Nonconventional Machining Processes PDF

CHAPTER 15 NONCONVENTIONAL MACHINING PROCESSES 15.1 Introduction The recent in the use of hard, high-strength, and temperature resistant materials in engineering has made in necessary to develop many new machining techniques. With the exception of grinding, convent...

CHAPTER 15 NONCONVENTIONAL MACHINING PROCESSES 15.1 Introduction The recent in the use of hard, high-strength, and temperature resistant materials in engineering has made in necessary to develop many new machining techniques. With the exception of grinding, conventional methods of removing material from a worpiece are not readily applicable to these new materials. Even when such machining is possible, it is usually slow and highly inefficient. Although most of the new machining processes have been developed specifically for materials that are difficult to machine, some of them have found use in the production of the complex shapes and cavities in softer, more readily machined materials. Descriptions of the main nonconventional machining processes are given in this chapter. Nonconventional machining process may be used as alternatives to the more traditional processes for a number of reasons, including the following: 1. Machinability of the workpiece material: Many workpiece materials are dificulut to machine by conventional methods because of high hardness, high thermal resistance, or high abrasive wear. For these materials many of the nonconventional machining processes are suitable. 2. Shape complexity of the workpiece: Many shape features are difficult or impossible to produce by conventional machining methods. For example, it is usually relatively simple to produce a cylindrical hole in a workpiece, but to produce a square, sharp-cornered hole of similar size is much more difficult. Nonconventional machining process are often capable of machining complex shape features with comparative ease-even in materials that are normally difficult to machine. 3. Surface integrity: Conventional machining processes can result in surface cracks and residual stresses in the workpiece. Many nonconventional machining processes can remove material without causing these effects. 4. Precision: Many nonconventional processes are capable of high levels of precision, which cannot be achieved by conventional machining. 5. Miniaturization: Some nonconventional processes are capable of producing very small shape features and fine detail not possible by conventional machining (e.d., the machining of deep holes with very small diameters). 6. Automatic data communication and computer integration: Many nonconventional processes can be computer-controlled and linked directly CAD/CAM systems in an integrated manufacturing environment. 15.2. Electrical Machining Processes Electrical discharge machining or EDM - is sometimes called, spark erosion machining. It uses a series of fine, electrical discharges or sparks to erode the workpiece material. The discharges pass from the tool (cathode) to the workpiece (anode) at a rate greater than 20,000 times per second. The workpiece material, which must be electrically conductive, melts or vaporizes at the point where it is touched by the spark. A dielectric fluid, usually kerosene, circulates between the electrode and the work. The fluid confines the spark, cools and solidifies molten material, and carries away the residue. The process is advantageous for hard, conductive material including hardened steel and carbide. There is no significant cutting force, so delicate shapes can be produced. The gap between the anode and the work is only about 0.002 in (0.05 mm) and is servo controlled. There is an overcut equivalent to the length of the spark from the electrode to the work. The cutting rate is slow compared with conventional machining but is still advantageous for Chapter 15 1 materials too hard or otherwise not easily machined. The rate of cutting and surface finish are controllable by varying the frequency, voltage, and current in the electrical pulses. Higher energy levels in the pulses produce faster erosion of the workpiece but a rougher surface finish. Typically, the rate of cutting is reduced toward the end of the operation, in order to provide smoother surfaces. Spark erosion produces small craters in the workpiece and these craters are manifested as a matte finish on the workpiece. The tool also is eroded since some spark action is in the reverse direction. Additionally, some secondary operations may be required to remove a hard, thin, re-cast surface layer, or fine surface cracks caused by thermal stresses, depending on the material used and the function of the part produced. 