三维微细超声波加工研究@中英文翻译@外文翻译

prcesses are mostly limited in generating two-dimensional 2D! fea-tures. It is very difficult to control the size and shape in the z-axis.The corresponding generated structures have a low aspect ratio57#. LIGA processes, which include X-ray lithography, electro-forming, and plastic molding, are used to produce structures witha high aspect ratio. LIGA can process materials such as copperand nickel in electroforming and plastic molding. However, thistechnique cannot process hard and brittle materials, such as siliconand glass 8#. Mechanical manufacturing methods, such as millingand turning, have an ability to machine complex three-dimensional 3D! shapes in many metallic and nonmetallic mate-rials. Usually, the tool material is diamond and the machinedworkpieces are convex shapes 9,10#. However, the direct me-chanical contact between the tool and workpiece induces me-chanical deformation, heat generation, and distortion. The corre-sponding tool wear results in poor accuracy and high cuttingforces. When a microhole or a concave cavity is machined, thesize of tool and rotation speed of spindle limit the cutting speed,resulting in difficulty of machining brittle and hard materials.Electrical discharge machining EDM! and electrochemical ma-with 5 mm in diameter. However, the tool design and fabricationsometimes of multiple tools! and the adverse effect of tool wearon the accuracy of machined components continue to pose chal-lenges in achieving the full potential of micro-USM process.These problems become more critical for meso- microscale com-ponents. Therefore, the objectives of this paper are to study mi-crohole drilling by USM, to understand the extent of tool wearand the feasibility of generating arbitrary 3D microshapes usingsimple shaped tools such as cylindrical shape! with a capabilityof self-compensation for tool wear. The basic study of microholedrilling by micro-USM is described in Section 2 to study theeffect of machining parameters on process performance and theextent of tool wear. Section 3 outlines the uniform wear methodand its integration with CAD/CAM for generating 3D complexmicrocavities. The experimental results are presented in Section 4.The summary of this study is given in the final section.Study of Microhole Drilling by USMIn USM, the abrasive grains are vibrated between the tool andworkpiece by a high-frequency vibratory system. The vibratedabrasive grains impact on the workpiece surface, cause cracks,and finally remove the material from the workpiece and to a cer-Contributed by the Manufacturing Engineering Division for publication in theJOURNAL OF MANUFACTURING SCIENCE AND ENGINEERING. Manuscript receivedFebruary 11, 2004; revised September 2, 2004. Associate Editor: K. Dohda.Journal of Manufacturing Science and Engineering NOVEMBER 2004, Vol. 126 727Z. Y. Yue-mail: K. P. RajurkarA. TandonDepartment of Industrial & ManagementSystems Engineering,University of Nebraska-Lincoln175 Nebraska Hall, Lincoln, NE 68588-0518Study ofMachiningMany manufacturingmachining (EDM), andthe meso- and microscalecrystal, and ceramics(MEMS) devices. Ultrasonicsome of the hard andchining adversely affectsand to compensate theapplying the uniformand its integration withaccurate three-dimensionalshape remains unchangedKeywords: Micromachining,IntroductionMinimization technologies and microproducts are rapidly gain-ing industrial importance due to an increasing need for the effi-cient use of space and savings in resources, such as material andenergy. The pursuit of product minimization has lead to the devel-opment of microelectromechanical system MEMS! devices withthe capability of sensing and actuating 1#. Many micromachiningtechnologies have been developed to fabricate MEMS compo-nents and devices 2#. Besides the silicon the most widely usedmaterial in MEMS!, other materials, such as glass and quartz crys-tal, are increasingly being used in many industries. Glass is usedfor fabricating optical components or reaction chambers in bio-logical science 3# and quartz crystal known for its piezoelectricbehavior! is used for making microactuator elements. Ceramicswith resistance to high temperature and corrosion is used in mak-ing several components used in the automotive industries 4#.Micromachining technologies, such as lithography and etching,are used in the microelectronic industry to produce integrated cir-cuits. These processes are suitable for mass production and easy tointegrate with electronic control circuits. However, these pro-3D Micro-Ultrasonicocesses, such as lithography, etching, laser, electrical dischargeelectrochemical machining (ECM), are being applied to produceparts and products. Materials such as silicon, glass, quartzare being increasingly used in microelectromechanical systemmachining (USM) offers an attractive alternative to machinebrittle materials. However, the tool wear in micro-ultrasonic ma-the machining accuracy. Therefore, it is necessary to account fortool wear during machining. This paper reports the feasibility ofwear method developed for micro electrical discharge machiningCAD/CAM to microultrasonic vibration process for generating(3D) microcavities. Experimental results show that the tooland the tool wear has been compensated.Ultrasonic Machining, 3D Microshapeschining ECM! have been used to generate microshapes rangingfrom microholes to 3D complex shapes in only electrical conduc-tive workpiece materials 1113#. Laser is used to remove