Unit 2 Machinability, Cutting Fluids, Tool Life & Wear, Tool Materials

Concept of machinability, machinability index, factors affecting machinability
Different mechanism of tool wear types of tool wear (crater, flank etc.), Measurement and control of tool wear
Concept of tool life, Taylor's tool life equation (including modified version)
Different tool materials and their applications including effect of tool coating
Introduction to economics of machining
Cutting fluids: types, properties, selection and application methods

1. Machining & Machining Tools Unit-2 1 Machinability, Tool Wear & Tool Life, Tool Materials, Economics of Machining, Cutting Fluids

2. Syllabus Concept of machinability, machinability index, factors affecting machinability Different mechanism of tool wear types of tool wear (crater, flank etc.), Measurement and control of tool wear Concept of tool life, Taylor's tool life equation (including modified version) Different tool materials and their applications including effect of tool coating Introduction to economics of machining Cutting fluids: types, properties, selection and application methods 2

3. Machinability 3

4. Machinability Machinability is the ease with which a given material may be worked with a cutting tool. The machinability of a material is usually defined in terms of four factors: Surface finish Tool life. Force and power required. The level of difficulty in chip control. Thus, good machinability indicates good surface finish and surface integrity, a long tool life, low force and power requirements and desired chip control in the cutting zone. 4

5. Machinability Index/Rating Because of the complex nature of cutting operations, it is difficult to establish relationships that quantitatively define the machinability of a particular material. The machinability rating of a material attempts to quantify the machinability of various materials.   where V 60 is the cutting speed for the target material that ensures tool life of 60 min, V 60R is the same for the reference material. Reference materials are selected for each group of work materials (ferrous and non-ferrous) among the most popular and widely used brands. If KM > 1, the machinability of the target material is better that this of the reference material, and vice versa. Note that this system can be misleading because the index is different for different machining processes. 5

6. Machinability Rating Example The reference material for steels, AISI 1112 steel has an index of 1. Machining of this steel at cutting speed of 0.5 m/s gives tool life of 60 min. Therefore, V 60R = 0.5 m/s. For the austenitic 302 SS steel The machinability index is K M = 0.23/0.5 = 0.46 (tool life of 60 min is reached at 0.23 m/s). For AISI 1045 steel The machinability index is K M = 0.36/0.5 = 0.72. (tool life of 60 min is reached at 0.36 m/s). So, we can rate these steels in a descending order of machinability: AISI 1112>AISI 1045>302 SS Note that Tool life cannot be considered as the only criteria for judging machinability as it is dependent on many factors other than cutting velocity. Keeping all such factors and limitations in view, Machinability can be tentatively defined as “ability of being machined” and more reasonably as “ease of machining”. 6

7. Machinability Factors The machinability is affected by following variables Aspects (a) Work material Aspects (b) Cutting tool Aspects (c) Process parameters Aspects (d) Machining environments Aspects 7

8. Work Material Aspects 8 Following work material properties govern machinability. Nature – Brittle/Ductile Microstructure-Coarse/Fine Mechanical strength – fracture or yield Hardness, hot strength and hot hardness Work hardenability Thermal conductivity Chemical reactivity Stickiness / self lubricity.

9. Work Material Aspects 9 Nature-Brittle/Ductile Brittle materials are easy to machine as the chip separation is due to brittle fracture requiring lesser energy of chip formation and further shorter chips causing lesser frictional force and heating at the rake surface Ductile materials like mild steel produce better surface finish but BUE, if formed, may worsen the surface finish. Also cutting forces increase with the increase in yield shear strength, τ s of the work material Microstructure –Coarse/Fine The value of shear strength and hence shear force of a given material depends on its microstructure. Coarse microstructure leads to lesser value of τs . Therefore, τs can be desirably reduced by either proper heat treatment like annealing of steels or controlled addition of materials like Sulphur (S), lead ( Pb ), Tellurium etc. leading to free cutting of soft ductile metals and alloys.

10. Work Material Aspects 10 Hardness, hot strength and hot hardness and work hardening Harder materials are obviously more difficult to machine for increased cutting forces and tool damage. Usually, with the increase in cutting velocity the cutting forces decrease to some extent making machining easier through reduction in τs and also chip thickness. τs decreases due to softening of the work material at the shear zone due to elevated temperature. Such benefits of increased temperature and cutting velocity are not attained when the work materials are hot strong and hard like Ti and Ni based superalloys and work hardenable like high manganese steel, Ni- hard, Hadfield steel etc. Stickiness/Self Lubricity Sticking of the materials (like pure copper, aluminium and their alloys) and formation of BUE at the tool rake surface also hamper machinability by increasing friction, cutting forces, temperature and surface roughness. Thermal Conductivity Lower thermal conductivity of the work material affects their machinability by raising the cutting zone temperature and thus reducing tool life.

11. Cutting Tool Aspects 11 Tool Materials: In machining a given material, the tool life is governed mainly by the tool material which also influences cutting forces and temperature as well as accuracy and finish of the machined surface. The composition, microstructure, strength, hardness, toughness, wear resistance, chemical stability and thermal conductivity of the tool material play significant roles on the machinability characteristics though in different degree depending upon the properties of the work material. High wear resistance and chemical stability of the cutting tools like coated carbides, ceramics, cubic Boron nitride ( cBN ) etc. also help in providing better surface integrity of the product by reducing friction, cutting temperature and BUE formation in high speed machining of steels. Very soft, sticky and chemically reactive material like pure aluminium attains highest machinability when machined by diamond tools. Figure 2.1: Role of cutting tool material on machinability /Tool life

12. Cutting Tool Aspects 12 Machinability is affected by tool geometry elements like rake angles ,clearance angles and nose radius. Increase in rake angle reduces cutting forces requirements but does not affect surface finish Too high rake angle decrease tool tip strength and hence decreases tool life.   Inadequate clearance angle reduces tool life and surface finish by tool work rubbing and again too large clearance reduces the tool strength and hence tool life. Proper tool nose radius improves machinability to some extent through increase in tool life by increasing mechanical strength and reducing temperature at the tool tip reduction of surface roughness  

13. Process Parameter Aspects 13 Proper selection of the cutting velocity, feed and depth of cut provide better machinability characteristics of a given work – tool pair even without sacrificing productivity or MRR. Amongst the process parameters, depth of cut plays least significant role and is almost invariable. Now increase in cutting velocity in general, reduces tool life but it also reduces cutting forces or specific energy requirement and improves surface finish through favorable chip-tool interaction. Some cutting tools especially ceramic tools perform better and last longer at higher V within limits. Increase in feed raises cutting forces proportionally but reduces specific energy requirement to some extent. Cutting temperature is also less affected by increase in feed than V. But increase in feed unlike V raises surface roughness. Therefore, proper increase in V, even at the expense of feed often can improve machinability quite significantly.

14. Work Environment Aspects 14 The basic purpose of employing cutting fluid is to improve machinability characteristics of any work – tool pair through : • improving tool life by cooling and lubrication • reducing cutting forces and specific energy consumption • improving surface integrity by cooling, lubricating and cleaning at the cutting zone The favorable roles of cutting fluid application depend not only on its proper selection based on the work and tool materials and the type of the machining process but also on its rate of flow, direction and location of application.

15. Possible ways of Improving Machinability 15 The machinability of the work materials can be more or less improved, without sacrificing productivity, by the following ways: Favorable change in composition, microstructure and mechanical properties by mixing suitable type and amount of additive(s) in the work material and appropriate heat treatment Proper selection and use of cutting tool material and geometry depending upon the work material and the significant machinability criteria under consideration Optimum selection of cutting velocity, feed and depth of cut based on the tool – work materials and the primary objectives. Proper selection and appropriate method of application of cutting fluid depending upon the tool – work materials, desired levels of productivity i.e., V C and f and also on the primary objectives of the machining work undertaken Proper selection and application of special techniques like dynamic machining, hot machining, cryogenic machining etc., if feasible, economically viable and eco-friendly.

