Lecture 2 edm wedm and medm


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Lecture 2 edm wedm and medm

  1. 1. Electrical Discharge Machining
  2. 2. Fundamentals of EDM• The process dates back to WW I & II when work as well as substantial tool material was removed due to manual feeding of electrode.• Later vibratory electrodes were used to control inter electrode gap.• Two Russian scientists developed R-C circuit and servo controller.• The Die sinking version of EDM was developed sometime in 1940s.• The process modeling involves understanding of complex hydrodynamic and thermodynamic behavior of the fluid.
  3. 3. Fundamentals of EDM Preparation PhasePhase of Discharge Interval Phase
  4. 4. Fundamentals of EDM Voltage –Current curves (Free, Normal, Stationary located, and Short circuit discharges) General observations  Difficult to start the process with very clean dielectric  Firing of high current discharges at same voltage is easy in contaminated dielectric  New ignition opt to ignite in prior discharge regions  Greater ignition preferences in more contaminated regions
  5. 5. Fundamentals of EDM• DC pulses of appropriate shape, frequency and duty cycle are used. This is used even for motor control now-a-days. Frequency is ~ 100,000 Hz.• Spark is initiated at the peak between the contacting surfaces and exists only momentarily. Spark temp is 12,000 C. Metal as well as dielectric will evaporate at this intense localized heat. A crater is caused by both due to the local evaporation as well as the vapor action.• Vapor quenches and next spark it at another narrow place. Thus, spark wanders throughout the surface making uniform metal removal for the desired finish.
  6. 6. Fundamentals of EDM• Material removal in EDM is based on erosion effect.• Several theories have been proposed: – Electro-mechanical theory: electric field force exceeds the cohesive force of lattice. – Thermo-mechanical theory: Melting of material by ‘flame-jets’. – Thermo-electric theory: Generation of extremely high temperature due to high intensity discharge current.
  7. 7. Fundamentals of EDMDebris and Bubble particles generatedby single spark Debris gathering at Bubble boundary
  8. 8. Fundamentals of EDM Large number of Spherical particles with few non- spherical particles Spherical particles are rich in workpiece material and non-spherical particles are rich in tool material Understanding of Erosion Mechanism and Oxide free power production Important parameters affecting Debris morphology are  Current  Voltage Input Energy  Pulse On-time  Capacitance
  9. 9. Fundamentals of EDM Micro analysis reveals that there is movement of material from workpiece to cathode and vice-versa Normal distribution of particle size (Stochastic nature) Structures of Debris-  Large Size & Small Size  Hollow & Solid Debris  Satellite structure  Hollow Spheres  Dents  Burnt Cores
  10. 10. Fundamentals of EDMMicroanalysis of Debris – Low Energy Densely populated, Small diameter, solid particles a)Dendrite structure; b)Solid sphere; c)Satellite formation; d) Non-spherical particles
  11. 11. Fundamentals of EDM Larger population of hollow satellites with dents, surface cracks, and burnt core a)Debris structure, b)Hollow sphere, c)Dendrite structure, d)Satellite with dent formation, e)Dent formation
  12. 12. Fundamentals of EDM Effect of Tool Rotation. Results in fine debris particles and improved processstability. Effect of Ultrasonic Vibrations.  Larger particles  Large number of particles with spherical geometry  More uniformity of spherical and non-spherical particles  Uniform mixing of materials  More collision between debris particles
  13. 13. Fundamentals of EDM
  14. 14. Fundamentals of EDM  A series of voltage pulses of magnitude about 20 to 120 V and frequency on the order of 5 kHz is applied between the two electrodes, which are separated by a small gap, typically 0.01 to 0.5 mm.  When using RC generators, the voltage pulses are responsible for material removal.
  15. 15. Breakdown of dielectric during one cycle  Temperatures of about 8000 to 12,000 C and heat fluxes up to 1017 W/m2 are attained during process
  16. 16. Breakdown of dielectric during one cycle  Explosion and implosion action of dielectric  EDM performance measures such as material removal rate, electrode tool wear, and surface finish, for the same energy, depends on the shape of the current pulses.
