CHAPTER 2 LITERATURE SURVEY

Transcription

1 20 CHAPTER 2 LITERATURE SURVEY 2.1 INTRODUCTION EDM is one of the emerging technologies for micro-fabrication methods on which research attempts are being made around the world in the scientific community to explore its potential. After going through related literature, the survey is broadly classified into three different categories such as (i) Machining performance (ii) Micro-hole machining by varying EDM Specifications and (iii) Process optimization. 2.2 MACHINING PERFORMANCE Chen et al. (1999) have investigated the machining characteristics of Ti 6A1 4V with kerosene and distilled water as the dielectrics. The results show that the MRR is greater and the relative electrode wear ratio is lower when machining in distilled water rather than in kerosene. By using X-ray diffraction, it is confirmed that Titanium carbide (TiC) and Titanium oxide (TiO) are formed on the workpiece surface when using kerosene and distilled water, respectively. The lower removal rate when machining Ti 6A1 4V alloy in kerosene can be explained by both the formation of TiC, which has a higher melting temperature and therefore requires a larger discharge energy, and carbon deposition on the electrode, causing further retardation of the discharge process. A larger amount

2 21 of debris and more micro cracks are also found when using distilled water as the dielectric. Yeo et al. (1999) investigated the current attempts to improve micro hole drilling. The focus is to develop a novel technique that enhances the capabilities of Micro-EDM for realization of micro holes with a high aspect ratio. Based on theories in fluidization engineering and ultrasonic degassing, a method of introducing ultrasonic vibrations into Micro-EDM processes is conceptualized and developed. A comparative analysis has revealed that the new technique can produce a significant increase in the aspect ratios over the current methods of micro hole drilling. The potential of the capabilities of ultrasonic vibrations in Micro-EDM drilling is thus emphasized. Masuzawa (2000) has summarized the basic concepts and applications of major methods of micromachining. The basic characteristics of each group of methods are discussed based on different machining phenomena. Capable methods are introduced in detail hinting at suitable areas of application. Finally, the present state of micromachining technologies is explained with examples of experimental and practical applications. Wansheng et al. (2002) have introduced ultrasonic vibration into micro-edm and analyzes the effect of ultrasonic vibration on the EDM process. A four-axis EDM machine tool which combines ultrasonic and micro-edm has been developed. With a cylindrical tool electrode, made of hard carbide, which has high stiffness, a single-side notch is made along the electrode. Ultrasonic vibration is then introduced into the micro-edm. Experiments have been carried out and results have shown that holes with a diameter of less than Ø0.2 mm and a

3 22 depth/diameter ratio of more than 15 can be drilled steadily using this equipment and technology. Yan et al. (2002) adopted a machining method that combined Micro Electrical Discharge Machining (MEDM) and Micro UltraSonic Machining (MUSM). They showed that the Diameter Variation between the Entrance and Exit (DVEE) could reach a value of about 2 mm in micro-holes with diameters of about 150 mm and depth of 500 mm. They obtained micro-holes with a roundness value of about 2 mm (the maximum radius minus the minimum radius). Kaminski and Capuano (2003) have reported that the parameters affecting the micro holes machining process when there are diameters smaller than 0.1 mm and aspect ratio bigger than 20 used by electro-erosion penetration process in sheets. And circularity deviation provided by the experiment is smaller than 0.01 for 0.1 mm diameter holes. Wong et al. (2003) have investigated the micro-edm material removal characteristics using single RC-pulse discharges. They have reported that the estimated erosion efficiency of material removal at low-energy (<50 J) discharges is found to be seven to eight times higher than that at higher-energy discharges. They have also observed that the volume and size of the microcraters are more consistent at lower energy discharge than at higher energy discharges. A single discharge erosion RC circuit with DC voltage power supply (0-300V) is developed.

4 23 Tsai and Masuzawa (2004) have discussed the machining accuracy of the EDM process. Although EDM is an efficient machining process for the fabrication of a micro-hole, tool wear leads to significant problem resulting in deterioration in the machining accuracy. The machining accuracy of EDM is limited by tool wear, which is unavoidable consequence in EDM process because the sparks generated for the machining remove the part of the electrode simultaneously. The tool wear is characterized by corner and end wear which mean tool material removal in radial and axial directions, respectively. Therefore, the tool wear deteriorates both the depth and the shape of a machined hole in EDM drilling. In particular, this problem becomes intensified in the fabrication of a blind hole. Electrode wear is inversely related to the melting point of the material used as the electrode that is the higher the melting point, the lower the tools wear. Singh et al. (2004) discuss the evolution of effect of the EDM current, pulse on-time and flushing pressure on MRR, TWR, Taper, ROC, and surface roughness on machining as-cast Al-MMC with 10% SiCp and use of metal matrix composites. ELEKTRAPLUS spark erosion machine is used for the purpose and jet flushing of the dielectric fluid, kerosene, is employed. Brass tool of diameter 2.7mm is chosen to drill the specimens. An L 27 OA, for the three machining parameters at three levels each, is opted to conduct the experiments. ANOVA is performed and the optimal levels for maximizing the responses are established. Scanning Electron Microscope (SEM) analysis is done to study the surface characteristics. Kunieda et al. (2005) aims to show the prospects of EDM technology by interrelating recent achievements in fundamental studies on EDM with newly developed advanced application technologies. Although gap phenomena in the