16. Machinability of Ferrous Metals 16 Steels- Carbon steels have a wide range of machinability, depending on their ductility and hardness. If carbon steel is too ductile, chip formation can produce built-up edge, leading to poor surface finish if the steel is too hard, it can cause abrasive wear of the tool because of the presence of carbides in the steel. Cold-worked carbon steels are desirable from a machinability standpoint. An important group of steels is free-machining steels , containing sulfur and phosphorus . Sulfur forms manganese-sulfide inclusions (second-phase particles), which act as stress raisers in the primary shear zone. As a result, the chips produced break up easily and are small, thus improving machinability. The size, shape, distribution, and concentration of these inclusions significantly influence machinability. Elements such as tellurium and selenium, both of which are chemically similar to sulfur, act as inclusion modifiers in resulfurized steels. Phosphorus in steels has two major effects: It strengthens the ferrite, causing increased hardness and resulting in better chip formation and surface finish, and It increases hardness and thus causes the formation of short chips instead of continuous stringy ones, thereby improving machinability.

17. Machinability of Ferrous Metals 17 Steels-Leaded steels In leaded steels, a high percentage of lead solidifies at the tips of manganese sulfide inclusions. In nonresulfurized grades of steel, lead takes the form of dispersed fine particles. Lead is insoluble in iron, copper, and aluminum and their alloys. Because of its low shear strength, lead acts as a solid lubricant and is smeared over the tool-chip interface during cutting. At high temperatures lead melts directly in front of the tool, acting as a liquid lubricant. Lead lowers the shear stress in the primary shear zone, thus reducing cutting forces and power consumption. Lead can be used with every grade of steel and is identified by the letter “L” between the second and third numerals in steel identification (e.g., 10L45). (Note that in stainless steels, a similar use of the letter L means “low carbon” Lead is a well-known toxin and a pollutant hence efforts being made to eliminate the use of lead in steels (lead-free steels). Bismuth and tin are substitutes for lead in steels, although their performance is not as good. Calcium-deoxidized steels contain oxide flakes of calcium silicates ( CaSO ) that reduce the strength of the secondary shear zone and decrease tool-chip interface friction and wear. Increases in temperature are reduced correspondingly. Consequently, these steels produce less crater wear, especially at high cutting speeds. Alloy steels can have a wide variety of compositions and hardness. Consequently, their machinability cannot be generalized, although they have higher levels of hardness and other mechanical properties. Alloy steels at hardness levels of 45 to 65 HRC can be machined with polycrystalline cubic boron-nitride cutting tools, producing good surface finish, integrity, and dimensional accuracy.

18. Machinability of Ferrous Metals 18 Effect of Various Elements in Steels The presence of aluminum and silicon in steels is always harmful, because these elements combine with oxygen to form aluminum oxide and silicates, which are hard and abrasive. As a result, tool wear increases and machinability is reduced. Carbon and manganese have various effects on the machinability of steels, depending on their composition. Plain low-carbon steels (less than 0.15% C) can produce poor surface finish by forming a built-up edge. Cast steels are more abrasive, although their machinability is similar to that of wrought steels. Tool and die steels are very difficult to machine and usually require annealing prior to machining. The machinability of most steels is improved by cold working, which hardens the material and reduces the tendency for built-up edge formation. Other alloying elements (such as nickel, chromium, molybdenum, and vanadium ) that improve the properties of steels generally reduce machinability. The effect of boron is negligible. Gaseous elements such as hydrogen and nitrogen can have particularly detrimental effects on the properties of steel. Oxygen has been shown to have a strong effect on the aspect ratio of the manganese-sulfide inclusions: The higher the oxygen content, the lower the aspect ratio, and the higher the machinability. In improving the machinability of steels, however, it is important to consider the possible detrimental effects of the alloying elements on the properties and strength of the machined part in service. At elevated temperatures, for example, lead causes embrittlement of steels although at room temperature it has no effect on mechanical properties. Sulfur can reduce the hot workability of steels severely because of the formation of iron sulfide (unless sufficient manganese is present to prevent such formation). At room temperature, the mechanical properties of resulfurized steels depend on the orientation of the deformed manganese-sulfide inclusions (anisotropy). Rephosphorized steels are significantly less ductile and are produced solely to improve machinability.

19. Machinability of Ferrous Metals 19 Stainless Steels Austenitic (300 series) steels generally are difficult to machine. Chatter can be a problem, necessitating machine tools with high stiffness. Ferritic stainless steels (also 300 series) have good machinability. Martensitic (400 series) steels are abrasive, tend to form a built-up edge, and require tool materials with high hot hardness and crater-wear resistance. Precipitation-hardening stainless steels are strong and abrasive, thus requiring hard and abrasion-resistant tool materials. Cast Irons Gray irons generally are machinable, but they can be abrasive depending on composition, especially pearlite. Free carbides in castings reduce their machinability and cause tool chipping or fracture. Nodular and malleable irons are machinable with hard tool materials.

20. Machinability of Non-Ferrous Metals 20 Aluminum is generally very easy to machine, although the softer grades tend to form a built-up edge, resulting in poor surface finish. Thus, high cutting speeds, high rake angles, and high relief angles are recommended. Wrought aluminum alloys with high silicon content and cast aluminum alloys are generally abrasive; hence, they require harder tool materials. Dimensional tolerance control may be a problem in machining aluminum, because it has a high thermal expansion coefficient and a relatively low elastic modulus. Beryllium generally is machinable, but because the fine particles produced during machining are toxic, it requires machining in a controlled environment. Cobalt-based alloys are abrasive and highly work hardening. They require sharp, abrasion-resistant tool materials and low feeds and speeds. Copper in the wrought condition can be difficult to machine because of builtup edge formation, although cast copper alloys are easy to machine. Brasses are easy to machine, especially with the addition of lead (leaded free-machining brass). Note, however, the toxicity of lead and associated environmental concerns. Bronzes are more difficult to machine than brass. Magnesium is very easy to machine, with good surface finish and prolonged tool life. However, care should be exercised because of its high rate of oxidation (pyrophoric) and the danger of fire. Molybdenum is ductile and work hardening. It can produce poor surface finish; thus, sharp tools are essential. Nickel-based alloys and super alloys are work hardening, abrasive, and strong at high temperatures. Their machinability depends on their condition and improves with annealing. Tantalum is very work hardening, ductile, and soft. It produces a poor surface finish, and tool wear is high. Titanium and its alloys have very poor thermal conductivity (the lowest of all metals), causing a significant temperature rise and built-up edge. They are highly reactive and can be difficult to machine. Tungsten is brittle, strong, and very abrasive; hence, its machinability is low, although it improves greatly at elevated temperatures. Zirconium has good machinability, but it requires a coolant-type cutting fluid because of the danger of explosion and fire.

21. Machinability of Miscellaneous Materials 21 Thermoplastics Generally have low thermal conductivity and a low elastic modulus, and they are thermally softening. Consequently, machining them requires sharp tools with positive rake angles (to reduce cutting forces), large relief angles, small depths of cut and feed, relatively high speeds, and proper support of the workpiece. External cooling of the cutting zone may be necessary to keep the chips from becoming gummy and sticking to the tools. Cooling usually can be achieved with a jet of air, a vapor mist, or water-soluble oils. Thermosetting plastics are brittle and sensitive to thermal gradients during cutting; Machining conditions generally are similar to those of thermoplastics. Polymer-matrix composites Very abrasive because of the fibers that are present; hence, they are difficult to machine. Fiber tearing, pulling, and edge delamination are significant problems and can lead to severe reduction in the load carrying capacity of the machined component. Machining of these materials requires careful handling and removal of debris to avoid contact with and inhaling of the fibers. Metal-matrix and ceramic-matrix composites can be difficult to machine, depending on the properties of the matrix material and the reinforcing fibers. Graphite Abrasive; it requires sharp, hard, and abrasion-resistant tools. Ceramics Steadily improved machinability, particularly with the development of machinable ceramics and Nano ceramics and with the selection of appropriate processing parameters. Wood Orthotropic material with properties varying with its grain direction. Type of chips and the surfaces produced also vary depending on the type of wood and its condition. Two basic requirements are generally sharp tools and high cutting speeds.

22. Tool Wear & Tool Life 22

23. Tool Wear 23 Definition: Gradual failure of cutting tools due to regular operations is known as tool wear. Modes of Cutting Tool Failures Fracture Failure: This mode of failure occurs due to mechanical breakage due to excessive forces and shocks at the tool point causing it to fail suddenly by brittle fracture. Also known as mechanical chipping. Such kind of tool failure is random and catastrophic in nature, results in premature loss of tool and hence is extremely detrimental. Temperature Failure: This failure occurs when the cutting temperature is too high for the tool material, causing the material at the tool point to soften, which leads to plastic deformation and loss of the sharp edge. This type of failure also occurs rapidly, results in premature loss of tool and is quite detrimental and unwanted. Note: Both of the above kinds of tool failure need to be prevented by using suitable tool materials and geometry depending upon the work material and cutting condition. Gradual Wear: Gradual wearing of the cutting edge causes loss of tool shape, reduction in cutting efficiency, an acceleration of wearing as the tool becomes heavily worn, and finally tool failure in a manner similar to a temperature failure. Gradual wear is preferred because it leads to the longest possible use of the tool, option of changing the tool before the final catastrophic loss of the cutting edge occurs, with the associated economic advantage of that longer use. Note: failure by gradual wear cannot be prevented but can be slowed down to enhance the service life of the tool.

