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Objectives Understand basic machining processes and operations. Understand the mechanics and significance of cutting tool properties and tool wear. Understand the role and types of cutting tool fluids and their characteristics. Understand basics behind the various machining processes. Understand and compare the various advanced machining processes. Unit 6 Copyright © MDIS. All rights reserved.

Machining Process Machining is a general term describing a group of processes that consist of the removal of material and modification of the surfaces of a workpiece after it has been produced by various methods. Thus, machining involves secondary and finishing operations. In general, however, resorting to machining suggests that a part could not have been produced to the final desired specifications by the primary processes used in making them and that additional operations are necessary. We again emphasize the importance of net-shape manufacturing, to avoid these additional steps and reduce production costs. Unit 6 Copyright © MDIS. All rights reserved.

Machining Process Material-removal processes have the following disadvantages: They waste material (although the amount may be relatively small). The processes generally takes longer than other processes. They generally require more energy than do forming and shaping They can have adverse effects on the surface quality and properties of the product. Unit 6 Copyright © MDIS. All rights reserved.

Machining Process Machining consists of several major types of material-removal processes: Cutting, typically involving single-point or multipoint cutting tools, each with a clearly defined shape. Abrasive processes, such as grinding and related processes. Advanced machining processes utilizing electrical, chemical, laser, thermal, and hydrodynamic methods to accomplish this task. Unit 6 Copyright © MDIS. All rights reserved.

Cutting Processes Cutting processes remove material from the surface of a workpiece by producing chips. Some of the more common cutting processes, are as follows: Turning, in which the workpiece is rotated and a cutting tool removes a layer of material as the tool moves to the left. Cutting off, in which the cutting tool moves radially inward and separates the right piece from the bulk of the blank. Slab milling, in which a rotating cutting tool removes a layer of material from the surface of the workpiece. End milling, in which a rotating cutter travels along a certain depth in the workpiece and produces a cavity. Unit 6 Copyright © MDIS. All rights reserved.

Examples of common machining operations
Unit 6 Copyright © MDIS. All rights reserved.

Cutting Temperature As in all metalworking processes where plastic deformation is involved, the energy dissipated in cutting is converted into heat that, in turn, raises the temperature in the cutting zone. Temperature rise is a very important factor in machining because of its major adverse effects, such as the following: Excessive temperature lowers the strength, hardness, stiffness, and wear resistance of the cutting tool; tools also may soften and undergo plastic deformation; thus, tool shape is altered. Increased heat causes uneven dimensional changes in the part being machined, making it difficult to control its dimensional accuracy and tolerances. An excessive temperature rise can induce thermal damage and metallurgical changes in the machined surface, adversely affecting its properties. Unit 6 Copyright © MDIS. All rights reserved.

Tool Wear and Failure We have seen that cutting tools are subjected to (a) high localized stresses at the tip of the tool, (b) high temperatures, especially along the rake face, (c) sliding of the chip along the rake face, and (d) sliding of the tool along the newly cut workpiece surface. These conditions induce tool wear, which is a major consideration in all machining operations, as are mould and die wear in casting and metalworking. Tool wear adversely affects tool life, the quality of the machined surface and its dimensional accuracy, and, consequently, the economics of cutting operations. Unit 6 Copyright © MDIS. All rights reserved.

Tool Wear and Failure Tool-life curves are plots of experimental data obtained by performing cutting tests on various materials under different cutting conditions, such as cutting speed, feed, depth of cut, tool material and geometry, and cutting fluids. Tool life decreases rapidly as the cutting speed increases, the condition of the workpiece material has a strong influence on tool life, and there is a large difference in tool life for different workpiece-material microstructures. Unit 6 Copyright © MDIS. All rights reserved.

Tool-condition Monitoring
With computer-controlled machine tools and automated manufacturing, the reliable and repeatable performance of cutting tools is a critical consideration. Modern machine tools operate with little direct supervision by a machine operator and generally are enclosed, making it impossible or difficult to monitor the machining operation and the condition of the tool. It is therefore essential to continuously and indirectly monitor the condition of the cutting tool so as to note, for example, wear, chipping, or gross tool failure. Unit 6 Copyright © MDIS. All rights reserved.