15.2.1. Ram EDM Ram EDM - is sometimes called “diesinker” EDM, and the principle is illustrated in Fig. 15.1. The electrode is shaped to fit the desired cavity and is fed into the workpiece, which is eroded to match the electrode shape. The electrode is made undersize to allow for the expected overcut, commonly 0.0005 to 0.020 in (0.013 to 0.5 mm). Feed is downward into the work, but CNC controls can also provide transverse movement of the electrode to produce special shapes in the cavity. Since the tool also erodes and becomes tapered, extra tools are often made. Graphite is a common electrode material, but copper, brass, aluminum, copper-tungsten, zinc-tin, and other alloys are also used. The process is mainly used to machine cavities in hardened steel dies and molds. It is also used in machining carbide, and in salvaging hardened parts or tools such as broken taps. Slots, nonround holes, small deep holes, and the machining of honeycomb and other fragile parts, are additional applications. Figure 15.1. Ram EDM. A series of fine, rapidly repeating electrical sparks from the electrode to the workpiece erode a cavity in the workpiece. The cavity matches the shape of the electrode. 15.2.2. Wire EDM Wire EDM - uses a constantly-moving wire instead of a shaped electrode. The wire, of 0.001 to 0.013 in (0.025 to 0.33 mm) diameter, passes through the work, with a vertical axis, (though it may be set at an angle when required when cutting apertures for stamping tools.) Tungsten, copper, and brass are common wire materials. The wire or the work, is fed horizontally as the cut progresses, to cut a slit or shaped through-hole in the workpiece. Chapter 15 2  Different wire material is constantly exposed to the spark, so wear of the wire is widely distributed and is not a problem. The process, shown in Fig. 15.2, is often used to cut die openings in hardened stock to produce dies and die components. A high level of accuracy and fine detail can be achieved. Figure 15.2. Wire EDM. Electrical sparks from the wire to the workpiece cut a contoured slit in the workpiece as the wire advances. Figure 15.3. Electrical discharge grinding (EDG) - is EDM with a rotating wheel electrode. Electric sparks from the electrode erode the workpiece. The shape of the periphery of the wheel is transferred to the ground surface of the workpiece. 15.2.3. Electrical discharge grinding (EDG) Electrical discharge grinding (EDG) - is similar to electrical discharge machining, except that the electrode is a rotating wheel instead of an electrode that is stationary except for its downfeed. The workpiece and wheel are immersed in dielectric fluid, and metal is removed by the same kind of spark erosion as that which occurs with EDM. The wiping action of the wheel produces better surface finishes than with ram-type EDM. The graphite wheel is dressed as necessary to Chapter 15 3 compensate for its wear in the process. The volume of material lost by the wheel from wear averages about 1/3 of that removed from the workpiece but is considerably less in many instances. Since the wheel wear is spread over its circumference, the amount of reduction of wheel diameter from dressing is normally small. The process is used in shaping carbide form tools and in grinding fragile or brittle materials. Material to be ground by EDG must be electrically conductive. There is a thin heat-affected layer from the process. The layer varies from 0.0001 to 0.0015 in (0.0025 to 0.038 mm) in depth. Fig. 15.3 illustrates the process. 15.3. Electrochemical machining (ECM) Electrochemical machining (ECM) - removes metal by a reverse electroplating process. The workpiece becomes the anode and the tool is the cathode of the electrolytic process. A highly conductive electrolytic fluid is pumped into the space between the workpiece and the tool, and highamperage current removes workpiece material by anodic dissolution. The workpiece takes a mirrorimage shape of the tool. The gap between the electrode (tool) and workpiece is a small as 0.001 in (0.025 mm) and pressure of the electrolyte must be high to insure adequate flow. Temperature control of the electrolyte, control of the gap and feed rate of the tool and maintenance of electrolyte cleanliness are important elements in the process. The electrolyte is circulated through equipment that removes the operational debris from the fluid. The process is particularly suited to hard materials and others that are difficult to machine by conventional methods. Workpiece hardness does not affect its machinability by the process, but the workpiece material must be electrically conductive. The tool does not wear and no stresses are induced in the workpiece by the operation. Common tool materials are copper, brass, bronze, stainless steel, and copper-tungsten. Metal removal rates are good compared to a number of other non-traditional machining