24. Tool Wear 24 Wear zones & Types of Gradual Wear Gradual wear occurs at three principal locations on a cutting tool. Accordingly, three main types of tool wear can be distinguished, Œ Crater wear (K T )  Flank wear (V B ) Ž Corner wear Types of wear observed in cutting tools

25. Tool Wear 25 Crater wear: It consists of a cavity or concave section on the tool face/rake face formed and grows from the action of the chip sliding against the surface. High stresses and temperatures characterize the tool–chip contact interface, contributing to the wearing action. The crater can be measured either by its depth or its area. Crater wear affects the mechanics of the process increasing the actual rake angle of the cutting tool and consequently, making cutting easier. At the same time, the crater wear weakens the tool wedge and increases the possibility for tool breakage. This wear predominates at high speed. In general, crater wear is of a relatively small concern . The crater wear is mainly caused due to The presence of friction between the chip-tool interface, The abrasion action of microchips present at the chip-tool interface. The abrasive action of fragments of Built up Edge (BUE) at the chip-tool interface and diffusion wear. The diffusion wears, due to the atomic attraction between the tool and work the atoms of the tool material will get diffused and deposited over the workpiece called diffusion wear.

26. Tool Wear 26 Flank wear It occurs on the tool flank as a result of friction between the machined surface of the workpiece and the tool flank. Flank wear appears in the form of so-called wear land and is measured by the width of this wear land, VB. Flank wear affects to the great extend the mechanics of cutting. An extreme condition of flank wear often appears on the cutting edge at the location corresponding to the original surface of the workpart. This is called notch wear. It occurs because the original work surface is harder and/or more abrasive than the internal material, which could be caused by work hardening from cold drawing or previous machining, sand particles in the surface from casting, or other reasons. As a consequence of the harder surface, wear is accelerated at this location. Cutting forces increase significantly with flank wear. If the amount of flank wear exceeds some critical value (VB > 0.5~0.6 mm) then the excessive cutting force may cause tool failure. This wear predominates at low speed.  The reasons for flank wear are: The presence of friction at the tool work interface. The abrasive action of microchips or powdered particles present at the tool work interface and diffusion wear. The diffusion wears, due to the atomic attraction between the tool and work the atoms of the tool material will get diffused and deposited over the workpiece called as diffusion wear.

27. Tool Wear 27 Corner wear/Nose wear It occurs on the tool corner. It can be considered as a part of the wear land and respectively flank wear since there is no distinguished boundary between the corner wear and flank wear land. We consider corner wear as a separate wear type because of its importance for the precision of machining. Corner wear actually shortens the cutting tool thus increasing gradually the dimension of machined surface and introducing a significant dimensional error in machining, which can reach values of about 0.03~0.05 mm. Figure: Top view showing the effect of tool corner wear on the dimensional precision in turning

28. Tool Wear Mechanisms 28 Abrasion This is a mechanical wearing action caused by hard particles in the work material gouging and removing small portions of the tool. This abrasive action occurs in both flank wear and crater wear; it is a significant cause of flank wear. Adhesion When two metals are forced into contact under high pressure and temperature, adhesion or welding occur between them. These conditions are present between the chip and the rake face of the tool. As the chip flows across the tool, small particles of the tool are broken away from the surface, resulting in attrition of the surface. Diffusion This is a process in which an exchange of atoms takes place across a close contact boundary between two materials. In the case of tool wear, diffusion occurs at the tool–chip boundary, causing the tool surface to become depleted of the atoms responsible for its hardness. As this process continues, the tool surface becomes more susceptible to abrasion and adhesion. Diffusion is believed to be a principal mechanism of crater wear. Chemical reactions The high temperatures and clean surfaces at the tool–chip interface in machining at high speeds can result in chemical reactions, in particular, oxidation, on the rake face of the tool. The oxidized layer, being softer than the parent tool material, is sheared away, exposing new material to sustain the reaction process. Plastic deformation The cutting forces acting on the cutting edge at high temperature cause the edge to deform plastically, making it more vulnerable to abrasion of the tool surface. Plastic deformation contributes mainly to flank wear. Most of these tool-wear mechanisms are accelerated at higher cutting speeds and temperatures. Diffusion and chemical reaction are especially sensitive to elevated temperature.

29. Tool Wear Curve 29 The general relationship of tool wear versus cutting time is shown in Figure 23.3. Although the relationship shown is for flank wear, a similar relationship occurs for crater wear. Three regions can usually be identified in the typical wear growth curve. Figure: Tool wear as a function of cutting time. Flank wear (FW) is used here as the measure of tool wear. Crater wear follows a similar growth curve. The first is the break-in period, in which the sharp cutting edge wears rapidly at the beginning of its use. This first region occurs within the first few minutes of cutting. The break-in period is followed by wear that occurs at a fairly uniform rate. This is called the steady-state wear region. In our figure, this region is pictured as a linear function of time, although there are deviations from the straight line in actual machining. Finally , wear reaches a level at which the wear rate begins to accelerate. This marks the beginning of the failure region, in which cutting temperatures are higher, and the general efficiency of the machining process is reduced. If allowed to continue, the tool finally fails by temperature failure..

30. Tool Life 30 Definition: Tool life is generally defined by the span of actual uninterrupted machining time through which the tool or tool-tip renders desired service and satisfactory performance and after which that tool needs replacement Tool Life Determination Process: Tool wear is a time dependent process. As cutting proceeds, the amount of tool wear increases gradually. Rather than operating the tool until final catastrophic failure an alternative way in which a level of tool wear (flank wear at 0.3 to 0.6 mm shown by horizontal line on wear curve) is set as safe limit. The safe limit is referred to as allowable wear land (wear criterion), V B . The cutting time required for the cutting tool to develop a flank wear land of width V B is called tool life, T, a fundamental parameter in machining. When wear curves intersects that line, the life of the corresponding tool is defined as ended and the tool is replaced.

31. Tool Life & Cutting Parameters 31 Wear and hence tool life of any tool for any work material is governed mainly by the level of the machining parameters i.e., cutting velocity, feed and depth of cut. Increased speed, feed, and depth of cut have a similar effect, with speed being the most important of the three. Cutting velocity affects maximum and depth of cut minimum. Harder work materials cause the wear rate (slope of the tool wear curve) to increase. The usual pattern of growth of cutting tool wear (mainly VB), principle of assessing tool life and its dependence on cutting velocity are schematically shown in Fig.3.2.3. The tool life obviously decreases with the increase in cutting velocity keeping other conditions unaltered as indicated in Fig. 3.2.3. Fig. 3.2.3 Growth of flank wear and assessment of tool life

32. Taylor’s Tool Life Equation 32 If the tool lives, T 1 , T 2 , T 3 , T 4 etc. are plotted against the corresponding cutting velocities, V 1 , V 2 , V 3 , V 4 etc. as shown in Fig. 3.2.4, a smooth curve like a rectangular hyperbola is found to appear. When F. W. Taylor plotted the same figure taking both V and T in log-scale, a more distinct linear relationship appeared as schematically shown in Fig. 3.2.5. With the slope, n and intercept, C, Taylor derived the simple equation as VT n = C where, n is called, Taylor’s tool life exponent. The values of both ‘n’ and ‘C’ depend mainly upon the tool-work materials and the cutting environment (cutting fluid application). The value of C depends also on the limiting value of VB undertaken (i.e., 0.3 mm, 0.4 mm, 0.6 mm etc.) These constants are well tabulated and easily available. Note that the magnitude of C is the cutting speed at T = 1 min. Figure: Cutting velocity – tool life relationship Figure: Cutting velocity versus tool life on a log-log scale

33. Taylor’s Tool Life Equation 33 Problem: If in turning of a steel rod by a given cutting tool (material and geometry) at a given machining condition (feed and depth of cut) under a given environment (cutting fluid application), the tool life decreases from 80 min to 20 min. due to increase in cutting velocity, V from 60 m/min to 120 m/min., then at what cutting velocity the life of that tool under the same condition and environment will be 40 min.?