Tool Wear and Failure The direct method for observing the condition of a cutting tool involves optical measurements of wear, such as the periodic observation of changes in the tool profile. This is a common and reliable technique and is done with a microscope (toolmakers’ microscope). However, this requires that the cutting operation be stopped for tool observation. Indirect methods of observing tool conditions involve the correlation of the tool condition with parameters such as cutting forces, power, temperature rise, workpiece surface finish, vibration, and chatter. A powerful technique is acoustic emission (AE), which utilizes a piezoelectric transducer mounted on a tool holder. The transducer picks up acoustic emissions (typically above 100 kHz), which result from the stress waves generated during cutting. By analysing the signals, tool wear and chipping can be monitored. Unit 6 Copyright © MDIS. All rights reserved.

Surface Finish and Integrity
Surface finish influences not only the dimensional accuracy of machined parts but also their properties and their performance in service. The term surface finish describes the geometric features of a surface, and surface integrity pertains to material properties, such as fatigue life and corrosion resistance, that are strongly influenced by the nature of the surface produced. The difference between finish machining and rough machining should be noted. In finish machining it is important to consider the surface finish to be produced, whereas in rough machining the main purpose is to remove a large amount of material at a high rate. Surface finish is not a primary consideration, since it will be improved during finish machining. Unit 6 Copyright © MDIS. All rights reserved.

Machinability The machinability of a material is usually defined in terms of four factors: Surface finish and surface integrity of the machined part. 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, and low force and power requirements. Unit 6 Copyright © MDIS. All rights reserved.

Cutting-Tool Materials and Cutting Fluids

Cutting Tool Characteristics
Hot hardness, so that the hardness, strength, and wear resistance of the tool are maintained at the temperatures encountered in machining operations. This property ensures that the tool does not undergo any plastic deformation and thus retains its shape and sharpness. Toughness and impact strength (or mechanical shock resistance), so that impact forces on the tool that are encountered repeatedly in interrupted cutting operations do not chip or fracture the tool. Thermal shock resistance, to withstand the rapid temperature cycling encountered in interrupted cutting. Wear resistance, so that an acceptable tool life is obtained before replacement is necessary. Unit 6 Copyright © MDIS. All rights reserved.

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. Unit 6 Copyright © MDIS. All rights reserved.

Cutting Fluids Cutting fluids have been used extensively in machining operations to achieve the following results: Reduce friction and wear, thus improving tool life and the surface finish of the workpiece. Cool the cutting zone, thus improving tool life and reducing the temperature and thermal distortion of the workpiece. Reduce forces and energy consumption. Flush away the chips from the cutting zone, thus preventing the chips from interfering with the cutting process, particularly in operations such as drilling and tapping. Protect the machined surface from environmental corrosion. Unit 6 Copyright © MDIS. All rights reserved.

Cutting Fluids Depending on the type of machining operation, the cutting fluid needed may be a coolant, a lubricant, or both. The effectiveness of cutting fluids depends on a number of factors, such as the type of machining operation, tool and workpiece materials, cutting speed, and the method of application. Water is an excellent coolant and can effectively reduce the high temperatures developed in the cutting zone. However water is not an effective lubricant; hence, it does not reduce friction. Furthermore, it can cause oxidation (rusting) of workpieces and machine-tool components. Unit 6 Copyright © MDIS. All rights reserved.

Cutting Fluids The need for a cutting fluid depends on the severity of the particular machining operation, which may be defined as the level of temperatures and forces encountered and the ability of the tool materials to withstand them, the tendency for built-up edge formation, the ease with which chips produced can be removed from the cutting zone, and how effectively the fluids can be applied to the proper region at the tool-chip interface. The relative severities of specific machining processes, in increasing order of severity, are as follows: sawing, turning, milling, drilling, gear cutting, thread cutting, tapping, and internal broaching. Unit 6 Copyright © MDIS. All rights reserved.

Cutting Fluids There are operations, however, in which the cooling action of cutting fluids can be detrimental. It has been shown that cutting fluids may cause the chip to become more curly and thus concentrate the heat closer to the tool tip, reducing tool life. More importantly, in interrupted cutting operations, such as milling with multiple tooth cutters, cooling of the cutting zone leads to thermal cycling of the cutter teeth, which can cause thermal cracks by thermal fatigue or thermal shock. Beginning with the mid-1990s, there has been a major trend toward near-dry machining, meaning a minimal use of cutting fluids, as well as toward dry machining. Unit 6 Copyright © MDIS. All rights reserved.

