英文原文State_of_the_art_in_wire_electrical_discharge_machining_(WEDM).pdf
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International Journal of Machine Tools & Manufacture 44 (2004) 12471259/locate/ijmactoolState of the art in wire electrical discharge machining (WEDM)K.H. Ho, S.T. Newman?, S. Rahimifard, R.D. AllenAdvanced Manufacturing Systems and Technology Centre, Wolfson School of Mechanical and Manufacturing Engineering,Loughborough University, Loughborough, Leicestershire LE11 3TU, UKReceived 13 October 2003; accepted 29 April 2004AbstractWire electrical discharge machining (WEDM) is a specialised thermal machining process capable of accurately machining partswith varying hardness or complex shapes, which have sharp edges that are very difficult to be machined by the main streammachining processes. This practical technology of the WEDM process is based on the conventional EDM sparking phenomenonutilising the widely accepted non-contact technique of material removal. Since the introduction of the process, WEDM hasevolved from a simple means of making tools and dies to the best alternative of producing micro-scale parts with the highestdegree of dimensional accuracy and surface finish quality.Over the years, the WEDM process has remained as a competitive and economical machining option fulfilling the demandingmachining requirements imposed by the short product development cycles and the growing cost pressures. However, the risk ofwire breakage and bending has undermined the full potential of the process drastically reducing the efficiency and accuracy of theWEDM operation. A significant amount of research has explored the different methodologies of achieving the ultimate WEDMgoals of optimising the numerous process parameters analytically with the total elimination of the wire breakages thereby alsoimproving the overall machining reliability.This paper reviews the vast array of research work carried out from the spin-offfrom the EDM process to the development ofthe WEDM. It reports on the WEDM research involving the optimisation of the process parameters surveying the influence ofthe various factors affecting the machining performance and productivity. The paper also highlights the adaptive monitoring andcontrol of the process investigating the feasibility of the different control strategies of obtaining the optimal machining conditions.A wide range of WEDM industrial applications are reported together with the development of the hybrid machining processes.The final part of the paper discusses these developments and outlines the possible trends for future WEDM research.# 2004 Elsevier Ltd. All rights reserved.Keywords: Wire electrical discharge machining (WEDM); Hybrid machining process; Process optimisation; Cutting rate; Matenal removal rate;Surface finish1. IntroductionWire electrical discharge machining (WEDM) is awidely accepted non-traditional material removal pro-cess used to manufacture components with intricateshapes and profiles. It is considered as a unique adap-tation of the conventional EDM process, which uses anelectrode to initialise the sparking process. However,WEDM utilises a continuously travelling wire electrodemade of thin copper, brass or tungsten of diameter0.050.3 mm, which is capable of achieving very smallcorner radii. The wire is kept in tension using a mech-anical tensioning device reducing the tendency of pro-ducing inaccurate parts. During the WEDM process,the material is eroded ahead of the wire and there is nodirect contact between the workpiece and the wire,eliminating the mechanical stresses during machining.In addition, the WEDM process is able to machineexotic and high strength and temperature resistive(HSTR) materials and eliminate the geometrical chan-ges occurring in the machining of heat-treated steels.WEDM was first introduced to the manufacturingindustry in the late 1960s. The development of the pro-cess was the result of seeking a technique to replace themachined electrode used in EDM. In 1974, D.H. Dule-bohnappliedtheoptical-linefollowersystemto?Corresponding author. Tel.: +44-1509-227660; fax: +44-1509-227648.E-mail address: s.t.newmanlboro.ac.uk (S.T. Newman).0890-6955/$ - see front matter # 2004 Elsevier Ltd. All rights reserved.doi:10.1016/j.ijmachtools.2004.04.017automatically control the shape of the component to bemachined by the WEDM process 1. By 1975, itspopularity was rapidly increasing, as the process andits capabilities were better understood by the industry2. It was only towards the end of the 1970s, whencomputer numerical control (CNC) system was initi-ated into WEDM that brought about a major evol-ution of the machining process. As a result, the broadcapabilities of the WEDM process were extensivelyexploited for any through-hole machining owing to thewire, which has to pass through the part to bemachined.ThecommonapplicationsofWEDMinclude the fabrication of the stamping and extrusiontools and dies, fixtures and gauges, prototypes, aircraftand medical parts, and grinding wheel form tools.This paper provides a review on the various academ-ic research areas involving the WEDM process, and isthe sister paper to a review by Ho and Newman 3 ondie-sinking EDM. It first presents the process overviewbased on the widely accepted