34. Modified Taylor’s Tool Life Equation 34 In Taylor’s tool life equation, only the effect of variation of cutting velocity, V on tool life has been considered. But practically, the variation in feed (f) and depth of cut (d) also play role on tool life to some extent. Taking into account the effects of all those parameters, the Taylor’s tool life equation has been modified as, Here- d is the depth of cut and f is the feed in mm/rev The exponents x and y must be determined experimentally for each cutting condition. These are now also available in machining handbooks. Here x>y as tool life is affected more by depth of cut then feed.

35. Tool Wear Control 35 The rate of tool wear strongly depends on the cutting temperature; therefore any measures which could be applied to reduce the cutting temperature would reduce the tool wear as well. Use of cutting fluids, lubricants is another method. Additional measures to reduce the tool wear include the application of advanced cutting tool materials, such as coated carbides, ceramics, etc. The figure shows the process parameters that influence the rate of tool wear:  

36. Tool Materials 36

37. Tool Materials 37 Needs of Cutting Tool Materials With the progress of the industrial world it has been needed to continuously develop and improve the cutting tool materials and geometry; to meet the growing demands for high productivity, quality and economy of machining to enable effective and efficient machining of the exotic materials that are coming up with the rapid and vast progress of science and technology for precision and ultra-precision machining For micro and even Nano machining demanded by the day and future. The relative contribution of the cutting tool materials on productivity, for instance, can be roughly assessed from Fig. 2.2 Figure 2.2: Productivity raised by cutting tool materials

38. Chronological Development of Cutting Tool Materials 38 Figure 2.3: Chronological development of cutting tool materials

39. Cutting Tool Characteristics 39 Hardness: The tool material must be harder than the work piece material. Higher the hardness, easier it is for the tool to penetrate the work material. Hot hardness: Hot Hardness is the ability of the cutting tool must to maintain its Hardness and strength at elevated temperatures. This property is more important when the tool is used at higher cutting speeds, for increased productivity. This property ensures that the tool does not undergo any plastic deformation and thus retains its shape and sharpness Toughness: Tool should have enough toughness to withstand the impact loads that come in the start of the cut to force fluctuations due to imperfections in the work material. Toughness of cutting tools is needed so that tools don’t chip or fracture, especially during interrupted cutting operations like milling. Wear Resistance: The tool-chip and chip-work interface are exposed to severe conditions that adhesive and abrasion wear is very common. Wear resistance means the attainment of acceptable tool life before tools need to be replaced. Low friction: The coefficient of friction between the tool and chip should be low. This would lower wear rates and allow better chip flow. Chemical stability and inertness with respect to the material being machined, to avoid or minimize any adverse reactions, adhesion, and tool-chip diffusion that would contribute to tool wear. Thermal characteristics: Since a lot of heat is generated at the cutting zone, the tool material should have higher thermal conductivity to dissipate the heat in shortest possible time; otherwise the tool temperature would become high, reducing its life.

40. Primary Cutting Tool Materials 40 Carbon and Medium alloy steels : These are the oldest of the tool materials dating back hundreds of years. In simple terms it is a high carbon steel (steel which contains about 0.9 to 1.3% carbon). Inexpensive, easily shaped, sharpened. No sufficient hardness and wear resistance. Limited to low cutting speed operation High Speed Steel (HSS-1905) Addition of alloying elements (manganese, chromium, tungsten, vanadium, molybdenum, cobalt, and niobium) to harden and strengthen the steel and make it more resistant to heat (hot hardness). They are of two types: Tungsten HSS (denoted by T), Molybdenum HSS (denoted by M). The basic composition of HSS (18-4-1) is 18% W (for hot hardness), 4% Cr (for deformation resistance), 1% V (for wear resistance), 0.7% C and rest Fe .

41. Primary Cutting Tool Materials 41 High Speed Steel (HSS-1905) Such HSS tool could machine (turn) mild steel jobs at speed only upto 20 ~ 30 m/min (which was quite substantial those days). However, HSS is still used as cutting tool material where; the tool geometry and mechanics of chip formation are complex, such as helical twist drills, reamers, gear shaping cutters, hobs, form tools, broaches etc. brittle tools like carbides, ceramics etc. are not suitable under shock loading the small scale industries cannot afford costlier tools the old or low powered small machine tools cannot accept high speed and feed. The tool is to be used number of times by resharpening. With time the effectiveness and efficiency of HSS (tools) and their application range were gradually enhanced by improving its properties and surface condition through - Refinement of microstructure Addition of large amount of cobalt and Vanadium to increase hot hardness and wear resistance respectively Manufacture by powder metallurgical process Surface coating with heat and wear resistive materials like TiC, TiN, etc by Chemical Vapour Deposition (CVD) or Physical Vapour Deposition (PVD)

42. Primary Cutting Tool Materials 42 Figure 2.4: Compositions and types of popular high speed steels Commonly Used Grades of HSS

43. Primary Cutting Tool Materials 43 Stellites This is a cast alloy of Co (40 to 50%), Cr (27 to 32%), W (14 to 19%) and C (2%) . Stellite is quite tough and more heat and wear resistive than the basic HSS (18 – 4 – 1) But such Stellite as cutting tool material became obsolete for its poor grindability and especially after the arrival of cemented carbides. Cemented/Sintered Tungsten carbides These are basically carbon cemented together by a binder. It is a powder metallurgy product and the binder mostly used is cobalt. The basic ingredient is tungsten carbide-82%, titanium carbide-10% and cobalt-8%. These materials possess high hardness and wear resistance Cutting speed 6 times higher than high speed steel (HSS).

44. Primary Cutting Tool Materials 44 Sintered Carbides Straight or single carbide Powder metallurgically produced by mixing, compacting and sintering 90 to 95% WC powder with cobalt. The hot, hard and wear resistant WC grains are held by the binder Co which provides the necessary strength and toughness. Suitable for machining grey CI, brass, bronze etc. which produce short discontinuous chips and at cutting velocities 2 to 3 times of HSS tools. Composite carbides The single carbide is not suitable for machining steels because of rapid growth of wear, particularly crater wear by diffusion of Co and carbon from the tool to the chip under the high stress and temperature bulk (plastic) contact between the continuous chip and the tool surfaces. Used for machining steels By adding (8 to 20%) a gamma phase to WC and Co mix. The gamma phase is a mix of TiC, TiN, TaC , NiC etc. which are more diffusion resistant than WC Mixed carbides Titanium carbide (TiC) is not only more stable but also much harder than WC. So for machining ferritic steels causing intensive diffusion and adhesion wear a large quantity (5 to 25%) of TiC is added with WC and Co to produce another grade called Mixed carbide. But increase in TiC content reduces the toughness of the tools.

45. Primary Cutting Tool Materials 45 Sintered Carbides Gradation of cemented carbides and their applications The standards developed by ISO for grouping of carbide tools and their application ranges are given in figure below Figure 2.5: Broad classification of carbide tools

46. Primary Cutting Tool Materials 46 Plain Ceramics It mainly consists of aluminum oxide (Al 2 O 3 ) and silicon nitride (Si 3 N 4 ). Ceramic cutting tools are hard with high hot hardness and do not react with the workpiece. They can be used at elevated temperature and cutting speed 4 times that of cemented carbide. These have low heat conductivity. Inherently high compressive strength, chemical stability and hot hardness of the ceramics led to powder metallurgical production of indexable ceramic tool inserts since 1950. Table below shows the advantages and limitations of alumina ceramics in contrast to sintered carbide. Figure 2.6: Cutting tool properties of alumina ceramics Alumina (Al2O3) is preferred to silicon nitride (Si3N4) for higher hardness and chemical stability. Si3N4 is tougher but again more difficult to process. The plain ceramic tools are brittle in nature and hence had limited applications . * Cutting tool should resist penetration of heat but should disperse the heat throughout the core .

