MANUFACTURING PROCESSES Tool Material & Cutting Fluid 1
CUTTING TOOL MATERIAL Success in metal cutting depends on the selection of the proper cutting tool (material and geometry) for a given work material. A cutting tool must have the following characteristics in order to produce good quality and economical parts: TOUGHNESS HOT HARDNESS WEAR RESISTANCE 2
TOUGHNESS To avoid fracture failure, the tool material must possess high toughness. Toughness is the capacity of a material to absorb energy without failing. It is usually characterized by a combination of strength and ductility in the material. 3
HOT HARDNESS Hot hardness is the ability of a material to retain its hardness at high temperatures. This is required because of the high-temperature environment in which the tool operates. 4
WEAR RESISTANCE Wear is the erosion of material from a solid surface by the action of another surface. The ability of a metal to resist the gradual wearing away caused by abrasion and friction is called wear resistance. All cutting-tool materials must be hard. However, wear resistance in metal cutting depends on more than just tool hardness like surface finish on the tool (a smoother surface means a lower coefficient of friction), chemistry of tool and work materials, and whether a cutting fluid is used. 5
Desirable characteristics of a cutting tool material 1. High hardness 2. High hardness temperature, hot hardness 3. Resistance to abrasion, wear due to severe sliding friction 4. Resistance to Chipping of the cutting edges 5. High toughness (impact strength) 6
Desirable characteristics of a cutting tool material 6. Strength to resist bulk deformation 7. Good chemical stability 8. Adequate thermal properties 9. High elastic modulus (stiffness) 10. Correct geometry and surface finish 7
TOOL MATERIALS CATEGORIES 1. High-speed Steels 2. Cast Cobalt Alloys (Stellite) 3. Carbides 4. Ceramics 5. Synthetic Diamond & CBN 8
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ALLOWABLE CUTTING SPEEDS 12
1. HIGH-SPEED STEEL(HSS) High-speed steel(hss) is a highly alloyed tool steel capable of maintaining hardness at elevated temperatures better than high carbon and low alloy steels. Its good hot hardness permits tools made of HSS to be used at higher cutting speeds. HSS is especially suited to applications involving complicated tool geometries, such as drills, taps, milling cutters, and broaches. 13
1. HIGH-SPEED STEEL(HSS) A wide variety of high-speed steels are available, but they can be divided into two basic types: Tungsten Type Molybdenum Type 14
1. HIGH-SPEED STEEL(HSS) (a) TUNGSTEN-TYPE Tungsten-type HSS contains tungsten (W) as its principal alloying ingredient. Additional alloying elements are chromium (Cr), and vanadium (V). One of the original and best known HSS grades is T1, or 18-4-1 high-speed steel, containing 18% W, 4% Cr, and 1% V. Grade C Cr Mo W V T1 0.7 4.0-18.0 1.0 15
1. HIGH-SPEED STEEL(HSS) (b) MOLYBDENUM - TYPE Molybdenum HSS grades contain combinations of tungsten and molybdenum (Mo), plus the same additional alloying elements as in the T-grades. Cobalt (Co) is sometimes added to HSS to enhance hot hardness. Of course, high-speed steel contains carbon, the element common to all steels. Grade C Cr Mo W V M2 0.8 4.0 5.0 6.0 2.0 16
2. CAST COBALT ALLOYS (Stellite) These alloys have the following composition 38% - 53% cobalt, 30% -33% chromium, and 10% - 20% tungsten. Because of their high hardness, they have good wear resistance and can maintain their hardness at elevated temperatures. They are not as tough as HSS and are sensitive to impact forces. 17
2. CAST COBALT ALLOYS (Stellite) Consequently, they are less suitable than highspeed steels for interrupted cutting operations. Commonly known as Stellite tools, these alloys are cast and ground into relatively simple tool shapes. Used only for special applications that involve deep, continuous roughing cuts at relatively high feeds and speeds -- as much as twice the rates possible with HSS. 18
3. CARBIDES (CEMENTED OR SINTERED CARBIDES) The two groups of tool materials described above cannot be used as effectively where high cutting speeds (High temp.) are involved. Introduced in the 1930 s CARBIDES have high hardness over a wide range of temperatures, so can be used for higher cutting speeds. High elastic modulus and thermal conductivity, and low thermal expansion. 19