principle of thermal con-duction and highlights some of its applications. Themain section of the paper focuses on the majorWEDM research activities, which include the WEDMprocess optimisation together with the WEDM processmonitoring and control. The final part of the paper dis-cusses these topics and suggests the future WEDMresearch direction.2. WEDMThis section provides the basic principle of theWEDM process and the variations of the process com-bining other material removal techniques.2.1. WEDM processThe material removal mechanism of WEDM is verysimilar to the conventional EDM process involving theerosion effect produced by the electrical discharges(sparks). In WEDM, material is eroded from the work-piece by a series of discrete sparks occurring betweenthe workpiece and the wire separated by a stream ofdielectricfluid,whichiscontinuouslyfedtothemachining zone 4. However, todays WEDM processis commonly conducted on workpieces that are totallysubmerged in a tank filled with dielectric fluid. Such asubmerged method of WEDM promotes temperaturestabilisation and efficient flushing especially in caseswheretheworkpiecehasvaryingthickness.TheWEDM process makes use of electrical energy generat-ing a channel of plasma between the cathode andanode 5, and turns it into thermal energy 6 at a tem-perature in the range of 800012,000vC 7 or as highas 20,000vC 8 initialising a substantial amount ofheating and melting of material on the surface of eachpole. When the pulsating direct current power supplyoccurring between 20,000 and 30,000 Hz 9 is turnedoff, the plasma channel breaks down. This causes asudden reduction in the temperature allowing the circu-lating dielectric fluid to implore the plasma channeland flush the molten particles from the pole surfaces inthe form of microscopic debris.While the material removal mechanisms of EDMand WEDM are similar, their functional characteristicsare not identical. WEDM uses a thin wire continuouslyfeeding through the workpiece by a microprocessor,which enable parts of complex shapes to be machinedwith exceptional high accuracy. A varying degree oftaper ranging from 15vfor a 100 mm thick to 30vfor a400 mm thick workpiece can also be obtained on thecut surface. The microprocessor also constantly main-tains the gap between the wire and the workpiece,which varies from 0.025 to 0.05 mm 2. WEDM elim-inates the need for elaborate pre-shaped electrodes,which are commonly required in EDM to perform theroughing and finishing operations. In the case ofWEDM, the wire has to make several machining passesalong the profile to be machined to attain the requireddimensional accuracy and surface finish (SF) quality.Kunieda and Furudate 10 tested the feasibility of con-ducting dry WEDM to improve the accuracy of the fin-ishing operations, which was conducted in a gasatmosphere without using dielectric fluid. The typicalWEDM cutting rates (CRs) are 300 mm2/min for a50 mm thick D2 tool steel and 750 mm2/min for a150 mm thick aluminium 11, and SF quality is as fineas 0.040.25 lRa. In addition, WEDM uses deionisedwater instead of hydrocarbon oil as the dielectric fluidand contains it within the sparking zone. The deionisedwater is not suitable for conventional EDM as it causesrapid electrode wear, but its low viscosity and rapidcooling rate make it ideal for WEDM 12.2.2. Hybrid machining processesThere are a number of hybrid machining processes(HMPs) seeking the combined advantage of WEDMwith other machining techniques. One such combi-nation is wire electrical discharge grinding (WEDG),which is commonly used for the micro-machining offine rods utilized in the electronic circuitry. WEDGemploys a single wire guide to confine the wire tensionwithin the discharge area between the rod and the frontedge of the wire and to minimise the wire vibration.Therefore, it is possible to grind a rod that is as smallas 5 lm in diameter 13 with high accuracy, goodrepeatability and satisfactory straightness 14. Otheradvantages of WEDG include the ability to machine arod with a large aspect ratio, maintaining the concen-tricity of the rod and providing a wider choice of com-plex shapes such as tapered and stepped shapes at1248K.H. Ho et al. / International Journal of Machine Tools & Manufacture 44 (2004) 12471259various sections 15. Several authors 1619 haveemployed the WEDG process in the micro-machiningof fine electrodes or pins with a large aspect-ratio,which are difficult to be machined by traditional pre-cision micro-machining methods such as Micro-EDM,LIGA and excimer laser drilling.Some of the HMPs seek to improve the WEDM per-formance measures such as the surface integrity andthe CR. For example, the ultrasonic vibration isapplied to the wire electrode to improve the SF qualitytogether with the CR and to reduce the residual stresson the machined surface 20. On the other hand, thewire electrochemical grinding (WECG) process replacesthe electrical discharge used in WEDG with an electro-chemical solution to produce high SF quality part for awide range of machining condition 15. Masuzawaet al. 13,15 compared the SF quality obtained fromthe WECG with WEDG, which is suitable for finishingmicro-parts. A rotary axis is also added to WEDM toachieve higher material removal rate (MRR) and toenable the generation