  17. 17. Voltage and Current characteristics Types of pulses  Effect of pulses  Pulse classification systems  Data acquisition and classification
  18. 18. EDM Schematics
  19. 19. Components of EDM
  20. 20. Tool Wear and Tool Materials Graphite is suitable material with good electrical conductivity and machinability Copper  WCu and WAg Brass
  21. 21. Corner wear ratio
  22. 22. FlushingThe main functions of the dielectric fluid are to1. Flush the eroded particles from the machining gap2. Provide insulation between the electrode and the workpiece3. Cool the section that was heated by the discharging effectThe main requirements of the EDM dielectric fluids are adequateviscosity, high flash point, good oxidation stability, minimum odor,low cost, and good electrical discharge efficiency
  23. 23. Parameters affecting EDM performance
  24. 24. Erosion Rate and Surface Finish
  25. 25. Effect of Pulse Current and Pulse on time
  26. 26. EDM hazards
  27. 27. Process Stability Indication of constantly moving spark Importance of Debris content in inter-electrode gap Discharge conduction through debris chain Effect on surface cracks Process stability primarily depends on discharge transitivity rather than breakdown strength Absence of Debris can be one of the causes of arching
  28. 28. Processing and Response parameters• Electrode material• Accuracy and finish of electrode manufacture• Current/ voltage• Frequency• Pulse width
  29. 29. Operating parameters• Current and voltage: As the voltage drops from A to B, the current increases because of the negative voltage-current relationship. At C, current is interrupted, and voltage goes to zero and reverses to D; but since there is no break down in opposite direction, no current reversal takes place. The voltage now returns to zero and waits for the next pulse.
  30. 30. Operating parameters• The energy dissipated in the system is voltage times current times time, it remains fairly constant.• At ‘A’ energy is zero.• ‘B’ represents the power going to the work.• ‘C’, ‘D’, ‘E’ and ‘F’ represent traces at where there are either voltage or current is zero, hence no power.• In section ‘B’ voltage times current is nearly constant, indicates a constant input of power during a current pulse.
  31. 31. Operating parameters• In the inter electrode gap, there is a mixture of electrons, ions, and neutral atoms in the gaseous form.• Cathode supplies electrons for the flow of current so should be enough to emit the electrons, also positive ions in front of cathode provide a pulling force.• Cathode material also matters – Cu is a low melting point alloy so it melts (at 1083 C) and emits electrons by heat and electric field.• Graphite, W, Mo emit electrons at the temperatures below there melting points hence are more stable as cathode.
  32. 32. Operating parameters• Resistance to the flow of current is higher near the electrodes.• The voltage drop near cathode is smaller as compared to that of anode. It helps electrons in achieving high speed to ionize the gases near cathode.• Cathode voltage drop ranges from 12V for Cu to 25V for graphite.• The plasma generated is at 6000 to 10,0000 C.• (+) ions and electrons (-), due to the mass difference ions move slowly therefore, 95% of the current is carried by electrons.• The electrons and ions provide major power input to the cathode and anode surfaces.• When the current is high, evaporation of material from anode occurs, the stream of atoms coming out of anode surface interferes with the electrons going to the anode.• Some ions get ionized at the near anode drop but the electrons get additional energy to cause more vaporization of anode.
  33. 33. Operating parameters• Straight polarity: in which electrode is usually a cathode (-). Here, work surface energy can be controlled by controlling the current so that anode drop energy provides proper wear and desired surface finish.• Reverse polarity: in which electrode anode (+) and work (-), in which rough cut higher cutting rates can be obtained with virtually no electrode wear.
  34. 34. Operating parameters• Electrode rotating: Improves flushing difficulties with speed of about 200 rpm max. It provides better surface finish.• Electrode orbiting: Electrode does not rotate but revolve in an orbit. Orbiting need not be restricted to round shape.• Both actions reduce electrode wear as it gets distributed uniformly.
  35. 35. Operating parameters• No Wear EDM: It is defined as the condition when the electrode to work wear ratio is 1% or less.• Effect of arc duration: Melting depth is a function of arc duration for a circular non expanding heat source.• The maximum melting depth occurs at different durations for different materials subjected to same energy. The melting depth reaches a peak value with an increase in arc duration, it reduces with further increase in the arc duration.• Thus, it should be possible to choose an arc duration which maximizes the work erosion while holding the electrode to some lesser value.• In Cu and steel system, at the arc duration suitable for maximum melting of steel, the melting of Cu is at the minimum.
