperature rise, workpiece surface finish, vibration, and chatter. A common technique is acoustic emis- sion (AE), which utilizes a piezoelectric transducer mounted on a toolholder. The transducer picks up acoustic emissions (typically above 100 kHz) which result from the stress waves generated during cutting. By an- alyzing the signals, tool wear and chipping can be monitored. This technique is effective particularly in precision-machining operations, where cutting forces are low (because of the small amounts of material re- moved). Another effective use of AE is in detecting the fracture of small carbide tools at high cutting speeds. A similar indirect technique consists of various sensors that are installed in the original machine tool, or are retrofitted on existing machines. The system continually monitors torque and forces during machin- ing. The signals are analyzed and interpreted to differentiate between events: tool breakage, tool wear, a missing tool, overloading of the machine tool, or colliding with machine components. This system also can compensate automatically for tool wear, and thus improve the dimensional accuracy of the part being machined. The design of these systems must be such that they are (a) nonintrusive to the machining operation, (b) accurate and repeatable in signal detection, (c) resistant to abuse, (d) robust for t