5 24 EDM are very complicated and hence not yet very well understood, recent improvements in computers and electronic measuring instruments are contributing to new discoveries and inventions in the EDM technology. Such newly acquired insight sometimes raises questions on the validity of the established theories of the EDM phenomena, and the EDM processes once believed to be impossible or unrealistic are now becoming practical. Rajurkar et al. (2006) have reported that there are worldwide technical developments and the state-of-the-art of electro-physical and chemical micromachining processes. Issues related to the supporting technologies such as standardization, metrology and equipment design are briefly discussed. Additionally non-technological issues including environmental effects and education are also discussed. Weng (2006) investigates the manufacturing of micro-parts with an array of micro holes using the machined micro-graphite copper electrode in the EDM. This electrode is fabricated by using electrochemical process of anodic etching combined with electroforming process. He indicates that yielding parts can be utilized as a micro-water spray nozzle in the biotechnology applications. Han et al. (2006) have developed a new transistor type iso-pulse generator for micro-edm. Their experiments show that the machining characteristics prove that the transistor type iso-pulse generator is suitable for micro-edm. Their experimental results reveal that the transistor type pulse train generator is unsuitable for micro-edm due to its low removal rate. The removal rate of the transistor type isopulse generator is two or three times higher than that of the traditional RC pulse generator.

6 25 Son et al. (2007) have investigated about the influences of EDM machining conditions on micro-edm characteristics. The pulse condition is particularly focused on the pulse duration and the ratio of off-time to on-time, and the machining properties are reported on tool wear, MRR, and machining accuracy. Their experimental results show that the voltage and current of the pulse exert strongly to the machining properties and the shorter EDM pulses is more efficient to make a precision part with a higher MRR. Bhattacharyya et al. (2007) have studied the white layer thickness which is formed on the machined surface due to intense heat generation in the EDM process. Li et al. (2007) have discussed the latest technologies in micromachining. Based on the study, they have concluded that challenges still exist in the fields like innovation in workpiece and tool material, and development of auxiliary tooling equipment together with processes combination. Nakaoku et al. (2007) have investigated about the basic characteristic of micro-edm by machining micro holes in sintered diamond. They have reported that micro-holes with a diameter of 50 m can be machined in sintered diamond and machining characteristic is similar to the tungsten carbide alloy. In addition, they have observed that side gap between electrode and hole wall increases with increasing open-circuit voltage. Pham et al. (2007) have studied the electrode wear during microelectrical discharge drilling with micro rod and micro tube electrodes. They have reported that wear measurement increases dramatically after a certain depth while rod electrode is used. This is mainly expected due to deteriorated flushing

7 26 conditions. Good flushing should be enabled to avoid from side sparking on the electrode. However, tubular electrode is more stabilized in deep depth when compared to the rod electrode. Ali et al. (2009) have discussed the characterization of form of microholes produced by micro Electrical Discharge Drilling (micro-edd) on beryllium copper alloy using tungsten carbide electrode of 300 m diameter. Using a fixed set of micro-edd parameters, micro-holes of different aspect ratio are drilled. The selected form characteristics diameter, roundness, and taper angle are investigated. The hole diameter and roundness are estimated by using SEM and graphical calculation. The micro-hole is sectioned to measure the depth and taper angle. The variations of these form characteristics are plotted against aspect ratio. This experimental study shows that diameter, roundness error, and taper angle of the micro-hole increase with the increase of aspect ratio almost at the same rate. The electrode wear ratio is not insignificant for low aspect ratio microhole. However, it increases sharply with the increase of aspect ratio. Pradhan et al. (2009) attempts to optimize the micro-edm process parameters for machining Ti-6Al-4V super alloy. To verify the optimal micro- EDM process parameters settings, MRR, TWR, OC and T are chosen as observed performance criteria. In addition, four independent parameters such as peak current, pulse-on time, flushing pressure, and duty ratio are adopted for evaluation by the Taguchi method. From the ANOVA and S/N ratio graph, the significant process parameters and the optimal combination level of machining parameters are obtained. It is seen that machining performances are affected mostly by the peak current and pulse-on time during micro-electro discharge machining of titanium alloy. The mathematical models have been developed to establish the relationship between various significant process parameters and