3. CARBIDES (CEMENTED OR SINTERED CARBIDES) The most important, versatile, and cost-effective tool and die materials for a wide range of applications. Two major groups of carbides used for machining operations are Tungsten Carbides Titanium Carbide 20
3.CARBIDES -TUNGSTEN CARBIDES (WC) A composite material consisting of WC particles bonded together in a cobalt matrix. These tools are manufactured with powder metallurgy techniques (sintered or cemented carbides). WC particles are first combined with cobalt in a mixer, resulting in a cobalt matrix surrounding the WC particles. These particles, which are 1-5 μm in size, are then pressed and sintered into the desired insert shapes. Typically, amount of cobalt is 6-16% 21
3.CARBIDES -TUNGSTEN CARBIDES (WC) As the cobalt content increases, the strength, hardness, and wear resistance of WC decrease, while its toughness increases because of the higher toughness of cobalt. Tungsten-carbide tools are generally used for cutting steels, cast irons, and abrasive nonferrous materials, and have largely replaced HSS tools because of their better performance. 22
3.CARBIDES -TUNGSTEN CARBIDES (WC) Composed of TiC with a nickel-molybdenum matrix. Has higher wear resistance than WC but is not as tough. TiC is suitable for machining hard materials (mainly steels and cast irons) and for cutting at speeds higher than those appropriate for WC. 23
3. CARBIDES - INSERTS Inserts are individual cutting tools with several cutting points A square insert has eight cutting points, and triangular insert has six. Inserts are usually clamped on the toolholder with various locking mechanisms. 24
3. CARBIDES - INSERTS 25
3. CARBIDES - INSERTS 26
3. CARBIDES - INSERTS 27
3. CARBIDES - INSERTS 28
3. CARBIDES - INSERTS 29
3. CARBIDES - INSERTS Methods of mounting inserts on toolholders: (a) clamping, and (b) wing lockpins. 30
3. CARBIDES - INSERTS Clamping is the preferred method of securing the insert because each insert has a number of cutting points, and after one edge is worn, it is indexed (rotated in its holder) to make available another cutting point. Carbides inserts are available in a variety of shapes (square, triangle, round, and diamond ). The strength of cutting edge of an insert depends on its shape. The smaller the included angle, the lower the strength of the edge. 31
3. CARBIDES - INSERTS 32
3. CARBIDES - COATED TOOLS 33
3. CARBIDES - COATED TOOLS 34
3. CARBIDES - COATED TOOLS Coated carbides are a cemented carbide insert coated with one or more thin layers of wearresistant material, such as titanium carbide (TiC), titanium nitride (TiN), and/or aluminum oxide (Al2O3). 35
3. CARBIDES - COATED TOOLS 36
3. CARBIDES - COATED TOOLS Titanium nitride coating low coefficient of friction Al2O3 Aluminum oxide 2nd layer chemical stability at high temperature resists abrasive wear Titanium carbide (TiCN) as first layer strength and wear resistance Carbide substrate 37
3. CARBIDES - COATED TOOLS The coating is applied to the substrate (the material on which a process is conducted) by chemical vapour deposition (CVD) or physical vapour deposition (PVD). The coating thickness is only 2.5 to 13 µm. It has been found that thicker coatings tend to be brittle, resulting in cracking, chipping, and separation from the substrate. 38
3. CARBIDES - COATED TOOLS Coatings have unique properties such as: Lower friction, Higher adhesion, Higher resistance to wear and cracking, Acting as a diffusion barrier, and Higher hot hardness and impact resistance. 39
3. CARBIDES - COATED TOOLS Because of such unique properties, coated tools can be used at high cutting speeds, reducing both the time required for machining operations and production costs. Coated tools can have tool lives 10 times longer than those of uncoated tools. 40
3. CARBIDES - COATED TOOLS Coated carbides are used to machine cast irons and steels in turning and milling operations. They are best applied at high cutting speeds in situations in which dynamic force and thermal shock are minimal. If these conditions become too severe, as in some interrupted cut operations, chipping of the coating can occur, resulting in premature tool failure. In this situation, uncoated carbides formulated for toughness are preferred. 41
It has 3. CARBIDES - COATED TOOLS Titanium-Nitride Coatings (TiN) low friction coefficient, high hardness, resistance to high temperature, and good adhesion to the substrate. Gold in color and perform well at higher cutting speeds and feeds. Flank wear is significantly lower than that of uncoated tools. 42
