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LECTURE-11NON-CONVENTIONAL MACHINING- ECM, EDM, LBM and EBMNIKHIL R. DHAR, Ph. DDepartment of Industrial & Production EngineeringBUET

Department of Industrial & Production Engineering33/2Electrochemical Machining (ECM) Electrochemical Machining is the controlled removal of metal by anodic dissolution in an electrolytic medium. It is based upon Faradays law of electrolysis. In this process the workpiece acts as the anode and the tool as cathode, as shown in Figure. The two electrodes are closely placed, with a gap of about 0.5 mm, and immersed in an electrolyte (a solution of sodium chloride).

Schematic illustration of the electrochemical-machining process. Department of Industrial & Production Engineering33/3When a potential difference is maintained between the electrodes, the ions existing in the electrolyte migrate towards them. Positively charged ions are attached towards the cathode and negatively charged ions are attached towards the anode. This initiates the flow of current in the electrolyte. The work is generally kept stationary and the tool is fed in a linear direction. The metal from the work is removed due to ion migration towards the tool. The tool is prevented from damage by pumping a strong stream of electrolyte at high pressure (15kg/cm2). No spark is produced in this process and the temperature generate is low enough to cause any metallurgical changes in the workpiece. On application of electrical energy, the metallic ion is pulled out of the workpiece. The positive metallic ion reacts with the negative ion present in the electrolytic solution forming metallic hydroxides and other components, resulting in ionic dissolution of metal with the formation of precipitates are washed away by the electrolyte.In this case, the tool does not come in contact with the workpiece, and the wear and tear of the tool is negligible. Machining takes place at low voltage and the metal removal rate is high. Dimension up to 0.05 mm can be easily controlled and the metal workpiece in not damaged thermally.Department of Industrial & Production Engineering33/4Examples of Parts Made by ECM

Typical parts made by electrochemical machining. (a) Turbine blade made of a nickel alloy (b) Thin slots on a 4340-steel roller-bearing cage. (c) Integral airfoils on a compressor disk.Department of Industrial & Production Engineering33/5Electrochemical Grinding (ECG)Wheel is a rotating cathode with abrasive particlesElectrochemical machining + conventional grinding

(a) Schematic illustration of the electrochemical-grinding process. (b) Thin slot produced on a round nickel-alloy tube by this process.Department of Industrial & Production Engineering33/6ElectrolytesFunctions of ElectrolyteIt carries the current between the tool and the work pieceIt removes the product of machining from the cutting regionIt dissipates heat produced in the operationIt helps the machining reactions necessary for anodic dissolution to take placeSelection of ElectrolytesChoice of proper electrolyte is of vital importance on the following considerations: Machining rateDimensional accuracySurface texture and Surface integrity Electrolyte should possess the following characteristicsIt should have high electrical conductivityIt should machine at high current efficiencyIt should produce good surface finish and integrityThe surface texture produced is dependent on the electrolyte composition and material structure.Department of Industrial & Production Engineering33/7Electrolyte Flow ArrangementCorrect electrolyte flow across the tool is essential for proper machining. Attention should be paid to the tool shape where cavitation of the electrolyte is likely to occur. Tool design must permit a uniform electrolyte flow in all machining areas. Excessive flows are not desirable as they cause tool erosion. Basically, two methods of flow are used, namely divergent flow and convergent flow. The convergent flow method provides a smooth flow of electrolyte. The electrolyte is admitted through a chamber called dam to pressurize the area outside the work and the tool. The advantages of the system are:More uniform and predictable side over cut and front machining gapImproved surface finishReduced possibility of arcingMuch cleaner operating conditionsElimination of undesirable machining due to stray currentsIt is, however, to be noted that cost of tooling with convergent flow is more than that with divergent flow system.Department of Industrial & Production Engineering33/8Advantages, Disadvantages and Applications AdvantagesECM has many advantages when compared to conventional machining.The components are not subject to either thermal or mechanical stress There is no tool wear during Electrochemical machining Non-rigid and open work pieces can be machined easily as there is no contact between the tool and workpieceComplex geometrical shapes can be machined repeatedly and accuratelyECM is a time saving process when compared with conventional machiningDuring drilling,deep holes can be made or several holes at once.ECM deburring can deburr difficult to access areas of parts.Fragile parts which cannot take more loads and also brittle material which tend to develop cracksduring machining can be machined easily through ECMSurface finishes of 25 in. can be achieved during ECMDepartment of Industrial & Production Engineering33/9Disadvantages The cost of the equipment is very high.Rigid fixturing is required to withstand the high electrolyte flow rates.Difficult in designing a proper tooling systemCorrosion-free materials requirement for the structure and electrolyte handling systemsThe tool is more difficult to make since it must be insulated to maintain correct conductive paths to the work piece.Hydrogen liberation at the tool surface may cause hydrogen-embitterment of the surfaceSpark damage may become sometimes more problematic Fatigue property of the machined component may reduce as compared to conventional techniques, so may need further treatments.Department of Industrial & Production Engineering33/10ApplicationECM is to be applied only in specialized areas where conventional machining is not feasible. One of the main applications of ECM is in the aerospace industry to machine difficult-to-machine materials and complex shaped parts.Various industrial techniques have been developed on the basis of this ECM principlesuch as:ElectrochemicalcuttingElectrochemicalECMElectrochemical broachingElectrochemical drillingElectrochemical deburringElectrochemical machining isused for the manufacture of dies, press and glass-making molds,turbine and compressor blades for gas-turbine engine, the generation of passages, cavities, holes and slots in parts. ECM deburring is used for deburring of gears, hydraulic and fuel-system parts, small electronic components andengine parts.Department of Industrial & Production Engineering33/11Electrical-Discharge Machining (EDM)EDM is a machining method primarily used for hard metals or those that would be impossible to machine with traditional techniques. One critical limitation, however, is that EDM only works with materials that are electrically conductive. EDM or Electrical Discharge Machining is especially well-suited for cutting intricate contours or delicate cavities that would be difficult to produce with a grinder, an end mill or other cutting tools. Metals that can be machined with EDM include hardened tool-steel, titanium, carbide, inconel and kovar.