Types of Cutting Fluids
Oils (also called straight oils), including mineral, animal, vegetable, compounded, and synthetic oils, typically are used for low-speed operations where temperature rise is not significant. Emulsions (also called soluble oils), a mixture of oil and water and additives, generally are used for high-speed operations because the temperature rise is significant. The presence of water makes emulsions highly effective coolants. The presence of oil reduces or eliminates the tendency of water to cause oxidation. Semisynthetic are chemical emulsions containing little mineral oil, diluted in water, and with additives that reduce the size of oil particles, making them more effective. Synthetics are chemicals with additives, diluted in water, and containing no oil. Unit 6 Copyright © MDIS. All rights reserved.

Near-dry and Dry Machining
For economic and environmental reasons, there has been a continuing worldwide trend since the mid-1990s to minimize or eliminate the use of metalworking fluids. This trend has led to the practice of near-dry machining (NDM), with major benefits such as the following: Alleviating the environmental impact of using cutting fluids, improving air quality in manufacturing plants, and reducing health hazards. Reducing the cost of machining operations, including the cost of maintenance, recycling, and disposal of cutting fluids. Further improving surface quality. Unit 6 Copyright © MDIS. All rights reserved.

Near-dry and Dry Machining
The principle behind near-dry cutting is the application of a fine mist of an air- fluid mixture containing a very small amount of cutting fluid, which may be reformulated to contain vegetable oil. 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. One of the functions of a metal-cutting fluid is to flush chips from the cutting zone. This function seems to be problematic with dry machining. However, tool designs have been developed that allow the application of pressurized air, often through the tool shank. The compressed air doesn’t serve a lubrication purpose and provides only limited cooling, but is very effective at clearing chips from the cutting interface. Unit 6 Copyright © MDIS. All rights reserved.

Cryogenic Machining More recent developments in machining include the use of cryogenic gases such as nitrogen or carbon dioxide as a coolant. With small-diameter nozzles and at a temperature of -200°C, liquid nitrogen is injected into the cutting zone. Because of the reduction in temperature, tool hardness is maintained and tool life is enhanced, thus allowing higher cutting speeds. Also, the chips are more brittle; hence, machinability is increased. Furthermore, the nitrogen simply evaporates and therefore has no adverse environmental impact. Unit 6 Copyright © MDIS. All rights reserved.

Machining Processes: Turning and Hole Making

Turning and Hole Making
One of the most basic machining processes is turning, meaning that the part is rotated while it is being machined. The starting material is generally a workpiece that has been made by other processes, such as casting, forging, extrusion, drawing, or powder metallurgy. Turning processes, which typically are carried out on a lathe or by similar mac/vine tools. Turning: to produce straight, conical, curved, or grooved workpieces, such as shafts, spindles, and pins. Facing: to produce a flat surface at the end of the part and perpendicular to its axis, useful for parts that are assembled with other components. Face grooving produces grooves for applications such as O-ring seats. Unit 6 Copyright © MDIS. All rights reserved.

Turning and Hole Making
Cutting with form tools: to produce various axisymmetric shapes for functional or aesthetic purposes. Boring: to enlarge a hole or cylindrical cavity made by a previous process or to produce circular internal grooves. Drilling: to produce a hole, which may be followed by boring to improve its dimensional accuracy and surface finish. Parting: also called cutting off, to cut a piece from the end of a part, as is done in the production of slugs or blanks for additional processing into discrete products Threading: to produce external or internal threads. Knurling: to produce a regularly shaped roughness on cylindrical surfaces, as in making knobs and handles. Unit 6 Copyright © MDIS. All rights reserved.

Machining Processes: Milling, Broaching, Sawing, Filing, and Gear Manufacturing

Milling Milling includes a number of highly versatile machining operations taking place in a variety of configurations with the use of a milling cutter-a multitooth tool that produces a number of chips in one revolution Unit 6 Copyright © MDIS. All rights reserved.

