Manufacturing 21

generates the fastest wear rates. Such curves often have three general regions, as shown in the figure.The central region is a steady-state region (or the region of secondary wear). This is the normal operating region for the tool. Such curves are typical for both flank wear and crater wear.When the amount of wear reaches the value wf, the permissible tool wear on the flank, the tool is said to be "worn out." wf is typically set at 0.025 to 0.030 in. for flank wear for high-speed steels and 0.008 to 0.050 for carbides, depending on the application. For crater wear, the depth of the crater is used to determine tool failure. Using the empirical tool wear data shown in Figure 21-17, which used the values of T (time in minutes) associated with V (cutting speed) for a given amount of tool wear, wf (see the dashed-line construction), Figure 21-18 was developed. When V and T are plotted on log-log scales, a linear relationship appears, described by the equation (21-3) This equation is called the Taylor tool life equation because in 1907, F.W. Taylor published his now-famous paper "On the Art of Cutting Metals" in ASME Transactions, wherein tool life (T) was related to cutting speed (V) and feed (f).This equation had the form (21-4) which over the years took the more widely published form where n is an exponent that depends mostly on tool material but is affected by work material, cutting conditions, and environment and C is a constant that depends on all the input parameters, including feed. Table 21-5 provides some data on Taylor tool life constants. Figure 21-19 shows typical tool life curves for one tool material and three work materials. Notice that all three plots have about the same slope, n. Typical values for n are 0.14 to 0.16 for HSS, 0.21 to 0.25 for uncoated carbides, 0.30 for TiC inserts, 0.33 for polydiamonds, 0.35 for TiN inserts, and 0.40 for ceramic-coated inserts. It takes a great deal of experimental effort to obtain the constants for the Taylor equation, as each combination of tool and work material will have different constants. Note that for a tool life of 1 minute,C V, or the cutting speed that yields about 1 minute of tool life for this tool. A great deal of research has gone into developing more sophisticated versions of the Taylor equation, wherein constants for other input parameters (typically feed, depth of cut, and work material hardness) are experimentally determined, for example,

speeds (25% higher) than HSS tools. Cast cobalt alloys are hard as cast and cannot be softened or heat treated. Cast cobalt alloys contain a primary phase of Co-rich solid solution strengthened by Cr and W and dispersion hardened by complex hard, refractory carbides of W and Cr. Other elements added include V, B, Ni, and Ta. The casting provides a tough core and elongated grains normal to the surface. The structure is not, however, homogeneous. Tools of cast cobalt alloys are generally cast to shape and finished to size by grinding. They are available only in simple shapes, such as single-point tools and saw blades, because of limitations in the casting process and the expense involved in the final shaping (grinding). The high cost of fabrication is primarily due to the high hardness of the material in the as-cast condition. Materials machinable with this tool material include plain-carbon steels, alloy steels, nonferrous alloys, and cast iron. Cast cobalt alloys are currently being phased out for cutting-tool applications because of increasing costs, shortages of strategic raw materials (Co, W, and Cr), and the development of other, superior tool materials at lower cost. CARBIDE OR SINTERED CARBIDES Carbide cutting-tool inserts are traditionally divided into two primary groups: 1. Straight tungsten grades, which are used for machining cast irons, austenitic stainless steel, and nonferrous and nonmetallic materials. 2. Grades containing major amounts of titanium, tantalum, and or columbium carbides, which are used for machining ferritic workpieces.There are also the titanium carbide grades, which are used for finishing and semifinishing ferrous alloys. The classification of carbide insert grades employs a C-classification system in the United States and ISO P and M classification system in Europe and Japan. These classifications are based on application, rather than composition or properties. Each cuttingtool vendor can provide proprietary grades and recommended applications. Carbides, which are nonferrous alloys, are also called sintered (or cemented) carbides because they are manufactured by powder metallurgy techniques.The P/M process is outlined in Figure 21-6. See Chapter 16 for details on powder metallurgy processes. These materials became popular during World War II, as they afforded a four- or fivefold increase in cutting speeds. The early versions had tungsten carbide as the major constituent, with a cobalt binder in amounts of 3 to 13%. Most carbide tools in use today

8. Adequate thermal properties 9. High elastic modulus (stiffness) 10. Correct geometry and surface finish Figure 21-5 compares these properties for various cutting-tool materials. Overlapping characteristics exist in many cases. Exceptions to the rule are very common. In many classes of tool materials, a wide range of compositions and properties are obtainable.

A chip groove (see Figure 21-7) with a positive rake angle at the tool tip may also be used to reduce cutting forces without reducing the overall strength of the insert significantly. The groove also breaks up the chips for easier disposal by causing them to curl tightly. For very low-speed cutting operations, the chips tend to weld to the tool face and cause subsequent microchipping of the cutting edge. Cutting speeds are generally in the range of 150 to 600 ft/min. Higher speeds (>1000 ft/min) are recommended for certain less-difficult-to-machine materials (such as aluminum alloys) and much lower speeds (100 ft/min) for more difficult-to-machine materials (such as titanium alloys). In interrupted cutting applications, it is important to prevent edge chipping by choosing the appropriate cutter geometry and cutter position with respect to the workpiece. For interrupted cutting, finer grain size and higher cobalt content improve toughness in straight WC-Co grades. After use, carbide inserts (called disposable or throwaway inserts) are generally recycled in order to reclaim the Ta, WC, and Co. This recycling not only conserves strategic materials but also reduces costs. A new trend is to regrind these tools for future use where the actual size of the insert is not of critical concern. COATED-CARBIDE TOOLS Beginning in 1969 with TiC-coated WC, coated tools became the norm in the metalworking industry because coating can consistently improve tool life 200 or 300% or more. In cutting tools, material requirements at the surface of the tool need to be abrasion resistant, hard, and chemically inert to prevent the tool and the work material from interacting chemically with each other during cutting.A thin, chemically stable, hard refractory coating of TiC, TiN, or AL2O3 accomplishes this objective. The bulk of the tool is a tough, shock-resistant carbide that can withstand high-temperature plastic deformation and resist breakage. The result is a composite tool as shown in Figure 21-8. To be effective, the coatings should be hard, refractory, chemically stable, and chemically inert to shield the constituents of the tool and the workpiece from interacting chemically under cutting conditions. The coatings must be fine grained, free of binders and porosity. Naturally, the coatings must be metallurgically bonded to the substrate. Interface coatings are graded to match the properties of the coating and the substrate. The coatings must be thick enough to prolong tool life but thin enough to prevent brittleness. Coatings should have a low coefficient of friction so the chips do not adhere to the rake face. Coating materials include single coatings of TiC, TiN, Al2O3, HfN, or HfC. Multiple coatings are used, with each layer imparting its own characteristic to the tool. Successful coating combinations include TiN/TiC/TiCN/TiN and TiC/Al2O3/TiN. Chemical vapor deposition is used to obtain coated carbides. The coatings are formed by chemical reactions that take place only on or near the substrate. Like electroplating, CVD is a process in which the deposit is built up atom by atom. It is therefore capable of producing deposits of maximum density and of closely reproducing fine detail on the substrate surface. Control of critical variables such as temperature, gas concentration, and flow pattern is required to assure adhesion of the coating to the substrate. The coating-tosubstrate adhesion must be better for cutting-tool inserts than for most other coatings applications to survive the cutting pressure and temperature conditions without flaking off. Grain size and shape are controlled by varying temperature and/or pressure. The purpose of multiple coatings is to tailor the coating thickness for prolonged tool life. Multiple coatings allow a stronger metallurgical bond between the coating and the substrate and provide a variety of protection processes for machining different work materials, thus offering a more general-purpose tool material grade.A very thin final coat of TiN coating (µm) can effectively reduce crater formation on the tool face by one to two orders of magnitude relative to uncoated tools. Coated inserts of carbides are finding wide acceptance in many metalcutting applications. Coated tools have two or three times the wear resistance of the best uncoated tools with the same breakage resistance.This results in a 50 to 100% increase in speed for the same tool life. Because most coated inserts cover a broader application range, fewer