EDM is sometimes called "spark machining" because it removes metal by producing a rapid series of repetitive electrical discharges. These electrical discharges are passed between an electrode and the piece of metal being machined. The small amount of material that is removed from the workpiece is flushed away with a continuously flowing fluid. The repetitive discharges create a set of successively deeper craters in the work piece until the final shape is produced.Department of Industrial & Production Engineering33/12There are two primary EDM methods:

Ram EDM and Wire EDM.

The primary difference between the two involves the electrode that is used to perform the machining. In a typical ram EDM application, a graphite electrode is machined with traditional tools. The specially-shaped electrode is connected to the power source, attached to a ram, and slowly fed into the workpiece. The entire machining operation is usually performed while submerged in a fluid bath. In wire EDM a very thin wire serves as the electrode. Special brass wires are typically used; the wire is slowly fed through the material and the electrical discharges actually cut the workpiece. Wire EDM is usually performed in a bath of water. Department of Industrial & Production Engineering33/13Ram Electro-Discharge Machining (EDM) Process: In this process, the control of erosion of the metal is achieved by the rapidly recurring spark discharges produced between two electrodes, one tool and the other work, and spark impinging against the surface of the workpiece which must be an electrically conducting body. A suitable gap (approximately 0.025 to 0.075 mm) known as spark gap, is maintained between the tool and the work by a servomotor which is actuated by the difference between a reference voltage and the gap breakdown voltage, which feeds the tool downwards towards the workpiece.

Department of Industrial & Production Engineering33/14The metal removal rate depends on the spark gap maintained. If both electrodes are made of same material, it has been found that the greatest erosion takes place upon the positive electrode (anode). Therefore, in order to remove maximum metal and have minimum wear on the tool, the tool is made cathode and the workpiece as anode. The two electrodes are separated by a dielectric fluid medium. The spark is a transient electric discharge across the gap between workpiece and tool. When the potential difference (voltage) across the gap becomes sufficiently large, the dielectric fluid becomes ionized and breaks down to produce an electrically conductive spark channel and the condensers discharge current across the channel in the form of a spark. When the voltage drops to about 12 volts, the spark discharge extinguishes and the dielectric fluid once again becomes deionized.