of free-form cylindrical geome-tries 21,22. The effects of the various process para-meters such as part rotational speed, wire feed rate andpulse on-time on the surface integrity and roundness ofthe part produced have been investigated in the samefeasibility study 23.3. WEDM applicationsThis section discusses the viability of the WEDMprocess in the machining of the various materials usedparticularly in tooling applications.3.1. Modern tooling applicationsWEDM has been gaining wide acceptance in themachining of the various materials used in moderntooling applications. Several authors 24,25 have inves-tigated the machining performance of WEDM in thewafering of silicon and machining of compacting diesmade of sintered carbide. The feasibility of using cylin-drical WEDM for dressing a rotating metal bond dia-mond wheel used for the precision form grinding ofceramics has also been studied 22. The results showthat the WEDM process is capable of generating pre-cise and intricate profiles with small corner radii but ahigh wear rate is observed on the diamond wheel dur-ing the first grinding pass. Such an initial high wheelwear rate is due to the over-protruding diamondgrains, which do not bond strongly to the wheel afterthe WEDM process 26. The WEDM of permanentNdFeB and soft MnZn ferrite magnetic materialsused in miniature systems, which requires small mag-netic parts, was studied by comparing it with thelaser-cuttingprocess27.ItwasfoundthattheWEDM process yields better dimensional accuracy andSF quality but has a slow CR, 5.5 mm/min for NdFeBand 0.17 mm/min for MnZn ferrite. A study was alsodone to investigate the machining performance ofmicro-WEDM used to machine a high aspect ratiomeso-scale part using a variety of metals includingstainless steel, nitronic austentic stainless, berylliumcopper and titanium 28.3.2. Advanced ceramic materialsThe WEDM process has also evolved as one of themost promising alternatives for the machining of theadvanced ceramics. Sanchez et al. 29 provided aliterature survey on the EDM of advanced ceramics,which have been commonly machined by diamondgrinding and lapping. In the same paper, they studiedthe feasibility of machining boron carbide (B4C) andsilicon infiltrated silicon carbide (SiSiC) using EDMand WEDM. Cheng et al. 30 also evaluated the possi-bility of machining ZrB2based materials using EDMand WEDM, whereas Matsuo and Oshima 31 exam-ined the effects of conductive carbide content, namelyniobium carbide (NbC) and titanium carbide (TiC), onthe CR and surface roughness of zirconia ceramics(ZrO2) during WEDM. Lok and Lee 32 have success-fully WEDMed sialon 501 and aluminium oxidetitanium carbide (Al2O3TiC). However, they realisedthat the MRR is very low as compared to the cuttingof metals such as alloy steel SKD-11 and the surfaceroughness is generally inferior to the one obtained withthe EDM process. Dauw et al. 33 explained that theMRR and surface roughness are not only dependenton the machining parameters but also on the materialof the part.An innovative method of overcoming the technologi-cal limitation of the EDM and WEDM processesrequiring the electrical resistivity of the material withthreshold values of approximately 100 X/cm 34 or300 X/cm 35 has recently been explored. There aredifferent grades of engineering ceramics, which Koniget al. 34 classified as non-conductor, natural-conduc-tor and conductor, which is a result of doping non-conductors with conductive elements. Mohri et al. 36brought a new perspective to the traditional EDMphenomenon by using an assisting electrode to facili-tate the sparking of highly electrical-resistive ceramics.Both the EDM and WEDM processes have been suc-cessfully tested diffusing conductiveparticles fromassisting electrodes onto the surface of sialon ceramicsassisting the feeding the electrode through the insulat-ing material. The same technique has also been experi-mented on other types of insulating ceramic materialsincluding oxide ceramics such as ZrO2and Al2O3,which have very limiting electrical conductive proper-ties 37.K.H. Ho et al. / International Journal of Machine Tools & Manufacture 44 (2004) 1247125912493.3. Modern composite materialsAmong the different material removal processes,WEDM is considered as an effective and economicaltool in the machining of modern composite materials.Several comparative studies 38,39 have been madebetween WEDM and laser cutting in the processing ofmetal matrix composites (MMC), carbon fibre andreinforced liquid crystal polymer composites. Thesestudies showed that WEDM yields better cutting edgequality and has better control of the process para-meters with fewer workpiece surface damages. How-ever, it has a slower MRR for all the tested compositematerials. Gadalla and Tsai 40 compared WEDMwith conventional diamond sawing and discovered thatit produces a roughness and hardness that is compara-ble to a low speed diamond saw but with a higherMRR. Yan et al. 41 surveyed the various machiningprocesses performed on the MMC and experimentedwith the machining of Al2O3/6061Al composite usingrotary EDM coupled with a disk-like electrode. Otherstudies 42,43 have been conducted on the WEDM ofAl2O3particulate reinforced composites investigatingthe effect of the process parameters on the WEDM per-formance