  36. 36. Operating parameters• Electrode polarity: The energy distribution between anode and cathode is a function of – – ratio of electron current to ion current at cathode – Physical constant (work function) of the cathode material. – In Cu as cathode current density decreases, the electron to ion current ratio also decreases. As the arc duration increases, the energy delivered to the gap concentrates at the cathode. Therefore, the electrode must be of positive duration if long arc durations are used to achieve the no-wear condition.• Electrode coating is observed in Cu-steel system. – Coating of electrodes with thin black film of carbon which has erosion resistance and tend to reduce electrode wear.
  37. 37. Processing and Response parameters Effect of CurrentAs current increases, the depth and width of the crater becomeslarger. So also the MRR. But this may result in rough surface.However, this can be used to our advantages to obtain mattysurface.
  38. 38. Processing and Response parameters Effect of FrequencyAs frequency increases, the depth and width of the crater becomes smalleralthough the MRR may not be affected as there will be more craters per unittime. However, frequency has a limit since initiation of spark requires certainminimum time required for the breakdown of the dielectric. Similarly thespark needs some time to quench. In principle, one should operate as high afreq as possible.
  39. 39. Processing and Response parameters Effect of VoltageGap ↓  Voltage ↓Voltage ↓  Current ↓Current ↓  MRR ↓Current ↓  Accuracy & finish ↑Gap ↓  Poor flow of dielectric.
  40. 40. Processing and Response parameters Effect on fatigue LifeA layer of resolidified metal of 0.002– 0.050 mm thick remains on thesurface. This may flake off duringcyclic loading. When high fatigue lifeis required, this layer must beremoved on a subsequent operationsuch as chemical etching.
  41. 41. Machine Construction
  42. 42. EDM process Variations 50Content Percentage 40 30 20 10 0 1 2 3 Group Number Normal Discharge Open Circuit Abnormal Discharge Group Number Group 1 Group 2 Group 3 Planetary Motion Yes No No Debris Layer Yes Yes No Input Voltage 15mV 15mV 15mV
  43. 43. EDM process Variations Modern controllers uses gap controlling strategy to control debris Dielectric flushing (injection, suction, & electrode jump) Jet sweeping Rotary Electrode/workpiece method. Without With Rotation Rotation
  44. 44. EDM process Variations Magnetic AssistanceUse of Magnetic field 1(05A,20µs), 2( 20A,350µs) Magnetic force used to change path of debris motion. Magnets attached on plates rotating under machining zone Magnetic force is useful not only at low energy but also at high energy inputs
  45. 45. EDM process VariationsVibration Assistance Condition of Adhesion Debris removal and SparkingThe combined process of EDM with USM had the potential to preventdebris accumulation, improve machining efficiency, and modify themachined surface.
  46. 46. Dielectric Fluid – Desirable properties• Break down characteristic: Non-conducting until breakdown and very high conduction through rapid ionization just after breakdown.• High latent heat – to minimize evaporation – to contain the spark in a narrow region for localized sparking• Low viscosity for ease of flow• Efficiency as coolant. It is kerosene or water.
  47. 47. Dielectric Fluid• Functions of Dielectric Fluid It acts as an insulator until sufficiently high potential is reached . Acts as a coolant medium and reduces the extremely high temp. in the arc gap. More importantly, the dielectric fluid is pumped through the arc gap to flush away the eroded particles between the work piece and the electrode which is critical to high metal removal rates and good machining conditions.
  48. 48. Dielectric FluidWork Material Fluid Medium ApplicationAluminumBrassMild Steel Hydrocarbon oilStainless or glycerin-water Submergedsteel (90:10)Tool steelTungstenCarbide Mineral oil
  49. 49. Dielectric Fluid• Dielectric fluids: should have very high flash point and very low viscosity. – Petroleum based hydrocarbons – Silicon fluids mixture with petroleum oils for machining of titanium, high MRR and good SF. – Kerosene, water-in oil emulsion, distilled water.• Cooling of dielectric is required sometimes while cutting with high amperage can be done by using heat exchangers.• Filtering of dielectric is necessary to filter out 2 – 5 µm particles.