3. CARBIDES - COATED TOOLS Titanium-Carbide Coatings (TiC) TiC coatings on tungsten-carbide inserts have high flank-wear resistance in machining abrasive materials. 43
3. CARBIDES - COATED TOOLS Ceramics Coatings have Chemical inertness. Ceramics Coatings Low thermal conductivity. Resistance to high temperature. Resistance to flank and crater wear. Most commonly used ceramic coating is aluminum oxide (Al2O3). However, oxide coating generally bond weakly to the substrate. 44
3. CARBIDES - COATED TOOLS Multiphase Coatings The desirable properties of the coatings can be combined and optimized with the use of multiphase coatings. Carbide tools are now available with two or three layers of such coatings and are particularly effective in machining cast irons and steels. 45
3. CARBIDES - COATED TOOLS Multiphase Coatings Typical applications of multiple-coated tools are: High-speed, continuous cutting: TiC/Al2O3. Heavy-duty, continuous cutting: TiC/Al2O3/TiN. Light, interrupted cutting: TiC/TiC + TiN/TiN. 46
3. CARBIDES - COATED TOOLS Multiphase Coatings 47
3. CARBIDES - COATED TOOLS Diamond Coatings Polycrystalline diamond is being used widely as a coating, particularly on tungsten-carbide and silicon-nitride inserts. Thin films are deposited on substrates with PVD and CVD techniques. Thick films are obtained by growing a large sheet of pure diamond, which is then laser cut to shape and brazed to a carbide shank. 48
3. CARBIDES - CERMETS Cermets ( ceramic and metal ) consist of ceramic particles in a metallic matrix. A typical cermet consists of 70% aluminum oxide and 30% titanium carbide; Other cermets contain molybdenum carbide, niobium carbide, and tantalum carbide. 49
3. CARBIDES - CERMETS They have chemical stability and resistance to build-up edge formation, The brittleness and high cost of cermets have been a limitation to their wider use. 50
4. CERAMICS Developed in 1970s, SiN based ceramics consist of silicon nitride with various additions of aluminum oxide, yttrium oxide, and titanium carbide. These tools have toughness, hot hardness, and good thermal-shock resistance. An example of a SiN-base material is sialon, composed of: silicon, aluminum, oxygen, and nitrogen. 51
4. CERAMICS It has higher thermal-shock resistance than silicon nitride and recommended for machining cast irons and nickel-based super-alloys at intermediate cutting speeds. Because of chemical affinity to iron, SiN-based tools are not suitable for machining steels. 52
5. DIAMOND Of all known materials, the hardest substance is diamond. As a cutting tool, it has low friction. High wear resistance. Ability to maintain sharp cutting edge. Used when good surface finish and dimensional accuracy are required particularly with soft nonferrous alloys, and abrasive non -metallic and metallic materials. 53
5. DIAMOND Synthetic or industrial diamonds now are used widely because natural diamond has flaws and its performance can be unpredictable. Diamond tools can be used at almost any speed, Most suitable for light uninterrupted finishing cuts. Diamond is not recommended for machining plain-carbon steels or titanium and cobalt-based alloys, because of its strong chemical Affinity at elevated temperatures. 54
5. CUBIC BORON NITRIDE Next to diamond, Cubic Boron Nitride (cbn) is the hardest material presently available. Made by bonding a 0.5-1-mm layer of polycrystalline cbn to a carbide substrate (the material on which a process is conducted) by sintering under high pressure and high temperature. While the carbide provides shock resistance, the cbn layer provides very high wear resistance and cutting-edge strength. 55
5. CUBIC BORON NITRIDE cbn tools are also made in small sizes without a substrate. Because cbn tools are brittle, stiffness of the machine tool and fixturing is important in order to avoid vibration and chatter. To avoid cracking due to thermal shock, machining should generally be performed dry, particularly in interrupted cutting operations such as milling. 56
CUTTING FLUIDS 57
CUTTING FLUIDS Cutting fluid is a type of coolant and lubricant designed specifically for metalworking and machining processes. 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. 58
CUTTING FLUIDS 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. 59