measures. It was found that the process para-meters have little influence on the surface roughnessbut have an adverse effect on CR.4. Major areas of WEDM researchThe authors have organised the various WEDMresearch into two major areas namely WEDM processoptimisation together with WEDM process monitoringand control.4.1. WEDM process optimisationToday, the most effective machining strategy isdetermined by identifying the different factors affectingthe WEDM process and seeking the different ways ofobtaining the optimal machining condition and per-formance. This section provides a study on the numer-ous machining strategies involving the design of theprocess parameter and the modelling of the process.4.1.1. Process parameters designThe settings for the various process parametersrequired in the WEDM process play a crucial role inproducing an optimal machining performance. Thissection shows some of the analytical and statisticalmethods used to study the effects of the parameters onthe typical WEDM performance measures such as CR,MRR and SF. Factors affecting the performance measures.WEDM is a complex machining process controlled bya large number of process parameters such as the pulseduration, discharge frequency and discharge currentintensity. Any slight variations in the process para-meters can affect the machining performance measuressuch as surface roughness and CR, which are two ofthe most significant aspects of the WEDM operation44. Suziki and Kishi 45 studied the reduction of dis-charge energy to yield a better surface roughness, whileLuo 46 discovered the additional need for a high-energy efficiency to maintain a high machining ratewithout damaging the wire. Several authors 47 havealso studied the evolution of the wire tool performanceaffecting the machining accuracy, costs and perform-ance measures.The selection of appropriate machining conditionsfor the WEDM process is based on the analysis relat-ing the various process parameters to different per-formance measures namely the CR, MRR and SF.Traditionally, this was carried out by relying heavily onthe operators experience or conservative technologicaldataprovidedbytheWEDMequipmentmanu-facturers, which produced inconsistent machining per-formance. Levy and Maggi 48 demonstrated that theparameter settings given by the manufacturers are onlyapplicable for the common steel grades. The settingsfor machining new materials such as advanced cer-amics and MMCs have to be further optimised exper-imentally. Effects of the process parameters on the cuttingrate. Many different types of problem-solving qualitytools have been used to investigate the significant fac-tors and its inter-relationships with the other variablesin obtaining an optimal WEDM CR. Konda et al. 49classified the various potential factors affecting theWEDM performance measures into five major cate-gories namely the different properties of the workpiecematerial and dielectric fluid, machine characteristics,adjustablemachiningparameters,andcomponentgeometry. In addition, they applied the design ofexperiments (DOE) technique to study and optimisethe possible effects of variables during process designanddevelopment,andvalidatedtheexperimentalresults using noise-to-signal (S/N) ratio analysis. Tarnget al. 50 employed a neural network system with theapplication of a simulated annealing algorithm forsolving the multi-response optimisation problem. Itwas found that the machining parameters such as thepulse on/offduration, peak current, open circuit volt-age, servo reference voltage, electrical capacitance andtable speed are the critical parameters for the esti-mation of the CR and SF. Huang et al. 51 arguedthat several published works 50,52,53 are concernedmostly with the optimisation of parameters for the1250K.H. Ho et al. / International Journal of Machine Tools & Manufacture 44 (2004) 12471259roughing cutting operations and proposed a practicalstrategy of process planning from roughing to finishingoperations. The experimental results showed that thepulse on-time and the distance between the wire per-iphery and the workpiece surface affect the CR and SFsignificantly. The effects of the discharge energy on theCR and SF of a MMC have also been investigated 54. Effects of the machining parameters on thematerial removal rate. The effects of the machiningparameters on the volumetric MRR have also beenconsidered as a measure of the machining performance.Scott et al. 52 used a factorial design requiring a num-ber of experiments to determine the most favourablecombination of the WEDM parameter. They foundthat the discharge current, pulse duration and pulse fre-quency are the significant control factors affecting theMRR and SF, while the wire speed, wire tension anddielectric flow rate have the least effect. Liao et al. 53proposed an approach of determining the parametersettings based on the Taguchi quality design methodand the analysis of variance. The results showed thatthe MRR and SF are easily influenced by the table feedrate and pulse on-time, which can also be used to con-trol the discharging frequency for the prevention ofwire breakage. Huang and Liao 55 presented the useof Grey relational and S/N ratio analyses, which alsodisplay similar results demonstrating the influence oftable feed