8 27 micro-edm performance criteria. In-depth studies have also been made to examine the influence of various process parameters on the white layer and surface topography through SEM micrographs of machined micro-hole. Nagahanumaiah et al. (2009) have presented the spectroscopic measurement of temperature and electron density in the micro-edm process. A systematic study using L 18 OA experiments based on the Taguchi method is conducted to understand the effect of varying process parameters including voltage, current, spark gap and electrode size on the plasma characteristics. The line pair method and the Stark broadening of the spectral line are used to compute plasma temperature and electron density, respectively. The spark gap and electrode size are found to have a significant influence on the plasma characteristics. The plasma produced by low energy discharge in micro-edm is more non-ideal, denser, and colder than the high-energy discharge plasma produced in the conventional EDM process. The inter particle distance is roughly equal to the Debye length, resulting in more electrostatic interactions between ions. Yu et al. (2009) have presented a new method of drilling high aspect ratio micro-holes by EDM, in which the planetary movement of an electrode, with enhancement from ultrasonic vibration, provides an unevenly distributed gap for the debris and bubbles to escape from the discharge zone easily. Microholes with aspect ratio of 29 have been drilled. Opoz et al. (2009) have investigated experimentally of the shape and dimensional geometry of blind micro-holes produced by micro-edm to explore the relational dependency with respect to machining time. For this purpose,

9 28 micro-holes are machined with machining times ranging from 1-80 minutes on plastic mould steel samples using tungsten carbide tool electrode and hydro carbide dielectric liquid composed of mineral and synthetic oils. All other operational machining parameters such as pulse time, gap voltage, average current, and so forth, are kept constant during machining. Depth, diametric expansions, end-tip shape, wall side parallelisms of holes are analyzed both on surface and cross-sections of the samples. Consequently, nine different end-tip shape formations are observed, and these shapes discussed on basis of dielectric liquid circulation and sparking conditions during machining. Enlargement in hole diameter is predicted by defining a band-width of 20 m hole expansion. Moreover, parallel wall-side length is expressed as the micro-hole depth where 5% reduction in entrance diameter is measured at the tapered section. Aligiri et al. (2010) have proposed a new tool wear compensation method in the micro-edm. Owing to the reduced tool area and poor flushing conditions in deep holes, tool wear in the micro-edm is more significant than in macro-edm. In micro-edm drilling, the z-axis of the tool position is monitored as machining progresses. However, due to significant electrode wear, the machined hole depth is not identical to the programmed depth of the hole, and thus this will result in geometrical inaccuracy. New micro-edm drilling method is used, in which the material removal volume is estimated as machining progresses. The compensation length is calculated and adjustment is made repeatedly along the tool path until the targeted material removal volume is reached. A real-time material removal volume estimator is developed based on the theoretical electro-thermal model, number of discharge pulse and pulse discrimination system. Under various energy input and machining depth settings, the experimental and estimated results are found to be in satisfactory agreement with average error lower than 14.3% for SS, titanium, and nickel alloy work materials. The proposed drilling method can compensate the tool wear and

10 29 produce more accurate micro-holes as compared to other methods. The experimental work also shows that the proposed method is more reliable as compared to the uniform wear method. In drilling micro-holes of 900 m depth, the depth error can be reduced to 4% using the proposed method. The micro-edm process has proved to be an appropriate nonconventional machining method for manufacturing accurate and complex three-dimensional structural micro-features which are difficult to be produced by conventional processes. However, the miniaturization of the EDM process needs special requirements on the machining equipment. The pulse generators which can produce small input energy pulses and high precision systems are the two major requirements. In this paper, newly developed technologies regarding these aspects are explored with the aid of a commercial micro-edm machine. By examining the pulses, innovative strategies implemented in the pulse generator are studied. The pulse measurements reveal the correlation between the discharge pulses and the machine parameters in order to provide an overview of process capability. The conclusions are applied on machining of a ceramic composite Si 3 N 4 -TiN and optimized machining settings for different machining conditions are achieved. Accordingly, applications of two- and three-dimensional microstructures on different types of materials such as a stainless steel microcompressor and a ceramic miniature gas turbine are demonstrated. By inspecting the machining geometry and surface integrity, process characteristics of micro- EDM are discussed (Liu et al 2010). Wang et al. (2011) investigates the micro-hole machining performance for Polycrystalline diamond (PCD) by micro-edm. The experimental results indicate that negative polarity machining is suitable for micro-edm of PCD because of the protection brought over by the adhesion sticking to the electrode.

11 30 An appropriate volume of adhesion on the tool electrode can help increase the MRR and reduce the relative TWR. By contrast, an excessive volume of adhesion can make the machining into a vicious circle in which micro-holes are drilled with overlarge diameters. An optimal set of machining conditions is chosen among the investigated ranges of nominal capacitance and electrode rotation speed. An exemplary PCD through-hole, machined under the chosen optimal machining conditions, shows satisfactory machining results. Jahan et al. (2011) have reviewed the conventional and micro-edm of tungsten carbide. The capability of machining intricate features with high dimensional accuracy in hard and difficult-to-cut material has made EDM process as an inevitable and one of the most popular non-conventional machining processes. In recent years, both EDM and micro-edm processes are being used extensively in the field of mould making, production of dies, cavities and complex 3D structures using difficult-to-cut tungsten carbide and its composites. The objective of this paper is to provide a state of the art in the field of EDM and micro-edm of tungsten carbide and its composites. The review begins with a brief introduction on the EDM and micro-edm processes. The research and developments in EDM of tungsten carbide are grouped broadly into conventional EDM of tungsten carbide, micro-edm of tungsten carbide and current research trends in EDM and micro-edm of tungsten carbide. The problems and challenges in the area of conventional and micro-edm of tungsten carbide and the importance of compound and hybrid machining processes are discussed. A summary of the future research directions based on the review is presented at the final section.