All of these PVD processes share the following common features: 1. The coating takes place inside a vacuum chamber under a hard vacuum with the workpiece heated to 200° to 405°C (400 to 900°F). 2. Before coating, all parts are given a final cleaning inside the chamber to remove oxides and improve coating adhesion. 3. The coating temperature is relatively low (for cutting and forming tools), typically about 842° F (450° C). 4. The metal source is vaporized in an inert gas atmosphere (usually argon), and the metal atoms react with gas to form the coating. Nitrogen is the reactive gas for nitrides, and methane or acetylene (along with nitrogen) is used for carbides. 5. All four are ion-assisted deposition processes.The ion bombardment compresses the atoms on the growing film, yielding a dense, well-adhered coating. A typical cycle time for the coating of functional tools, including heat-up and cool down, is about six hours. Of the three, PVD arc evaporation, shown in Figure 21-13, is the most recent development.The plasma sources are from several arc evaporators located on the sides and top of the vacuum chamber. Each evaporator generates plasma from multiple arc spots. In this way a highly localized electrical arc discharge causes minute evaporation of the material of the cathode, and a self-sustaining arc is produced that generates a highenergy and concentrated plasma. The kinetic energy of deposition is much greater than that found in any other PVD method. During coating, this energy is of the order of 150 electron volts and more. Therefore, the plasma is highly reactive and the greater percentage of the vapor is atomic and ionized. Coating temperatures can be selected and controlled so that metallurgy is preserved. This enables a coating of a wide variety of sintered carbide tools, for example, brazed tools, solid carbide tools such as drills, end mills, form tools, and inserts.The PVD arc evaporation process will preserve substrate metallurgy, surface finish, edge sharpness, geometrical straightness, and dimensions. CVD AND PVD—COMPLEMENTARY PROCESSES CVD and PVD are complementary coating processes.The differences between the two processes and resultant coatings dictate which coating process to use on different tools. Since CVD is done at higher temperatures, the adhesion of these coatings tends to be superior to a PVD-CVD-deposited coating. CVD coatings are normally deposited thicker than PVD coatings (6 to 9 µm for CVD, 1 to 3 µm for PVD). See Figure 21-14.

Although many formulations are used, a typical composition is that of the 18-4-1 type (tungsten 18%, chromium 4%, vanadium 1%), called T1. Comparable performance can also be obtained by the substitution of approximately 8% molybdenum for the tungsten, referred to as a tungsten equivalent (Weq). High-speed steel is still widely used for drills and many types of general-purpose milling cutters and in single-point tools used in general machining. For high-production machining, it has been replaced almost completely by carbides, coated carbides, and coated HSS. HSS main strengths are: • Great toughness—superior transverse rupture strength • Easily fabricated • Best for sever applications where complex tool geometry is needed (gear cutters, taps, drills, reamers, dies) High-speed steel tools are fabricated by three methods: cast, wrought, and sintered (using the powder metallurgy technique). Improper processing of cast and wrought products can result in carbide segregation, formation of large carbide particles and significant variation of carbide size, and nonuniform distribution of carbides in the matrix. The material will be difficult to grind to shape and will cause wide fluctuations of properties, inconsistent tool performance, distortion, and cracking. To overcome some of these problems, a powder metallurgy technique has been developed that uses the hot-isostatic pressing (HIP) process on atomized, prealloyed tool steel mixtures. Because the various constituents of the P/M alloys are "locked" in place by the compacting procedure, the end product is a more homogeneous alloy, Figure 21-4. P/M high-speed steel cutting tools exhibit better grindability, greater toughness, better wear resistance, and higher red (or hot) hardness; they also perform more consistently. They are about double the cost of regular HSS. TIN-COATED HIGH-SPEED STEELS Coated high-speed steel provides significant improvements in cutting speeds, with increases of 10 to 20% being typical. First introduced in 1980 for gear cutters (hobs) and in 1981 for drills,TiN-coated HSS tools have demonstrated their ability to more than pay for the extra cost of the coating process. In addition to hobs, gear-shaper cutters, and drills, HSS tooling coated by TiN now includes reamers, taps, chasers, spade-drill blades, broaches, bandsaw and circular saw blades, insert tooling, form tools, end mills, and an assortment of other milling cutters. Physical vapor deposition has proved to be the most viable process for coating HSS, primarily because it is a relatively low-temperature process that does not exceed the tempering point of HSS.Therefore, no subsequent heat treatment of the cutting tool is required. Films 0.0001 to 0.0002 in. in thickness adhere well and withstand minor elastic, plastic, and thermal loads. Thicker coatings tend to fracture under the typical thermomechanical stresses of machining. There are many variations to the PVD process, as outlined in Table 21-1. The process usually depends on gas pressure and is performed in a vacuum chamber. PVD processes are carried out with the workpieces heated to temperatures in the range of 400° to 900°F. Substrate heating enhances coating adhesion and film structure. Because surface pretreatment is critical in PVD processing, tools to be coated are subject to a vigorous cleaning process. Precleaning methods typically involve degreasing, ultrasonic cleaning, and Freon drying. Deburring, honing, and more active cleaning methods are also used. The advantages of TiN-coated HSS tooling include reduced tool wear. Less tool wear results in less stock removal during tool regrinding, thus allowing individual tools to be reground more times. For example, a TiN hob can cut 300 gears per sharpening; the uncoated tool would cut only 75 parts per sharpening. Therefore the cost per gear is reduced from 20 cents to 2 cents. Naturally, reduced tool wear means longer tool life. Higher hardness, with typical values for the thin coatings,"equivalent" to Rc 80-85, as compared to Rc 65-70 for hardened HSS, means reduced abrasion wear. Relative inertness (i.e., TiN does not react significantly with most workpiece materials) results in