mate-rial from a workpiece regardless of its conductivity and hardness14#.ACO2or Nd:YAG laser can drill microholes or machinemicroslots. The difficulties of the z-axis control and the low aspectratio hole generation due to limitations of removing molten ma-terial from a deep hole! pose challenges in its application. Exci-mar or Femtosecond lasers can vaporize material without leavingany heat affected zone. However, similar to other beam machiningmethods, such as electron beam and focused ion beam, these pro-cesses face the problems of low machining efficiency and highinvestment costs for equipment 14#.Ultrasonic machining USM! is known for its ability to ma-chine brittle and hard materials, such as glass, silicon, quartz crys-tal, nitride, sapphire, ferrite, and fiber optics. A tool vibrating at afrequency of 20100k Hz with an amplitude of 0.025 mm gener-ates accurate, mirror-image-shaped cavities. In USM, the materialis removed by the impact of abrasive grains to which kinetic en-ergy is given by the ultrasonic vibration of an acoustic system.Micro USM has also been successfully applied to drill microholesTable 1 Machining conditions for micro hole drillingtain extent from the tool 15#. Figure 1 shows the basic principleof USM process. Usually, the sinking process is used in ultrasonicmachining, especially in the machining of 3D shapes. The tool hasthe same shape and size as that of the designed part 16#. Cur-rently, micro-USM is mainly applied to drill microholes 17,18#.The ultrasonic vibration is transmitted to the abrasive grainthrough an assembly of transducer, concentrator, and other relatedparts, resulting in a large eccentricity of tool rotation. The work-piece vibration has been proposed to solve this problem, and mi-croholes with 5 mm in diameter have been drilled successfully19#.In order to conduct the micro-USM drilling experiments, amicro-USM system has been designed and assembled as shown inFig. 2. The workpiece fixed on the top of the transducer is vi-brated by an ultrasonic vibrating system that includes an ultra-sonic vibration generator and a transducer at frequency of 39.5kHz. The slurry, a mixer of abrasive grains and water, is added onthe workpiece surface. An electronic balance serves as the staticload sensor with a resolution of 0.1 mg (9.831027mN). The 3Dtool-path movement is controlled by the X, Y, and Z mini-stagesand their controller. When a tool is controlled to move along adesigned tool path, the static load is monitored during machining.If the static load is larger than the set value, the tool is lifted up toa certain height to avoid the tool breakage because of overloading.Many factors have been found to influence the performance ofmacro-ultrasonic machining. The static load, amplitude of vibra-tion, type and size of abrasive grains, material and size of tool,and slurry concentration significantly affect the material removalrate and surface roughness. In micro-USM, however, it is verydifficult to maintain the slurry concentration at the same levelbecause the water easily vaporizes during micro-USM under ul-trasonic vibrations. Additionally, the external flushing causes thevibration of microtool and the variation of static load during ma-chining. Therefore, for these preliminary studies, the slurry con-centration was not considered as one of the input parameters. Thediamond powder is selected as the abrasive because it has highhardness and it does not fracture under the impact of ultrasonicvibrations.Experiments of microhole drilling by micro-USM were con-ducted to understand the influence of static load and tool size onthe material removal rate MRR!, tool wear ratio, and gap. Thetool tungsten! was brought down to touch the workpiece siliconwafer! surface, and this position was set as the starting point forhole drilling. The machining was completed when the set totaltool feed was reached. The machining conditions used in drillingby micro-USM are listed in Table 1. The tool was moved to theFig. 1 Diagram of USM principleFig. 2 Structure of experimental equipment728 Vol. 126, NOVEMBER 2004same reference point at the workpiece surface before and aftermachining. The tool was fed down to touch the surface at thestatic load of 10 to 20 mg (9.831025mN to 1.9631024mN)without vibrations. The tool wear length was calculated by ob-serving the difference in the z-axis. The tool diameter was mea-sured using an optical microscope. The hole volume was calcu-lated by measuring the tool diameter and the thickness of thesilicon wafer. The static load along the longitudinal axis wasmonitored and recorded by an electronic balance.Figures 3 and 4 show a microhole drilled by USM and themicrotool after machining, respectively. The hole diameter is 66mm. The total tool feed was 200 mm. Tool wear length was 11.2mm. Thus the gap which is half the difference between the tooldiameter and the measured diameter of the drilled hole! is about 8mm, which is almost double the largest grain diameter. The edgeof the hole appears to have irregular cracks, which may be be-cause of the crystalline fracture of silicon. It may be reduced ifsmaller abrasive grains are used although additional experimentalwork is needed to ascertain it. The diameter of the tool remainsconstant throughout the length. However, the tool tip becomesround due to wear during machining.Figure 5 shows that the MRR increases with an increase ofstatic load and tool diameter. The larger static load means largerpressure on the abrasive grains and eventually on the workpiece,resulting in an increase of MRR. The increase of the tool sizeFig. 