47. Primary Cutting Tool Materials 47 Plain Ceramics Basically three types of ceramic tool bits are available in the market Plain alumina with traces of additives: these white or pink sintered inserts are cold pressed and are used mainly for machining cast iron and similar materials at speeds 200 to 250 m/min Alumina with or without additives : hot pressed, black color, hard and strong, used for machining steels and cast iron at speeds = 150 to 250 m/min Carbide ceramic (Al2O3 + 30% TiC) cold or hot pressed, black color, quite strong and enough tough – used for machining hard cast irons and plain and alloy steels at 150 to 200 m/min. The plain ceramic outperformed the then existing tool materials in some application areas like high speed machining of softer steels mainly for higher hot hardness as indicated in Fig. 2.7 However , the use of those brittle plain ceramic tools, until their strength and toughness could be substantially improved since 1970, gradually decreased for being restricted to uninterrupted machining of soft cast irons and steels only relatively high cutting velocity but only in a narrow range (200 ~ 300 m/min) requiring very rigid machine tools Advent of coated carbide capable of machining cast iron and steels at high velocity made the then ceramics almost obsolete. Figure 2.7: Hot hardness of the different commonly used tool materials.

48. Advanced Cutting Tool Materials 48 Coated carbides Cermets Coronite High Performance Ceramics (HPC) Cubic Boron Nitride (cBN) Diamond  

49. Advanced Cutting Tool Materials 49 Coated Carbides The properties and performance of carbide tools could be substantially improved by Refining microstructure Manufacturing by casting: expensive and uncommon Surface coating – made remarkable contribution . Thin but hard coating of single or multilayers of more stable and heat and wear resistive materials like TiC, TiCN, TiOCN , TiN, Al 2 O 3 etc. on the tough carbide inserts (substrate) by processes like chemical Vapour Deposition (CVD), Physical Vapour Deposition (PVD) at controlled pressure and temperature enhanced MRR and overall machining economy remarkably enabling, reduction of cutting forces and power consumption increase in tool life (by 200 to 500%) for same V or increase in V (by 50 to 150%) for same tool life improvement in product quality effective and efficient machining of wide range of work materials pollution control by less or no use of cutting fluid through reduction of abrasion, adhesion and diffusion wear reduction of friction and BUE formation heat resistance and reduction of thermal cracking and plastic deformation Fig. 2.8 Machining by coated carbide insert. The cutting velocity range in machining mild steel could be enhanced from 120 ~ 150 m/min to 300 ~ 350 m/min by properly coating the suitable carbide inserts. About 50% of the carbide tools being used at present are coated carbides The properties and performances of coated inserts and tools are getting further improved by; Refining the microstructure of the coating Multilayering (already up to 13 layers within 12 ~ 16 μm) Direct coating by TiN instead of TiC, if feasible Using better coating materials

50. Advanced Cutting Tool Materials 50 Cermets These sintered hard inserts are made by combining ‘ cer ’ from ceramics like TiC, TiN or TiCN and ‘met’ from metal (binder) like Ni, Ni-Co, Fe etc. Since around 1980, the modern Cermets providing much better performance are being made by TiCN which is consistently more wear resistant, less porous and easier to make. The characteristic features of such Cermets, in contrast to sintered tungsten carbides, are: The grains are made of TiCN (in place of WC) and Ni or Ni-Co and Fe as binder (in place of Co) Harder , more chemically stable and hence more wear resistant More brittle and less thermal shock resistant Wt % of binder metal varies from 10 to 20% Cutting edge sharpness is retained unlike in coated carbide inserts Can machine steels at higher cutting velocity than that used for tungsten carbide, even coated carbides in case of light cuts. TiCN based Cermets with bevelled or slightly rounded cutting edges are suitable for finishing and semi-finishing of steels at higher speeds, stainless steels Not suitable for jerky interrupted machining and machining of aluminium and similar materials.

51. Advanced Cutting Tool Materials 51 Coronite It is already mentioned earlier that the properties and performance of HSS tools could have been improved by refinement of microstructure, powder metallurgical process of making and surface coating. tool material Coronite has been developed for making the tools like small and medium size drills and milling cutters etc. which were earlier essentially made of HSS. Coronite is made basically by combining HSS for strength and toughness and tungsten carbides for heat and wear resistance. Microfine TiCN particles are uniformly dispersed into the matrix. Unlike a solid carbide, the coronite based tool is made of three layers; the central HSS or spring steel core a layer of Coronite of thickness around 15% of the tool diameter a thin (2 to 5 μm) PVD coating of TiCN. Such tools are not only more productive but also provide better product quality. The Coronite tools made by hot extrusion followed by PVD-coating of TiN or TiCN outperformed HSS tools in respect of cutting forces, tool life and surface finish.

52. Advanced Cutting Tool Materials 52 High Performance Ceramics (HPC) Ceramic tools as such are much superior to sintered carbides in respect of hot hardness, chemical stability and resistance to heat and wear but lack in fracture toughness and strength . Through last few years remarkable improvements in strength and toughness and hence overall performance of ceramic tools could have been possible by several means which include; Sinterability, microstructure, strength and toughness of Al 2 O 3 ceramics were improved to some extent by adding TiO 2 and MgO Transformation toughening by adding appropriate amount of partially or fully stabilized zirconia in Al 2 O 3 powder Isostatic and hot isostatic pressing (HIP) – these are very effective but expensive route Introducing nitride ceramic (Si 3 N 4 ) with proper sintering technique – this material is very tough but prone to built-up-edge formation in machining steels Developing SIALON – deriving beneficial effects of Al 2 O 3 and Si 3 N 4 Adding carbide like TiC (5 ~ 15%) in Al 2 O 3 powder – to impart toughness and thermal conductivity Reinforcing oxide or nitride ceramics by SiC whiskers, which enhanced strength, toughness and life of the tool and thus productivity spectacularly. But manufacture and use of this unique tool need specially careful handling Toughening Al 2 O 3 ceramic by adding suitable metal like silver which also impart thermal conductivity and self-lubricating property; this novel and inexpensive tool is still in experimental stage.

53. Advanced Cutting Tool Materials 53 High Performance Ceramics (HPC) The enhanced qualities of the unique high performance ceramic tools, specially the whisker and zirconia based types enabled them machine structural steels at speed even beyond 500 m/min and also intermittent cutting at reasonably high speeds, feeds and depth of cut. Such tools are also found to machine relatively harder and stronger steels quite effectively and economically. The HPC tools can be broadly classified into two groups as: Fig. 2.9 Classification of HPC Tools

54. Advanced Cutting Tool Materials 54 High Performance Ceramics (HPC) Nitride based ceramic tools Plain nitride ceramics tools Compared to plain alumina ceramics, Nitride (Si 3 N 4 ) ceramic tools exhibit more resistance to fracturing by mechanical and thermal shocks due to higher bending strength, toughness and higher conductivity. More suitable for rough and interrupted cutting of various material excepting steels, which cause rapid diffusional wear and BUE formation. The fracture toughness and wear resistance can be increased by adding zirconia and coating the finished tools with high hardness alumina and titanium compound. Nitride ceramics cannot be easily compacted and sintered to high density. Sintering with the aid of ‘reaction bonding’ and ‘hot pressing’ may reduce this problem to some extent. SIALON tools (Si-Al-O-N) By Hot pressing and sintering of an appropriate mix of Al 2 O 3 and Si 3 N 4 powders an excellent composite ceramic tool called SIALON is made. Very hot hard, quite tough and wear resistant. Can machine steel and cast irons at high speed (250–300 m/min). But machining of steels by such tools at too high speeds reduces the tool life by rapid diffusion. SiC reinforced Nitride tools The toughness, strength and thermal conductivity and hence the overall performance of nitride ceramics could be increased remarkably by adding SiC whiskers or fibers in 5 – 25 volume %. The SiC whiskers add fracture toughness mainly through crack bridging, crack deflection and fiber pull-out. Such tools are very expensive but extremely suitable for high production machining of various soft and hard materials even under interrupted cutting.

