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... of powder EDM process 17 2.9.1 Mechanism of powder EDM process 17 2.9.2 Influence of powder EDM in surface roughness 20 2.9.3 Influence of powder EDM in surface modification 22 2.10 Conclusion... voltage and capacitance in powder mixed scanning micro- EDM 73 Figure 4.44 Cross section profile of maximum Peak to valley distance in (a) powder mixed die sinking, (b) powder mixed scanning micro- EDM. .. normal EDM while only 25 minutes in EDM with Al powder suspension The spark gap distance depends on powder concentration, type of powder and electrical condition In general an increase in powder concentration
AN EXPERIMENTAL INVESTIGATION ON SURFACE CHARACTERISTICS OF TOOL STEEL MACHINED BY POWDER MIXED MICRO-EDM MOHAMMED MUNTAKIM ANWAR NATIONAL UNIVERSITY OF SINGAPORE 2008 AN EXPERIMENTAL INVESTIGATION ON SURFACE CHARECTERISTICS OF TOOL STEEL MACHINED BY POWDER MIXED MICRO-EDM MOHAMMED MUNTAKIM ANWAR (B.Sc. in Mechanical Engineering, BUET) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING DEPARTMENT OF MECHANICAL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2008 Acknowledgements Acknowledgements I would like to express my deepest and heartfelt gratitude and appreciation to my Supervisors, Professor Wong Yoke San and Professor Mustafizur Rahman for their valuable guidance, continuous support and encouragement throughout the tenure. They have provided valuable suggestions from the development of my thesis concept to the fulfillment of my research work. Without their continuous supervision it would have been impossible to manage the research within this short period. I would like to take this opportunity to thank National University of Singapore (NUS) for providing me with research scholarship and sufficient funds for purchasing different materials to carry out my research work. The rich state of the art facilities and support of Advanced Manufacturing Lab (AML) and Micro Fabrication Lab provided the opportunity to carry out experiments and their analysis smoothly. . I also would like to take this opportunity to thank the following staff for their sincere help, guidance and advice without which this project would not be successfully completed: Mr. Tan Choon Huat, Mr. Lim Soon Cheong and Mr Wong Chian Long from Advanced Manufacturing Lab (AML) and Mr Lee Chiang Soon from Workshop 2. Special thanks go to Mr Abu Bakar Md Ali Asad, an M.Eng candidate of NUS, and Atikur Rahman from Mikro tools, a NUS spin off company, for their help with the machine set-up and for their guidance and technical assistance provided during different period of my research. I would also like to thank Mr. Muhammad Pervej Jahan, a PhD candidate of NUS, for his valuable advice and encouragement during my experimental work. I Acknowledgements I would like to offer my appreciation for the support and encouragement during various stages of this research work to my labmates and friends. My appreciation goes to Mohammad Ahsan Habib, Chandra Nath, Woon Keng Soon, Sadiq Mohammad Alam, Mohammad Sazedur Rahman, Masheed Ahmad, Angshuman Ghosh, Md. Saiful Karim, Saira Sanjida, Md. Ershadul Alam, Mohammad Iftekhar Hossain, Abdullah Al Mamun and many more. Special thanks to all of them for being so supportive for the past two years. Last but not the least, my heartfelt gratitude goes to my mother, Ms Shirin Akhter, for her loving encouragement and best wishes throughout the whole period and my father, Mr. Manjour Murshed Anwar, for his mental support and encouragement. I would also like to convey my sincere gratitude to my loving sister, Ms Nawrin Anwar for her inspiration and my aunty, Ms Rumaisa Samad, for always being there. II Table of contents Table of Contents ACKNOWLEDGEMENTS TABLE OF CONTENTS SUMMARY I III VII LIST OF TABLES X LIST OF FIGURES XI CHAPTER 1 INTRODUCTION 1 1.1 Motivation 1 1.2 Objectives of the research 3 1.3 Organization of the thesis 4 CHAPTER 2 LITERATURE REVIEW 6 2.1 Introduction 6 2.2 Historical Background of EDM 7 2.3 Principles of EDM 8 2.4 Micro-EDM and its type 9 2.5 Micro-EDM compared to other Micro-machining processes 10 2.5.1 Advantages of micro-EDM over other micromachining processes 10 2.5.2 Compatibility of micro-EDM with other micromachining processes 10 III Table of contents 2.6 Key system components of Micro-EDM 11 2.7 Types of pulse generator 12 2.8 Surface characteristics after EDM 14 2.9 Characteristics of powder EDM process 17 2.9.1 Mechanism of powder EDM process 17 2.9.2 Influence of powder EDM in surface roughness 20 2.9.3 Influence of powder EDM in surface modification 22 2.10 Conclusion 23 CHAPTER 3 EXPERIMENTAL DETAILS 26 3.1 Introduction 26 3.2 Experimental setup 26 3.2.1 Multi-purpose miniature machine 26 3.2.2 Workpiece material 29 3.2.3 Tool material 30 3.2.4 Dielectric 30 3.2.5 Powder material 31 3.2.6 Magnetic filter 32 3.3 Experimental procedure 33 3.3.1 Electrode dressing 33 3.3.2 Machining parameter 36 3.3.3 Machining details 36 IV Table of contents 3.4 Measurement apparatus 3.4.1 Atomic force microscope (AFM) 37 37 3.4.2 Scanning electron microscope (SEM) and Energy Dispersive X-ray (EDX) machine 38 3.4.3 Keyence VHX digital microscope 38 CHAPTER 4 RESULTS AND DISCUSSIONS 40 4.1 Introduction 40 4.2 Micro-EDM of SKH-51 tool steel 41 4.2.1 Analysis for die sinking micro-EDM 41 4.2.1.1 Surface topography 41 4.2.1.2 Surface roughness 44 4.2.1.3 Peak to valley height 47 4.2.2 Analysis for scanning micro-EDM 48 4.2.2.1 Surface topography 48 4.2.2.2 Surface roughness 53 4.2.2.3 Peak to valley height 56 4.3 Powder mixed micro-EDM of SKH-51 tool steel 4.3.1 Analysis for powder mixed die sinking micro-EDM 58 58 4.3.1.1 Surface topography 58 4.3.1.2 Surface roughness 61 4.3.1.3 Peak to valley height 64 V Table of contents 4.3.2 Analysis for powder mixed scanning micro-EDM 67 4.3.2.1 Surface topography 67 4.3.2.2 Surface roughness 70 4.3.2.3 Peak to valley height 72 CHAPTER 5 CONCLUSION AND RECOMMENDATIONS 5.1 Conclusion 75 5.2 Recommendations 79 BIBLIOGRAPHY 81 LIST OF PUBLICATIONS 89 Appendix A A-1 Appendix B B-1 Appendix C C-1 VI Summary Summary Electrical Discharge Machining (EDM) is a non traditional machining process that has become a well established machining option in manufacturing industries throughout the world and has replaced drilling, milling, grinding and other traditional machining operations. EDM is capable of machining geometrically complex or hard material components that are precise and difficult to machine. In recent years numerous developments in EDM focused in the fabrication of micro tools, micro components and parts with micro features. Due to negligible forces and good repeatability of the process, micro-EDM has become the best means for achieving high aspect ratio micro features. For the fabrication of complex 3-D molds using tough die materials, micro-EDM is one of the successfully applied machining processes. Since the surface finish of the micro mold cavity affects the surface quality of the finished products, it is necessary to perform investigation about how to improve surface roughness using micro-EDM. Moreover fabrication of parts smaller than several micrometers requires pulse duration of several dozen of nanoseconds. Since RC type pulse generator can generate such small energy by simply minimizing the capacitance, it is necessary to investigate the effect of such pulse generators in fine finish micro-EDM. In view of this ongoing challenge to improve surface finish by micro-EDM, a series of experiments were conducted using tungsten electrode of 500µm diameter