Milling In peripheral milling (also called plain milling), the axis of cutter rotation is parallel to the workpiece Conventional or milling: The cutter rotation can be either clockwise or counter- clockwise; this is significant in the operation surface. Unit 6 Copyright © MDIS. All rights reserved.

Milling In face milling, the cutter is mounted on a spindle having an axis of rotation perpendicular to the workpiece surface. End milling is an important and common machining operation because of its versatility and capability to produce various profiles and curved surfaces. The cutter, called an end mill has either a straight shank (for small cutter sizes) or a tapered shank (for larger sizes) and is mounted into the spindle of the milling machine. End mills may be made of high-speed steels or with carbide inserts, similar to those for face milling. The cutter usually rotates on an axis perpendicular to the workpiece surface, and it also can be tilted to conform to machine-tapered or curved surfaces. Unit 6 Copyright © MDIS. All rights reserved.

Abrasive Machining and Finishing Operations

Abrasive Machining There are many situations in manufacturing where the processes described thus far cannot produce the required dimensional accuracy or surface finish for a part, or the workpiece material is too hard or too brittle to process. One of the most common methods for producing such demanding characteristics on parts is abrasive machining. An abrasive is a small, hard particle having sharp edges and an irregular shape, unlike the cutting tools described earlier. Abrasives are capable of removing small amounts of material from a surface through a cutting process that produces tiny chips. Abrasives also are used to hone, lap, buff, and polish workpieces. With the use of computer-controlled machines, abrasive processes are now capable of producing (a) a wide variety of workpiece geometries and (b) very fine dimensional accuracy and surface finishes. Unit 6 Copyright © MDIS. All rights reserved.

Abrasive Machining Abrasives that are used most commonly in abrasive-machining operations are as follows: Conventional abrasives: Aluminium oxide (Al2O3), Silicon carbide (SiC) Superabrasives: Cubic boron nitride (cBN), Diamond. Unit 6 Copyright © MDIS. All rights reserved.

Ultrasonic Machining In ultrasonic machining (UM), material is removed from a surface by micro chipping and erosion with loose, fine abrasive grains in a Water slurry. The tip of the tool (called a sonotrode) vibrates at a frequency of 20 kHz and a low amplitude of to mm. This vibration imparts a high velocity to abrasive grains between the tool and the workpiece. The stress produced by the impact of abrasive particles on the workpiece surface is high because (a) the time of contact between the particle and the surface is very short (10 to 100 /us) and (b) the area of contact is very small. In brittle materials, these impact stresses are sufficiently high to cause micro chipping and erosion of the workpiece surface. Unit 6 Copyright © MDIS. All rights reserved.

Ultrasonic Machining Ultrasonic machining is best suited for materials that are hard and brittle, such as ceramics, carbides, precious stones, and hardened steels. A special tool is required for each shape to be produced; hence it is also called a form tool. Unit 6 Copyright © MDIS. All rights reserved.

Finishing Operations Several processes that utilize fine abrasive grains are used on workpieces as the final finishing operation. However, these operations can significantly affect production time and product cost. Thus, they should be specified with due consideration to their costs and benefits. Some common types are: Coated Abrasives: Common examples of coated abrasives are sandpaper and emery cloth. Belt Grinding: Coated abrasives also are used as belts for high-rate material removal with good surface finish. Wire Brushing: The workpiece is held against a circular wire brush that rotates at speeds ranging from 1750 rpm for large wheels to 3500 rpm for small wheels. Unit 6 Copyright © MDIS. All rights reserved.

Advanced Machining Processes

The need for Advanced Machining Processes
The machining processes described in the preceding chapters involved material removal by mechanical means: chip formation, abrasion, or micro chipping. However, there are situations in which mechanical methods are not satisfactory, economical, or even possible, for the following reasons: The strength and hardness of the workpiece material are very high, typically above 400 HB The workpiece material is too brittle to be machined without damage to the workpiece. This is typically the case with highly heat treated alloys, glass, ceramics, and powder-metallurgy parts. Unit 6 Copyright © MDIS. All rights reserved.