The condensers start to recharge and the process repeats itself. The spark occurs in an interval of from 10 to 30 microseconds and with a current density of approximately 15-500 amp/mm2. The repetitive sparks release their energy in the form of local heat, as a result of which, local temperature of the order of 12000C is reached at the spot hit by electrons, and at such a high pressure and temperature some metal is melted and eroded. Some of it is vaporized and under it fine material particles are carried away by dielectric medium (liquid) circulated around it, forming a crater on the workpiece. The time interval between the sparks is so short that the heat is unable to conduct into the tool and work. Fig.4.12 shows the schematic representation of the process illustrating the various components involved in the process. Department of Industrial & Production Engineering33/15Tool MaterialThe selection of the tool material depends upon many factors such as:It should have low erosion rate or good work to tool wear ratioIt should be electrically conductiveIt should have good machinabilityIt should have low electrical resistanceIt should have high melting pointIt should have high electron emission.One of the major draw back of EDM is the wear ratio of the tool. The wear ratio may be defined as:

The less the wear ratio, the better it is. Wear ratio for brass electrode is 1:1. For most other metallic electrodes, it is about 3:1 or 4:1. With graphite (with the highest melting point, 3500OC), the wear ratio may be range from 5:1 upto 50:1.Department of Industrial & Production Engineering33/16Tool WearDuring the EDM process the tool (i.e. the cathode) also gets eroded which is undesirable no doubt but unavoidable too. However the wear of the cathode is much less than that of the anode, the reasons for this are given below:

The positive ions of the dielectric fluid strike the cathode whereas the electrons to the anode. The mass of electrons is much less than that of the ions but it moves with much greater velocity than the ions do. So the cathode gets eroded much less than the anode.

Due to the spark a compressive force is created on the cathode which helps in reduction of cathode erosion.

The dielectric fluids are usually hydrocarbons. Due to its pyrolysis, gases evolve which produce carbon particles. These particles get deposited on the heated cathode as a thin layer which protects the tool from wear. Department of Industrial & Production Engineering33/17Dielectric Fluid PurposeIt acts as a coolant for the workpiece and the toolIt acts as an insulating medium during charging operation of the condenser and provides the correct condition for efficient spark discharge and its conduction when ionized (during discharging). It carries away the eroded metal particles. It acts as a coolant in quenching the spark and helps arcing to be prevented.Essential RequirementsIt should have an optimum viscosity. Because lower viscosity unable the fluid to carry away the metal particle efficiently at a flow velocity. Whereas higher viscosity imposes restriction on the velocity of liquid itself.It should not react with the work material, the tool or the container etc.It should be inflammable, cheap and easily availableIt should not evolve gases and toxic vapors during operationIt must be a hydrocarbon. The various dielectric fluids are; kerosene, transformer oil, white spirit, oil etc: Some conducting powers such as aluminum or fine and light density graphite if added to the dielectric fluid, the metal removal rate increases.Department of Industrial & Production Engineering33/18Advantages, Disadvantage and ApplicationsAdvantagesMetal of any hardness, toughness or brittleness could be machined by this process provided they are conductor of electricity.Dies of harder materials like alloy steels tungsten carbide etc., for molding, forging, extrusion and press tools could be reproduced.Dies can be machined even in the hardened stateAny complicated shape that can be made on the tool can be reproduced on a workpieceVery fine holes can be drilled accurately since the cutting forces are too smallThe accuracy of work produced can be as high as 0.005 mm on finishing operationsMore suitable for producing surfaces that are to be used for wear resistance because the surface produced has micro-craters (appear as shot blasted surface) which can contain lubricants effectivelyThere is no physical connection between the tool and the workpiece. There is no cutting force except the blasting pressure. So cylinder and fragile workpieces could be machined without causing any damage to themThe machining time especially for harder work materials is much less than conventional machining process. Department of Industrial & Production Engineering33/19DisadvantagesThe power requirement is very high compared to conventional processes (120 J/mm2)Some of the materials may become brittle at room temperature and there is some chance of surface crackingSometimes a layer of 0.01 to 0.10 mm containing 4% carbon may get deposited on steel workpiecesThe metal removal rate is comparatively low (75 mm3/sec)It is difficult to reproduce sharp cornersIn some cases the microstructure of the workpiece surface gets distorted necessitating subsequent etching. ApplicationsEDM is widely used for machining burr free intricate shapes, narrow slots and blind cavities etc., for example, sinking of dies for molding, die casting, plastic molding, wire drawing, compacting, cold heading, forging, extrusion and press tools. Almost any geometry (negative of tool geometry) can be generated on a workpiece if a suitable tool can be fabricated (the use of punch as a tool to machine its own mating die is commonly employed in EDM method). EDM is particularly useful for machining of small holes, orifices or slot in diesel-fuel injection nozzles, or in aircraft engines, air brake valves etc.Department of Industrial & Production Engineering33/20Wire Electrical Discharge Machining: The wire-cut EDM uses a very thin wire 0.02 to 0.3 mm in diameter as an electrode and machines a workpiece with electrical discharge like a band saw by moving either the workpiece or wire. Erosion of the metal utilizing the phenomenon of spark discharge is the very same as in conventional EDM. The prominent feature of a moving wire is that a complicated cutout can be easily machined without using a forming electrode.

