RESEARCH OF MICRO ELECTRO DISCHARGE MACHININGEQUIPMENT AND PROCESS TECHNIQUES

CHINESE JOURNAL OF MECHANICAL ENGINEERINGVolume 1Number 22002●177●RESEARCH OF MICRO ELECTRO DISCHARGE MACHINING .EQUIPMENT AND PROCESS TECHNIQUES'Li Yong Guo Min Li Fang Zhou Zhaoying T6|ADepartment of Precision Instruments and Mechanology, Tsinghua University, Beiing 100084, ChinaAbstract: Micro electro discharge machining (micro EDM) is a fcasible way to manufacture micro structures and haspotential application in advanced industrial fields. For the realization of micro EDM, it is necessary to pay careful atten-tion to is equipment design and the development of process techniques. The present status of research and developmentof micro EDM equipment and process tchniques is overviewed. A micro electro discharge machine incorporated with aninchworm type of micro feed mechanism is introduced, and a micro electro dischange machine for dilig micro holessuitable to industial use is also introduced. Some of the machining experiments crried out on the micro EDM prototypesare shown and the feasbility of the micro EDM technology to practical use is discussed.Key words: Micro elcto discharge machining Inchworm mechanism Friction drive mechanism Micro electrodeMicro hole Micro structuredue to the reduction of discharge gap in micro EDM0 INTRODUCTIONprocess is focused on. Impact drive mechanism/2] anddirect drive method!] for micro feed of the electrodeThe essential process mechanism of electro dis-were developed based on piezoelectric actuation.charge machining (EDM) is known as that the pulsedThis paper presents the development of a microdischarge between electrode and workpiece is used toelectro discharge machine incorporated with an inch-melt and evaporate the surface material with very highworm type of micro feed mechanism, and an electrotemperature and pressure, therefore remove it out fromdischarge machine for drilling micro holes to industrialthe workpiece. Due to the fact of non-contact ma-use. Then the machining experiments on the microchining and without cutting force, EDM process isEDM prototypes are carried out, and some examplesconsidered suitable to micro machining and to be oneof micro electrode rods, micro holes and micro stuc-of the feasible ways to produce microstructures.tures are shown.For the realization of micro EDM, it is necessaryto develop key technologies on both equipment and1 SYSTEMATIC DESIGN OF MICRO EDMprocess techniques. The research and development ofWITH INCHWORM MECHANISMmicro EDM technology have got great progress inboth machining size and accuracy in recent years. TheThe design configuration of micro electro dis-minimum size down to several μm order and the sur-face roughness down to 0.1 um are obtained. Thecharge machine with inchworm mechanism is depictedprocess techniques to shape micro electrode rods withas Fig.1l. The inchworm mechanism is used to moveeither circular cross-section or non-circular cCrOSS-the rotating unit forward and backward in feed direc-section not only to drill micro holes with high aspecttion for the electrode feeding. The wire electro dis-ratio but also to manufacture 3-dimension microcharge grinding unit together with the rotating unit areused to manufacture micro electrode. RC circuity isstructures have been developed.On equipment technologies, the development ofadopted for micro electro-discharge generation and awire electro discharge grinding (WEDG) methodcomputer cotrolled system is constructed to detectopened up a better way to manufacture a micro elec-and control machining process.trode in-process!"!. And micro feed of the electrode :中国煤化工large gap betweenelect2)M demands moreSelected from Procecdings of 2000 the First IntemationalfMH.CNMHGeding.AndontheConference on Mocbanical Engineering; revised manuscriptother hand, adding vibration or rotation onto electrodeAugust 22, 2001 ..178 Li Yong. et al: Research of micro clectro discharge machining equipment and process techniqueswhile it is feeding is certainly helpful to achieve stableThe minimization of Eap，down to the order ofand efficient machining. Based on such viewpoints, an0.1~ l.0μJ, is needed for micro electro-dischargeinchworm type of micro feed mechanism is designedmachining. RC circuity is a feasible selection for theto drive the rotating unit directly