as a tool and SKH-51 tool steel as workpiece to machine blind holes of 5 µm depth on the workpiece using RC type pulse generator. The aim is to find the correct combinations VII Summary of parameters that result in better surface characteristics. The effect of voltage and capacitance on surface topography and surface roughness were investigated. Since most of the study on surface characteristics of EDM has been conducted using die sinking method, it is important to adopt scanning micro-EDM to evaluate if there is any change in surface characteristics. In this point of view experiments are conducted using 500 µm tungsten electrodes to machine 1 mm slot at a depth of 5 µm on SKH-51 tool steel. The combination of voltage and capacitance settings for best surface finish is evaluated. The results were compared with die-sinking micro-EDM. With the continuous process improvement in EDM, the demand for high machining precision with low surface roughness at relatively high machining rates arise in die, mold and tool manufacturing industries. To fulfill this requirement a relatively new advancement in the direction of process capabilities is the addition of powder in the dielectric fluid of EDM. The results show that powder mixed EDM (PMEDM) can distinctly improve the surface finish and surface quality. Following the results on studies conducted on the use of PMEDM, an extension of this technique to microEDM is essential. In this aspect graphite powder with average particle size of 55 nm was mixed with EDM oil at a concentration of 2 g/l and die sinking micro-EDM were conducted to find the correct combination of voltage capacitance setting that results in improved surface quality. The results were compared with die-sinking micro-EDM conducted without powder. VIII Summary In the final part of the study a combination of powder added dielectric and tool movement were applied to study the effect of powder mixed scanning micro-EDM on the surface roughness. The results were compared with scanning micro-EDM without powder additives. Therefore in this study effort has been made to employ the micro-EDM process in surface study considering different types of dielectric using both die sinking and scanning micro-EDM. The results show that powder added dielectric and scanning method together generates the best surface finish in micro-EDM. Few recommendations for further improvements are also put forward. IX List of tables List of Tables Table 2.1 Overview of the micro-EDM varieties 10 Table 2.2 Compatibility of machining technologies with different materials 11 Table 3.1 Chemical Composition of SKH-51 tool steel 29 Table 3.2 Heat treatment and Hardness of SKH-51 tool steel 29 Table 3.3 Properties of Tungsten Tool Electrode 30 Table 3.4 Properties of EDM 3 Dielectric fluid 31 Table 3.5 Properties of graphite powder 32 Table 3.6 Specifications of magnetic filter 33 Table 3.7 Parameters used in machining 36 X List of figures List of figures Figure 2.1 Different types of pulse generators (a) RC type and (b) Transistor type 12 Figure 2.2 Schematic representation of layer-by-layer scanning µEDM 24 Figure 3.1 Block diagram of Multi-Purpose Miniature Machine Tool used for micro-EDM 27 Figure 3.2 Multi-purpose Miniature Machine Tool with micro-EDM attachment 27 Figure 3.3 Detailed view of the setup with micro-EDM attachment 28 Figure 3.4 Magnetic filter used in powder mixed micro-EDM 32 Figure 3.5 Optical image of electrode surface after cutting from bulk 33 Figure 3.6 Optical image of the electrode after first step of dressing 34 Figure 3.7 Electrode dressing methods (a) first step with plate micro-EDM, (b) second step with scanning micro-EDM 35 Figure 3.8 SEM images of electrode after dressing (a) electrode surface after first step dressing, (b) electrode surface after second step dressing 35 Figure 3.9 Atomic Force Microscope 37 Figure 3.10 Scanning Electron Microscope (SEM) also with Energy Dispersive X-ray (EDX) device 38 Figure 3.11 Keyence VHX Digital Microscope 39 Figure 4.1 Optical images of surface after die sinking micro-EDM at 120 volt setting using (a) 10 pf, (b) 100 pf 42 Figure 4.2 SEM images of surface after die sinking micro-EDM at 120 volt setting using (a) 10 pf, (b) 100 pf 42 Figure 4.3 EDX analysis of surface after die sinking micro-EDM at 10 pf-80 volt setting 43 Figure 4.4 EDX analysis of surface after die sinking micro-EDM at 10 pf-140 volt setting 44 Figure 4.5 AFM images of surface after die sinking micro-EDM at 10 pf-120 volt setting (a) surface profile, (b) three dimensional profile 45 XI List of figures Figure 4.6 AFM images of surface after die sinking micro-EDM at 100 pf-120 volt setting (a) surface profile, (b) three dimensional profile 45 Figure 4.7 Variation of surface roughness with voltage and capacitance in die sinking micro-EDM 46 Figure 4.8 Peak-to-valley distance across the line drawn on the surface profile using 100 pf and 120 volt in die sinking micro-EDM (a) surface profile, (b) cross section profile 47 Figure 4.9 Variation of distance between highest peak and lowest valley with voltage and capacitance in die sinking micro-EDM 48 Figure 4.10 Optical images of surface after scanning micro-EDM at 120 volt setting using (a) 10 pf, (b) 100 pf 49 Figure 4.11 SEM images of surface after scanning micro-EDM at 120 volt setting using (a) 10 pf, (b) 100 pf 49 Figure 4.12 SEM images of surface after micro-EDM at 47 pf-80 volt setting using (a) die sinking, (b) scanning 50 Figure 4.13 SEM images of surface after micro-EDM at 47 pf-120 volt setting using (a) die sinking, (b) scanning 51 Figure 4.14 EDX analysis of surface after die sinking micro-EDM at 47 pf-120 volt setting 52 Figure 4.15 EDX analysis of surface after scanning micro-EDM at 47 pf-120 volt setting 52 Figure 4.16 AFM images of surface after die scanning micro-EDM at 10 pf-120 volt setting (a) surface profile, (b) three dimensional profile 53 Figure 4.17 AFM images of surface after scanning micro-EDM at 100 pf-120 volt setting (a) surface profile, (b) three dimensional profile 53 Figure 4.18 Surface profiles of AFM images of surface at 10 pf-80 volt setting after micro-EDM in (a) die sinking, (b) scanning 54 Figure 4.19 Variation of surface roughness with voltage and capacitance in scanning micro-EDM 55 Figure 4.20 Comparison of variation of surface roughness with voltage at 10 pf capacitance at die sinking and scanning micro-EDM 56 Figure 4.21 Variation of distance between highest peak and lowest valley with voltage and capacitance in scanning micro-EDM 56 XII List of figures Figure 4.22 Cross section profile of maximum Peak to valley distance in (a) die sinking ,(b) scanning micro-EDM 57 Figure 4.23 Optical images of surface after powder mixed die sinking micro-EDM at 120 volt setting using (a) 10 pf, (b) 100 pf 59 Figure 4.24 SEM images of surface after powder mixed die sinking micro-EDM at 120 volt setting using (a) 10 pf, (b) 100 pf 59 Figure 4.25 SEM images of surface after micro-EDM at 10 pf-80 volt setting using (a) die sinking, (b) powder mixed die sinking 60 Figure 4.26 EDX analysis of surface after die sinking micro-EDM at 10 pf-80 volt setting 60 Figure 4.27 EDX analysis of surface after powder mixed die sinking micro-EDM at 10 pf-80 volt setting 61 Figure 4.28 Variation of surface roughness with voltage and capacitance in powder mixed die sinking micro-EDM 61 Figure 4.29 AFM images of surface after die sinking micro-EDM at 47 pf-80 volt setting (a) surface profile, (b) three dimensional profile 62 Figure 4.30 AFM images of surface after powder mixed die sinking micro-EDM at 47 