Department of Industrial & Production Engineering33/21Wire-cut EDM machine basically consists of a machine proper composed of a workpiece contour movement control unit (NC tension; a machining power supply which applies electrical energy to the wire electrode; and a unit or copying unit), workpiece mounting table and wire driver section for accurately moving the wire at constant tension; a machine power supply which applies electrical energy to the wire electrode; and a unit which supplies a dielectric fluid (distilled water) with constant specific resistance. The various features of wire cut EDM process are:

Forming electrode adapted to product shape is not requiredElectrode wear is negligibleMachined surfaces are smoothGeometrical and dimensional tolerances are tight Relative tolerance between punch and die is extremely high and die life is extendedStraight holes can be produced to close toleranceEDM machine can be operated unattended for long time at high operating rateMachining is done without requiring any skill.Department of Industrial & Production Engineering33/22AdvantagesSaving of stages in sequential tools, due to absence of split lines in the die, hence permitting more punch opening per stage. Molded parts will not have flashes, as the moulds with draught can be made without vertical divisions. Tool manufacturing and storage is not required. Heat treatment distortions are totally avoided, as the workpieces are hardened before cutting. Cycle time for die manufacture is shorter, as the whole work is done on one machine.Inspection time is reduced, due to single piece construction of dies with high positioning accuracy. The time utilization of wire cut EDM is high, as it can cut right through the day Economical, even for small batch production, including prototypes, as most of the programming can be easily done.High surface finishes, with low thermal affected zone depths are obtained. This reduces the manual finishing operation timeAvoids rejections, due to initial planning and checking the programDepartment of Industrial & Production Engineering33/23Laser Beam Machining (LBM) In laser beam machining, the source of energy is a laser (Light Amplification by Simulated Emission of Radiation), which focuses optical energy on the surface of the workpiece. A laser beam can melt and vaporize diamond when focused by lens system, the energy density being of the order of 105 kW/cm2. Such tremendous energy release is due to certain atoms which have higher energy level and oscillate with particular frequency.

There are several types of lasers used in manufacturing operations, e.g., solid state, gas, liquid and semi conductor. For machining and welding, high power lasers are required and, in general, only the solid state lasers can provide the required power levels.

The most commonly used solid-state laser is the ruby laser (crystalline aluminium oxide or sapphire). These lasers are fabricated into rods about 150 mm long and their ends are furnished to close optical tolerances. Figure in the next slide shows a schematic view of laser beam machining process. Department of Industrial & Production Engineering33/24