12 MICRO-HOLE MACHINING BY VARYING EDM SPECIFICATIONS Micro-EDM is one of the most powerful methods for drilling microholes in any electrically conductive materials, since circular rod electrode with a very small diameter of 5 m can be fabricated by using WEDG unit placed on the micro-edm machine (Masuzawa 1997). The depth of the holes which can be drilled will be rather limited, since the side gap between electrode and workpiece is very narrow in micro-edm. This is an obstacle to remove away debris from sparking area, and result in bad discharge called arc and short circuit. The recent study in micro-edm is generally condensed to get high aspect ratio in microholes, inner and exit diameter differences, surface and geometrical shape quality, and minimum achievable size in micro-hole manufacturing. A literature survey on micro-hole machining by using micro-edm with different decisive specifications is presented below. Her et al. (2001) have studied on micro-hole machining of copper with traditional EDM machine using a tungsten carbide tool electrode. They reported that electrode wear and hole enlargement are both smaller, and a better profile of micro hole can be obtained when positive polarity machining is selected, however, electrode wear is higher when positive polarity machining is selected. In addition, the author has also emphasized that MRR is higher when negative polarity machining and tungsten carbide electrode are selected. Yan et al. (2002) have studied precise micro-hole machining with high aspect ratios in borosilicate glass. They use a machining method which combines micro-edm and MUSM. Their experimental results show that using appropriate machining parameters; the diameter variations between the entrances and exits

13 32 (DVEE) can be decreased to nearly 2 m in micro holes with a diameter of 150 m and depth of 500 m. In that paper, authors claim that DVEE can be enhanced by improving an ultrasonic amplitude or rotational speed and they have also reported that the degree of micro-hole roundness is mostly related to the machining tool rotation speed. Yu et al. (2002) have proposed a model which provides planetary movement to the electrode. The planetary movement in micro-edm results in sufficient gap to remove debris during drilling high aspect ratio micro holes, blind noncircular micro holes as well. Their approach is verified by manufacturing micro holes with an aspect ratio of 18 and blind noncircular micro holes with sharp corners and edges. Li et al. (2002) have proposed an inchworm type micro feeding mechanism in micro-edm. With the aid of this mechanism, micro holes and micro-rods with an aspect ratio of more than 20 and 10 are obtained respectively. Wansheng et al. (2002) have introduced the ultrasonic vibration into micro-edm when they make their experiments with a single side notched cylindrical tool electrode. They have reported that holes with a diameter of less than Ø0.2 mm and aspect ratio of more than 15 can be machined steadily using this technology. Kaminski and Capuano (2003) have reported that the parameters affecting the micro holes machining process when diameter smaller than 0.1 mm and aspect ratio bigger than 20 by electro-erosion penetration process in sheets. And circularity deviation provided by the experiment is smaller than 0.01 for 0.1 mm diameter holes.

14 33 Diver et al. (2004) investigated of the fabrication of reverse tapered holes in EDM systems. They have emphasized that the existing EDM system is not fully qualified and produce low quality reverse tapered holes. According to their novel technique, tapered holes with a entry diameter of 100 m and exit diameter of 160 m can be produced. The other alternative ways of producing tapered holes are listed below; Modify the EDM parameters (e.g. voltage, frequency, current, gap, gain, and pulse width) during machining to remove more material radially as the depth of machining increases. Move the workpiece relative to the electrode to achieve the desired hole taper. Change the electrode angle and position radially during drilling. Feed and rotate the electrode at the angle required to achieve the desired hole taper. Yeo et al. (2004) have investigated of feasibility of a magnetic field to obtain higher aspect holes on hardened tool steel using micro-edm process. They have reported that the magnetic field can assist to improve debris circulation, and they achieve to machine a hole with a depth of 1177 m in 360 min, which is 26% higher than that of the achievement in conventional micro- EDM.