and pulse on-time on the MRR. An experi-mental study to determine the MRR and SF for vary-ing machining parameters has also been conducted 56.The results have been used with a thermal model toanalyse the wire breakage phenomena. Effects of the process parameters on the surfacefinish. There are also a number of published works thatsolely study the effects of the machining parameters onthe WEDMed surface. Go kler and Ozano zgu 57 stud-ied the selection of the most suitable cutting and offsetparameter combination to get a desired surface rough-ness for a constant wire speed and dielectric flushingpressure. Tosun et al. 58 investigated the effect of thepulse duration, open circuit voltage, wire speed anddielectric flushing pressure on the WEDMed workpiecesurface roughness. It was found that the increasingpulse duration, open circuit voltage and wire speedincreases with the surface roughness, whereas theincreasing dielectric fluid pressure decreases the surfaceroughness. Anand59used a fractionalfactorialexperiment with an orthogonal array layout to obtainthe most desirable process specification for improvingthe WEDM dimensional accuracy and surface rough-ness. Spedding and Wang 60 optimised the processparameter settings by using artificial neural networkmodelling to characterise the WEDMed workpiece sur-faces, while Williams and Rajurkar 61 presentedthe results of the current investigations into the char-acteristics of WEDM generated surfaces.4.1.2. Process modellingIn addition, the modelling of the WEDM process bymeansofmathematicaltechniqueshasalsobeenapplied to effectively relate the large number of processvariables to the different performance of the process.Spedding and Wang 62 developed the modelling tech-niques using the response surface methodology andartificial neural network technology to predict the pro-cess performance such as CR, SF and surface wavinesswithin a reasonable large range of input factor levels.Liu and Esterling 63 proposed a solid modellingmethod, which can precisely represent the geometry cutby the WEDM process, whereas Hsue et al. 64developed a model to estimate the MRR during geo-metrical cutting by considering wire deflection withtransformed exponential trajectory of the wire centre.Spur and Scho nbeck 65 designed a theoretical modelstudying the influence of the workpiece material andthe pulse-type properties on the WEDM of a work-piece with an anodic polarity. Han et al. 66 developeda simulation system, which accurately reproduces thedischarge phenomena of WEDM. The system alsoapplies an adaptive control, which automatically gen-erates an optimal machining condition for high pre-cision WEDM.4.2. WEDM process monitoring and controlThe application of the adaptive control systems tothe WEDM is vital for the monitoring and control ofthe process. This section investigates the advancedmonitoring and control systems including the fuzzy,the wire breakage and the self-tuning adaptive controlsystems used in the WEDM process.4.2.1. Fuzzy control systemThe proportional controllers have traditionally beenused in the servo feed control system to monitor andevaluate the gap condition during the WEDM process.However, the performance of the controllers was lim-ited by the machining conditions, which considerablyvary with the parameters settings. Kinoshita et al. 67investigated the effects of wire feed rate, wire windingspeed, wire tension and electrical parameters on thegap conditions during WEDM. As a result, many con-ventional control algorithms based on explicit math-ematical and statistical models have been developed forEDM or WEDM operations 6872. Several authors73,74 have also developed a pulse discrimination sys-tem providing a means of analysing and monitoringthe pulse trains under the various WEDM conditionsquantitatively.Althoughthesetypesofcontrolsystems can be applied to a wide range of machiningK.H. Ho et al. / International Journal of Machine Tools & Manufacture 44 (2004) 124712591251conditions, it cannot respond to the gap conditionwhen there is an unexpected disturbance 75.In recent years, the fuzzy control system has beenapplied to WEDM process to achieve optimum andhighly efficient machining. Several authors claimed thatthe fuzzy logic control system implements a controlstrategy, which captures the experts knowledge oroperatorsexperienceinmaintainingthedesiredmachining operation 76. In addition, the fuzzy logiccontroller does not require any comprehensive math-ematical models adapting to the dynamic behaviour ofthe WEDM operation 77. Several authors 75,78 pro-posed the sparking frequency monitoring and adaptivecontrol systems based on the fuzzy logic control andthe adjusting strategies, which can be applied to a widerange of machining conditions. Liao and Woo 79 alsodesigned a fuzzy controller with an online pulse moni-toring system isolating the discharging noise and dis-criminating the ignition delay time of each pulse. EDMpulses can be classified into open, spark, arc, offorshort, which are dependent on the ignition delay time,and have a direct influence on the MRR, SF, electrodewear and accuracy of the part 80,81.4.2.2. Wire inaccuracy adaptive control systemsThe occurrence of wire breakage during WEDM isone of the most undesirable machining characteristicsgreatly