pf-80 volt setting (a) surface profile, (b) three dimensional profile 63 Figure 4.31 Comparison of variation of surface roughness with voltage at 47 pf capacitance in die sinking and powder mixed die sinking micro-EDM 64 Figure 4.32 Variation of distance between highest peak and lowest valley with voltage and capacitance in powder mixed die sinking micro-EDM 65 Figure 4.33 Cross section profile of maximum Peak to valley distance in (a) die sinking ,(b) powder mixed die sinking micro-EDM 66 Figure 4.34 Optical images of surface after powder mixed scanning micro-EDM at 120 volt setting using (a) 10 pf, (b) 100 pf 67 Figure 4.35 SEM images of surface after powder mixed scanning micro-EDM at 120 volt setting using (a) 10 pf, (b) 100 pf 67 Figure 4.36 SEM images of surface after micro-EDM at 100 pf-1000 volt setting using (a) powder mixed die sinking, (b) powder mixed scanning 68 XIII List of figures Figure 4.37 EDX analysis of surface after powder mixed die sinking micro-EDM at 100 pf-1000 volt setting 69 Figure 4.38 EDX analysis of surface after powder mixed scanning micro-EDM at 100 pf-1000 volt setting 69 Figure 4.39 Variation of surface roughness with voltage and capacitance in powder mixed die sinking micro-EDM 70 Figure 4.40 AFM images of surface after powder mixed die sinking micro-EDM at 10 pf-80 volt setting (a) surface profile, (b) three dimensional profile 71 Figure 4.41 AFM images of surface after powder mixed scanning micro-EDM at 10 pf-80 volt setting (a) surface profile, (b) three dimensional profile 71 Figure 4.42 Comparison of variation of surface roughness with voltage at 10 pf capacitance at powder mixed die sinking and powder mixed scanning micro-EDM 72 Figure 4.43 Variation of distance between highest peak and lowest valley with voltage and capacitance in powder mixed scanning micro-EDM 73 Figure 4.44 Cross section profile of maximum Peak to valley distance in (a) powder mixed die sinking, (b) powder mixed scanning micro-EDM 74 XIV Introduction Chapter 1 Introduction 1.1 Motivation Electrical Discharge Machining (EDM) is one of the most extensively used non conventional material removal processes. It uses thermal energy to machine electrically conductive parts regardless of hardness. In addition there is no direct contact between the electrode and workpiece. This characteristics of EDM eliminates mechanical stresses, chatter and vibration problems during machining. With growing demands for micro parts and the development of the micro electro mechanical system (MEMS), micro-EDM is becoming increasingly important. MicroEDM is considered as one of the most promising methods in terms of size and precision. It has advantage over other fabrication process, such as liga, laser, ultrasonic ion beam etc. because of its lower cost. Also the majorities of such non conventional processes are slower and limited in planar geometries. There have been several successful attempts in producing micro parts such as micro pins, micro nozzles and micro cavities using micro-EDM. In conventional EDM two types of pulse generators are used: RC type pulse generator and Transistor type pulse generator. The fabrication of parts smaller than several micrometers require minimization of pulsed energy supplied into the gap between the workpiece and electrode. This means that finishing by micro-EDM requires pulse 1 Introduction duration of several dozen nano seconds. Since RC type pulse generators can generate such small discharge energy simply by minimizing capacitance in the circuit, it is widely used in micro-EDM. However most of the surface study on EDM has been done using transistor type pulse generator. Therefore it is important to investigate the effect of RC type pulse generator in finishing and micro-machining operation since it is difficult to obtain significantly short pulse duration with constant pulse energy using transistor type pulse generators. While using RC type pulse generators, the variation of discharge energy can be done by using different gap voltage and capacitance setting. However at lower gap voltage, the gap between electrode and workpiece is very small. Therefore debris produced in EDM can not be flushed out properly, resulting in arcing and damaging the surface. Therefore the correct combination of voltage and capacitance is required for better surface finish. Two types of micro-EDM can be used in surface finish operation. In die sinking microEDM an electrode with micro features is employed to produce its mirror image in the workpiece. In scanning micro-EDM or micro-EDM milling, micro electrodes adopt a movement strategy similar to conventional milling and produce 3-D cavities. It is required to understand the effectiveness of both these processes in obtaining cavities with micro features. Surfaces produced by micro-EDM are generally known to have a matt appearance and required polishing before most practical applications. But apart from enormous time spent in polishing to reach at an acceptable finish, polishing sometimes become impossible in case of micro parts. This drawback can be overcome to a great extent through various changes in EDM procedures. Therefore scanning micro-EDM can be 2 Introduction used to observe any changes in surface finish of micro-parts relative to die-sinking micro-EDM. A new approach to a practical and efficient finish process comes in the form of machining in presence of suspended powder dispersed uniformly in the dielectric medium. Powders such as graphite, silicon and aluminium suspended in the dielectric have been found to generate fine surfaces over large working areas in high machining rate in case of conventional EDM. However the effectiveness of such a method in case of micro-EDM needs to be investigated. 1.2 Objectives of the research The aim of this project is to make a comprehensive study on the surface of tool steel and to investigate the parameters that results in better surface finish using die-sinking micro-EDM. Another purpose of this project is to find the feasibility of venturing variation in the micro-EDM process by employing tool movement to improve the surface finish. While pursuing this, other possibilities, such as the effect of using suspended powder in the dielectric in modifying surface roughness is also investigated. The following objectives are to be achieved in this study: • To perform die sinking micro-EDM to find out the parameters that results in better surface roughness. • To perform scanning micro-EDM to find out the parameters that results in good surface finish and to compare the results with die-sinking micro-EDM • To use suspended powder in dielectric and perform die sinking micro-EDM to investigate the surface characteristics and compare the results with die sinking micro-EDM without powder additives. 3 Introduction • To use suspended powder in dielectric and incorporate tool movement and investigate whether there is any improvement in surface finish and compare the results with suspended powder micro-EDM without the tool movement. 