15 34 Kim et al. (2006) have proposed a method to reduce the difference between the entrance and exit diameters of the hole. By reducing secondary discharge, electrode wear, and capacitance when the tool is closed to the exit of the hole by introducing ultrasonic vibration to the EDM, a mostly straight hole is fabricated and inner shape of hole is not influenced from the proposed method. Hung et al. (2006) have used a helical micro-tool electrode to drill and finish micro-holes by using micro-electro-discharge machining combined with ultrasonic vibration. They have reported that this method can substantially reduce the EDM gap, taper and machining time for deep micro-hole drilling. Moreover, using a helical micro-tool with micro ultrasonic vibration finishing, good surface quality and less taper of the hole wall can be obtained. Weng (2006) has investigated the manufacturing of microparts with an array of micro holes using the machined micrographite copper electrode in EDM. This electrode is fabricated by using electrochemical process of anodic etching combined with electroforming process. He indicates that yielding parts can be utilized as a microwater spray nozzle in the biotechnology applications. Nakaoku et al. (2007) have investigated that the basic characteristic of micro-edm by machining micro holes in sintered diamond. They have reported that micro-holes with a diameter of 50 m can be machined in sintered diamond and machining characteristic is similar to the tungsten carbide alloy. In addition, they have observed that side gap between electrode and hole wall increases with increasing open-circuit voltage.

16 PROCESS OPTIMIZATION Lin et al. (2000) have described the application of the Taguchi method with fuzzy logic for optimizing the EDM process with multiple performance characteristics. Based on the experiments, the performance characteristics such as electrode wear rate and MRR are improved through this approach. Hence the optimization methodology developed with this study is useful in improving multiple performance characteristics in the EDM operations. Huang et al. (2003) have applied Grey Relational Analysis (GRA) to determine the optimal selection of machining parameters for the Wire-EDM process. Based on Taguchi quality design concept, an L 18 mixed-oa table is chosen for the experiments. With both GRA and a statistical method, it is found that the table feed rate has a significant influence on the metal removal rate, whilst the gap width and surface roughness are mainly influenced by pulse on time. Moreover, the optimal machining parameters setting for maximum metal removal rate and minimum surface roughness (or a desired surface roughness or a desired gap width) could be obtained by this approach. Fung (2003) has studied the optimization of injection moulding process parameters using the GRA method. Nine experimental runs based on the Taguchi method of OA s are performed to determine the best factor level condition. The wear volume losses of fiber-reinforced polybutylene terephthalate in different sliding directions are selected to be the quality targets. The degree of influence that the controllable process factors exert on the wear volume losses is studied by investigating the correlation between them. By analyzing the grey relational grade matrix, the most influential process factor and the most easily influenced wear property could be picked.

17 36 Lin (2004) has addressed an approach based on the Taguchi method with GRA for optimizing turning operations with multiple performance characteristics. A grey relational grade obtained from the GRA is used to solve the turning operations with multiple performance characteristics. Optimal cutting parameters can then be determined by the Taguchi method using the grey relational grade as the performance index. Tool life, cutting force, and surface roughness are important characteristics in turning. Using these characteristics, the cutting parameters, including cutting speed, feed rate, and depth of cut are optimized in the study. Experimental results have been improved through this approach. Singh et al. (2004) have suggested that OA with GRA is useful for optimization of multiple response characteristics which are more complex compared to optimization of single performance characteristics. They have obtained optimal EDM parameters settings of MRR, TWR, DOC, T and surface roughness during the EDM of Al-10%SiCP as cast metal matrix composites. Chiang and Chang (2006) have presented an effective approach for the optimization of the wire Electric Discharge Machining (WEDM) process of Al 2 O 3 particle-reinforced material (6061 alloy) with multiple performance characteristics based on the GRA. The machining information for the difficultcutting particle-reinforced material is inadequate and complicated. The response table and response graph for each level of the machining parameters are obtained from the grey relational grade, and select the optimal levels of machining parameters. In this study, the machining parameters namely the cutting radius of working piece, the on time of discharging, the off time of discharging, the arc on time of discharging, the arc off time of discharging, the servo voltage, the wire feed and water flow are optimized with considerations of multiple performance

18 37 characteristics, such as the surface removal rate and the maximum surface roughness. It is clearly shown that the above performance characteristics in the WEDM process are great improved together through this approach. Rajurkar et al. (2006) have been reviewed the micro-edm research. Research on micromachining has been pursued only by few research groups worldwide. A recent review of -EDM reported in the CIRP proceedings has concluded that the gap phenomena in -EDM are not fully understood, and there exists a need for significant advancements in the theoretical understanding of - EDM technology at the micro-nano scale, the influence of the material s grain size, crystallographic orientation, melting/boiling point, elastic strength, the effect of residual stress, environmental control with dielectrics or under nearly dry EDM machining conditions, and the scaling up of these processes for parallel processing Zhang et al. (2007) have studied the Taguchi design application to optimize surface quality in a CNC face milling operation. This study has included feed rate, spindle speed and depth of cut as control factors, and the noise factors are the operating chamber temperature and the usage of different tool inserts in the same specification, which introduces tool condition and dimensional variability. An OA of L 9 is used; ANOVA analyses are carried out to identify the significant factors affecting surface roughness, and the optimal cutting combination is determined by seeking the best surface roughness (response) and S/N ratio. Finally, confirmation tests verify that the Taguchi design is successful in optimizing milling parameters for surface roughness.