moving in electrodemicro electro discharge generation by decreasing thefeeding direction. The inchworm mechanism is fea-pulse width.tured with enlarged thrust capbility so as to meet theOn systematic construction, the in-process detec-requirement in micro EDM. lts concrete design andtion of the machining situation and the feeding controlprototype performance will be described in the fol-of eletrode are very important for micro EDM. In thislowing section.design, the sampled mean values of charge and dis-charge current are taken to monitor the machiningInchwormprocess. The curent signal capable of distinguishingmechanism'~normal machining, circuit short or circuit open is A/Dedconverted and fed back to the control computer. An，Pilara.adaptive control strategy is developed to optimize thelotatinganqpor↓l DIAfeeding response of the inchwom mechanism.head一ControlEloctrode刘这computer2 INCHWORM TYPE OF MICRO FEEDAIDMECHANISM__.兰_. Wie 、WEDO unit'ower Detectiog|supply| circuitryThe inchwom mechanism mainly consists of aElectrodemoving shaft, a tubular stacked piezoelectric element,xY stageWorkpieceand two radial clamping mechanisms mounted on twoFig. 1 Design configuration of micro EDMguide sleeves'1. The operating principle of the inch-with inchworm mechanismworm mechanism is fundamentally the same as usualones. Its clamping mechanism designed in order toWith regard to microelectrode making, there areincrease the thrust force of the inchworm mechanismseveral traditional methods such as wire redressing,is depicted in Fig.2. It is comprised of three groups ofcapillary tube drawing and reverse EDM. But com-levers distributed along circumference and connectedpared with those, the wire electro discharge grinding isone another by flexure hinges. The clamping mecha-capable of getting micro electrode with much smalernism grips the shaft by tight fit at the initial state.size and higher accuracy both in size and shape. AsWhen voltage is appied, electromagnetic forceF ex-shown in Fig.1, a micro electrode is shaped by a com-erts on 6 drive arms in 3 pairs. Thus each pair of drivepound movement of wire moving, electrode feedingarms is pulled separately along the circumferentialand rotating, based on electro-discharge machiningdirection of arc levers, and this makes the circumfe-principle.ence constructed with 3 arc levers increase. Conse-Thereically material volume rermoved by a sin-quently the 3 clamp pads gripping the shaft are movedgle pulse in EDM process is proportional to its dischar-away in radial direction and releases the shaft.ge energy. And discharge energy Esp of a single pulseSuppose that ! is the wbole length of drive arm, hcan be expressed asis the length of drive arm inside the arc levers, andnisthe number of groups of distributed levers, Then totalEap= ju()i()dtclamping force EF. exerted on the shaft has tbe fol-where bp t一Pulse width and discharge durationlowin中国煤化工time respectivelyTYHCNMHGFu(), i()Discharge voltage and curent re-The inchworm mechanism's output thrust force inspectivelyCHINESE JOURNAL OF MECHANICAL ENGINEERING●179.the feeding direction can be proportionally increasedrotating and feeding unit of wire electrode is so des-with the clamping force amplified, because the inch-igned as to enable the electrode to feed continuouslywom mechanism's motion is transferred based on the and compensate its length loss via two clampingfrictional force between the shaft and the clamp inmechanisms' switching control, under simultaneousgripping state.rotation. It consists of rotating shaft and clampingswitching mechanism for electrode feding. A cylindersupported by two bearings and belt-driven, serves asDriverotating carrier to rotate together with clampingswitching mechanisms and electrode inside it. The twoClampFlcxureclamping switching mechanisms, normally the closedone is fixed on the fore end of the piston moving inaxial direction, and another normally opened one is onthe back of flange without axial movement. In ma-chining process, the normally closed clamp enclaspsShaftthe electrode and feeds it in axial direction, togetherFig.2 Clamping mechanismwith the piston driven by friction feed mechanism.The prototype of inchworm mechanism developedAfter one feeding stroke, the switching signals areis able to generate static thrust force up to 50 