  50. 50. Dielectric Fluid• Insulation and conduction: Insulating characteristic is measured by the maximum voltage that can be applied before ionization.• Cooling: ability to resolidify vaporized material into chips , thermal transfer capability.• Flushing: Sufficiently viscous to pass through a small gap &remove debris.• Methods of fluid application – Normal flow – Reverse flow – Jet flushing – Immersion flushing
  51. 51. Workpiece and Tool MaterialElectrode Materials ApplicationsBrass High Accuracy for most metalsCopper Smooth finish Low accuracy for holesZinc Alloys Commonly used for steel, forging cavitiesCopper-Graphite General Purpose workSteel Used for nonferrous metalsCopper Tungsten High accuracy for detail workGraphite Large volume/fine details  Low wear Excellent machinability
  52. 52. Workpiece and Tool Material• Tool electrodes transport current to the work surface.• Graphite – Coarse (for large volume) or fine (for fine finish). – Normally used for steel provides large MRR/A as compared to other metallic electrodes. – When used for WC, deposits of carbon on work leads to flow of current without ionization of dielectric and hence arcing. High density, fine particles preferred. – Average surface finish using graphite electrodes:0.5 µm Ra.• Copper Graphite – For rough and finish machining of WC.
  53. 53. Workpiece and Tool Material• Copper – When smoothest surface finish is required. – In no-wear mode, copper works best under low ampere and long spark times. – Tellurium increases the machinability of copper. – Free machining brass is used for making complex shaped electrodes. – Copper tungsten (70% W) for fine detail and high-precision EDM. High density, strength, thermal and electrical conductivity.• Tungsten – Tungsten carbide is used for cutting steel and WC. – Small holes of deeper dimensions.
  54. 54. Workpiece and Tool Material• Electrical conductivity Tool W/P rw• Less wear due to the spark Brass Brass 0.5 (Low rw) Brass Hard C.S. 1.0• Good machinability Brass WC 3.0• Good surface finish on w/p Loss of material from the tool Wear ratio rw = Loss of material from the work piecerw increases with material hardness and decreases with the increase in melting point of the tool material.
  55. 55. Advantages Any material that is electrically conductive can be cut Hardened work pieces can be machined eliminating the deformation caused by heat treatment. Complex dies sections and molds can be produced accurately, faster, and at lower costs. The EDM process is burr-free. Thin fragile sections such as webs or fins can be easily machined without deforming the part.
  56. 56. Disadvantages High specific energy consumption (about 50 times that in conventional machining) When force circulation of dielectric is not possible, removal rate is quite low Surface tends to be rough for larger removal rates EDM process is not applicable to non-conducting materials
  57. 57. Applications• Mold and die making, slowly becoming a production process.• Machining of ‘difficult-to-machine’ materials.• Miniature and fragile parts that can not withstand the force of conventional cutting. Holes of 0.05 mm, slots of 0.3 mm• As EDM is a very slow process, it can be justified only where the hardness is too high or the features cannot be realized by other means.• Tool making: sharp corners, small features, deep features etc. With the advent of hard cutting tools, full sinking is out of fashion.• Removal of broken drills or fasteners• Deep hole drilling of small holes. Eg.: turbine blades, fuel injection nozzles, inkjet printer head etc.
  58. 58. Wire-EDM
  59. 59. Wire EDM• This process is similar to contour cutting with a band saw.• Slow moving wire travels along a prescribed path, cutting the work piece with discharge sparks.• Wire should have sufficient tensile strength and fracture toughness.• Wire is made of brass, copper or tungsten. (about 0.25mm in diameter).