55. Advanced Cutting Tool Materials 55 High Performance Ceramics (HPC) Oxide based ceramic tools Zirconia (or partially stabilized Zirconia-PSZ) toughened alumina (ZTA) ceramic Fine powder of partially stabilized zirconia (PSZ) is mixed in proportion of ten to twenty volume percentage with pure alumina, then either cold pressed and sintered at 1600–1700 o C or hot isostatically pressed (HIP) under suitable temperature and pressure. The phase transformation of metastable tetragonal zirconia (t-Z) to monoclinic zirconia (m-Z) during cooling of the composite (Al 2 O 3 + ZrO 2 ) inserts after sintering or HIP and during polishing and machining imparts the desired strength and fracture toughness through volume expansion (3 – 5%) and induced shear strain (7%). Their hardness have been raised further by proper control of particle size and sintering process. Hot pressing and HIP raise the density, strength and hot hardness of ZTA tools but the process becomes expensive and the tool performance degrades at lower cutting speeds. However such ceramic tools can machine steel and cast iron at speed range of 150 – 500 m/min. Alumina ceramic reinforced by SiC whiskers Alumina based ceramic tools have been improved by drastic increase in fracture toughness (2.5 times) and bulk thermal conductivity without sacrificing hardness and wear resistance. This is achieved by mechanically reinforcing the brittle alumina matrix with extremely strong and stiff silicon carbide whiskers. The randomly oriented, strong and thermally conductive whiskers enhance the strength and toughness mainly by crack deflection and crack-bridging and also by reducing the temperature gradient within the tool. After optimization of the composition, processing and the tool geometry, such tools can machine wide range of materials, over wide speed range (250 – 600 m/min) even under large chip loads. Manufacturing of whiskers need careful handling and precise control Costlier than zirconia toughened ceramic tools. Silver toughened alumina ceramic Alumina-metal composites have been studied primarily using addition of metals like aluminium, nickel, chromium, molybdenum, iron and silver. Compared to zirconia and carbides, metals were found to provide more toughness in alumina ceramics. Again compared to other metal-toughened ceramics, the silver-toughened ceramics can be manufactured by simpler and more economical process routes like pressure less sintering and without atmosphere control. Such HPC tools can suitably machine with large MRR and V (250 – 400 m/min) and long tool life even under light interrupted cutting like milling. Such tools also can machine steels at speed from quite low to very high cutting velocities (200 to 500 m/min).

56. Advanced Cutting Tool Materials 56 Cubic Boron Nitride (cBN) Next to diamond, cubic boron nitride is the hardest material presently available. Only in 1970 and onward cBN in the form of compacts has been introduced as cutting tools. It is made by bonding a 0.5 – 1 mm layer of polycrystalline cubic boron nitride to cobalt based carbide substrate at very high temperature and pressure . It remains inert and retains high hardness and fracture toughness at elevated machining speeds. It shows excellent performance in grinding any material of high hardness and strength. The extreme hardness, toughness, chemical and thermal stability and wear resistance led to the development of cBN cutting tool inserts for high material removal rate (MRR) as well as precision machining imparting excellent surface integrity of the products. Such unique tools effectively and beneficially used in machining wide range of work materials covering high carbon and alloy steels, non-ferrous metals and alloys, exotic metals like Ni-hard, Inconel, Nimonic etc and many non-metallic materials which are as such difficult to machine by conventional tools. It is firmly stable at temperatures upto 1400o C. The operative speed range for cBN when machining grey cast iron is 300 ~ 400 m/min . Speed ranges for other materials are as follows : Hard cast iron (> 400 BHN) : 80 – 300 m/min Superalloys (> 35 RC) : 80 – 140 m/min Hardened steels (> 45 RC) : 100 – 300 m/min In addition to speed, the most important factor that affects performance of cBN inserts is the preparation of cutting edge. It is best to use cBN tools with a honed or chamfered edge preparation, especially for interrupted cuts. Like ceramics, cBN tools are also available only in the form of indexable inserts. The only limitation of it is its high cost .

57. Advanced Cutting Tool Materials 57 Cubic Boron Nitride (cBN) Next to diamond, cubic boron nitride is the hardest material presently available. Only in 1970 and onward cBN in the form of compacts has been introduced as cutting tools. It is made by bonding a 0.5 – 1 mm layer of polycrystalline cubic boron nitride to cobalt based carbide substrate at very high temperature and pressure. It remains inert and retains high hardness and fracture toughness at elevated machining speeds. It shows excellent performance in grinding any material of high hardness and strength. The extreme hardness, toughness, chemical and thermal stability and wear resistance led to the development of cBN cutting tool inserts for high material removal rate (MRR) as well as precision machining imparting excellent surface integrity of the products. Such unique tools effectively and beneficially used in machining wide range of work materials covering high carbon and alloy steels, non-ferrous metals and alloys, exotic metals like Ni-hard, Inconel, Nimonic etc and many non-metallic materials which are as such difficult to machine by conventional tools. It is firmly stable at temperatures upto 1400o C. The operative speed range for cBN when machining grey cast iron is 300 ~ 400 m/min . Speed ranges for other materials are as follows : Hard cast iron (> 400 BHN) : 80 – 300 m/min Superalloys (> 35 RC) : 80 – 140 m/min Hardened steels (> 45 RC) : 100 – 300 m/min In addition to speed, the most important factor that affects performance of cBN inserts is the preparation of cutting edge. It is best to use cBN tools with a honed or chamfered edge preparation, especially for interrupted cuts. Like ceramics, cBN tools are also available only in the form of indexable inserts. The only limitation of it is its high cost .

58. Advanced Cutting Tool Materials 58 Diamond Tools Single crystalline Diamond (SCD ) Single crystal diamond (natural or synthetic) are used as tips/edge of cutting tools. Extreme hardness and sharp edges, natural single crystal is used for many applications, particularly where high accuracy and precision are required. Their important uses are: Single point cutting tool tips and small drills for high speed machining of non-ferrous metals, ceramics, plastics, composites, etc. and effective machining of difficult-to-machine materials Drill bits for mining, oil exploration, etc. Tool for cutting and drilling in glasses, stones, ceramics, FRPs etc. Wire drawing and extrusion dies Super abrasive wheels for critical grinding. Limited supply, increasing demand, high cost and easy cleavage of natural diamond has led to the manufacture of artificial diamond grits with desired control on various parameters. Polycrystalline Diamond (PCD) PCD tools consist of a layer (0.5 to 1.5 mm) of fine grain size, randomly oriented diamond particles sintered with a suitable binder (usually cobalt) and then metallurgically bonded to a suitable substrate like cemented carbide or Si 3 N 4 inserts. PCD exhibits excellent wear resistance, hold sharp edge, generates little friction in the cut, provide high fracture strength, and had good thermal conductivity. These properties contribute to PCD tooling’s long life in conventional and high speed machining of soft, non-ferrous materials (aluminium, magnesium, copper etc.), advanced composites and metal-matrix composites, super alloys, and non-metallic materials. PCD is particularly well suited for abrasive materials (i.e. drilling and reaming metal matrix composites) where it provides 100 times the life of carbides. PCD is not usually recommended for ferrous metals because of high solubility of diamond (carbon) in these materials at elevated temperature. However, under special conditions can be used; for example, light cuts are being successfully made in grey cast iron. The main advantage of such PCD tool is the greater toughness due to finer microstructure with random orientation of the grains and reduced cleavage. But such unique PCD also suffers from some limitations like: High tool cost Presence of binder, cobalt, which reduces wear resistance and thermal stability Complex tool shapes like in-built chip breaker cannot be made Size restriction, particularly in making very small diameter tools

59. Advanced Cutting Tool Materials 59 Diamond Tools Diamond coated carbide tools Since the invention of low pressure synthesis of diamond from gaseous phase, continuous effort has been made to use thin film diamond in cutting tool field. These are normally used as thin (<50 μm) or thick (> 200 μm) films of diamond synthesized by CVD method for cutting tools, dies, wear surfaces and even abrasives for Abrasive Jet Machining (AJM) and grinding. Thin film is directly deposited on the tool surface. Thick film ( > 500 μm) is grown on an easy substrate and later brazed to the actual tool substrate and the primary substrate is removed by dissolving it or by other means. Thick film diamond finds application in making inserts, drills, reamers, end mills, routers. CVD coating has been more popular than single diamond crystal and PCD mainly for : Free from binder, higher hardness, resistance to heat and wear more than PCD and properties close to natural diamond Highly pure, dense and free from single crystal cleavage Permits wider range of size and shape of tools and can be deposited on any shape of the tool including rotary tools Relatively less expensive However, achieving improved and reliable performance of thin film CVD diamond coated tools; (carbide, nitride, ceramic, SiC etc ) in terms of longer tool life, dimensional accuracy and surface finish of jobs essentially need: Good bonding of the diamond layer Adequate properties of the film, e.g. wear resistance, micro-hardness, edge coverage, edge sharpness and thickness uniformity Ability to provide work surface finish required for specific applications. While cBN tools are feasible and viable for high speed machining of hard and strong steels and similar materials, Diamond tools are extremely useful for machining stones, slates, glass, ceramics, composites, FRPs and nonferrous metals specially which are sticky and BUE former such as pure aluminium and its alloys. CBN and Diamond tools are also essentially used for ultraprecision as well as micro and nano machining.