1.3 Organization of the thesis There are five chapters in this dissertation. In chapter 2, a comprehensive review is given, which includes historical background of EDM, principles of EDM, micro-EDM and its types, micro-EDM compared to other micro machining processes, key system components of micro-EDM, types of pulse generator, surface characteristics after EDM, characteristics of powder EDM process. Chapter 3 describes the experimental details. This is done in thrre parts. In the first part, details of the experimental set-up are first given. It also illustrates the details for the experiments done in micro-EDM, i.e., selection of tool and workpiece materials, the specifications of the dielectric used, the details for experiments done with powder suspended dielectrics such as powder specifications and properties and specifications of magnetic filter. The second part illustrates the experimental procedure followed throughout the course of study. It gives a brief description of electrode dressing, a summary of the different machining parameter settings and machining details used throughout the experiments. The third part gives brief descriptions of the different measuring equipment used. Chapter 4 describes the results obtained from the experiments done without powder additives and with powder additives using die-sinking and scanning micro-EDM and discuses these results. This gives detailed analysis of surface topography, surface 4 Introduction roughness and peak to valley height of machined surfaces. This also highlights the parameters that results in better surface finish. Chapter 5 summarizes the conclusions derived from the experimental analysis and guides for possible future work that can be incorporated. 5 Literature Review Chapter 2 Literature Review 2.1 Introduction Electrical discharge machining (EDM) is potentially an important, well-established and cost-effective method for manufacturing geometrically complex or hard material parts that are extremely difficult-to-machine by the conventional machining processes. The non-contact machining process has been endlessly evolving from a mere tool and dies making process to a micro-scale application machining. In micro-EDM the discharge energy is reduced in order to minimize the unit material removal. Since micro-EDM provides such advantages as the ability to manufacture complicated shapes with high accuracy, and can process any conductive materials regardless of hardness, it has become one of the most important methods for manufacturing microfeatures and parts with sub-micrometer order size. However, a number of issues remain to be solved before micro-EDM can become a reliable process with repeatable results and its full capabilities as a micro-manufacturing technology can be realised. This chapter gives an overview of the whole EDM process, then focuses on studies on surface characteristics by using EDM and also focuses on the different characteristics of EDM with powder mixed dielectric. Section 2.2 gives a brief history of EDM. In section 2.3, principles of EDM process is illustrated while in section 2.4, the different types of micro-EDM are discussed. Section 2.5 compares micro-EDM with other micro-machining processes. Key system components of micro-EDM and types of pulse generators have been discussed in sections 2.6 and 2.7 respectively. Section 2.8 deals 6 Literature Review with the surface characteristics after EDM, while section 2.9 discusses the characteristics of powder EDM process. 2.2 Historical Background of EDM In 1770, English chemist Joseph Priestly came to discover that electrical discharge or sparks had erosive effects and it is believed to be the basis of EDM (Kalpajian and Schmid, 2003). In 1943, B.R. Lazarenko and N.I. Lazarenko at the Moscow University were able to use this sparks and developed resistance-capacitance type power supply to use with Lazarenko EDM System, which was able to machine difficult to machine materials in a controlled process by vaporizing material from workpiece surface (Webzell, 2001). This RC type power supply was extensively used at EDM machines in 1950s. At about the same time three American employees used electrical discharges to remove broken taps and drills from hydraulic valves. They were able to use electronic-circuit servo system which maintained space between electrode and workpiece automatically for sparks to occur (Jameson, 2001). In 1980, the introduction of CNC in EDM has automated the EDM process. Therefore after inserting the electrodes in tool changer, there is no requirement to monitor the process till the final product is ready (Houman, 1983). Since then EDM has been used in manufacturing industries and has become an issue of research. Through the years, the machines have improved drastically – progressing from RC (resistor capacitance or relaxation circuit) power supplies and vacuum tubes to solid-state transistors with nanosecond pulsing, from crude hand-fed electrodes to modern CNC-controlled simultaneous six-axis machining. 7 Literature Review 2.3 Principles of EDM Electrical discharge machining (EDM) is based on the material erosion of electrically conductive materials. It is given through the series of spatially discrete high-frequency electrical discharges (sparks) between the tool and the workpiece (Llanes et al., 2001). When sparks are generated the electrode materials will erode and in this way material removal is realized. Every discharge (or spark) melts a small amount of material from both of them. Part of this material is removed by the dielectric fluid and the remaining solidifies on the surface of the electrodes. The net result is that each discharge leaves a small crater on both workpiece and tool electrode (Allen et al., 1996). In the EDM process, as the electrode charged with a high-voltage potential, come close to the workpiece, an intense electromagnetic flux or ‘energy column’ is formed and eventually breakdown the insulating properties of the dielectric fluid (Guitrau, 1997). The voltage then drops as current is produced, and the spark vaporizes anything in contact with it, including the dielectric fluid. The area struck by the spark will be vaporized and melted, resulting in crater being formed. Thus metal is predominantly removed by the effect of intense heat locally generated and collapse of the vaporized dielectric. Melting and vaporization actions are the causes of removing material in the EDM process. 8 Literature Review 2.4 Micro-EDM and its types The basic mechanism of the micro-EDM process is essentially similar to that of the EDM process with the main difference being in the size of the tool used, the power supply of discharge energy and the resolution of the X-, Y- and Z-axis movement (Masuzawa, 2000). In micro-EDM the discharge energy is reduced to the order of 10-6 to 10-7 Joules in order to minimize the unit material removal. Current micro-EDM technology used for manufacturing micro-features can be categorized into four different types (Pham et al., 2004): Die-sinking micro-EDM, where an electrode with micro-features is employed to produce its mirror image in the workpiece. Micro-wire EDM, where a wire of diameter down to 0.02mm is used to cut through a conductive workpiece. Micro-EDM drilling, where micro-electrodes (of diameters down to 5–10μm) are used to ‘drill’ micro-holes in the workpiece. Micro-EDM milling, where micro-electrodes (of diameters down to 5–10μm) are employed to produce 3D cavities by adopting a movement strategy similar to that in conventional milling. There is another variant of micro-EDM called micro- WEDG (wire electro-discharge grinding) in which grinding is done using EDM mechanism. An overview of the capabilities of micro-EDM is provided in the table 2.1 (Masuzawa, 2000): 9 Literature Review Table 2.1 Overview of the micro-EDM varieties Micro-EDM Geometric Minimum Maximum Surface quality variant Complexity feature size aspect ratio Ra (µm) Drilling 2D 5 µm ~ 25 0.05 – 0.3 Die-sinking 3D ~ 20 µm ~ 15 0.05 – 0.3 Milling 3D ~ 20 µm ~ 10 0.5 – 1 WEDM 2½D ~ 30 µm ~ 100 0.1 – 0.2 WEDG Axi-sym. 3 µm 30 0.8 2.5 Micro-EDM compared