affecting the machining accuracy and perform-ance together with the quality of the part produced.Many attempts have made to develop an adaptive con-trol system providing an online identification of anyabnormal machining condition and a control strategypreventing the wire from breaking without compromis-ing the various WEDM performance measures. Thissection reports research from a collection of publishedwork involving the adaptive control of wire breakage,wire lag and wire vibration. Wire breakage. A wide variety of the controlstrategies preventing the wire from breaking are builton the knowledge of the characteristics of wire break-age. Kinoshita et al. 82 observed the rapid rise inpulse frequency of the gap voltage, which continues forabout 540 ms before the wire breaks. They developeda monitoring and control system that switches offthepulse generator and servo system preventing the wirefrom breaking but it affects the machining efficiency.Several authors 83,84 also suggested that the concen-tration of electrical discharges at a certain point of thewire, which causes an increase in the localised tempera-ture resulting in the breakage of the wire. However, theadaptive control system concentrating on the detectionof the sparking location and the reduction of the dis-charge energy was developed without making any con-siderations to the MRR. The breakage of the wire hasalso been linked to the rise in the number of short-circuit pulses lasting for more than 30 ms until the wirebroke 85.Other authors 86 argued that the wire breakage iscorrelated to the sudden increase in sparking fre-quency. It was also found that their proposed monitor-ing and control system based on the online analysis ofthe sparking frequency and the real-time regulation ofthe pulse off-time affects the MRR. Liao et al. 87 rem-edied the problem by relating the MRR to the machin-ing parameters and using a new computer-aided pulsediscrimination system based on the pulse train analysisto improve the machining speed. Whereas Yan andLiao 88,89 applied a self-learning fuzzy control strat-egy not only to control the sparking frequency but alsoto maintain a high MRR by adjusting in real time theoff-time pulse under a constant feed-rate machiningcondition.The breaking of the wire is also due to the excessivethermal load producing unwarranted heat on the wireelectrode. Most of the thermal energy generated duringthe WEDM process is transferred to the wire while therest is lost to the flushing fluid or radiation 86. How-ever, when the instantaneous energy rate exceeds a cer-tain limit depending on the thermal properties of thewire material, the wire will break. Several authors9092investigatedtheinfluenceofthevariousmachining parameters on the thermal load of the wireand developed a thermal model simulating the WEDMprocess. In addition to the sparking characteristics orthe temperature distribution, the mechanical strengthof the wire also has a significant effect on the occur-rence of the wire breakage. Luo 93 claimed that thewire material yielding and fracture contribute to thewire breakage, whilst an increase in temperature aggra-vates the failure process. Wire lag and wire vibration. The main factorscontributing to the geometrical inaccuracy of theWEDMed part are the various process forces acting onthe wire causing it to depart for the programmed path.These forces include the mechanical forces produced bythe pressure from the gas bubbles formed by theplasma of the erosion mechanism, axial forces appliedto straighten the wire, the hydraulic forces induced bythe flushing, the electro-static forces acting on the wireand the electro-dynamic forces inherent to the sparkgeneration 94,95.As a result, the static deflection in the form of a lageffect of the wire is critically studied in order to pro-duce an accurate cutting tool path. Several authors93,96,97 performed a parametric study on the geo-metrical inaccuracy of the part caused by the wire lagand attempted to model WEDM process mathemat-ically. Whereas Beltrami and Dauw 98 monitored andcontrolled the wire position online by means of anoptical sensor with a control algorithm enabling vir-1252K.H. Ho et al. / International Journal of Machine Tools & Manufacture 44 (2004) 12471259tually any contour to be cut at a relatively high cuttingspeed. A number of geometric tool motion compen-sation methods, which increase the machining gap andprevent gauging or wire breakages when cutting areaswith high curvatures such as corners with small radiihave also been developed 99,100. Lin et al. 101developed a control strategy based on the fuzzy logicto improve the machining accuracy and concentratedsparking at corner parts without affecting the cuttingfeed rates.In addition, the dynamic behaviour of the wire dur-ing WEDM is also restrained to avoid cutting inac-curacies. There are a few discussions on the design anddevelopment of a monitoring and control system forcompensating the behaviour of the wire vibration86,102. Dauw et al. 103 also reported that thevibration of the wire can be substantially reduced whenthe wire and the wire guides are completely submergedin the working tank filled with deionised water. Severalauthors 104 derived a mathematical model analysingthe transient response of the wire