CUTTING FLUIDS Cutting fluid may be a coolant, a lubricant or a mixture of both. Water is an excellent coolant (reduce high temperature effectively) but it is not an effective lubricant (it does not reduce friction). Also, water causes rusting of workpiece and machine-tool components. 60
COOLANTS Coolants are cutting fluids designed to reduce the effects of heat in the machining operation. They have a limited effect on the amount of heat energy generated in cutting; instead, they carry away the heat that is generated, thereby reducing the temperature of tool and workpiece. This helps to prolong the life of the cutting tool. The capacity of a cutting fluid to reduce temperatures in machining depends on its thermal properties. 61
COOLANTS Coolant-type cutting fluids seem to be most effective at relatively high cutting speeds, in which heat generation and high temperatures are problems. They are most effective on tool materials that are most susceptible to temperature failures, such as high-speed steels, and are used frequently in turning and milling operations, in which large amounts of heat are generated. Coolants are formulated with ingredients that help reduce friction also. 62
COOLANTS Coolants are formulated with ingredients that help reduce friction also. Cutting Fluids for Turning & Milling 95% Coolant 5% Lubricant 63
LUBRICANTS Lubricants are usually oil-based fluids (because oils possess good lubricating qualities) formulated to reduce friction at the tool chip and tool work interfaces. Lubricant cutting fluids operate by extreme pressure lubrication, a special form of lubrication that involves formation of thin solid salt layers (films) on the hot, clean metal surfaces through chemical reaction with the lubricant. 64
LUBRICANTS These extreme pressure films are significantly more effective in reducing friction in metal cutting than conventional lubrication, which is based on the presence of liquid films between the two surfaces. Lubricant-type cutting fluids are most effective at lower cutting speeds. 65
LUBRICANTS They tend to lose their effectiveness at high speeds (above about 120 m/min ) because the motion of the chip at these speeds prevents the cutting fluid from reaching the tool chip interface. In addition, high cutting temperatures at these speeds cause the oils to vaporize before they can lubricate. 66
LUBRICANTS Machining operations such as drilling and tapping usually benefit from lubricants. In these operations, built-up edge formation is retarded, and torque on the tool is reduced. Although the principal purpose of a lubricant is to reduce friction, it also reduces the temperature in the operation. 67
CUTTING FLUIDS FUNCTIONS Reduce friction and wear thus improving tool life. Cool the cutting zone, thus reducing workpiece temperature and thermal distortion of the workpiece. Reduce forces and energy consumption. Prevents built-up edge chip formation To provide a good surface finish on the workpiece. 68
CUTTING FLUIDS FUNCTIONS Flush away chips from the cutting zone, and thus chips from interfering with cutting process. Protect machined surface from environmental corrosion. 69
Cutting Fluid Requirements A good cutting fluid should be Non-toxic No fire hazard Non-corrosive/rusting or chemical attack Not harmful to the lubricating system Cheap and easily available 70
Cutting Fluids : Method of Application 1. Manual application 2. Flooding 3. Jet application 4. Through the cutting tool 5. Mist applications 71
1. MANUAL APPLICATION 72
1. MANUAL APPLICATION Application of a fluid from a can manually by the operator. It is not acceptable even in job-shop situations except for tapping and some other operations where cutting speeds are very low and friction is a problem. In this case, cutting fluids are used as lubricants. 73
2. FLOODING 74
2. FLOODING In flooding, a steady stream of fluid is directed at the chip or tool-workpiece interface. Most machine tools are equipped with a recirculating system that incorporates filters for cleaning of cutting fluids. Flow rates range from 10 L/min for single-point tools to 225 L/min per cutter for multiple-tooth cutters, such as milling. 75
2. FLOODING Fluid pressures, range from 700 to 14,000 KPa, are used to flush away the chips. 76
3. JET APPLICATION 77