CERAMICS Ceramics are made of pure aluminum oxide, Al2O3, or Al2O3 used as a metallic binder. Using P/M, very fine particles are formed into cutting tips under a pressure of 20 to 28 tons/in.2 (267 to 386 MPa) and sintered at about 1800°F (1000°C). Unlike the case with ordinary ceramics, sintering occurs without a vitreous phase. Ceramics are usually in the form of disposable tips. They can be operated at two to three times the cutting speeds of tungsten carbide.They almost completely resist cratering, run with no coolant, and have about the same tool life at their higher speeds as tungsten carbide does at lower speeds. As shown in Table 21-2, ceramics are usually as hard as carbides but are more brittle (lower bend strength) and therefore require more rigid tool holders and machine tools in order to take advantage of their capabilities. Their hardness and chemical inertness make ceramics a good material for high-speed finishing and/or high-removal-rate machining applications of superalloys, hard-chill cast iron, and high-strength steels. Because ceramics have poor thermal and mechanical shock resistance, interrupted cuts and interrupted application of coolants can lead to premature tool failure. Edge chipping is usually the dominant mode of tool failure. Ceramics are not suitable for aluminum, titanium, and other materials that react chemically with alumina-based ceramics. Recently, whisker-reinforced ceramic materials that have greater transverse rupture strength have been developed. The whiskers are made from silicon carbide. CERMETS Cermets are a new class of tool materials best suited for finishing. Cermets are ceramic TiC, nickel, cobalt, and tantalum nitrides.TiN and other carbides are used for binders. Cermets have superior wear resistance, longer tool life, and can operate at higher cutting speeds with superior wear resistance. Cermets have higher hot hardness and oxidation resistance than cemented carbides. The better finish imparted by a cermet is due to its low level of chemical reaction with iron [less cratering and built-up edge (BUE)]. Compared to carbide, the cermet has less toughness, lower thermal conductivity, and greater thermal expansion, so thermal cracking can be a problem during interrupted cuts. Cermets are usually cold pressed, and proper processing techniques are required to prevent insert cracking. New cermets are designed to resist thermal shocking during milling by using high nitrogen content in the titanium carbonitride phase (produces finer grain size) and adding WC and TaC to improve shock resistance. PVD-coated cermets have the wear resistance of cermets and the toughness range of a coated carbide, and they perform well with a coolant. Figure 21-9 shows a comparison of speed feed coverage of typical cermets compared to ceramics, carbides, and coated carbides. The values illustrate that cermets can clearly cover a wide range of important metalcutting applications. DIAMONDS Diamond is the hardest material known. Industrial diamonds are now available in the form of polycrystalline compacts, which are finding industrial application in the ma

Figure 21-3 compares various tool materials on the basis of hardness, the most critical characteristic, and hot hardness (hardness decreases slowly with temperature). Figure 21-4 compares hot hardness with toughness, or the ability to take impacts during interrupted cutting. Naturally, it would be wonderful if these materials were also easy to fabricate, readily available, and inexpensive, since cutting tools are routinely replaced, but this is not usually the case. Obviously, many of the requirements conflict and therefore tool selection will always require trade-offs. ■ 21.2 CUTTING-TOOL MATERIALS In nearly all machining operations, cutting speed and feed are limited by the capability of the tool material. Speeds and feeds must be kept low enough to provide for an acceptable tool life. If not, the time lost changing tools may outweigh the productivity gains from increased cutting speed. Coated high-speed steel (HSS) and uncoated and coated carbides are currently the most extensively used tool materials. Coated tools cost only about 15 to 20% more than uncoated tools, so a modest improvement in performance can justify the added cost. About 15 to 20% of all tool steels are coated, mostly by the physical vapor deposition (PVD) processes. Diamond and CBN are used for applications in which, despite higher cost, their use is justified. Cast cobalt alloys are being phased out because of the high raw-material cost and the increasing availability of alternate tool materials. New ceramic materials called cermets (ceramic material in a metal binder) are having a significant impact on future manufacturing productivity. Tool requirements for other processes that use noncontacting tools, as in electrodischarge machining (EDM) and electrochemical machining (ECM), or no tools at all (as in laser machining), are discussed in Chapter 20. Grinding abrasives will be discussed in Chapter 29. TOOL STEELS Carbon steels and low-/medium-alloy steels,called tool steels, were once the most common cutting-tool materials. Plain-carbon steels of 0.90 to 1.30% carbon when hardened and tempered have good hardness and strength and adequate toughness and can be given a keen cutting edge. However, tool steels lose hardness at temperatures above 400°F because of tempering and have largely been replaced by other materials for metal cutting. The most important properties for tool steels are hardness, hot hardness, and toughness. Low-/medium-alloy steels have alloying elements such as Mo and Cr, which improve hardenability, and W and Mo, which improve wear resistance. These tool materials also lose their hardness rapidly when heated to about their tempering temperature of 300° to 650°F, and they have limited abrasion resistance. Consequently, low-/medium-alloy steels are used in relatively inexpensive cutting tools (e.g., drills, taps, dies, reamers, broaches, and chasers) for certain low-speed cutting applications when the heat generated is not high enough to reduce their hardness significantly. High-speed steels, cemented carbides, and coated tools are also used extensively to make these kinds of cutting tools. Although more expensive, they have longer tool life and improved performance. These steels greatly benefit from P/M manufacturing due to uniformly distributed carbides. HIGH-SPEED STEELS First introduced in 1900 by F. W.Taylor and White, high-alloy steel is superior to tool steel in that it retains its cutting ability at temperatures up to 1100°F, exhibiting good "red hardness." Compared with tool steel, it can operate at about double or triple cutting speeds to about 100 sfpm with equal life,resulting in its name:high-speed steel,often abbreviated HSS. Today's high-speed steels contain significant amounts of W, Mo, Co, V, and Cr besides Fe and C. W, Mo, Cr, and Co in the ferrite as a solid solution provide strengthening of the matrix beyond the tempering temperature, thus increasing the hot hardness. Vanadium (V), along with W, Mo, and Cr, improves hardness (Rc 65-70) and wear resistance. Extensive solid solutioning of the matrix also ensures good hardenability of these steels.