N, andapplied to make the normally opened clamp hold thereaches 30 N in feeding movement. In the inchwormelectrode in its position, then let normally closedmechanism's movement operated at the stepping mode,clamp loose the electrode and back to its initial posi-the pitch of each step is about 6 pum with applied volt-tion with piston quickly. On this moment, if the nor-age 100~ 300 V and the velocity is determined by themally closed clamp enclasps the electrode and thestepping pitch and frequency. The resolution of itsnormally opened clamp looses the electode again, themovement reaches to 50 om by subdividing the ap-feeding and machining will be continued for the nextstroke.plied voltage during each step.Friction dive3 ELECTRO DISCHARGE MACHINE FORDRILLING MICRO HOLESF.The mechanical design of the electro dischargemachine for driling micro holes is depicted in Fig.3. Itconsists of a fiction drive micro feed mechanism, aMagotie coplirmagnetic coupling for feeding motion transfer, a r-tating and feeding unit of electrode, and machiningBeltsection.Friction drive mechanism is capable of operatingRouning gundwith high speed, small noise, no backlash, and runninglding umitsmoothly. And its simple configuration has advantage toenhance tranmission accuracy. In the friction drive feedmechanism, stepping motor rotates friction wheels andthen drives linear motion output bar in feeding direction,finally controls tool electrode feeding foward or back-中国煤化工wmward according to machining situation.MHCN M H Gipise|The wire with circular cross section is used as thetool electrode to save electrode exchange time. TheFig.3 Schematic view of EDM for dilling micro holes.180 Li Yong. etal: Research of micro clectro discharge machining equipment and process tchniquesIn machining process, a pulse power supply withagainst the exerted pre-press. Under the conditions ofhigh frequency is used to generate the required pulsemaximum contact stress 400 MPa, Young's moduluswave which is applied over to the electrode and work-215 GPa of the material, the possible maximum pre-piece. At the same time, the electrode is fed near to thepress is 495 N. The output thrust force of the frictionworkpiece till the discharge occurs. The electro dis-drive mechanism is about 50N, while the friction .charge machining situation is monitored through thefactor supposed to be 0.1.voltage or current sampling detection, and is fed backto the control software. Then the computer adjusts and5 MACHINING EXPERIMENTSoptimizes the factors of the pulse power supply or thefeeding speed of electrode, so as to reach a better ma-In all of the following machining experimentschining state.carried out on the micro electro discbarge machinewith inchworm mechanism, kerosene serves as dielec-tric liquid, brass wire is used for WEDG, and the elec-4 FRICTION DRIVE MICRO FEEDtrode rod's material is red copper. The output voltageMECHANISMof power supply is kept on 100 V and the rotatingspeed of electrode rod is set on 1 200 r/min.The friction drive mechanism for precise feed isFig.4 shows the SEM photographs of a rod withshown as Fig.3. The output thrust force of the frictioncircular cross -section and size of φ38 umX 600 um,drive mechanism depends on the normal press applieda rod with square cross-section and size of 55 pμmXon the contact surface of the friction fit. Two elastic55 um X 400 um, manufactured by WEDG methodstructures to provide prc-exerted normal press arewhere the capacitance for pulse electo discharge 1 000 pFsymmtrically set on the two outsides of the wholeand 100 pF are used respectively.friction drive chain. This makes the resultant force innormal direction onto supporting bearings of frictionwheels balanced to be zero, therefore minimize theaffection of rotating error of bearings to frictiontransmission accuracy.In the friction drive mechanism, suppose the step(向)HV: 20kV0.128 mmangle of drive motor is a, the diameter of drive sbaft,driven wheel and deceleration wheel are d, D and drespectively, then the output moving resolution of thelinear motion bar is calculated as follows.4= gndz_ ad,d22πD 2DThen there are two ways to obtain fine resolution,(6)HV: 20kV0.103 mmdividing step angle by electrical circuit or increasingFig4 SEM photographs of elecrode rodsthe transmission ratio. Here the stepping motor withtwo phases and 1.8° step angle is used. The step angleIn machining electrode rod with circular cross-is further divided into 25.9" by electrical circuit. Andsection, the rotation of the electrode rod is beneficialthe transmission ratio is selected as 10: 1 while theto its circularity. The movement of the wire along thedriven wheel's size is considered as compact as possi-guideway in WEDG unit can compensate the radialble. The calculated output moving resolution of theloss of the wire itself. and keep the macbining acura-中国煤化工lectrode rod withlinear motion bar is about 0.1um, while D, d and hcyinnon-Ciare 160 mm, 16 mm and 16 mm respectively.