to other Micro-machining processes 2.5.1 Advantages of micro-EDM over other micromachining processes Compared to traditional micromachining technologies micro-EDM has several advantages such as (Reynaerts et al., 1997): EDM requires a low installation cost compared to lithographic techniques. EDM is very flexible, thus making it ideal for prototypes or small batches of products with a high added value. EDM requires little job overhead (such as designing masks, etc.). EDM can easily machine complex (even 3D) shapes. Shapes that prove difficult for etching are relatively easy for EDM. 2.5.2 Compatibility of micro-EDM with other micromachining processes Another aspect of comparison between the micromachining technologies is of course the compatibility of the machining technology with the material to be machined. Micro-EDM has the advantage of machining of all types of conductive materials such as: metals, metallic alloys, graphite, or even some ceramic materials, of whatsoever 10 Literature Review hardness (Masuzawa, 2000). The following table gives an overview of the compatibility of the different micromachining technologies (Reynaerts et al., 1997). Table 2.2 Compatibility of machining technologies with different materials Micromachining Technology Feasible Materials LIGA metals, polymers, ceramic materials Etching metals, semiconductors Excimer-LASER metals, polymers, ceramic materials Micro-milling metals, polymers Diamond cutting Non-Ferro metals, polymers Micro-stereolithography Polymers Micro-EDM metals, semiconductors, ceramics 2.6 Key system components of micro-EDM Micro-EDM is a version of the conventional die-sinking EDM. Initially, this type of equipment was used as a slicing machine for thin-walled structure. With the help of computer numerical control, complex shapes can be cut without using special electrodes. The narrow spark gap and dimensional accuracy of the process make it possible to provide close fitting parts. A typical micro-EDM set-up consists of the following parts: Controller circuit Main spindle unit Workpiece holder and base Dielectric fluid circulation unit 11 Literature Review 2.7 Types of pulse generator The controller circuit can have two types of pulse generators – Resistance-Capacitance (RC) or Relaxation type and Transistor type pulse generator. Based on the research by Han et al. (2004) and review by Kunieda et al. (2005) a description is given here on these different types of pulse generators. Two kinds of pulse generators: relaxation or RC type pulse generator and transistor type pulse generator shown in Figures 2.1 (a) and (b) respectively. (a) (b) Figure 2.1 Different types of pulse generators (a) RC type and (b) Transistor type The fabrication of parts smaller than several micro meters requires minimization of the pulse energy supplied into the gap between the workpiece and electrode. This means that finishing by micro-EDM requires pulse duration of several dozen nanoseconds. Since the RC pulse generator can generate such small discharge energy simply by minimizing the capacitance in the circuit, it is widely applied in microEDM. RC pulse generator results in extremely low material removal rate from its low discharge frequency due to the time needed to charge the capacitor. 12 Literature Review The transistor type pulse generator is on the other hand widely used in conventional EDM. Compared with the RC pulse generator, it provides a higher removal rate due to its high discharge frequency because there is no need to charge any capacitor. Moreover, the pulse duration and discharge current can arbitrarily be changed depending on the machining characteristics required. However, the relaxation type pulse generators are used in finishing and micromachining because it is difficult to obtain significantly short pulse duration with constant pulse energy using the transistor type pulse generator. If the transistor type is used, it takes at least several tens of nano-seconds for the discharge current to diminish to zero after detecting the occurrence of discharge because the electric circuit for detecting the occurrence of discharge, the circuit for generating an output signal to switch off the power transistor and the power transistor itself have a certain amount of delay time. Hence, it is difficult to keep the constant discharge duration shorter than several tens of ns using the transistor type pulse generator. But with RC type pulse generator, uniform surface finish is difficult to obtain because the discharge energy varies depending on the electrical charge stored in the capacitor before dielectric breakdown. 13 Literature Review 2.8 Surface characteristics after EDM There has been extensive use of EDM technology in tool, die and mould making industries, which requires tool steels with high precision and better surface finish, since conventional machining cannot machine these difficult-to-cut materials economically (Singh et al., 2004) Amorim et al.(2003) conducted experiments on AISI P200 tool steel with a view to analyze material removal rate, volumetric relative wear and workpiece surface roughness by varying discharge current, discharge duration, pulse interval time, gap voltage, electrode polarity and generator mode. It shows that negatively polarized electrode results in better surface finish. Singh et al. (2004) carried out experiments on En-31 tool steel with different electrodes and reported the effects of pulsed current on material removal rate, overcut, electrode wear and surface roughness. The results revealed that output parameters of EDM increases with an increase in pulse current. Guu (2005) worked on AISI D2 Tool steel to analyze surface roughness (Ra) by varying pulsed current and pulse on duration. AFM study of the specimen shows that the higher discharge energy, the poorer the surface finish. It was reported that surface roughness varied in a range of 103-172 nm, whereas peak to valley height varied in a range of 1272-1873 nm. Puertas and Luis (2003) used a prismatic electrode made of copper with a square cross-section of 25mm × 25mm to perform EDM on soft steel.(F-1110). It was observed, the pulse on time (ti) and pulse off time (to) have little influence on the variation of surface roughness (Ra). On the contrary, the discharge current intensity (I) has a great deal of influence on the Ra parameter. Moreover, there is a great decrease of Ra when 14 Literature Review increasing the I parameter and the same tendency but in a much lower rate can be considered for the ti parameter. This behavior is just the opposite of what is expected but considering that the variation range of Ra is a narrow one, it is considered that this effect is due to a better arc stability causing more uniform pulses. There is an interaction between the I factor and the ti factor. The best Ra value will be obtained when increasing the I factor and maintaining the ti factor at its low value. The copper electrode was kept at positive polarity and the specimen was kept at negative polarity during the EDM process of AISI D2 tool steel (Guu et al., 2003). EDM surfaces were measured. The surface roughness on the machined surface varied from 1.3 to 11.0 µm. The results exhibit that a higher pulse current causes a poorer surface finish. An excellent machined finish can be obtained by setting the machine parameters at a low pulse current and small pulse-on duration. The pulse current has a more dominant effect on the surface roughness compared to the pulse-on duration. It can be attributed to the fact that a higher pulse current may cause