The need for Advanced Machining Processes
The workpiece is too flexible or too slender to withstand forces in machining or grinding, or the parts are difficult to clamp in fixtures and work-holding devices. The shape of the part is complex, including such features as internal and external profiles or holes with high length-to-diameter ratios in very hard materials. Special surface finish and dimensional tolerance requirements exist that cannot be obtained by other manufacturing processes or are uneconomical through alternative processes. The temperature rise during processing and residual stresses developed in the workpiece are not desirable or acceptable. Unit 6 Copyright © MDIS. All rights reserved.

Chemical Machining Chemical machining (CM) was developed from the observation that chemicals attack and etch most metals, stones, and some ceramics, thereby removing small amounts of material from the surface. The CM process is carried out by chemical dissolution using reagents or etchants, such as acids and alkaline solutions. Chemical machining is the oldest of the advanced machining processes and has been used in engraving metals and hard stones, in deburring, and in the production of printed-circuit boards and microelectronic devices Unit 6 Copyright © MDIS. All rights reserved.

Electrochemical Machining
Electrochemical machining (ECM) is basically the reverse of electroplating. An electrolyte acts as current carrier, and the high rate of electrolyte movement in the tool-workpiece gap washes metal ions away from the workpiece (anode) before they have a chance to plate onto the tool (cathode). Unit 6 Copyright © MDIS. All rights reserved.

Electrochemical Grinding
Electrochemical grinding (ECG) combines electrochemical machining with conventional grinding. The equipment used is similar to a conventional grinder, except that the wheel is a rotating cathode embedded with abrasive particles. The wheel is metal bonded with diamond or aluminium-oxide abrasives and rotates at a surface speed from 1,200 to 2,000 m/min. Unit 6 Copyright © MDIS. All rights reserved.

Electrical-discharge Machining
The principle of electrical-discharge machining (EDM) (also called electro discharge or spark-erosion machining) is based on the erosion of metals by spark discharges. We know that when two current-conducting wires are allowed to touch each other, an arc is produced. If we look closely at the point of contact between the two wires, we note that a small portion of the metal has been eroded away, leaving a small crater. Unit 6 Copyright © MDIS. All rights reserved.

Laser-beam Machining In laser-beam machining (LBM), the source of energy is a laser (an acronym for light amplification by stimulated emission of radiation), which focuses optical energy on the surface of the workpiece. The highly focused, high-density energy source melts and evaporates portions of the workpiece in a controlled manner. This process (which does not require a vacuum) is used to machine a variety of metallic and non-metallic materials. Unit 6 Copyright © MDIS. All rights reserved.

Water-jet Machining In water-jet machining (WJM) (also called hydrodynamic machining), water jet is used in cutting and deburring operations. The water jet acts like a saw and cuts a narrow groove in the material. A pressure level of about 400 MPa is generally used for efficient operation, although pressures as high as 1400 MPa can be generated. ]et-nozzle diameters range between 0.05 and 1 mm. A variety of materials can be cut, including plastics, fabrics, rubber, wood products, paper, leather, insulating materials, brick, and composite materials. Unit 6 Copyright © MDIS. All rights reserved.

Abrasive Water-jet Machining.
In abrasive water-jet machining (AWJM), the water jet contains abrasive particles (such as silicon carbide or aluminium oxide), which increase the material-removal rate above that of water-jet machining. Unit 6 Copyright © MDIS. All rights reserved.

Abrasive-jet Machining
In abrasive-jet machining (AJM), a high-velocity jet of dry air, nitrogen, or carbon dioxide containing abrasive particles is aimed at the workpiece surface under controlled conditions. The impact of the particles develops a sufficiently concentrated force to perform operations such as (a) cutting small holes, slots, or intricate patterns in very hard or brittle metallic and non-metallic materials, (b) deburring or removing small flash from parts, (C) trimming and bevelling, (d) removing oxides and other surface films, and (e) generally cleaning components with irregular surfaces. Unit 6 Copyright © MDIS. All rights reserved.

Hybrid Machining Systems
A more recent development in manufacturing is the concept of hybrid machining systems. Two or more machining processes are combined into one system to take advantage of the ca abilities of each process increasing production speed and thus improving the efficiency of the operation. The system is able to handle a variety of materials, including metals, ceramics, polymers, and composites. Unit 6 Copyright © MDIS. All rights reserved.

Copyright © MDIS. All rights reserved.

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