60. Cutting Tool Materials 60 Figure 2.10: (A) Hardness of various cutting-tool materials as a function of temperature. (B) Ranges of properties of various groups of materials.

61. Economics of Machining 61 Cutting conditions in a machining operation consist of speed, feed, depth of cut, and cutting fluid . Cutting Fluids: Requirements and type are determined by tooling considerations. Depth of cut : It is often predetermined by workpiece geometry and operation sequence like roughing operations (high depth of cut) followed by a final finishing operation (low depth of cut). The problem then reduces to selection of feed and speed. In general, values of these parameters should be decided in the order: feed first, speed second. Feed rate for a given machining operation depends on the following factors: Tooling: What type of tooling will be used? Harder tool materials (e.g., cemented carbides, ceramics, etc.) tend to fracture more readily than high-speed steel. These tools are normally used at lower feed rates. HSS can tolerate higher feeds because of its greater toughness. Roughing or finishing: Roughing operations involve high feeds, typically 0.5 to 1.25 mm/rev for turning; finishing operations involve low feeds, typically 0.125 to 0.4 mm/rev for turning. Constraints on feed in roughing: If the operation is roughing, how high can the feed rate be set? To maximize metal removal rate, feed should be set as high as possible. Upper limits on feed are imposed by cutting forces, setup rigidity, and sometimes horsepower. Surface finish requirements in finishing: If the operation is finishing, what is the desired surface finish? Feed selected affects the surface finish. The problem now reduces to selection of speed which is discussed now.

62. Optimizing Cutting Speed 62 It means choosing a speed that provides a high metal removal rate along with long tool life. Optimal cutting speed for a machining operation can be determined if the various time and cost components of the operation are known. The original derivation of these machining economics equations was done by W. Gilbert. The formulas allow the optimal cutting speed to be calculated for either of two objectives: (1) Maximum production rate (2) Minimum unit cost. Both objectives seek to achieve a balance between material removal rate and tool life.

63. Maximizing Production Rate Model 63 Objective: To determine optimum speed that minimizes machining time per workpiece i.e. maximizes production rate. Assumptions: The formulas are based on Taylor tool life equation. Feed, depth of cut, and work material have already been set. The derivation is for a turning operation. Time Elements: Total production cycle time for one part T c is consists of- Part handling time T h : Time for loading & unloading of part on machine. Machining time T m : Actual tool cutting time during the cycle. Tool change time T t : Changing worn tool takes time (transfer for t) which is apportioned over the number of parts cut during the tool life. Let n p the number of pieces cut in one tool life thus, the tool change time per part = T t /n p

64. Maximizing Production Rate Model 64 The sum of these three time elements gives the total time per unit product for the operation cycle T c is a function of cutting speed. As cutting speed is increased, T m decreases and T t /n p increases; This unaffected by speed. The cycle time per part is minimized at a certain value of cutting speed. This optimal speed can be identified by recasting total cycle time equation as a function of speed.   Machining time in a straight turning operation is given by- Where T m = machining time per part, min D= workpart diameter, mm L =workpart length, mm f=feed, mm/rev v=cutting speed, mm/min  

65. Maximizing Production Rate Model 65 The number of pieces per tool n p is also a function of speed. It can be shown that- Where T =tool life, min/tool T m =machining time per part, min/pc. Both T and T m are functions of speed; hence, the ratio is a function of speed Solving this equation yields the cutting speed for maximum production rate in the operation   The cycle time per piece is a minimum at the cutting speed at which the derivative of (1) is zero The corresponding tool life for maximum production rate is

66. Minimizing Cost per Unit Model 66 Objective: To determine optimum speed that minimizes production cost per piece for the operation Assumptions : The formulas are based on Taylor tool life equation. Feed, depth of cut, and work material have already been set. The derivation is for a turning operation. Cost Elements: Total cost of producing one part during a turning operation is consists of- Cost of part handling time: Cost of the time in loading and unloading the part. Let C o =the cost for the operator and machine. Thus the cost of part handling time =C o T h Cost of machining time : This is the cost of the time the tool is engaged in machining. Using C o again to represent the cost per minute of the operator and machine tool, the cutting time cost=C o T m Cost of tool change time: The cost of tool change time = C o T t /n p Tooling cost: In addition to the tool change time, the tool itself has a cost that must be added to the total operation cost. This cost is the cost per cutting edge C t divided by the number of pieces machined with that cutting edge n p . Thus, tool cost per workpiece is given by C t /n p Note: Variation in Tooling cost For disposable inserts (e.g., cemented carbide inserts), tool cost is determined as where C t =cost per cutting edge, Rs/tool life P t = price of the insert, Rs/insert n e =number of cutting edges per insert This depends on the insert type; for example, triangular inserts that can be used only one side (positive rake tooling) have three edges/insert; if both sides of the insert can be used (negative rake tooling), there are six edges/insert; and so forth. For regrindable tooling (e.g., high-speed steel solid shank tools, brazed carbide tools), the tool cost includes purchase price plus cost to regrind : Where C t =cost per tool life, Rs/tool life P t =purchase price of the solid shank tool or brazed insert, Rs/tool n g =number of tool lives per tool, which is the number of times the tool can be ground before it can no longer be used (5 to 10 times for roughing tools and 10 to 20 times for finishing tools); T g =time to grind or regrind the tool, min/tool life C g =grinder’s rate, Rs/min.

67. Minimizing Cost per Unit Model 67 The sum of the four cost components gives the total cost per unit product C c for the machining cycle:   Cc is a function of cutting speed, just as Tc is a function of v. The relationships for the individual terms and total cost as a function of cutting speed are shown in Figure Eq. (1) can be rewritten in terms of v to yield: The cutting speed that obtains minimum cost per piece for the operation can be determined by taking the derivative of above Eq . with respect to v, setting it to zero, and solving for v min The corresponding tool life is given by  

68. Cutting Fluids 68

69. Cutting Fluids: Definition & Purpose 69 Definition: A cutting fluid is any liquid or gas that is applied directly to the machining operation to improve cutting performance. Purpose: Act as lubricant: Reduce friction and wear by acting as a film and hence also reduce welding tendency. Act as coolant: Cooling of cutting zone and hence increasing tool life & improving dimensional stability, Reducing the temperature of the workpart for easier handling Reduce forces and energy consumption. Flush away the chips from the cutting zone to avoid interference in cutting Protect the machined surface from environmental corrosion (weakening and depletion of surface by deposition of other matter like rust) & contamination by the gases like SO 2 , O 2 , H 2 S, and N x O y present in the atmosphere

70. Essential Properties of Cutting Fluids 70 For cooling: High specific heat (high heat absorbing capacity), thermal conductivity and film coefficient for heat transfer spreading and wetting ability For lubrication: High lubricity without gumming (accumulation & sticking of dirt & dust) and foaming Wetting and spreading High film boiling point Friction reduction at extreme pressure and temperature Chemical stability, non-corrosive to the materials of the system less volatile (tendency of evaporation) and high flash point (temperature at which fluid vapor ignite by some source) high resistance to bacterial growth odorless and also preferably colorless nontoxic in both liquid and gaseous stage Easily available and low cost. It should permit clear view of the work operation.

71. Types of Cutting Fluids 71 Air blast or compressed air only Grey cast iron use no cutting fluid in liquid form as graphite flakes in the cast iron acts as a lubricant in itself by sliding over each other and producing short discontinuous chips. In such cases only air blast is recommended for cooling and cleaning Water For its good wetting and spreading properties and very high specific heat, water is considered as the best coolant and hence employed where cooling is most urgent. Cutting oils or Straight oils Compounds of mineral oil with vegetable, animal or marine oils added for improving spreading, wetting and lubricating properties. As and when required some EPA additive including Sulphur, chlorine, and phosphorus are also mixed to reduce friction, adhesion and BUE formation in heavy cuts. Additives react chemically with the chip and tool surfaces to form solid films (extreme pressure lubrication) that help to avoid metal-to-metal contact between the two. Advantages :: excellent lubrication, good corrosion protection, easy maintenance Limitation & Disadvantages : poor heat removal, toxic mist, high viscosity, flammable, expensive, not suitable for high speed machining

72. Types of Cutting Fluids 72 Emulsified oils or soluble oils Oil droplets suspended in water Water acts as the best coolant but does not lubricate and also induce rusting. So oil containing some emulsifying agent to improve blending and stability (An emulsion is a mixture of two or more liquids that are normally immiscible (unmixable or unblendable)) and extreme pressure additive (EPA) like Sulphur, chlorine, and phosphorus are mixed with water in a suitable ratio (1:30). It look likes white milk, used widely, have less lubrication qualities but have good cooling ability. Advantages : good lubrication, good cooling capability, some corrosion protection, low cost, nonflammable. Disadvantages : anti-bacteria additives and maintenance are needed, toxic mist, susceptible to hard water (may form insoluble precipitates).