more frequent cracking of the dielectric fluid, there is more frequent melt expulsion resulting in the poorer surface finish. Ghanema et al. (2003) reported that surfaces obtained by EDM of X155CrMoV12 steel exhibit a characteristic aspect consisting of the superposition of craters due to metal evaporation during machining. No evidence of preferential material removal directions was observed (no ridges were seen).The metal ejected by erosion is resolidified and redeposited at the surface as spheroidal particles of different sizes. The evolution of the surface roughness is mainly affected by the machining intensity. Qualitative energy dispersive X-ray (EDX) analysis, performed in the SEM on specimen surfaces of hardenable steels before and after EDM. Comparison between these profiles clearly shows enrichment in carbon and hydrogen in the outer layer of EDM specimens. This phenomenon is due to the diffusion of carbon and hydrogen 15 Literature Review atoms produced by the evaporation of the dielectric liquid and the graphite of the tool. This evaporation occurs during the electric discharge. Lee et al. (1988) showed that EDMed surfaces usually have a matt appearance covered by shallow craters, globules of debris, and pockmarks formed by entrapped gases escaping from the redeposited material. After EDM of AISI D2 tool steel it has been found that at low discharge energy, the craters are shallow and the density of global appendages and pockmarks is low. Whereas at higher discharge energy, the craters are deeper and global appendages are most evident. These global appendages are molten metals which were expelled randomly during the discharge and later solidified on the workpiece surfaces. The edge of some of the pockmarks is thin and sharp suggesting that the molten metal must have solidified at an extremely high rate. For otherwise the surface tension of the molten metal would have caused the sharp edges to be rounded off. Another common feature on EDMed surfaces is the abundance of cracks, especially at high discharge energies. These cracks are formed as a result of the exceedingly high thermal stresses prevailing at the specimen surface as the latter was cooled at a fast rate after the discharge. It is interesting that under identical discharge conditions, the surface roughnesses of all the steels are quite similar. Their values, however, depend on both the pulse energy and current in a complex manner. By and large, at a given pulse energy, a poorer surface finish is obtained with a higher pulse current. This implies that the pulse current has a more dominant effect on the surface roughness compared to the pulse-on duration. It is possible that a higher pulse current (or input power) may cause more frequent cracking of the dielectric, giving rise to more frequent melt expulsion. This in turn will result in a higher density of global appendages and poorer surface finish. 16 Literature Review The surface texture in EDM is characterized by discharge craters. The surface roughness is therefore mainly determined by the electric pulse parameters: discharge current, discharge duration, and polarity. Very fine roughness and shiny surfaces are hard to obtain directly by sinking EDM. Smooth or shiny surfaces (with Ra < 1 µm) can only be obtained with low discharge energy and inverse polarity (with the tool electrode as cathode), resulting in smooth surface craters (Bleys et al., 2006).When the tool is kept at negative polarity, it is exposed to more heat than workpiece. Optimal pulse duration produces sufficient heat for controlled melting of workpiece and removes fine amounts of material resulting in better surface appearance (Wong et al. 1998).Wu et al. (2005) reported that discharge current in EDM is composed of ions and electrons. With increase in pulse duration time, the proportion of ion flow increases. So if the workpiece is kept at negative polarity the ion flow will impact violently on the workpiece resulting in poor surface roughness. 2.9 Characteristics of powder EDM process There have been various attempts to improve the surface finish after EDM by polishing and other means. But if it is possible to improve the surface during machining it will shorten the machining time. From this viewpoint powder EDM is one of these processes. However suspended powder EDM not only imparts fine surface finish but also modifies the surface. However, it is necessary to understand the mechanism of powders EDM process. 2.9.1 Mechanism of Powder EDM process Suspended powders increase the spark gap distance due to their presence between tool and workpiece. It has two outcomes: firstly, increased spark gap is useful in effective 17 Literature Review removal of debris from the gap; secondly, it makes the powder EDM process highly stable with effective discharge dispersion throughout the gap. An increase in the distance decreases the electrostatic capacitance of the gap .Therefore, it becomes possible to discharge micro-current at potential micro-peaks throughout the work surface. To prove this phenomenon, the electrode was divided into two units of equal areas and connected to two current transducers. Waveforms were monitored on oscilloscope (CRT).In case of powder EDM, discharge was observed in both electrodes at the same time. But in normal EDM almost all discharges occur at only one of the two electrodes at any point in time. Efficient discharge dispersion not only forms uniform work surface but also prevents the occurrence of concentrated arc discharge and hence reduces finishing time. Suspended powder EDM reduces machining time significantly for fine finishing compared to normal EDM methods. It has been reported that to obtain 5 µm Rmax on SKH-51 tool steel, it requires 5hrs in normal EDM while only 25 minutes in EDM with Al powder suspension. The spark gap distance depends on powder concentration, type of powder and electrical condition. In general an increase in powder concentration tends to increase the spark gap distance. Al powder suspension of 30g/l increases the spark gap by factor of ten than normal EDM (Narumiya et al., 1989). Zhao et al. (2002) reported that powder particles in the gap are conducting and the role these powders play causes aberration in electric field. Under applied gap voltage, positive and negative charges gather on to the top and bottom of powder particles respectively. When the electric field density surpasses the breakdown voltage, discharge breakdown starts at the points which are near to the top or bottom as these points have higher electric charge density. It results in short circuit between two powder particles and causes redistribution of electrical charges. More electric charges gather at points which are near to tool and workpiece. It 18 Literature Review results in series discharge between two powders and other powders and accordingly the discharge breakdown between tool and workpiece. This enlarges the discharge gap. Moreover electrodes and ions collide with powder particles and release more electrons and ions. This results in increase in carriers (ions and electrons) and widening of the discharge passage. This widened discharge passage enlarges the area that is influenced by discharge heat resulting in reduced discharge density. This improves the surface finish. Yih-fong and Fu-chen (2005) described the material removal mechanism and mentioned the different factors that affect the surface roughness in powder EDM. In case of normal EDM, working fluid evaporates and creates gas explosion that results in mechanical thrust. In case of powder