For carbide tools, inserts for different work materials and tool holders can be supplied with several standard values of back rake angle: 6° to 6°. The side rake angle and the back rake angle combine to form the effective rake angle.This is also called the true rake angle or resultant rake angle of the tool. True rake inclination of a cutting tool has a major effect in determining the amount of chip compression and the shear angle. A small rake angle causes high compression, tool forces, and friction, resulting in a thick, highly deformed, hot chip. Increased rake angle reduces the compression, the forces, and the friction, yielding a thinner, less deformed, and cooler chip. Unfortunately, it is difficult to take much advantage of the desirable effects of larger positive rake angles, since they are offset by the reduced strength of the cutting tool, due to the reduced tool section, and by its greatly reduced capacity to conduct heat away from the cutting edge. To provide greater strength at the cutting edge and better heat conductivity, zero or negative rake angles are commonly employed on carbide, ceramic, polydiamond, and PCBN cutting tools.These materials tend to be brittle, but their ability to hold their superior hardness at high temperatures results in their selection for high-speed and continuous machining operations. While the negative rake angle increases tool forces, it keeps the tool in compression and provides added support to the cutting edge. This is particularly important in making intermittent cuts and in absorbing the impact during the initial engagement of the tool and work. In general, the power consumption is reduced by approximately 1% for each 1° in alpha (a). The end relief angle is called gamma (g). The wedge angle determines the strength of the tool and its capacity to conduct heat and depends on the values of a and g. The relief angles mainly affect the tool life and the surface quality of the workpiece. To reduce the deflections of the tool and the workpiece and to provide good surface quality, larger relief values are required. For high-speed steel, relief angles in the range of 5° to 10° are normal, with smaller values being for the harder work materials. For carbides, the relief angles are lower to give added strength to the tool. The side and end cutting-edge angles define the nose angle and characterize the tool design.The nose radius has a major influence on surface finish. Increasing the nose radius usually decreases tool wear and improves surface finish. Tool nomenclature varies with different cutting tools, manufacturers, and users. Many terms are still not standard because of all this variety. The most common tool terms will be used in later chapters to describe specific cutting tools. The introduction of coated tools has spurred the development of improved tool geometries. Specifically, low-force groove (LFG) geometries have been developed that reduce the total energy consumed and break up the chips into shorter segments. These grooves effectively increase the rake angle, which increases the shear angle and lowers the cutting force and power.This means that higher cutting speeds or lower cutting temperatures (and better tool lives) are possible. As a chip breaker, the groove deflects the chip at a sharp angle and causes it to break into short pieces that are easier to remove and are not as likely to become tangled in the machine and possibly cause injury to personnel. This is particularly important on high-speed, mass-production machines. The shapes of cutting tools used for various operations and materials are compromises, resulting from experience and research so as to provide good overall performance. For coated tools, edge strength is an important consideration.A thin coat enables the edge to retain high strength, but a thicker coat exhibits better wear resistance. Normally, tools for turning have a coating thickness of 6 to 12 µm. Edge strength is higher for multilayer coated tools. The radius of the edge should be 0.0005 to 0.005 in. ■ 21.4 TOOL COATING PROCESSES The two most effective coating processes for improving the life and performance of tools are the chemical vapor deposition and physical vapor deposition of titanium nitride (TiN) and titanium carbide (TiC).The selection of the cutting materials for cutting tools depends on what property you are seeking. If you want

Further definitions are being developed based on the probabilistic nature of the tool failure, in which machinability is defined by a tool reliability index. Using such indexes, various tool replacement strategies can be examined and optimum cutting rates obtained. These approaches account for the tool life variability by developing coefficients of variation for common combinations of cutting tools and work materials. The results to date are very promising. One thing is clear, however, from this sort of research: although many manufacturers of tools have worked at developing materials that have greater tool life at higher speeds, few have worked to develop tools that have less variability in tool life at all speeds. The reduction in variability is fundamental to achieving smaller coefficients of variation, which typically are of the order of 0.3 to 0.4. This means that a tool with a 100-min average tool life has a standard deviation of 30 to 40 min, so there is a good probability that the tool will fail early. In automated equipment, where early, unpredicted tool failures are extremely costly, reduction of the tool life variability will pay great benefits in improved productivity and reduced costs. ■ 21.8 CUTTING FLUIDS From the day that Frederick W.Taylor demonstrated that a heavy stream of water flowing directly on the cutting process allowed the cutting speeds to be doubled or tripled, cutting fluids have flourished in use and variety and have been employed in virtually every machining process.The cutting fluid acts primarily as a coolant and secondly as a lubricant, reducing the friction effects at the tool-chip interface and the work flank regions. The cutting fluids also carry away the chips and provide friction (and force) reductions in regions where the bodies of the tools rub against the workpiece. Thus in processes such as drilling, sawing, tapping, and reaming, portions of the tool apart from the cutting edges come in contact with the work, and these (sliding friction) contacts greatly increase the power needed to perform the process, unless properly lubricated. The reduction in temperature greatly aids in retaining the hardness of the tool, thereby extending the tool life or permitting increased cutting speed with equal tool life. In addition, the removal of heat from the cutting zone reduces thermal distortion of the work and permits better dimensional control. Coolant effectiveness is closely related to the thermal capacity and conductivity of the fluid used. Water is very effective in this respect but presents a rust hazard to both the work and tools and also is ineffective as a lubricant. Oils offer less effective coolant capacity but do not cause rust and have some lubricant value. In practice, straight cutting oils or emulsion combinations of oil and water or wax and water are frequently used.Various chemicals can also be added to serve as wetting agents or detergents, rust inhibitors, or polarizing agents to promote formation of a protective oil film on the work. The extent to which the flow of a cutting fluid washes the very hot chips away from the cutting area is an important factor in heat removal. Thus the application of a coolant should be copious and of some velocity. The possibility of a cutting fluid providing lubrication between the chip and the tool face is an attractive one. An effective lubricant can modify the process, perhaps producing a cooler chip, discouraging the formation of a built-up edge on the tool, and promoting improved surface finish. However, the extreme pressure at the tool-chip interface and the rapid movement of the chip away from the cutting edge make it virtually impossible to maintain a conventional hydrodynamic lubricating film at the tool-chip interface. Consequently, any lubrication action is associated primarily with the formation of solid chemical compounds of low shear strength on the freshly cut chip face, thereby reducing chip-tool shear forces or friction. For example, carbon tetrachloride is very effective in reducing friction in machining several different metals and yet would hardly be classified as a good lubricant in the usual sense. Chemically active compounds, such as chlorinated or sulfurized oils, can be added to cutting fluids to achieve such a lubrication effect. Extreme-pressure lubricants are especially valuable in severe operations, such as internal threading (tapping), where the extensive tool-work contact results in high friction with limited access for a fluid. In addition to functional effectiveness as coolant and lubricant, cutting fluids should be stable in use and storage, noncorrosive to