(a) Schematic illustration of the laser-beam machining process. (b) and (c) Examples of holes produced in nonmetallic parts by LBM.Department of Industrial & Production Engineering33/25Laser Beam Machining (LBM) The ruby crystal is doped with a small amount of chromium oxide. The laser is pumped by a flash of high intensity light (xenon-filled flash lamp). The xenon lamp is fired by discharging a large capacitor through it (electric power of 250 to 1000 watts may be needed for this). The intense radiation from the lamp excites fluorescent impurity atoms (chromium atoms) to a higher energy level. When the atoms fall back to the original energy level through a series of energy levels, an intense beam of visible light is emitted. When this light is reflected back from the coated rod ends, more atoms are excited and stimulated to return to their ground level. This chain reaction results in a stimulated avalanche of light, some of which is transmitted through the reflecting coatings (about 80% reflective). This light is highly coherent in time and space, that is, it has a very narrow frequency band, is highly in phase, and is quite parallel. When this light is focused with ordinary lenses at spot on the workpiece, high energy density is obtained which will melt and vaporize the metal.Department of Industrial & Production Engineering33/26Advantages, Disadvantages and Applications AdvantagesThere is no direct contact between tool and workpiece. As such no tool wear problems are faced. Metal, non-metal irrespective of their brittleness and hardness, and even soft metals like plastics and rubber can be machined. Laser beam can be sent to longer distances, without diffraction. It can also be focused at one place thereby generating lot of heat. It is thus possible to weld, drill and cut areas not readily accessible. The advantages of laser welding are that heat treated and magnetic material can be welded without losing their properties all over the material except a small region of heat-affection. Laser welding is possible in any environment through transparent materials and magnetic fields as well. Distortion is negligible and any two materials can be joined together. However, it is important that the vaporization of the metal must be avoided. Micro-sized holes can be laser drilled in difficult-to-machine or refractory materials. Precision location is ensured by focusing of the beam. Deep holes of very short diameter can be drilled by using unidirectional multiple pulses. Beam configuration and size of exposed area can be easily controlled. Department of Industrial & Production Engineering33/27DisadvantagesHigh initial cost and short life of flash lampSafety procedures to be followed strictlyOver-all low efficiency (0.3% to 0.5%)Very low material removal rateNotable to drill too deep holesMachined holes not round and straight and No possibility of machining some plastics which bum or char. ApplicationsUsed for making very small holes (holes in rubber baby bottle nipples), difficult welding of non-conductive and refractory materials, cutting complex profiles in thin and hard materials. Also used for partial cutting or engraving.Can be used for mass micro-machining production.Can also be used for selective heat treating of materialsIt is also sometimes used for dynamic balancing of rotating parts.It is very useful for producing very fine and minute holes etc.Department of Industrial & Production Engineering33/28Electron Beam Machining (EBM) The source of energy in electron beam machining is high velocity electrons, which strikes the surface of the workpiece and generate heat. In electron beam machining, electrons emitted by a hot surface and accelerated by a voltage of 50 to 200 kV are focused to a very small areas on the workpiece. This stream of high energy electrons posses a very high energy density (of the order of 104 kW / mm2) and when this narrow stream strikes the workpiece (by impact), the kinetic energy of the electrons is converted to powerful heat energy which is quite sufficient to melt and vaporize any material. Even though, the electrons can penetrate metals to a depth of only a few atomic layers and can melt metal to a depth of 25 mm or more. The electron beam which travels at about half to three-fourth the velocity of sound is focused on the workpiece by electro-static or electro-magnetic lenses. Electron beam machining done in a high vacuum chamber to eliminate the scattering of the electron beam as it contacts the gas molecules on the workpiece. Figure in the next slide shows schematic view of electron beam machining process.Department of Industrial & Production Engineering33/29

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.Department of Industrial & Production Engineering33/30For observing the process of machining an optical viewing system consisting of lens and prism is also incorporated. The beam can be controlled very accurately and focused on a width as small as 0.002 mm. The electrons on impingement over the workpiece heat it up and raise its temperature to a value as high as 5000C. Due to this the material melts and vaporizes locally.

Recent developments have made it possible to machine outside the vacuum chamber. In this arrangement, the necessary vacuum is maintained within the electron gun proper by removing gases as soon as they enter. The fully vacuum system is more costly, but it has the advantage that no contaminating gases are present and the electron gun can be located at a considerable distance from the workpiece. Department of Industrial & Production Engineering33/31Advantages, Disadvantages and Applications AdvantagesVery hard, heat resistant materials could be machined or welded easilyNo physical or metallurgical damage results in the workpiece.Close dimensional tolerance could be achieved since there is no cutting tool wear.In electron beam welding there is virtually no contamination and close control of penetration is possible.Holes as small as 0.002 mm diameter could be drilled. Disadvantages The equipment costs high and operator of high skill is required for carrying out operations.The power consumption is exceedingly highIt is not very suitable for sinking deep holes, if the sides must be parallel. In other words, it is not possible to have perfectly cylindrical deep holes by this method. Unless special care is taken the bottom of a thorough hole would become cone-shaped. It is most suitable for machining operation where much less material is to be removed. The material removal rate being of the order of a fraction of a milligram per sec.The electron beam operation can be carried out only in vacuum. Department of Industrial & Production Engineering33/32ApplicationsIt is used for drilling synthetic jewels in the watch industry.Holes as small as 0.002 mm diameter can be produced in hard synthetic sapphires.Electron beam can be suitably used for welding small pieces of highly reactive and refractory metals.For making fine gas orifices in space nuclear reactors and turbine blades for supersonic aero engines, it is used Wire drawing dies, flow orifices could be produced by this process. Fine copper wire can be welded to in transistors. Department of Industrial & Production Engineering