processes, and average 1 in3 (16 cm3) per minute per 10,000 amperes of current. The process is used for die sinking, manufacture of jet engine parts, cam profiling, and the machining of small, deep holes. ECM is most advantageous for materials that are difficult to machine by conventional methods. A typical electrochemical machining set up is illustrated in Fig. 15.4. Figure 15.4. Electrochemical machining (ECM). Heavy electrical current passing through the electrolyte between the electrode and the workpiece causes anodic dissolution of the workpiece material. Chapter 15 4  15.3.1. Electrochemical grinding (ECG) Electrochemical grinding (ECG) - is similar to electrochemical machining but replaces the relatively stationary tool with a rotating conductive grinding wheel. The wheel normally consists of aluminum oxide abrasive bonded to a metal wheel, which acts as the cathode of the electrolytic circuit. Electrolytic fluid circulates in the area where the abrasive contacts the work. A combination of electrolytic and mechanical action removes material from the workpiece but the electrolytic action predominates, accounting for about 90 percent of metal removal. Anodic dissolution of the workpiece metal leaves surface metal oxides. In conventional ECM, the flushing action of the electrolyte removes these oxides. In electrochemical grinding, the abrasive mainly functions to remove the oxide film, exposing a new metal surface to the electrolyte. The abrasive also separates the metal wheel from the work, preserving a fine (0.001 in or 0.025 mm) gap between the two. It also carries the electrolyte solution to the gap. The process has the advantage of relatively high metal removal rates for hard metals, freedom from heat damage to the workpiece, and the ability to grind fragile parts. Plunge, surface, cylindrical, and internal grinding are all feasible with the process. However, capital costs are high and the electrolyte can be corrosive to the equipment and workpiece. The process is commonly used in sharpening carbide cutting tools, avoiding the high wear rates of expensive diamond-abrasive wheels that would otherwise be required. It is also used for grinding surgical needles, honeycomb structures, and other fragile parts. ECG is illustrated by Fig. 15.5. Figure 15.5. Electrochemical grinding. Most of the metal removal results from electrolytic dissolution of the workpiece caused by flow of electrical current in the electrolyte between the conductive grinding wheel and the workpiece. Abrasive grains on the wheel remove surface oxides and expose more metal to the electrolyte. 15.3.2. Electrochemical turning (ECT) Electrochemical turning (ECT) - is another application of electrochemical machining. The workpiece rotates as in conventional turning but the cutting tool is replaced by an electrode. Electrolytic fluid is directed to the gap between the tool and the work, and material is removed by electrolytic action between the workpiece (anode) and the electrode (cathode). Facing and turning cuts can be made. Disc forgings and bearing races are machined by the process. 15.3.3. Electrochemical discharge grinding (ECDG) Electrochemical discharge grinding (ECDG) - is sometimes called, electrochemical discharge machining. It is a combination of electrochemical grinding (ECG) and electrical discharge grinding (EDG). Stock removal is primarily by ECG but oxides from ECG are then removed by intermittent spark discharges instead of by abrasive particles. The wheel is conductive (normally made of Chapter 15 5 graphite) and has no added abrasive. The fluid used is a highly-conductive electrolyte in contrast with EDG, which uses a dielectric fluid. The wheel rotates rapidly (4000 to 6000 ft/min - 1200 to 1800 m/min), bringing fresh electrolyte between the workpiece (the anode) and the wheel (the cathode). Spark discharges occur randomly when the breakdown voltage of the oxide film is exceeded. Alternating current or pulsating direct current are used. Current densities are less than in ECG (to avoid cratering of the workpiece and wheel), and metal-removal rates are considerably slower. However, the wheel cost for the graphiteonly wheels is less than that required for abrasive wheels used with ECG. The process is used in the grinding of carbide cutters. It can also be used with hardened tool steels, nickel alloys, and parts that are heat-sensitive or fragile. Honeycomb and other fragile parts are advantageous because they are finished free from burrs and stresses. Form grinding can be used if the wheel has a profile. 