In metalcutting, the failure of the cutting tool can be classified into two broad categories, according to the failure mechanisms that caused the tool to die (or fail): 1. Physical failures mainly include gradual tool wear on the flank(s) of the tool below the cutting edge (called flank wear) or wear on the rake face of the tool (called crater wear) or both. 2. Chemical failures, which include wear on the rake face of the tool (called crater wear) are rapid, usually unpredictable, and often catastrophic failures resulting from abrupt, premature death of a tool. Other modes of failure are outlined in Figure 21-15.The selection of failure criteria is also widely varied. Figure 21-15 also shows a sketch of a "worn" tool, showing crater wear and flank wear, along with wear of the tool nose radius and an outer-diameter groove (the DCL groove). Tools also fail by edge chipping and edge fracture. As the tool wears, its geometry changes.This geometry change will influence the cutting forces, the power being consumed, the surface finish obtained, the dimensional accuracy, and even the dynamic stability of the process.Worn tools are duller, creating greater cutting forces and often resulting in chatter in processes that otherwise are usually relatively free of vibration.The actual wear mechanisms active in this high-temperature environment are abrasion, adhesion, diffusion, or chemical interactions. It appears that in metalcutting, any or all of these mechanisms may be operative at a given time in a given process. Tool failure by plastic deformation, brittle fracture, fatigue fracture, or edge chipping can be unpredictable. Moreover, it is difficult to predict which mechanism will dominate and result in a tool failure in a particular situation. What can be said is that tools, like people, die (or fail) from a great variety of causes under widely varying conditions

Once criteria for failure have been established, tool life is that time elapsed between start and finish of the cut, in minutes. Other ways to express tool life, other than time, include: 1. Volume of metal removed between regrinds or replacement of tool 2. Number of pieces machined per tool 3. Number of holes drilled with a given tool Drilling tool failure is discussed more in chapter 23 and is very complex because of the varied and complex geometry of the tools and as shown here in Figure 21-21, the tool material. RECONDITIONING CUTTING TOOLS In the reconditioning of tools by sharpening and recoating, care must be taken in grinding the tool's surfaces. The following guidelines should be observed: 1. Resharpen to original tool geometry specifications.Restoring the original tool geometry will help the tool achieve consistent results on subsequent uses. Computer numerical control (CNC) grinding machines for tool resharpening have made it easier to restore a tool's original geometry. 2. Grind cutting edges and surfaces to a fine finish. Rough finishes left by poor and abusive regrinding hinder the performance of resharpened tools. For coated tools, tops of ridges left by rough grinding will break away in early tool use, leaving uncoated and unprotected surfaces that will cause premature tool failure. 3. Remove all burrs on resharpened cutting edges. If a tool with a burr is coated, premature failure can occur because the burr will break away in the first cut, leaving an uncoated surface exposed to wear. 4. Avoid resharpening practices that overheat and burn or melt (called glazed over) the tool surfaces, as this will cause problems in coating adhesion. Polishing or wire brushing of tools causes similar problems

Oxidation and corrosion resistance; high-temperature stability select Al2O3, TiN, TiC Crater resistance, select Al2O3, TiN, TiC Hardness and edge retention, select TiC, TiN, Al2O3 Abrasion resistance and flank wear, select Al2O3, TiN, TiC Low coefficient of friction and high lubricity, select TiN, Al2O3, TiC Fine grain size, select TiN, TiC, Al2O3 The CVD process, used to deposit a protective coating onto carbide inserts, has been benefiting the metal removal industry for many years and is now being applied with equal success to steel.The PVD processes have quickly become the preferred TiN coating processes for high-speed steel and carbide-tipped cutting tools. CVD Chemical vapor deposition is an atmosphere-controlled process carried out at temperatures in the range of 950° to 1050°C (1740° to 1920°F). Figure 21-12 shows a schematic of the CVD process. Cleaned tools ready to be coated are staged on precoated graphite work trays (shelves) and loaded onto a central gas distribution column (tree). The tree loaded with parts to be coated is placed inside the retort of the CVD reactor. The tools are heated under an inert atmosphere until the coating temperature is reached.The coating cycle is initiated by the introduction of titanium tetrachloride (TiCL4), hydrogen, and methane (CH4) into the reactor.TiCL4 is a vapor and is transported into the reactor via a hydrogen carrier gas; CH4 is introduced directly. The chemical reaction for the formation of TiC is: (21-1) To form titanium nitride, a nitrogen-hydrogen gas mixture is substituted for methane. The chemical reaction for TiN is: (21-2) PVD The simplest form of PVD is evaporation, where the substrate is coated by condensation of a metal vapor. The vapor is formed from a source material called the charge, which is heated to a temperature less than 1000°C. PVD methods currently being used include reactive sputtering, reactive ion plating, low-voltage electron-beam evaporation, triode high-voltage electron-beam evaporation, cathodic evaporation, and arc evaporation. In each of the methods, the TiN coating is formed by reacting free titanium ions with nitrogen away from the surface of the tool and relying on a physical means to transport the coating onto the tool surface.

Since diamond and PCBN are extremely hard but brittle materials, new demands are being placed on the machine tools and on machining practice in order to take full advantage of the potential of these tool materials. These demands include: • Use of more rigid machine tools and machining practices involving gentle entry and exit of the cut in order to prevent microchipping • Use of high-precision machine tools, because these tools are capable of producing high finish and accuracy • Use of machine tools with higher power, because these tools are capable of higher metal removal rates and faster spindle speeds ■ 21.3 TOOL GEOMETRY Figure 21-11 shows the cutting-tool geometry for a single-point tool (HSS) used in turning.The back rake angle affects the ability of the tool to shear the work material and form the chip. It can be positive or negative. Positive rake angles reduce the cutting forces, resulting in smaller deflections of the workpiece, tool holder, and machine. In machining hard work materials, the back rake angle must be small, even negative for carbide and diamond tools. Generally speaking, the higher the hardness of the workpiece, the smaller the back rake angle. For high-speed steels, back rake angle is normally chosen in the positive range, depending on the type of tool (turning, planing, end milling, face milling, drilling, etc.) and the work material