  60. 60. Wire EDM Process• Thin wire of as low as 0.03mm dia is used as the tool.• For through features  dies for punching, blanking and piercing; templates and profile gauges; extruder screws etc.• Taper also possible• Upto 4 axes available.• Water is the common di-electric
  61. 61. WEDM machine classification
  62. 62. WEDM Process• Machining of hard and complex shapes with Sharp corners.• Risk of wire breakage and bending has undermined the full potential of the process drastically reducing the efficiency and accuracy of the WEDM operation• WEDM utilizes a continuously travelling wire electrode made of thin copper, brass or tungsten of diameter 0.05– 0.3 mm, which is capable of achieving very small corner radii• The material is eroded ahead of the wire and there is no direct contact between the workpiece and the wire, eliminating the mechanical stresses during machining• Machining of EXOTIC and HSTR alloys
  63. 63. WEDM Process• The material removal mechanism of WEDM is very similar to the conventional EDM process involving the erosion effect produced by the electrical discharges (sparks)• The WEDM process makes use of electrical energy generating a channel of plasma between the cathode and anode, and turns it into thermal energy at a temperature in the range of 8000–12,000 C or as high as 20,000 C• A varying degree of taper ranging from15 degree for a 100 mm thick to 30 degree for a 400 mm thick workpiece can also be obtained on the cut surface.• The microprocessor also constantly maintains the gap between the wire and the workpiece, which varies from0.025 to 0.05 mm
  64. 64. WEDM Process• Number of passes are required to achieve the required degree of accuracy and surface finish• Dry WEDM (in gas) to achieve the high degree of surface finish• The typical WEDM cutting rates (CRs) are 300 mm2/min for a 50 mm thick D2 tool steel and 750 mm2/min for a 150 mm thick aluminium , and SF quality is as fine as 0.04–0.25 µRa• The deionised water is not suitable for conventional EDM as it causes rapid electrode wear, but its low viscosity and rapid cooling rate make it ideal for WEDM
  65. 65. Hybrid WEDM Process• WEDG – machining of fine rods used in electronic circuits; machining of electrodes as small as 5 micron in diameter advantages of WEDG include the ability to machine a rod with a large aspect ratio, maintaining the concentricity of the rod and providing a wider choice of complex shapes such as tapered and stepped shapes at various sections.• Ultrasonic Vibrations to wire to improve surface finish and cutting ratios• Wire electrochemical grinding
  66. 66. WEDM Applications• Modern tooling applications - wafering of silicon and machining of compacting dies made of sintered carbide• For dressing a rotating metal bond diamond wheel used for the precision form grinding of ceramics• Advanced ceramic materials – other common machining processes for machining ceramics are diamond grinding and lapping.• Machining of boron carbide and silicon carbide• MRR and surface roughness depends on processing parameters as well as workpiece material• Machining of naturally non-conductor by doping with conducting material• Machining of modern composite materials• MMC and carbon fiber polymers
  67. 67. Major Research issues• WEDM process optimizationFactors affecting performance measures – pulse duration, dischargefrequency and discharge current intensityCutting ratio – Factors affecting CR are properties of the workpiecematerial and dielectric fluid, machine characteristics, adjustablemachining parameters, and component geometry. Use of DOE, ANN.It was found that the machining parameters such as the pulse on/offduration, peak current, open circuit voltage, servo reference voltage,electrical capacitance and table speed are the critical parameters for theestimation of the CR and SF.MRR - discharge current, pulse duration and pulse frequency are thesignificant control factors affecting the MRR and SF, while the wirespeed, wire tension and dielectric flow rate have the least effectSurface finish – all the electrical parameters have a significant effect onthe surface finish
  68. 68. Major research issues• Wire EDM process monitoring and control  Fuzzy control system - proportional controls were used traditionally control the gap. Conventional control algorithms based on explicit mathematical and statistical models have been developed for EDM or WEDM operations  Pulse discrimination system  Knowledge system  Ignition delay based system  Wire breakage - rapid rise in frequency is observed before wire breaks; control strategy to switch off the generator at high frequency, localized high temperature causes wire breakage, excessive thermal force  Wire material breakage and fracture  Wire lag and wire vibrations- plasma and material erosion forces, hydraulic forces due to dielectric flow
  69. 69. ApplicationsThe common applications of WEDM include the fabrication of thestamping and extrusion tools and dies, fixtures and gauges,prototypes, aircraft and medical parts, and grinding wheel formtools.