19 38 Nalbant et al. (2007) have used Taguchi method to find the optimal cutting parameters for surface roughness in turning operations of AISI 1030 steel bars using TiN coated tools. Three cutting parameters, namely, insert radius, feed rate, and depth of cut, are optimized with considerations of surface roughness. However, very few studies have been conducted to investigate roundness under different turning parameter. Sundaram et al. (2008) investigates of the optimal combinations of process parameters to maximize the MRR and minimize the tool wear by applying Taguchi experimental design. Experimental investigation of ultrasonic assisted micro EDM is performed by introducing ultrasonic vibration to workpiece. Analysis of variance (ANOVA) is performed and signal-to-noise (S/N) ratio is determined to know the level of importance of the machining parameters. Based on ANOVA, ultrasonic vibration at 60% of the peak power with capacitance of 3300PF is found to be significant for best MRR. The machining time plays a significant role in the tool wear. The results are confirmed experimentally at 95% confidence interval. Tzeng et al. (2009) have investigated the optimization of CNC turning operation parameters for SKD11 using the GRA method. Nine experimental runs based on an OA of Taguchi method are performed. The surface properties of roughness average and roughness maximum as well as the roundness are selected as the quality targets. An optimal parameter combination of the turning operation is obtained via GRA. By analyzing the Grey relational grade matrix, the degree of influence for each controllable process factor onto individual quality targets can be found. The depth of cut is identified to be the most influential on the roughness average and the cutting speed is the most influential factor to the roughness maximum and the roundness. Additionally, the analysis of variance is

20 39 also applied to identify the most significant factor; the depth of cut is the most significant controlled factors for the turning operations according to the weighted sum grade of the roughness average, roughness maximum and roundness. Patel et al. (2009) have investigated of EDM-machinable ceramic composite material (Al2O3 SiCw TiC). Experiments are conducted using discharge current, pulse-on time, duty cycle and gap voltage as typical process parameters. The GRA is adopted to obtain grey relational grade for EDM process with multiple characteristics namely MRR and surface roughness. ANOVA is used to study the significance of process variables on grey relational grade which shows discharge current and duty cycle to be most significant parameters. Other than discharge current and duty cycle, pulse-on time and gap voltage have also been found to be significant. To validate the study, confirmation experiment has been carried out at optimum set of parameters and predicted results have been found to be in good agreement with experimental findings. Pradhan et al. (2009) have optimized micro-edm process parameters for machining Ti-6Al-4V super alloy. To verify the optimal micro-edm process parameters settings, MRR, TWR, OC and T are chosen as observed performance criteria. In addition, four independent parameters such as peak current, pulse-on time, flushing pressure, and duty ratio are adopted for evaluation by the Taguchi method. From the ANOVA and S/N ratio graph, the significant process parameters and the optimal combination level of machining parameters are obtained. It is seen that machining performances are affected mostly by the peak current and pulse-on time during micro-edm of titanium alloy. Mathematical models have been developed to establish the relationship between various significant process parameters and micro-edm performance criteria. In-depth studies have also been made to examine the influence of various process

21 40 parameters on the white layer and surface topography through SEM micrographs of machined micro-hole. Bhaduri et al. (2009) have discussed that the effects of EDM process parameters on MRR, electrode wear rate, radial OC, and taper angle while machining TiN Al 2 O 3 composite. The characteristic features of the EDM process are explored through Taguchi L9 OA design based experimental studies with various process parametric combinations. Finally, optimum parameter settings for each response factor are obtained and tested through verification experiments. The whole experimental study indicates that EDM has a very good potential in machining of TiN Al 2 O 3 ceramic composite in some particular ranges of process parameters. Lin et al. (2009) have optimized the magnetic force assisted EDM using Taguchi technique. Based on the experimental results, the magnetic force assisted EDM has a higher MRR, a lower electrode wear ratio, and a smaller surface roughness as compared to standard EDM. Ponappa et al. (2010) evaluates the effects of EDM parameters on drilled-hole quality such as taper and surface finish. Microwave-sintered magnesium nano composites are used as work materials. Experiments are conducted using Taguchi methodology to ascertain the effects of EDM process parameter. The process parameters such as pulse-on time, pulse-off time, voltage gap, and servo speed are optimized to get better surface finish and reduced taper. ANOVA analyses are carried out to identify the significant factors that affect the hole accuracy and the surface roughness. Pulse-on time and the servo speed are identified as major response variables. Micro structural changes and the effects of