15.3.4. Electrochemical honing (ECH) Electrochemical honing (ECH) - is electrolytic action added to conventional honing, and is quite similar to electrochemical grinding. Material is removed from the workpiece by a combination of electrolytic action and mechanical abrasion. The electrolytic action removes most of the material, and the abrasive action of the honing stones removes the oxides produced by the electrolytic action. The equipment provides both reciprocating and rotational movement of the honing stones, as in conventional honing. However, electrolytic fluid handling is added. The tool that holds the honing stones is hollow and fluid passes from the tool into the gap between the stones and the workpiece. The gap between the tool and the work is approximately 0.003 to 0.005 in (0.08 to 0.13 mm) at the start of the operation, increasing to about 0.020 in (0.5 mm) at the conclusion, but the stones remain in contact with the workpiece, normally the surface of a machined hole. (The wedging action of a conical piece in the tool forces the stones outward as the honing takes place.) The operation provides metal removal three to five times as fast as regular honing, while the wear of the honing stones is considerably less. Deburring action is also superior and the work is more apt to be free from stress-induced or heat damage. However, the operation requires more complex equipment due to the need for control of the electrolytic action and the corrosiveness of the electrolyte. Therefore it is not necessarily more economical than conventional honing. The process is used to refine the bores of hardened parts such as gears and pump components, particularly when production quantities are large. 15.3.5. Electro etching Electro etching - is a method for marking workpieces that are electrically conductive. A stencil is placed on the workpiece and a pad containing an electrolyte is placed or dabbed on the stencil. The workpiece and the pad are connected to sources of direct electrical current. The current removes workpiece material electrolytically in the areas corresponding to openings in the stencil. The method is useful for placing identification markings on metal parts with fairly smooth surfaces. Electro etching is normally performed manually and is a suitable method when production quantities are modest. 15.4. Water Jet Machining (Hydrodynamic Machining) This process uses a narrow, high-velocity jet of liquid as a cutting agent. The jet travels at a speed of up to 3000 ft/s (900 m/s ) and is from 0.002 to 0.040 in (0.05 to 1.0 mm) wide. The liquid is primarily water, but polyethylene oxide or other polymers may be added to keep the stream coherent. Cutting occurs where the jet strikes the work material. Although thin soft metals can be cut with the process, it is best adapted to non-metallic materials such as wood, rubber, plastics, fabric, gypsum board, leather, acoustic tile, paperboard, and various food products. The process is mostly used to cut out parts from materials in sheet form or to slit web materials. Cutting disposable diapers is a significant application, as are gasket, shoe sole, and carpet cutting. Water jet is also Chapter 15 6 used for wire stripping, cutting of foods, separating printed circuit boards, paint stripping, and cleaning. Noise from the jet is a disadvantage of the process which is illustrated in Fig. 15.6. Figure 15.6. Schematic illustration of water-jet machining. Figure 15.7. Schematic illustration of the abrasive-jet machining process. Abrasive water jet machining (AWJ) - adds abrasive particles to the water jet to aid the cutting action. The abrasive is added after the stream has left the orifice. This enables the process to be used for cutting of a wide range of ferrous and non-ferrous metals and non-metallics. Because the process is sensitive to variations in process parameters such as the type and amount of abrasive, water pressure and flow rates, tool traverse rate, and material thickness, automatic computer control is vital. Cutting is normally carried out under water, which eliminates objectionable noise that would otherwise accompany the process. Many but not all composite materials can be cut with the process without delamination of the material. The process is also used to blank parts from sheet and plate materials. Materials from 1/16 to 3 in (1.6 to 75 mm) thick are commonly cut. Removal of sprues, risers, and gates from castings, other trimming of castings, forgings, and other parts and beveling of edges prior to welding are other applications. The process is slower but more accurate than plasma cutting and much faster but not as accurate as wire-EDM cutting. There is no heat affected zone and the cut edge is smooth. Fig. 15.7 illustrates the process. Some plastics that give off toxic fumes when heated, can be cut with this method without ill-effects. Aluminum, which can give problems with laser cutting because of its reflective surface, can also be cut advantageously. Stainless