  70. 70. END
  71. 71. “Micro-EDM processes” 71
  72. 72. Outline  Principle of EDM process  Characteristics of EDM process  Control of Discharge location  Micro-manufacturing  Scope of micromachining  Classification of micromachining processes  Role of micro-EDM in micromachining  Micro-reverse EDM  Research issues in micro-EDM related processes  Experiments I micro-reverse EDM  Future of micromachining 72
  73. 73. Electrode gap monitoring and control 10 MHz • Mathematical adaptive control theory • Advances in computer technology and advanced algorithms for machine control (Artificial intelligence, ANN) 73
  74. 74. Micro-Manufacturing - What is it? Manufacture of products with the following features:  about 100 µm to about 10 mm in size  contain very complex 3-D (free-form) surfaces 70 µm - Human Hair  employ a wide range of engineering materials 25 µm - Characters  possess extremely high relative accuracies in the 10-3 to 10-5 range Micro-milling Fanuc - Japan Zeiss - Germany Micro-partsMicro-structures manufactured by micro-SLA Micro-EDM Klocke NanotechnikJapan NTU - Taiwan Micro-Motor 74
  75. 75. Why Miniaturization? • Minimizing energy and materials used for the manufacture of devices • Integration with electronics; simplifying systems • Cost/performance advantages • Faster devices • Increased selectivity and sensitivity • Drawback-Size effect in mechanical micromachining 75
  76. 76. Scope of micromachining processes MICRO MACHINING Micro Machining Removal of material at micro level Macro components but material removal is at micro/nano level Micro/nano components and material removal is at micro/nano level Unfortunately, the Definition present day notion is Material removal is micro/nano level with no constraint on the size of the component Machining of highly miniature components with miniature features – NOT CORRECT 76
  77. 77. Classification of micromachining processes FABRICATION Macro-fabrication Micro-fabrication Hybrid Micro-machining µ-nano finishing Processes Mechanical - µ Beam energy based Chem. & EC -µ machining - µ machining machining USM EBM PCMM AJM LBM ECMM AWJM EDM WJM IBM PBM
  78. 78. Micromachining processes Energy Used Principle Processes and FeaturesMechanical Material removal via highly Cutting, grinding, sandblasting.Force concentrated force UR ~ 100 nm, edge radius<1 µmMelting and Material removal via melting EDM, LBM, EBM. Small UR byvaporization and/or vaporization and reduced the pulse energy, debris by high pressure gas concentration of energy via ultra short pulse duration and/or sharply focused beam by opticsAblation Decomposition of atoms Excimer/Femto second laser. High using incident photon energy dimensional accuracy, less HAZ but or direct vaporization of low machining speed and high cost material via high energy of equipment pulsesSolidification Liquid or paste is solidified in Injection molding, die casting, etc. a mold and shape of the mold curing may be required after molding is replicated and porosity 78
  79. 79. Micromachining processes Energy Used Principle Processes and Features Dissolution Chemical or electrochemical Chemical, PCM and ECM. Small UR, reaction based ionic negligible force. Inter-electrode gap, dissolution flow of electrolyte influences accuracy Plastic Shape of the product Micro-punching, extrusion, etc. Deformation specified by die/punch/mold No UR is involved, high speed, spring-back and difficulties in die or mold making Lamination Material in solid powder or Stereolithography, internal as well as liquid form is solidified layer- external profiles can be formed by-layer. easily. 79
  80. 80. Role of EDM in micromachining  Non-contact machining  3D machining  Physical characteristics such as hardness, brittleness dose not affect the process  Use of deionized water as dielectric  Absence of Size Effect 80
  81. 81. Comparison of EDM and micro-EDM The Resistance Capacitance Relaxation (RC- relaxation) circuit used in EDM is replaced by the RC- pulse circuit in micro-EDM. In the RC-relaxation circuit, current and gap voltage are controlled at a pre-defined level throughout the pulse on-time but in modeling attempts in micro- EDM based on RC pulse circuits, the current and voltage are frequently assumed to be constant. On the other hand, in a single discharge of RC-pulse generator, the voltage and current are not maintained to any pre-defined level but depend upon the capacitor charge state at any instant. E = V I Duty cycle E = ½ CV^2 81
  82. 82. Comparison of EDM and micro-EDM EDM Micro-EDM Circuitry Elements • RC relaxation type • RC single pulse discharge • Single spark process • Single spark process • Forced process for constant voltage • Single capacitance discharge, no and current const V and I • User defined pulse on time • No control – gap characteristics Scaling Effects • Interelectrode gap is 10’s of µm • Interelectrode gap is 1-5 µm • Low efficiency • High efficiency Typical single spark crater 82