22 41 nano particle reinforcement in the drilled hole are studied through SEM micrographs. Jaswin and Lal (2010) have investigated the optimization of the deep cryogenic treatment for En 52 valve steel using the Taguchi method in combination with the GRA. The factors considered for the optimization are the cooling rate, soaking temperature, soaking period, and tempering temperature, each at three different levels. The mechanical properties such as the tensile strength, hardness, and wear resistance are selected as the quality targets. Nine experimental runs based on L 9 OA of the Taguchi method are performed. An optimal parameter combination of the deep cryogenic treatment is obtained via the GRA. The ANOVA is applied to identify the most influential factor and it is found that the soaking period is the most influential factor for the deep cryogenic treatment of En 52 valve steel. The results of the confirmation tests prove that the tensile strength, hardness, and wear resistance of the deep cryotreated En 52 valve steel samples have improved simultaneously through the optimal combination of the deep cryogenic treatment parameters obtained from the proposed method. The improvement in the tensile strength, hardness, and wear resistance of the deep cryotreated samples at the optimized treatment condition on the samples without deep cryogenic treatment is 7.84%, 11.16%, and 46.51%, respectively. Through the deep cryogenic treatment the wear resistance of the En 52 valve steel has improved more compared to the other responses. Kao et al. (2010) have discussed parameter optimization of the electrical discharge machining process to Ti 6Al 4V alloy considering multiple performance characteristics using the Taguchi method and GRA. The process parameters selected in this study are discharge current, open voltage, pulse duration and duty factor. Experiments based on the appropriate OA are

23 42 conducted first. The normalized experimental results of the performance characteristics are then introduced to calculate the coefficient and grades according to GRA. The optimized process parameters simultaneously leading to a lower electrode wear ratio, higher MRR and better surface roughness are then verified through a confirmation experiment. The validation experiments show an improved electrode wear ratio of 15%, MRR of 12% and surface roughness of 19% when the Taguchi method and GRA are used. Jagan et al. (2010) have investigated the feasibility of machining deep micro-holes in two difficult-to-cut materials: cemented carbide (WC-Co) and austenitic stainless steel (SUS 304) using the micro-edm drilling. The effect of discharge energy and electro-thermal material properties on the performance of the two work materials during the micro-edm drilling has also been investigated. The micro-edm drilling performance of two materials has been assessed based on the quality and accuracy of the produced micro-holes, machining stability, MRR, and electrode wear ratio. The results show that deephole micro-edm drilling is technically more feasible in WC-Co as it offers micro-holes with smooth and burr-free surfaces at the rim in addition to improved circularity and lower OC than those provided by SUS 304. Moreover, WC-Co exhibits better machinability during the deep-hole micro-edm drilling, providing relatively higher MRR and stable machining. Jung and Kwon (2010) have resolved the complicated interrelationships among the multiple performance characteristics by GRA. They have attempted to find the optimal machining conditions under which the microhole can be formed to a minimum diameter and a maximum aspect ratio. The Taguchi method is used to determine the relations between machining parameters and process characteristics. It is found that electrode wear and the entrance and

24 43 exit clearances have a significant effect on the diameter of the micro-hole when the diameter of the electrode is identical. The GRA is used to determine the optimal machining parameters, among which the input voltage and the capacitance are found to be the most significant. 2.5 NEED FOR THIS RESEARCH WORK From the analysis of aforementioned literature survey, it can be concluded that EDM technique is evolving rapidly and it requires further research to optimize various process variables for effective utilization in micromachining domain. The machined surface of micro-hole suffers from thermal stresses, formation of recast layer, OC, white layer thickness and geometrical inaccuracy. To minimize such problematic issues heuristic study of the process parameters is needed to achieve higher productivity i.e. higher MRR, least tool wear, lesser OC and higher dimensional accuracy. Researchers have studied the influence of various process parameters such as machining voltage, pulse ontime, pulse off-time, current, and tool vibrations on machining accuracy and machining performance. To exploit full potential of EDM, a thorough research is needed to improve the machining performance, surface quality and accuracy by optimizing the various EDM process parameters. The optimal use of the EDM process for higher production rate with better accuracy demands proper control of machining parameters. Therefore, by controlling the various process parameters, optimal quality of the workpiece can be obtained. This research attempts to conduct experiments in micro-edm machine and two commercially available sinking EDM machines such as ELECTRONICA and SPARKONIX using SS. These EDM machines have many undisclosed parameters which are effective in the determination of machining

25 44 characteristics. In order to reveal the machining characteristics and predict the machined shape of holes, this research aims to disclose the parameters which are more effective for finding out MRR, TWR, OC and Taper. In EDM, the debris particles produced during machining are to be flushed out properly from the machining area in the views of increasing the machining rate and minimizing the OC and avoiding workpiece tool contact. To avoid such problems, tube electrode is used to remove debris particles. Hence, analysis based on Taguchi method with GRA has been carried out to obtain significant process parameters by using their optimal combination level for effective micro-hole generation in 304 & 316 SS. 2.6 SCOPE OF THIS RESEARCH The present state of research indicates a potential scope for production of micro-turbine impeller and micro-compressor, nozzle for diesel fuel injectors, inkjet printer manufacturing, cooling holes in turbine blades and the production of microgrooves for self-acting fluid film bearings and complex shapes. Furthermore optimization of the process parameters has been carried out to achieve higher productivity i.e. higher MRR, lesser tool wear, lesser OC and higher dimensional accuracy.