So Vm represents a cutting speed that will minimize the cost per unit, as depicted in Figure 21-22. However, a word of caution here is appropriate. Note that this derivation was totally dependent upon the Taylor tool life equation. Such data may not be available because they are expensive and time consuming to obtain. Even when the tool life data are available, this procedure assumes that the tool fails only by whichever wear mechanism (flank or crater) was described by this equation and by no other failure mechanism. Recall that tool life has a very large coefficient of variation and is probabilistic in nature.This derivation assumes that for a given V, there is one T—and this simply is not the case, as was shown in Figure 21-16. The model also assumes that the workpiece material is homogeneous, the tool geometry is preselected, the depth of cut and feed rate are known and remain unchanged during the entire process, sufficient horsepower is available for the cut at the economic cutting conditions, and the cost of operating time is the same whether the machine is cutting or not cutting. Another example of tooling economics is summarized in Table 21-6, where a comparison is made between four different tools, all used for turning hot-rolled 8620 steel with triangular inserts. Operating costs for the machine tool are $60 per hour. The lowforce groove insert has only three cutting edges available instead of six. It takes 3 minutes to change inserts and half a minute to unload a finished part and load in a new 6-in.-diameter bar stock. The length of cut is about 24 in. The student should study and analyze this table carefully so that each line is understood. Note that the cutting-tool cost per piece is three times higher for the low-force groove tool over the carbide but really of no consequence, since the major cost per piece comes from two sources: the machining cost per piece and the nonproductive cost per piece. MACHINABILITY Machinability is a much maligned term that has many different meanings but generally refers to the ease with which a metal can be machined to an acceptable surface finish. The principal definitions of the term are entirely different, the first based on material properties, the second based on tool life, and the third based on cutting speed. 1. Machinability is defined by the ease or difficulty with which the metal can be machined. In this light, specific energy, specific horsepower, and shear stress are used as

Success in metal cutting depends on the selection of the proper cutting tool (material and geometry) for a given work material. A wide range of cutting-tool materials are available with a variety of properties, performance capabilities, and costs.These include highcarbon steels and low-/medium-alloy steels, high-speed steels, cast cobalt alloys, cemented carbides, cast carbides, coated carbides, coated high-speed steels, ceramics, cermets, whisker-reinforced ceramics, sialons, sintered polycrystalline cubic boron nitride (CBN), sintered polycrystalline diamond, and single-crystal natural diamond. Figure 21-1 shows some of these common tool materials ranked by the cutting speeds used to machine a unit volume of steel materials, assuming equal tool lives. As the speed (feed rate and DOC) increases, so does the metal removal rate. The time required to remove a given unit volume of material therefore decreases. Notice the fivefold increase in speed that the AL2O3-coated carbide has over the WC/Co tool (250 → 1200 sfpm).Today, approximately 85% of carbide tools are coated, almost exclusively by the chemical vapor deposition (CVD) process. The cutting tool (material and geometry) is the most critical aspect of the machining process. Clearly, the cutting-tool material, cutting parameters,

Table 21-3 is a cost comparison between the ceramic and PCBN insert.The PCBN insert used in the application is a full-top PCBN insert, meaning that the entire top of the insert is a layer of PCBN material. At first glance the PCBN tool appears to be extremely expensive. Each insert costs $208.00 and provides only three usable edges, whereas the ceramic insert costs $14.90 and provides six usable edges. However, the ceramic tool must be indexed every 35 pieces.The PCBN tool is indexed every 500 pieces.The cost per bore, including insert cost and the cost of labor to perform indexing, comes to $0.125 per bore for the ceramic tool and $0.142 per bore for the PCBN tool.This appears to make the ceramic tool more cost-effective, but downtime for indexing has not been accounted for. In this application, the ceramic insert required 10.75 hours of downtime for indexing, whereas the PCBN tool required only 0.75 hour of downtime for indexing.Use of the PCBN cutting tool significantly reduces the total cost per piece by eliminating 10 hours of downtime of the machine. Later in this chapter the economics of machining will be addressed again. The two predominant wear modes of PCBN tools are notching at the depth-ofcut line (DCL) and microchipping. In some cases, the tool will exhibit flank wear of the cutting edge.These tools have been used successfully for heavy interrupted cutting and for milling white cast iron and hardened steels using negative lands and honed cutting edges. See Table 21-4 for suggested applications of CBN and diamonds along with carbides and ceramics

The cost of each recoating is about one-fifth the cost of purchasing a new tool. By recoating, the tooling cost per workpiece can be cut by between 20 and 30%, depending on the number of parts being machined. ■ 21.7 ECONOMICS OF MACHINING The cutting speed has such a great influence on the tool life compared to the feed or the depth of cut that it greatly influences the overall economics of the machining process. For a given combination of work material and tool material, a 50% increase in speed results in a 90% decrease in tool life, while a 50% increase in feed results in a 60% decrease in tool life. A 50% increase in depth of cut produces only a 15% decrease in tool life. Therefore, in limited-horsepower situations, depth of cut and then feed should be maximized while speed is held constant and horsepower consumed is maintained within limits.As cutting speed is increased, the machining time decreases but the tools wear out faster and must be changed more often. In terms of costs, the situation is as shown in Figure 21-22, which shows the effect of cutting speed on the cost per piece. The total cost per operation is comprised of four individual costs: machining costs, tool costs, tool-changing costs, and handling costs. The machining cost is observed to decrease with increasing cutting speed because the cutting time decreases. Cutting time is proportional to the machining costs. Both the tool costs and the tool-changing costs increase with increases in cutting speeds. The handling costs are independent of cutting speed. Adding up each of the individual costs results in a total unit cost curve that is observed to go through a minimum point. For a turning operation, the total cost per piece C equals (21-7) Expressing each of these cost terms as a function of cutting velocity will permit the summation of all the costs. where Co operating cost ($/min) Tm cutting time (min/piece) where T tool life (min/tool) Ct initial cost of tool ($) where tc time to change tool (min) number of tool changes per piece C4 labor, overhead, and machine tool costs consumed while part is being loaded or unloaded, tools are being advanced, machine has broken down, and so on. , by rewriting equation 21-3, and using "K" for the constant "C", the cost per unit, C, can be expressed in terms of V: (21-8) To find the minimum, take dc/dV 0 and solve for V: Vm = c (21-9) 1 n - 1 d c Ct + 1Co * tc2 Co d

The examination of the data from a large number of tool life studies in which a variety of steels were machined shows that regardless of the tool material or process, tool life distributions are usually log normal and typically have a large standard deviation. As shown in Figure 21-20, tool life distributions have a large coefficient of variation, so tool life is not very predictable. Other criteria that can be used to define tool death in addition to wear limits are: • When surface finish deteriorates unacceptably • When workpiece dimension is out of tolerance • When power consumption or cutting forces increase to a limit • Sparking or chip discoloration and disfigurement • Cutting time or component quantity In automated processes, it is very beneficial to be able to monitor the tool wear online so that the tool can be replaced prior to failure, wherein defective products may also result. The feed force has been shown to be a good, indirect measure of tool wear. That is, as the tool wears and dulls, the feed force increases more than the cutting force increases.