  83. 83. Micro-analysis of Debris  Large number of Spherical particles with few non- spherical particles  Spherical particles are rich in workpiece material and non-spherical particles are rich in tool material  Understanding of Erosion Mechanism and Oxide free power production  Important parameters affecting Debris morphology are  Current  Voltage  Pulse On-time Input Energy  Capacitance
  84. 84. Micro-analysis of Debris Micro analysis reveals that there is movement of material from workpiece to cathode and vice-versa Normal distribution of particle size (Stochastic nature) Structures of Debris Low Energy  Large Size & Small Size  Hollow & Solid Debris  Satellite structure  Hollow Spheres  Dents  Burnt Cores High Energy
  85. 85. EDM process stability How will you measure?  Ignition delay time Effect of Tool Rotation Effect of Ultrasonic Vibrations Effect of workpiece-tool materialcombination Effect of polarity PMEDM Effect of dielectric Group Number Group 1 Group 2 Group 3 Planetary Motion Yes No No External material layer Yes Yes No
  86. 86. Micro-EDM process stability Indication of constantly moving spark Importance of eroded material in inter-electrode gap Discharge conduction through debris chain Effect on surface cracks Process stability primarily depends on discharge transitivity rather than breakdown strength – Yo et al. Absence of metallic particles can be one of the causes of arching 1 –Low energy 2 – High Energy
  87. 87. Variants of micro-EDM  Figure : Micro rods machining processes Process Capability Limitation BEDG Min. 3 µm diameter electrode, maximum 10 Only single electrodes can be machined aspect ratio, 0.6 µRa surface finish Micro-WEDG Min. 5 µm diameter electrode, maximum 10 Cylindrical electrodes as well as arrayed aspect ratio, 0.8 µRa surface finish electrodes can’t be machined Micro-WEDM Best results obtained are 10x10 square array (23 Cylindrical arrayed structures can’t be µm width, 700 µm height), minimum machining machined size achievable is 20 µm, surface finish 0.07-0.35 µm Ra, and maximum aspect ratio 100 Diamond milling micro tower of 1 mm in height and 25 μm square Mechanical process involves machining stresses 87 87
  88. 88. Research issues in micro-EDM Micro-EDM Research Areas Handling Electrode and Machining Measurement workpiece Process preparation Surface Electrode Off-machine electrode Process Sources of quality preparation Parameters Errors Parts Dimensions Mfg. Micro 3D Machine Drilling, electrode threading Electrode holes (WEDM) Jigs and Fixture On-machine electrode Electrode Stationery Guided wear and Rotating Disk block running wire machining strategies Uniform wear Multi Wear Z-compensation method electrode monitoring system 88
  89. 89. Applications Machining of mould and die in high strength materials (Carbides, die steel, conducting ceramics) – Recently replaced by high speed milling process Chemical aspects of EDM – Production of fine particle powders – RESA (for ultrafine powders)- Reactive Electrode Submerged Arc EDM – Diamond like carbon and nano-tubes (solidification of evaporated material) – Large amount of energy is consumed in the chemical action during EDM – Supplying oxygen can enhance the MRR during the process 89
  90. 90. Machining of arrayed micro-structures by REDM  Reverse replication of arrayed hole on the plate electrode to the bulk material by change aa) Normal EDM in the polarity Bulk Rod  Machined structures have a dimensions ab) Reverse EDM Micro-rods equal to the original Figure : Working of micro and reverse micro EDM processes dimension of pocket minus interelectrode gap  Important operating parameters are voltage , capacitance, threshold, and the feed Figure : a) array of 4 microrod machined, b) plate used as 90 a tool during machining
  91. 91. Machining of arrayed micro-structures by REDM Problem Statement : Machining of high aspect ratio arrayed microstructures by micro reverse EDM process. 91 Figure : set up of the micro-REDM process
  92. 92. Applications of micro-REDM Applications Mechanical MEMS Biomedical Micromachining Arrayed holes for passing As a interface device for As a electrode in wires in MEMS devices capturing neural signals arrayed hole/cavity Thin wall structures as a Brain neural activity machining cooling devices in MEMS recording Mask preparation system Arrayed microholes as a As a tool for generating Shaft for micro robots spray nozzels in the stable plasma micro actuator biotechnology applications Microneedels- syringe Heat Exchanging Holding sights for the Hexagonal and thin wall testing reagents structures Automobile Micronozzels 92
  93. 93. Components fabricated by micro-REDM 93
  94. 94. Reverse-micro Wire EDM 94
  95. 95. Experiments in micro-REDM Workpiece geometry : Machining of 400 µm square and 200 µm cylindrical electrodes, machined length 1 mm Images of the micro rods machined in each run of experiment 95
  96. 96. Surface Morphology Surface near tip exhibits number of craters , whereas the surface at Root Surface the root is relatively smooth. Smooth surface with almost no pits is observed near the root in the magnified image of fabricated structure Tip Surface A Sample 3 A 96
  97. 97. Arrayed structures machined at MTL IIT Bombay 97
  98. 98. 98