26 CONCLUDING REMARKS Literature review on machining performance, micro-hole machining by varying EDM specifications and process optimization can be summarized in the form of table is shown in Table follows: Table 2.1 Summary of literature review S. Author & No. Year of publication 1 Chen et al. & Wong et al. & Singh et al. & Han et al. & 2006 Parameters considered kerosene and distilled water as the dielectrics Investigated the micro-edm material removal characteristics using single RC-pulse discharges. current, pulse on-time and flushing pressure A new transistor type iso-pulse generator for micro-edm. Novelty of the work MRR is greater and the relative electrode wear ratio is lower when machining in distilled water rather than in kerosene. Estimated erosion efficiency of material removal at low-energy (<50 J) discharges is found to be seven to eight times higher than that at higher-energy discharges. MRR, TWR, Taper, ROC, and surface roughness on Al-MMC with 10% SiCp metal matrix composites. The removal rate of the transistor type isopulse generator is two or three times higher than that of the traditional RC pulse generator.

27 46 5 Son et al. & Pradhan et al. & Diver et al. & Kim et al. & Singh et al. (2004) 10. Rajurkar et al. (2006) pulse duration and the ratio of off-time to on-time, peak current, pulseon time, flushing pressure, and duty ratio voltage, frequency, current, gap, gain, and pulse width secondary discharge, electrode wear, and capacitance pulse on time, current, pulse off time, voltage reviewed the micro- EDM research Machining properties are reported on tool wear, MRR, and machining accuracy. Machining performances are affected mostly by the peak current and pulse-on time during microelectro discharge machining of titanium alloy. Fabrication of reverse tapered holes in EDM systems. mostly straight hole is fabricated Optimal EDM parameters settings of MRR, TWR, DOC, T and surface roughness during the EDM of Al-10%SiCP as cast metal matrix composites have been obtained. A need for significant advancements in the theoretical understanding of -EDM technology at the micro-nano scale, the influence of the material s grain size, crystallographic orientation, melting/boiling point, elastic strength, and the effect of residual stress.

28 Sundaram et al. (2008) 12. Patel et al. (2009) 13. Pradhan et al. (2009) 14. Bhaduri et al. (2009) 15. Ponappa et al. (2010) 16. Kao et al. (2010) 17. Jagan et al. (2010) Capacitance, voltage, and vibration frequency. discharge current, pulse-on time, duty cycle and gap voltage peak current, pulseon time, flushing pressure, and duty ratio machining of TiN Al2O3 ceramic composite pulse-on time, pulseoff time, voltage gap, and servo speed Discharge current, open voltage, pulse duration and duty factor. Voltage, Discharge energy per pulse, Resistance and capacitance Ultrasonic vibration at 60% of the peak power with capacitance of 3300PF is found to be significant for best MRR. The machining time plays a significant role in the tool wear. discharge current and duty cycle are found to be most significant parameters Machining performances are affected mostly by the peak current and pulse-on time during micro- EDM of titanium alloy. EDM has a very good potential in machining of TiN Al2O3 ceramic composite Pulse-on time and the servo speed are identified as major response variables. The optimized process parameters simultaneously leading to a lower electrode wear ratio, higher MRR and better surface roughness are then verified through a confirmation experiment. Deep-hole micro-edm drilling is technically more feasible in WC- Co as it offers micro-holes with smooth and burr-free surfaces at

29 Jung and Kwon (2010) Input voltage, current and capacitance the rim in addition to improved circularity and lower OC than those provided by SUS 304. Moreover, WC-Co exhibits better machinability during the deep-hole micro-edm drilling, providing relatively higher MRR and stable machining. The GRA is used to determine the optimal machining parameters, among which the input voltage and the capacitance are found to be the most significant. It is observed that many researchers have studied the influence of various process parameters such as pulse-on time, voltage, current, and frequency on MRR, TWR and accuracy using micro-edm machine. Very few have studied the performance using conventional EDM machine. Hence further studies are required to make the micro-holes and study the performance using conventional EDM. The optimal use of the EDM process for higher production rate with better accuracy demands proper control of machining parameters. Therefore, by controlling the various process parameters, optimal quality of the workpiece can be obtained. However, till date, the findings available in the field of drilling of micro-holes using EDM with regard to optimization of process parameters are

30 49 not adequate as there are no established optimal machining parameters. Thus, there is a great demand for extensive research of finding out the reliable and repeatable optimal combinational machining parameters. Taguchi method is very effective to deal with response influenced by multi-variables. This method focuses on minimizing the effect of causes of variation. In comparison with a traditional full factorial design of experiments, Taguchi s methods in general, provide a significant reduction in the size of experiments thereby speeding up the experimental process. Two important techniques used in Taguchi design are OA s and S/N ratios. The grey relational theory provides an efficient management upon the uncertain data, multi-input and discrete data. On the other hand, the GRA reveals the necessary information of the interactions among parameters. It provides a solution of a system in which the model is not sure or the information is incomplete. Hence, analysis based on Taguchi method with GRA has been carried out to obtain significant process parameters and their optimal combination level for effective micro-hole generation in 304 &316 SS.