steel, tool steel, Inconel, brass, titanium, glass, ceramics, marble, and carbon fiber reinforced materials are also cut with abrasive water jet. Chapter 15 7  15.5. Electron Beam Machining (EBM) EBM is essentially the same process as the more common electron beam welding, except that the beam is used to cut instead of to fuse. The beam size, power, and dwell time are set to provide a cutting action. Magnetic coils focus and direct the high-energy electron beam. The electrons striking the workpiece melt and vaporize the workpiece material. Any material can be processed. Very narrow slits and small holes can be machined, as narrow as 0.0005 to 0.001 in (0.013 to 0.025 mm). Depth-to-diameter ratios of 100 to 1 are possible. Holes can be drilled in steel up to about 0.3 in (7.5 mm) thick. When holes and slits are machined, a backing material is placed under the workpiece. When the beam penetrates the workpiece, this backing material vaporizes, expelling the molten workpiece material from the hole. The process takes place in a vacuum of 10-5 mm/Hg. The machining part of the operation is very rapid but the part must be placed in a sealed chamber and a vacuum must be drawn. The size of the workpiece that can be machined is limited by the size of the chamber. Equipment costs for this process are high and there must be protection against the x-rays that are generated when the electron beam strikes the workpiece. The operation is normally computer controlled. There is a small heat-affected zone and a thin recast layer at the cut. Machining of semiconductors and sapphire bearings are two commercial applications. Filters and screens are made by drilling multiple holes in sheet material. Figure 15.8. Schematic illustration of the electron-beam machining process. Unlike LBM, this process requires a vacuum, so workpiece size is limited to the size of the vacuum chamber. 15.6. Laser Beam Machining The laser beam, used for welding, is also adaptable to machining (cutting) operations. The heat generated by a powerful beam of coherent light, melts and vaporizes workpiece material. The process is best suited for drilling very small [0.005 in (0.13mm)] holes, but is also used increasingly for cutting flat stock. Depth-to-diameter ratios of 10 are feasible in laser-machined holes. The process works best with materials less than 0.2 in (5 mm) thick but can be used with greatly reduced cutting speeds with metals up to about 0.5 in ( 13 mm) in thickness and non-metals up to about 1 in (25 mm) thick. With thin materials, the process is faster than mechanical cutting. Holes can also be drilled rapidly. Chapter 15 8  CO2 or another gas may be blown from the nozzle to assist in removal of melted material from the cut. There is a thin recast layer and zone of heat effect where the cut takes place. The surface of the cut can be irregular, and there tends to be some taper in it. The depth of cut in blind holes and grooves is difficult to control. Typical applications of laser cutting are the machining of carbides and diamond drawing dies. Scribing, engraving, perforating, slitting, trimming and deburring are other operations. Turning, threading and milling of difficult to machine materials are feasible. Machining silicon wafers and other electronics components is a common application. Figure 15.9. (a) Schematic illustration of the laser-beam machining process. (b) and (c) Examples of holes produced in nonmetallic parts by LBM. The process has grown as a desirable method for cutting out sheet material components, particularly when quantities are too low to justify the expense of blanking dies. Steel, Inconel, Hastalloy, titanium, and stainless steel, and various plastics and other non-metallic materials are cut with the process. The degree of light reflectance of the material affects the type of laser that is best to use. Reflective metals such as copper, aluminum, gold, and silver do not work as well. Fig. 15.9. illustrates the process. Laser-assisted hot machining (LAM) - uses laser processing combined with milling or turning to machine materials that may not be easy to machine by normal methods. The laser and cutter work together; laser energy heats the workpiece material locally at the point of cutting, softening the material and facilitating the cutting action. Tool wear is reduced substantially, cutting forces are lessened, and higher speeds can be used. Titanium, cast iron, steel, and silicon carbide ceramic have been cut experimentally with this approach. Chapter 15 9  15.7. Range of Surface Roughnesses and Tolerances Figure 15.10. Surface roughness and tolerances obtained in various machining processes. Note the wide range within each process. Source: Machining Data Handbook, 3rd ed. Copyright ©1980. Chapter 15 10