EDM the striking impact of powder particles is added with this mechanical thrust to remove materials. However the grinding effects of particles are negligible in case of material removal. It is reported that smallest particle yields best surface finish as they produce fine cutting effects. Low electrical resistivity, high thermal conductivity and low density of the powder particles are essential for better surface finish. Due to low resistivity higher spark gap is created and for high thermal conductivity more heats are taken away resulting lower discharge density. Lower discharge density generates low gas explosion and shallow craters are produced on the surface. Moreover low density particles result in low explosive impact on the surface resulting better surface finish. Narumiya et al. (1989) reported that with the increase in powder concentration the discharge gap also increases. But after a certain concentration, the gap distance does not increase with further increase in concentration. An excessive amount of powder additives is likely to display similar characteristics as an excessive accumulation of debris resulting in short circuit and thus damaging the machined surface and deteriorating machining stability. 19 Literature Review 2.9.2 Influence of Powder EDM in surface roughness Erden and Bilgin (1980) added powdered form of copper, iron, aluminium and carbon into commercial kerosene oil as impurities. Brass-steel and copper-steel pairs were used as electrode and workpiece. The results indicated that added powder improves the breakdown characteristics of dielectric fluid. Machining rate increases with the increase in powder concentration as there is a decrease in time lags at high purity concentration. However excessive powder concentration makes the machining unstable as there occurs short-circuit. Impurity concentrations improved the surface quality and gap size. Jeswani (1981) added graphite powder into kerosene oil at a concentration of 4g/l and reported that the interspace for electric discharge initiation was increased while breakdown voltage was lowered. An improvement in machining process stability resulted in 60% increase in MRR and 28% reduction in WR. Mohri et al. (1991) added silicon powder of 10-30µm size uniformly in dielectric fluid and performed machining at discharge current of 0.5-1 A for discharge duration less than 3 µs using negative polarity. It was able to produce surface with roughness less than 2 µm. However to achieve this performance even distribution of debris and short discharge time are required. Narimuya et al. (1989) reported that Aluminium and Graphite powder generates better surface roughness than silicon powder at specific machining conditions .Aluminium and graphite powder having diameter less than 15µm and at a concentration of 2 to 15 g/l yield surface roughness of less than 2 µm. Kansal et al. (2005) reported that the concentration of powder should be 2g/l to obtain better surface finish. Kobayashi et al. (1992) reported that silicon powder improves the surface finish of SKD-61 tool steel. Yan and Chen (1994) added aluminium and silicon carbide powder with dielectric and performed EDM on SKD-11 and Ti-6Al-4V and reported 20 Literature Review improvement in MRR and increase in surface roughness. Uno and Okada (1997) reported that silicon powder mixed with dielectric reduces the impact force acting on the workpiece and results in smaller undulation of craters, hence glossy surface. Wong et al. (1998) used fine powders of silicon, graphite, molybdenum, aluminium and silicon carbide with dielectric .Aluminium powders produce mirror finish on SKH-51 workpiece but failed to produce mirror finish on SKH-54 material. Rather semiconductive silicon and carbon powders were able to produce very fine finish condition. It was observed that electrode polarity (negative), pulse parameter and powder characteristics play a major role in producing very fine finish. Rahuman (1994) reported that with low powder feed rate (2 ml/s) graphite powder at a concentration of 2 gm/l was able to yield Ra value of 0.0931 µm on SKH-54 tool steel. Wang et al. (2001) mixed Al and Cr powder in kerosene fluid which reduces isolation, increases the gap between tool and workpiece and thus stabilize the process. As small size particles have higher concentration, they would have higher suspension effect in dielectric fluid. Therefore there is higher probability that the gap will be bridged and uniform discharge dispersion will result in good surface finish. Chow et al. (2000) added SiC and Aluminium powders into kerosene to machine micro-slit on titanium alloy by EDM. The addition of powders increased the inter-electrode gap that facilitates debris removal and also causes extension of slit expansion. Moreover uniform dispersion of discharge results in several discharge trajectories from single input impulse. Therefore several discharging spots are created resulting in increased MRR and better surface finish. It was also observed that material removal depth is better using SiC than using aluminium powder. Tzeng and Lee (2001) added Al, Cr, Cu and Sic with working fluid and performed EDM on SKD-11 material. It was reported that machining performance is significantly affected by concentration, size, 21 Literature Review density, electrical resistivity and thermal conductivity of added powder. It was found that for a fixed concentration, highest MRR was achieved with smallest size particles. Moreover it was also revealed that due to its higher density Cu powder has almost no effect in EDM. Pecas and Henriques (2003) added silicon powder at a concentration of 2g/l and found that operating time and surface roughness decreases. It was also reported that average surface roughness depends on machining area and machining time. The variation in surface roughness is in the range of 0.09 to 0.57 µm for the area range of 1-64 cm2. 2.9.3 Influence of Powder EDM in surface modification Powder EDM also modifies the surface properties. These modifications result in high corrosion resistance, high oxidation temperature and high wear resistance. EDM with silicon electrode produces fine surface structure in stainless steel workpieces and hence imparts new properties. In plastic deformation test it has been revealed that EDMed surface of stainless steel has a strong bonding with the parent surface (Mohri et al., 1988).There have been reports that specimens machined in graphite, silicon and aluminium powder suspensions exhibits low level of corrosion compared to those machined in normal working fluid. The results have been confirmed by tests in aqua regia. The absence of heat affected areas in powder mixed EDM are mainly responsible for this effect. The surfaces generated in powder EDM are found to withstand hard corrosion and wear. Uno et al. (1998) observed that nickel powder mixed with working fluid deposits a layer on EDMed surface of aluminium bronze components and makes the surface abrasion resistant. Okada et al. (2000) performed EDM with titanium electrode and silicon powder mixed dielectric and was able to form a thick and smooth layer of titanium carbide on the workpiece. The hardness and wear 22 Literature Review resistance of the coated layer tend to be much higher than that of base material. Furutani et al. (2001) used Titanium powder of size less than 36 µm at a concentration of 50g/l with EDF-K (Mitsubishi oil).Gap voltage was maintained at 320V with negative polarity. A smooth layer of titanium carbide accretion was observed with thickness of 150 µm and hardness of 1600 Hv on surface of carbon steel with an electrode of 1 mm diameter. 