vibration based onthe force acting on the tool wire in a single dischargeprocess. A number of authors 105,106 reviewed theresearch and development of the various advancedmonitoring and control systems used in EDM andWEDM processes.4.2.3. Self-tuning adaptive control systemsIn recent years, the WEDM research and develop-ment has explored control strategies adjusting to thevariation in the power density required in machining aworkpiecewithvaryingthickness.Severalauthors82,85 found out that a change in the workpiece thick-ness during machining leads to an increase in the wirethermal density and an eventual breaking of the wire.Rajurkar et al. 107,108 proposed an adaptive controlsystem with a multiple input model that monitors andcontrols the sparking frequency according to the onlineidentifiedworkpieceheight.Otherauthors72developed a system that involves an explicit mathemat-ical model requiring a number of experiments and stat-istical techniques. Yan et al. 109 used the neuralnetworks to estimate the workpiece height and thefuzzy control logic to suppress the wire breakage whena workpiece with variable height is machined.The application of a knowledge-based control systemto control the adverse WEDM conditions has also beenexperimented. Snoeys et al. 110 proposed a knowl-edge-based system, which comprises of three modules,namely work preparation, process control and operatorassistance or fault diagnosis, enabling the monitoringand control of the WEDM process. The work prep-aration moduledetermines theoptimalmachiningparameter settings, while the operator assistance andfault diagnostics databases advise the operators anddiagnose the machining errors. Thus, the capabilities ofthese modules increase the amount of autonomy givento the WEDM machine. Huang and Liao 111 havealso indicated the importance of the operator assist-ance and fault diagnostics systems for the WEDM pro-cess.Theyproposedaprototypeartificialneuralnetwork-basedexpertsystemforthemaintenanceschedule and fault diagnosis of the WEDM. Dekeyseret al. 112 developed a thermal model integrated withan expert system for predicting and controlling thethermal overload experienced on the wire. Althoughthe model increases the level of machine autonomy, itrequires a large amount of computation, which slowsdown the processing speed and undermines the onlinecontrol performance.5. Discussion and future research directionsThe authors have classified the wide range of pub-lished works relating to the WEDM process into threemajor areas, namely optimising the process variables,monitoring and control the process, and WEDM devel-opments. This section discusses the classified WEDMresearch areas and the possible future research direc-tions, illustrated in Fig. 1.5.1. Optimising the process variablesTheoptimisationoftheWEDMprocessoftenproves to be a difficult task owing to the many regulat-ing machining variables. A single parameter changewill influence the process in a complex way 52. Thus,the various factors affecting the process has to beunderstood in order to determine the trends of the pro-cess variation, as discussed in Section 4.1.1. The selec-tion of the best combination of the process parametersfor an optimal machining performance involves ana-lytical and statistical methods. However, it is very com-plicated to relate the input process parameter with theoutput performance measures and derive an optimalresult using a simulated algorithm. The CR, MRR andSF are usually opted as the measures of the processperformance. Nevertheless, these methods provide aneffective means of identifying the variables affecting themachining performance.In addition, the modelling of the process is also aneffective way of solving the tedious problem of relatingthe process parameters to the performance measures.As mentioned in Section 4.1.2, several attempts havebeen carried out to model the process investigating intothe influence of the machining parameters on WEDMperformance and identifying the optimal machiningcondition from the infinite number of combinations. Asa result, it provides an accurate dimensional inspectionand verification of the process yielding a better stabilityand higher productivity for the WEDM process. How-K.H. Ho et al. / International Journal of Machine Tools & Manufacture 44 (2004) 124712591253ever, the complex and random nature of the erosionprocess in WEDM requires the application of determi-nistic as well as stochastic techniques 61. Therefore,the optimisation of the WEDM process will remain akey research area matching the numerous process para-meters with the performance measures.5.2. Monitoring and control the processOver the years, the monitoring and control systemshave made an important contribution in minimisingthe effect of disturbances on the WEDM performance.The multi-parameter machining settings have made itdifficult to clearly understand and obtain the optimalmachining conditions. It requires a control algorithmthat is often based on explicit mathematical and stat-istical models to cope with the machining process.However, the application of the fuzzy control logic hasbrought about a drastic change to the conventionalway of monitoring and controlling the WEDM pro-cess. The fuzzy control logic is able to consider