|YHCNMHGedoalybyuti.The output thrust force capability is mostly r-ing the wire movement in WEDG unit, but no rotationstricted by the bearing ability of surface materialof the electrode rod in machining process.CHINESE JOURNAL OF MECHANICAL ENGINEERING●181●Fig.5 shows a SEM photograph of micro bholesquick response required in micro gap between electro-drilled on stainless steel plate by using a micro electrodede and workpiece. And the combination of inchwormrod with circular cross-section under rotation. Capacitancemechanism, rotating unit of electrode and WEDG unit100 pF for pulse electro discharge is used.form the core motion system of the micro EDM. Inthis micro EDM equipment, RC circuitry is used forelectro-discharge with micro energy, and machiningsituation is detected and controlled by utilizing themean current in electro-charge and discharge.The machining experiments carried out on themicro EDM verify some key techniques to manufac-ture micro electrode rods, micro holes and microstructures on stainless steel and single crystalline sili-con materials. Micro electrode rods as small as φ25μm and micro holes with minimum size of less than0HV.20kV 342 m)φ50 μm are obtained.Fig.5 SEM photograph of micro holesTo industrial use, both machining eficiency andmachining accuracy, also the relability should beTo machine more complicated shape or even 3-taken into account in micro EDM equipment design.dimension, micro structures the process techrique ofAn electro discharge machine for drilling microscanning machining method by concentrating clectro-holes is developed, which is characterized with adischarge on the fore end of a simple shape electrodepulse power supply with high frequency for microis a better way$9. In this process, the electrode is fedelectro discharge generation, a micro feed mech a-step by step in z direction, and is moved to scan in xynism based on friction drive, a rotating and feedingplane between each step. The feeding step is set veryunit of wire electrode, and a subsystem to detect andsmall in z direction, so the fore end of electrode in-control machining process. Its preliminary machiningvolved in machining is totally lost in the scanning ofexperiments have shown the feasibility of the equip-one step. Thus the affection of the radial loss changingment design for practical application.shape is erased.ReferencesThe machining experiments are also carried out onthe electro discharge machine for dilling micro holes.1 Masuzawa T, Fujio M, Kobayashi K, et al. Wire electro-The machining conditions of power voltage 100 ~120 V,discharge grinding for micro. machining. Annals of thered copper wire electrode with diameter 0.2~ 0.4 mm,CIRP, 1985, 34(1): 431~434nd pure water as dielectric liquid are used. The pre-2 Higuchi T, Furutani K, Yamagata Y, etal. Development ofliminary experiments have verified the feasibility of thepocket-size eletro discharge machine. Annals of the CIRP,constructed prototype system. Further optimized control1991, 40(1): 203~206of machining process and experimental research are3 Mohr: N, Furutani K, Shirai K, et al. Forced dischargebeing made to achieve better machining accuracy anddispersion by dot-matrix method. Annals of the CIRP, 1997,efficiency.46(1): 139~142李勇，胡敏，周兆英，等，提高输出推力的蠕动式微进6 CONCLUSIONS .给定位机构。压电与声光，199 21(5): 407~4105 YuZ y, Masuzawa T, Fujino M. Micro-EDM for three-A prototype of micro electr-discharge machinedimensional cavities- -Development of uniform wear method.中国煤化工>~172with an inchworm type of micro feed mechanism isdeveloped. The inchworm mechanism for electrode CorrYHC N M H 8-1027222feeding has features of high feeding accuracy andE-mail: 1iy@pim.tsinghua.edu.cn