2.10 Conclusion SKH-51 tool steel has high wear resistance compared to conventional mold materials for processing thermoplastic, thermosetting or glass-fiber resins. These molds can be used several times compared to conventional products. Moreover mold maintenance requirements are reduced and the difference in total cost becomes significant when small parts are fabricated. However for micro slots and micro molds in tool steels, the optimum parameters for surface roughness using micro-EDM are not completely investigated yet. Moreover most of the works in EDM have been carried out using transistor type pulse generator to find the effect of discharge energy on the surface roughness. Since RC type pulse generator can produce very small discharge energy it is used in finishing operation. Therefore it is necessary to investigate the effect of RC type pulse generator on the surface characteristics of SKH-51 tool steel. Moreover in conventional EDM most of the work has been done using die sinking EDM. In die sinking EDM, the tool electrode has the complementary form of the finished workpiece and literally sinks into the workpiece. On the other hand, scanning EDM, more commonly known as EDM milling, is a newer process which is mainly 23 Literature Review used when large and complex geometries are required. In contrast to die-sinking EDM, scanning EDM eliminates the need for complex-shape electrodes. In this process, usually tubular or cylindrical micro-electrodes are employed to produce the desired complex shape by scanning. A cylindrical electrode rotates around its axis (Z-axis) with the scanning movements in X and Y directions. The contour of a particular layer is specified in the part program of CNC. However, electrode compensation is an important factor to consider as electrode length is reduced after scanning every layer. This study uses a common wear compensation method for layer-by-layer milling EDM indicated as “anticipated wear compensation” as shown in Fig.2.2 (Bleys et al., 2004). Electrode wear was compensated by down-motion (sW) of the electrode. When machining with a cylindrical electrode, such a continuous down-motion of the electrode, combined with occurring tool wear, causes a rapid stabilization of the electrode shape thus maintaining steady-state shape of the tool electrode tip. Figure 2.2 presents the schematic diagram of the scanning EDM process. So the effectiveness of scanning micro-EDM in modifying surface roughness is also investigated. Figure 2.2 Schematic representation of layer-by-layer scanning µEDM Following the results of studies conducted on the use of powder suspended dielectrics for conventional EDM, which indicated the possibility of reducing surface roughness while ensuring a limited increase in gap distance through the use of selected semi- 24 Literature Review conductive and non-conductive powder materials, an extension of this technique to micro-EDM is essential. Such work is necessary since the introduction of nanopowders suspended in dielectric is not well understood. In micro-EDM, studies on the use of powder suspended dielectrics for machining is limited. Tan et al. (2007) reported that using silicon carbide (particle size 45-55 nm) and aluminium oxide (particle size 40-47 nm) in the micro-EDM of AISI 420 stainless mould steel, the smallest Ra is achieved at about 0.04 g/l for both SiC ([...]... variation of surface roughness with voltage at 10 pf capacitance at powder mixed die sinking and powder mixed scanning micro- EDM 72 Figure 4.43 Variation of distance between highest peak and lowest valley with voltage and capacitance in powder mixed scanning micro- EDM 73 Figure 4.44 Cross section profile of maximum Peak to valley distance in (a) powder mixed die sinking, (b) powder mixed scanning micro- EDM. .. voltage and capacitance in powder mixed die sinking micro- EDM 70 Figure 4.40 AFM images of surface after powder mixed die sinking micro- EDM at 10 pf-80 volt setting (a) surface profile, (b) three dimensional profile 71 Figure 4.41 AFM images of surface after powder mixed scanning micro- EDM at 10 pf-80 volt setting (a) surface profile, (b) three dimensional profile 71 Figure 4.42 Comparison of variation of. .. images of surface after powder mixed die sinking micro- EDM at 47 pf-80 volt setting (a) surface profile, (b) three dimensional profile 63 Figure 4.31 Comparison of variation of surface roughness with voltage at 47 pf capacitance in die sinking and powder mixed die sinking micro- EDM 64 Figure 4.32 Variation of distance between highest peak and lowest valley with voltage and capacitance in powder mixed. .. SEM images of surface after micro- EDM at 100 pf-1000 volt setting using (a) powder mixed die sinking, (b) powder mixed scanning 68 XIII List of figures Figure 4.37 EDX analysis of surface after powder mixed die sinking micro- EDM at 100 pf-1000 volt setting 69 Figure 4.38 EDX analysis of surface after powder mixed scanning micro- EDM at 100 pf-1000 volt setting 69 Figure 4.39 Variation of surface roughness... three dimensional profile 53 Figure 4.17 AFM images of surface after scanning micro- EDM at 100 pf-120 volt setting (a) surface profile, (b) three dimensional profile 53 Figure 4.18 Surface profiles of AFM images of surface at 10 pf-80 volt setting after micro- EDM in (a) die sinking, (b) scanning 54 Figure 4.19 Variation of surface roughness with voltage and capacitance in scanning micro- EDM 55 Figure... Figure 4.20 Comparison of variation of surface roughness with voltage at 10 pf capacitance at die sinking and scanning micro- EDM 56 Figure 4.21 Variation of distance between highest peak and lowest valley with voltage and capacitance in scanning micro- EDM 56 XII List of figures Figure 4.22 Cross section profile of maximum Peak to valley distance in (a) die sinking ,(b) scanning micro- EDM 57 Figure 4.23... setting (a) surface profile, (b) three dimensional profile 45 Figure 4.7 Variation of surface roughness with voltage and capacitance in die sinking micro- EDM 46 Figure 4.8 Peak-to-valley distance across the line drawn on the surface profile using 100 pf and 120 volt in die sinking micro- EDM (a) surface profile, (b) cross section profile 47 Figure 4.9 Variation of distance between highest peak and lowest... before micro- EDM can become a reliable process with repeatable results and its full capabilities as a micro- manufacturing technology can be realised This chapter gives an overview of the whole EDM process, then focuses on studies on surface characteristics by using EDM and also focuses on the different characteristics of EDM with powder mixed dielectric Section 2.2 gives a brief history of EDM In section... machining conditions Aluminium and graphite powder having diameter less than 15µm and at a concentration of 2 to 15 g/l yield surface roughness of less than 2 µm Kansal et al (2005) reported that the concentration of powder should be 2g/l to obtain better surface finish Kobayashi et al (1992) reported that silicon powder improves the surface finish of SKD-61 tool steel Yan and Chen (1994) added aluminium and... growing demands for micro parts and the development of the micro electro mechanical system (MEMS), micro- EDM is becoming increasingly important MicroEDM is considered as one of the most promising methods in terms of size and precision It has advantage over other fabrication process, such as liga, laser, ultrasonic ion beam etc because of its lower cost Also the majorities of such non conventional processes