73. Types of Cutting Fluids 73 Chemical fluids or synthetic fluids Blended chemicals with additives, diluted in water, and containing no oil. Synthetic fluids are water based solutions (or emulsions) of synthetic lubricants (soaps and other wetting agents), corrosion inhibitors, water softeners, Extreme pressure additives (EPA), anti-bacteria additives (biocides), glycols and other additives. Synthetic fluids are supplied in form of concentrates, which are mixed with water before use. Synthetic fluids are used in a wide variety of metalworking operations including poorly machinable alloys, heavy duty grinding, high speed cutting. Advantages of synthetic fluids: very good cooling capability, good lubrication properties, good stability in hard water, quick wetting ability, low surface tension so good spreading, good corrosion protection, easy handling, cleaning and maintenance. Disadvantages/Limitations of synthetic fluids: some toxicity, easily contaminated by foreign oils, relatively high cost.

74. Types of Cutting Fluids 74 Semi-Synthetic or Semi-Chemical Fluids These will contain small amounts of oil and other additives to enhance lubrication while providing maximum cooling. Semi-synthetic fluids are water based mixture (solution and emulsion) of synthetic lubricants, additives, emulsifiers and some amount (2%-30%) of mineral oil. Semi-synthetic fluids combine advantages (and disadvantages at some extent) of mineral emulsions and synthetic fluids Advantages: They possess better corrosion protection than synthetic fluids and better cooling and wetting capabilities, easier handling and maintenance than mineral emulsions. Disadvantages/Limitations : misting, relatively poor stability in hard water, contaminated by foreign oils, some toxicity, lower lubrication ability, possible skin irritants, and less corrosion protection. Solid or Semi-Solid Lubricant Paste, waxes, soaps, graphite, Moly- disulphide (MoS 2 ) may also often be used, either applied directly to the workpiece or as an impregnant in the tool to reduce friction and thus cutting forces, temperature and tool wear. Cryogenic cutting fluid Extremely cold (cryogenic) fluids (often in the form of gases) like liquid CO 2 or N 2 are used in some special cases for effective cooling without creating much environmental pollution and health hazards.

75. Selection of Cutting Fluids 75 For high speed machining of not-difficult-to-machine materials greater cooling type fluids are preferred For low speed machining of both conventional and difficult-to-machine materials greater lubricating type fluid is preferred. Grey cast iron: Generally dry for its self-lubricating property Air blast for cooling and flushing chips Steels: Soluble oil for cooling and flushing chips in high speed machining and grinding If machined by HSS tools, sol. Oil (1: 20 ~30) for low carbon and alloy steels and neat oil with EPA for heavy cuts If machined by carbide tools thinner sol. Oil for low strength steel, thicker sol. Oil ( 1:10 ~ 20) for stronger steels and straight sulphurised oil for heavy and low speed cuts and EP cutting oil for high alloy steel. Often steels are machined dry by carbide tools for preventing thermal shocks. Aluminum and its alloys: Preferably machined dry Light but oily soluble oil Straight neat oil or kerosene oil for stringent cuts. Copper and its alloys: Water based fluids are generally used Oil with or without inactive EPA for tougher grades of Cu-alloy. Stainless steels and Heat resistant alloys: High performance soluble oil or neat oil with High concentration with chlorinated EP additive. The brittle ceramics and cermets should be used either under dry condition or light neat oil in case of fine finishing. Grinding at high speed needs cooling (1: 50 ~ 100) soluble oil. For finish grinding of metals and alloys low viscosity neat oil is also used .

76. Application Methods of Cutting Fluids 76 Drop-by-drop under gravity In the form of liquid jet(s) Flooding or Flood Cooling (under gravity) Most common method also called flood as used with coolant-type cutting fluids. A flood of cutting fluid is applied at the tool–work or tool–chip interface Mist (atomized oil) with compressed air Supplies fluid to inaccessible areas in the form of a high-speed mist using pressurized air stream. Provides better visibility of the workpiece being machined It is effective with water-based fluids at air pressures ranging from 70 to 600 kPa . Limited cooling capacity. Requires venting to prevent the inhalation of airborne fluid particles by operator Manual Operation: Manual application by means of a paint brush Used in tapping and other operations in which cutting speeds are low and friction is a problem. Not preferred by most production machine shops because of its variability in application.

77. Application Methods of Cutting Fluids 77 High-pressure systems These systems use high-pressure refrigerated coolant systems to increase the rate of heat removal from the cutting zone. High pressures also are used in delivering the cutting fluid via specially designed nozzles that aim a powerful jet of fluid to the zone, particularly into the clearance or relief face of the tool. The pressures employed, which are usually in the range from 5.5 to 35 MPa , act as a chip breaker in situations where the chips produced would otherwise be long and continuous, interfering with the cutting operation. In order to avoid damage to the workpiece surface by impact from any particles present in the high-pressure jet, contaminant size in the coolant should not exceed 20 um. Proper and continuous filtering of the fluid also is essential to maintain quality .

78. Application Methods of Cutting Fluids 78 Through the cutting tool system Narrow passages can be produced in cutting tools, as well as in tool holders, through which cutting fluids can be applied under high pressure. Two applications of this method are Gun drilling, with a long, small hole through the body of the drill itself Boring bars, where there is a long hole through the shank (tool holder), to which an insert is clamped. Delivering cutting fluids through the spindle of the machine tool is also developed. Z-Z method – centrifugal through the grinding wheels (pores) Figure 2.13 Application of cutting fluid by hole in the tool Figure 2.14: Z-Z Method in grinding

79. Cutting Fluid Filtration 79 Cutting fluids become contaminated over time by various substances like tramp oil (machine oil, hydraulic fluid, etc.), garbage (cigarette butts, food, etc.), small chips, molds, fungi, and bacteria. In addition to causing odors and health hazards, contaminated cutting fluids do not perform their lubricating function as well. Alternative ways of dealing with this problem are to: replace the cutting fluid at regular and frequent intervals use a filtration system to periodically clean the fluid Dry machining; that is, machine without cutting fluids. Because of growing concern about environmental pollution and associated legislation, disposing old fluids is a major concern. Filtration systems are being installed which have advantages of prolonged cutting fluid life between changes, reduced fluid disposal cost, cleaner cutting fluid for better working environment and reduced health hazards, lower machine tool maintenance & longer tool life. Several techniques of filtration are settling, skimming, centrifuging, and filtering . Recycling involves treatment of the fluids with various additives, agents, biocides, and deodorizers, as well as water treatment (for water-based fluids).

80. Dry Machining 80 No cutting fluid is used. It has advantages of addressing pollution concern, cost reduction & improving surface quality. Associated problems are overheating the tool, operating at lower cutting speeds and production rates to prolong tool life, and absence of chip removal benefits in grinding and milling. Application of a fine mist of an air-fluid mixture having very small amount of cutting fluid. The mixture is delivered to the cutting zone through the spindle of the machine tool, typically through a 1-mm-diameter nozzle and under a pressure of 600 kPa . It is used at rates on the order of 1 to 100 cc/ hr , which is estimated to be (at most) one ten-thousandth of that used in flood cooling. Consequently, the process is also known as minimum-quantity lubrication (MQL). Viable alternative in various machining operations (especially turning, milling, and gear cutting) on steels, steel alloys, and cast irons, but generally not for aluminum alloys. Flushing of chips is achieved by pressurized air, often through the tool shank which doesn’t serve a lubrication purpose and provides only limited cooling

81. Cutting Fluid Maintenance 81 Maintenance and monitoring includes concentration checks using the appropriate test, including: Refractometers which are used to determine the total amount of soluble in a solution. Titration Kits which are used to analyze fluid concentration in metal cutting fluids contaminated with tramp oils. Tests for PH levels and alkalinity (acid splits) are also useful. The fluid manufacturer’s product data sheets should always be consulted and rigidly followed.