Therefore, tool life should be treated as a random variable, or probabilistically, not as a deterministic quantity. ■ 21.6 FLANK WEAR During machining, the tool is performing in a hostile environment in which high contact stresses and high temperatures are commonplace, and therefore tool wear is always an unavoidable consequence. At lower speeds and temperatures, the tool most commonly wears on the flank. Suppose that the tool wear experiment were to be repeated 15 times without changing any of the input parameters. The result would look like Figure 21-16, which depicts the variable nature of tool wear and shows why tool wear must be treated as a random variable. In Figure 21-16 the average time is denoted as mT and the standard deviation as sT, where the wear limit criterion was 0.025 in. At a given time during the test, 35 minutes, the tool displayed flank wear ranging from 0.013 to 0.021 in, with an average of mw 5 0.00175 in. with standard deviation sw 0.001 in. In Figure 21-17 four characteristic tool wear curves (average values) are shown for four different cutting speeds, V1 through V4; V1 is the fastest cutting speed and therefore

With CVD multiple coatings, layers may be readily deposited but the tooling materials are restricted. CVD coated tools must be heat treated after coating.This limits the application to loosely toleranced tools. However, the CVD process, being a gaseous process, results in a tool that is coated uniformly all over; this includes blind slots and blind holes. Since PVD is mainly a line-of-sight process, all surfaces of the part to be coated may be masked. PVD also requires fixturing of each part in order to effect the substrate bias. APPLICATIONS Applications for the two different processes are as follows: CVD • Loosely toleranced tooling • Piercing and blanking punches, trim dies, phillips punches, upsetting punches • AISI A, D, H, M, and air hardening and tool steel parts • Solid carbide tooling PVD • All HSS, solid carbide, and carbide-tipped cutting tools • Fine blanking punches, dies (0.001 in. tolerance or less) • Non-composition-dependent process; virtually all tooling materials, including mold steels and bronze

als at higher speeds (fivefold) and with a higher removal rate (fivefold) than cemented carbide, and with superior accuracy, finish, and surface integrity. PCBN tools are available in basically the same sizes and shapes as sintered diamond and are made by the same process.The cost of an insert is somewhat higher than either cemented carbide or ceramic tools, but the tool life may be five to seven times that of a ceramic tool.Therefore, to see the economy of using PCBN tools, it is necessary to consider all the factors. Here is an industrial example of analysis of tooling economics, where a comparison is being made between two tool materials (insert tools). A manufacturer of diesel engines is producing an in-line six-cylinder engine block that is machined on a transfer line. Each cylinder hole must be bored to accept a sleeve liner. This operation has a depth of cut of 0.062 in. per side, for a total of 0.125 in. stock removal. The tolerance on this bore is ±0.001 in. and the spindle is operating at 2000 sfpm. Ceramic inserts are used on this operation, but with these inserts, wear was severe enough to require indexing after only 35 pieces. The ceramic insert was replaced with PCBN inserts made of a highcontent BCN. Both inserts had a 0.001- to 0.002-in. radius hone for edge preparation.

and tool geometry selected directly influence the productivity of the machining operation. Figure 21-2 outlines the input variables that influence the tool material selection decision. The elements which influence the decision are: • Work material characteristics, hardness, chemical and metallurgical state, • Part characteristics (geometry, accuracy, finish, and surface-integrity requirements) • Machine tool characteristics, including the workholders (adequate rigidity with high horsepower, and wide speed and feed ranges) • Support systems (operator's ability, sensors, controls, method of lubrication, and chip removal) Tool material technology is advancing rapidly, enabling many difficult-to-machine materials to be machined at higher removal rates and/or cutting speeds with greater performance reliability. Higher speed and/or removal rates usually improve productivity. Predictable tool performance is essential when machine tools are computer controlled with minimal operator interaction. Long tool life is desirable especially when machines become automatic or are placed in cellular manufacturing systems. The cutting tool is subjected to severe operating conditions.Tool temperatures of 1000°C and high local stresses require that the tool have these characteristics. 1. High hardness (Figure 21-3) 2. High hardness temperature, hot hardness (refer to Figure 21-3) 3. Resistance to abrasion, wear due to severe sliding friction 4. Chipping of the cutting edges 5. High toughness (impact strength) (refer to Figure 21-4) 6. Strength to resist bulk deformation 7. Good chemical stability (inertness or negligible affinity with the work material)

are either straight WC or multicarbides of W-Ti or W-Ti-Ta, depending on the work material to be machined. Cobalt is the binder. These tool materials are much harder, are chemically more stable, have better hot hardness, have high stiffness, have lower friction, and operate at higher cutting speeds than HSS. They are more brittle and more expensive and use strategic metals (W, Ta, Co) more extensively. Cemented carbide tool materials based on TiC have been developed primarily for auto industry applications using predominantly Ni and Mo as a binder. These are used for higher-speed (>1000 ft/min) finish machining of steels and some malleable cast irons. Cemented carbide tools are available in insert form in many different shapes: squares, triangles, diamonds, and rounds. They can be either brazed or mechanically clamped onto the tool shank. Mechanical clamping (Figure 21-7) is more popular because when one edge or corner becomes dull, the insert is rotated or turned over to expose a new cutting edge. Mechanical inserts can be purchased in the as-pressed state or the insert can be ground to closer tolerances. Naturally, precision-ground inserts cost more.Any part tolerance less than ±0.003 normally cannot be manufactured without radial adjustment of the cutting tool, even with ground inserts. If no radial adjustment is performed, precision-ground inserts should be used only when the part tolerance is between ±0.006 and ±0.003. Pressed inserts have an application advantage because the cutting edge is unground and thus does not leave grinding marks on the part after machining. Ground inserts can break under heavy cutting loads because the grinding marks on the insert produce stress concentrations that result in brittle fracture. Diamond grinding is used to finish carbide tools.Abusive grinding can lead to thermal cracks and premature (early) failure of the tool. Brazed tools have the carbide insert brazed to the steel tool shank.These tools will have a more accurate geometry than the mechanical insert tools, but they are more expensive. Since cemented carbide tools are relatively brittle, a 90° corner angle at the cutting edge is desired. To strengthen the edge and prevent edge chipping, it is rounded off by honing, or an appropriate chamfer or a negative land (a Tland) on the rake face is provided. The preparation of the cutting edge can affect tool life. The sharper the edge (smaller edge radius), the more likely the edge is to chip or break. Increasing the edge radius will increase the cutting forces, so a trade-off is required. Typical edge radius values are 0.001 to 0.003 in.