3. JET APPLICATION Delivering cutting fluid using specially designed nozzles that aim a powerful jet of fluid. Pressures employed (range of 5.5 MP to 35MP) act as a chip breaker for long and continuous chips. This method helps the cutting fluid to go close to the chip tool interface or work tool interfaces by slightly lifting or shifting the chip. 78
3. JET APPLICATION So this makes the cutting fluid action very effective in the form of jet but the consumption is high and here it is little expensive. 79
4. THROUGH THE CUTTING TOOL Tool bit Workpiece Cutting fluid Cutting fluid mixed with chips is collected for reclaiming 80
4. THROUGH THE CUTTING TOOL For a more effective application, narrow passages can be produced in cutting tools, as well as in toolholders, through which cutting fluids can be applied under high pressure. Some tools, especially drills for deep drilling, are provided with axial holes through the body of the tool so that the cutting fluid can be pumped directly to the tool cutting edge/ cutting section/zone. 81
5. MIST APPLICATIONS 82
5. MIST APPLICATIONS Fluid droplets suspended in air provide effective cooling by evaporation of the fluid. Mist application in general is not as effective as flooding, But can deliver cutting fluid to inaccessible areas that cannot be reached by conventional flooding. It also provides better visibility of the workpiece being machined. 83
5. MIST APPLICATIONS Effective with water-based fluids at air pressures 70 to 600 kpa. Limited cooling capacity. This is applicable for lubricating purpose mainly Requires venting to prevent inhalation of airborne fluid particles. Fluid consumption is low as compared to the other methods 84
Types of Cutting Fluids There are three basic types of cutting fluids used in metal cutting: Water based emulsions Mineral oils Oils with additives 85
Water Based Emulsions Pure water is by far the best cutting fluid available because of its highest heat carrying (high specific heat) capacity. Beside this, it is very cheap and easily available. Its low viscosity makes it to flow at high rates through the cutting fluid system and also penetrates the cutting zone. However, water corrodes the work material very quickly, particularly at high temperatures in the cutting zone as well as machine tool parts on which it is spill. 86
Water Based Emulsions Hence other materials would be added to water to improve its wetting characteristics, rust inhibitors and any other additives to improve the lubricating characteristics. These are also called water soluble oils. 87
Mineral Oils These are pure mineral oils without any additives. Their main function is lubrication and rust prevention. They are chemically stable. Their effectiveness is limited to light duty applications only. 88
Oils With Additives This is by far the largest variety of cutting fluids available. A number of additives have been developed which when added to mineral oils will produce the desirable characteristics for different machining situations. These lubricants generally termed as neat oils. These additives generally improve the load carrying capacity. Fatty oils are generally used for adding the load carrying capacity. 89
Oils With Additives Another class of additives termed as EP (extreme pressure) additives are used for more difficult to machine situations. This EP agent comes into effect whenever minute high-spot on the mating surface breaks through the oil film and rub together to set-up localized high temperature spots. This high temperature causes the EP additives to react with the adjacent metal and create an antiwelding layer of solid lubricant precisely where it is required. 90
Oils With Additives The layer is continuously broken by the severe rubbing action between the chip and the tool. EP additives are generally chlorine and sulphur or a combination of both. As a result the anti welding compounds formed in the cutting zone are iron chloride and iron sulphide, both of which have very low strength. 91
Situations where cutting fluids can be Harmful: If a cutting fluid is very effective as a coolant, it could lead to thermal shock in interrupted cutting operations. Fluids may cause the chip to become more curly, thus concentrating the heat closer to the tool tip, which reduces tool life Lubricants used on the machines may get a change in their viscosity and lubricating capabilities due to coming in contact with cutting fluids. 92
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