severalmachiningvariables,weighthesignificantfactorsaffecting the process and make changes to the machin-ing conditions without applying the detailed mathemat-ical model, as mentioned in Section 4.2.1. In addition,the feasibility of applying the expert system capable ofgiving advice and solving problems has also beenexplored 110. Such a system would greatly appeal totheshopflooroperationalneedsdemandingunattended WEDM operation.The risk of the wire breakage and the bending of thewire have also limited the efficiency and accuracy of theWEDM process. The occurrence of the wire breakagedirectly reduces the already low machining speed affect-ing the overall productivity of the machining process.Although, the control strategies reported in Section4.2.2 are designed to solve the problems of wire break-age, it solely relies on the indication of the possibleoccurrence and generates inadequate results investigat-ing the root cause of the wire breakage phenomenon.These strategies may therefore be deemed to be a set-backwhenmachiningaworkpiecewithvariableheights requiring a drastic change in the machiningconditions.In addition, the wire vibrational behaviour and staticdeflection easily influence the geometric accuracy of thepart produced. The typical solutions to these problemsare often very conservative in nature by increasing themachining gap or reducing the discharge energy, whichis regarded to be a main drawback for the WEDMprocess efficiency. Fig. 2 shows the huge amount ofresearch work concentrating on the improvement ofFig. 1.Classification of major WEDM research areas (corresponding section numbers are in brackets).Fig. 2.Distribution of the collection WEDM research publications.1254K.H. Ho et al. / International Journal of Machine Tools & Manufacture 44 (2004) 12471259the inaccuracy caused by the wire through the appli-cation of an adaptive control system. Jennes and Snoey113 believed that the traditional research purpose wasnot to improve machining efficiency, but to preventfromwireruptureduringthemachiningprocess.Hence, one possible new WEDM challenge and futurework area will be steered towards attaining highermachining efficiency by acquiring a higher CR andMRR with a low wire consumption and frequency ofwire breakage.5.3. WEDM developmentsThe WEDM process is a suitable machining optionin meeting the demands of todays modern applica-tions. It has been commonly used in the automotive,aerospace, mould, tool and die making industries.WEDM applications can also be found in the medical,optical, dental, jewellery industries, and in the auto-motive and aerospace R&D areas 114. Its large poolof applications, as shown in Fig. 2, is largely owed tothe machining technique, which is not restricted by thehardness, strength or toughness of the workpiecematerial. As mentioned in Section 3, the WEDM of theHSTR, modern composite and advanced ceramic mate-rials, which is showing a growing tend in many engin-eering applications, has also been experimented. It hasreplaced the conventional means of machining cer-amics, namely the ultrasonic machining and laser beammachining, which are not only costly to machine butdamage the surface integrity of the ceramic component.However, with the introduction of over 20 non-tra-ditional machining processes in the past 50 years andthe rapid growth in the development of harder, tougherand stronger workpiece materials 115, the WEDMprocess inevitably has to be constantly rejuvenated inorder to compete and satisfy the future crucial machin-ing requirements.In addition, the WEDM process has sought the ben-efits of combining with other material removal meth-ods to further expand its applications and improve themachining characteristics. The authors have classifiedthe WEDM machine into the various physical char-acteristics, which clearly distinguishes the differenttypes of machine features affecting the performancemeasures, machining capacity and auxiliary facilities,as shown in Fig. 3. One of the most practical and pre-cision HMP arrangements is the WEDG process usedmainly to produce small size and complicated shapethin rod, which can be easily bent or broken by the lat-eral force when using conventional grinding process.The precision of the CNC system is also partly respon-sible for the accuracy of the WEDG 116. Therefore,the HMP processes, in particular the WEDG process,will continue to receive intense research attentionFig. 3.Classification of wire-cutting EDM machine.K.H. Ho et al. / International Journal of Machine Tools & Manufacture 44 (2004) 124712591255especially in the growing field of micro-electronics cir-cuitry manufacturing.There is also a major push toward an unattendedWEDM operation attaining a machining performancelevel that can be only achieved by a skilled operator.Such a goal has been partly fulfilled through the appli-cation of the CNC to control the machining strategies,to prevent the wire breakage and to automate the self-threading systems. An environmentally friendly andhigh-capacitydielectricregenerationsystem,whichautonomously maintains the quality of the dielectriccirculating within the WEDM machine, has also beenexperimented 117. However, due consideration stillhas to be given to improve the WEDM performanceand enhance the le
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