chining of aluminum, bronze, and plastics, greatly reducing the cutting forces as compared to carbides. Diamond machining is done at high speeds, with fine feeds for finishing, and produces excellent finishes. Recently,single-crystal diamonds, with a cutting-edge radius of 100 Å or less, have been used for precision machining of large mirrors. However, single-crystal diamonds have been used for years to machine brass watch faces, thus eliminating polishing.They have also been used to slice biological materials into thin films for dega-c21_560-597-hr 1/9/07 4:08 PM Page 573 574 CHAPTER 21 Cutting Tools for Machining viewing in transmission electron microscopes. (This process, known as ultramicrotomy, is one of the few industrial versions of orthogonal machining in common practice.) The salient features of diamond tools include high hardness; good thermal conductivity; the ability to form a sharp edge of cleavage (single-crystal, natural diamond); very low friction; nonadherence to most materials; the ability to maintain a sharp edge for a long period of time, especially in machining soft materials such as copper and aluminum; and good wear resistance. To be weighed against these advantages are some shortcomings, which include a tendency to interact chemically with elements of Group IVB to Group VIII of the periodic table. In addition, diamond wears rapidly when machining or grinding mild steel. It wears less rapidly with high-carbon alloy steels than with low-carbon steel and has occasionally machined gray cast iron (which has high carbon content) with long life. Diamond has a tendency to revert at high temperatures (700°C) to graphite and/or to oxidize in air. Diamond is very brittle and is difficult and costly to shape into cutting tools, the process for doing the latter being a tightly held industry practice. The limited supply of, increasing demand for, and high cost of natural diamonds led to the ultra-high-pressure (50 Kbar), high-temperature (1500°C) synthesis of diamond from graphite at the General Electric Company in the mid-1950s and the subsequent development of polycrystalline sintered diamond tools in the late 1960s. Polycrystalline diamond (PCD) tools consist of a thin layer (0.5 to 1.5 mm) of fine grain size diamond particles sintered together and metallurgically bonded to a cemented carbide substrate. A high-temperature/high-pressure process, using conditions close to those used for the initial synthesis of diamond, is needed. Fine diamond powder (1 to 30 mm) is first packed on a support base of cemented carbide in the press. At the appropriate sintering conditions of pressure and temperature in the diamond stable region, complete consolidation and extensive diamond-to-diamond bonding take place. Sintered diamond tools are then finished to shape, size, and accuracy by laser cutting and grinding. See Figure 21-10. The cemented carbide provides the necessary elastic support for the hard and brittle diamond layer above it. The main advantages of sintered polycrystalline tools over natural single-crystal tools are better quality, greater toughness, and improved wear resistance, resulting from the random orientation of the diamond grains and the lack of large cleavage planes. Diamond tools offer dramatic performance improvements over carbides.Tool life is often greatly improved, as is control over part size, finish, and surface integrity. Positive rake tooling is recommended for the vast majority of diamond tooling applications. If BUE is a problem, increasing cutting speed and using more positive rake angles may eliminate it. If edge breakage and chipping are problems, one can reduce the feed rate. Coolants are not generally used in diamond machining unless, as in the machining of plastics, it is necessary to reduce airborne dust particles. Diamond tools can be reground. There is much commercial interest in being able to coat HSS and carbides directly with diamond, but getting the diamond coating to adhere reliably has been difficult. Diamond-coated inserts would deliver roughly the same performance as PCD tooling when cutting nonferrous materials but could be given more complex geometries and chip breakers while reducing the cost per cutting edge. POLYCRYSTALLINE CUBIC BORON NITRIDES Polycrystalline cubic boron nitride (PCBN) is a man-made tool material widely used in the automotive industry for machining hardened steels and superalloys. It is made in a compact form for tools by a process quite similar to that used for sintered polycrystalline diamonds. It retains its hardness at elevated temperatures (Knoop 4700 at 20°C, 4000 at 1000°C) and has low chemical reactivity at the tool-chip interface.This material can be used to machine hard aerospace materials like Inconel 718 and René 95 as well as chilled cast iron. Although not as hard as diamond, PCBN is less reactive with such materials as hardened steels, hard-chill cast iron, and nickel- and cobalt-based superalloys. PCBN can be used efficiently and economically to machine these difficult-to-machine materi

grades are needed, and therefore inventory costs are lower. Aluminum oxide coatings have demonstrated excellent crater wear resistance by providing a chemical diffusion reaction barrier at the tool-chip interface, permitting a 90% increase in cutting speeds in machining some steels. Coated-carbide tools have progressed to the place where in the United States about 80 to 90% of the carbide tools used in metalworking are coated.

greater tool life through a reduction in adhesion.TiN coatings have a low coefficient of friction. This can produce an increase in the shear angle, which in turn reduces the cutting forces, spindle power, and heat generated by the deformation processes. PVD coatings generally fail in high-stress applications such as cold extrusion, piercing, roughing, and high-speed machining. CAST COBALT ALLOYS Cast cobalt alloys, popularly known as stellite tools, are cobalt-rich, chromium- tungsten-carbon cast alloys having properties and applications in the intermediate range between high-speed steel and cemented carbides. Although comparable in roomtemperature hardness to high-speed steel tools, cast cobalt alloy tools retain their hardness to a much higher temperature. Consequently, they can be used at higher cutting

measures, and, in general, the larger the shear stress or specific power values, the more difficult the material is to machine, requiring greater forces and lower speeds. In this definition, the material is the key. 2. Machinability is defined by the relative cutting speed for a given tool life while cutting some material, compared to a standard material cut with the same tool material. As shown in Figure 21-23, tool life curves are used to develop machinability ratings. In steels, the material chosen for the standard material was B1112 steel, which has a tool life of 60 min at a cutting speed of 100 sfpm. Material X has a 70% rating, which implies that steel X has a cutting speed of 70% of B1112 for equal tool life. Note that this definition assumes that the tool fails when machining X by whatever mechanism dominated the tool failure when machining the B1112. There is no guarantee that this will be the case. ISO standard 3685 has machinability index numbers based on 30 min of tool life with flank wear of 0.33 mm. 3. Cutting speed is measured by the maximum speed at which a tool can provide satisfactory performance for a specified time under specified conditions. See ASTM standard E 618-81: "Evaluating machining performance of ferrous metals using an automatic screw bar machine." 4. Other definitions of machinability are based on the ease of removal of the chips (chip disposal), the quality of the surface finish of the part itself, the dimensional stability of the process, or the cost to remove a given volume of metal.

where n, m, and p are exponents and K' is a constant. Equations of this form are also deterministic and determined empirically. The problem has been approached probabilistically in the following way. Since T depends on speed, feed, materials, and so on, one writes (21-6) where K is now a random variable that represents the effects of all unmeasured factors and is an input variable. The sources of tool life variability include factors such as: 1. Variation in work material hardness (from part to part and within a part) 2. Variability in cutting-tool materials, geometry, and preparation 3. Vibrations in machine tool, including rigidity of work and tool-holding devices 4. Changing surface characteristics of workpieces

work and machines, and nontoxic to operating personnel. The cutting fluid should also be restorable by using a closed recycling system that will purify the used coolant and cutting oils. Cutting fluids become contaminated in three ways (Table 21-7.) All these contaminants can be eliminated by filtering, hydrocycloning, pasteurizing, and centrifuging. Coolant restoration eliminates 99% of the cost of disposal and 80% or more of new fluid purchases. See Figure 21-24 for a schematic of a coolant recycling system.

Manufacturing 21

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