3 USM-generated microhole drilling diam 66 mm Fig. 4 Tool after hole drilling diam 50 mm Amplitude of vibration 5 mmWorkpiece material SiliconTool material TungstenTool size 50, 100, 150 mmAbrasive grain type Polycrystalline diamond powderSize of abrasive grains 13 mmTotal tool feed 515 mmTransactions of the ASMEenlarges the working area and more abrasive grains get involvedin the material removal. The tool is also impacted by the abrasivegrain, which causes the local deformation at the tool working area,followed by the crack generation and material removal from thetool, leading to the tool wear. When the static load increases, theforce acting on the tool increases, resulting in the increase of thetool wear as shown in Figs. 6 and 7. The generated cracks at thesharp-edge portion of a tool lead to tool fracture at the edge por-tion more easily than the center part of the tool. The proportion ofthe wear on the edge part in a smaller size tool is higher than alarge size tool. Therefore, the tool wear increases with the de-crease in tool size as shown in Figs. 6 and 7. It is observed that thegap decreases with the increase of static load Fig. 8!. The staticload may also suppress the lateral vibration of the tool when thestatic load increases.The above-mentioned experimental results indicate the extentof tool wear during micro-USM. When a 3D microshape is ma-chined, a tool with microfeatures is needed to generate corre-sponding microfeatures. However, the preparation of a complexshaped microtool is a difficult task. Additionally, it is necessary tofabricate several complex shaped microtools to realize the accu-Fig. 5 MRR versus average static loadFig. 6 Tool wear length versus average static loadFig. 7 Tool wear ratio versus average static loadJournal of Manufacturing Science and Engineeringracy requirements of a design part because of the tool wear duringmachining 20,21#. The use of multiple tools leads to tool align-ment problems. Therefore, it is difficult and uneconomical to fab-ricate and use multiple microtools. To solve these problems, asimple shaped tool, such as a cylinder or square, can be used tomove along a designed tool path to generate 3D microshapes. TheWire Electrical Discharge Grinding WEDG! method is used toobtain simple shaped tools with various cross sections 22#.Ahigh tool wear occurring during machining prohibits the use of asingle simple-shaped tool traveling along the CNC generated pathto machine complex 3D microcavities. Therefore, an approach ofintegrating the Uniform Wear Method with CAD/CAM software,which compensates tool wear and generates desired tool paths, isused in the following attempt to generate 3D microcavities byUSM. This approach has been successfully demonstrated for mi-cro EDM 23,24#.3D Microcavities by USMTo generate complex 3D cavities with desired accuracy, it isnecessary to compensate the tool wear and generate the tool pathaccordingly. An introduction to the Uniform Wear Method and itsintegration with a commercial CAD/CAM software is presentedbelow from 23,24#!.Uniform Wear Method. The static load used to remove ma-terial from workpiece also causes wear of the microtool, resultingin local deformation and cracks on the microtool surface. To gen-erate accurate 3D microshapes it is necessary to compensate thetool wear in micro-USM. The basic principle of Uniform WearMethod is that under certain conditions, the shape of the tool isregained due to the tool wear after machining one layer. Thus, itmay become shorter in length but is able to regain the shape. Toenhance this phenomenon, the tool path design must include fol-lowing rules.Layer-by-Layer Machining. The 3D microshapes are ma-chined layer-by-layer in the z-axis using simple-shaped tools.With a small tool feed to each layer, the shape of tool tip caneasily be recovered after one layer machining without a deterio-rating effect on the machining surface.To-and-From Scanning. In one layer machining, the machinedsurface inclines from the start point of tool paths to the end pointof tool paths because the tool length becomes shorter due to thewear in machining. Reversing the tool paths, i.e., to-and-fromscanning, is helpful in reducing the inclination of generated sur-face. This can be verified using the tool wear model. To furtherimprove the machining accuracy, the cutting angle, i.e., the direc-tion of main paths is also changed.Tool Paths Overlapping. The edges and corners of a tool tipare worn more easily than the center part under the impact ofultrasonic vibration. The rounded edges and corners will be re-flected on the machined surface when the tool moves along theFig. 8 Gap versus average static loadNOVEMBER 2004, Vol. 126 729designed tool paths. Overlapping tool paths is to avoid the forma-tion of unmachined ridges at the surface due to the rounded edgesand corners of the tool.Machining the Central Part and the Boundary of the MachinedSurface Alternately. When the boundary is being machined, theedges of the tool become round due to wear. However, when thecentral part of the layer is machined, the static load at the center ofthe tool bottom is larger than the rounded edges, which are farfrom the machined surface. It causes the tool wear at the center ofthe tool more than at the edges of the tool. This help in recoveringthe original shape of the tool and a sharp corner can be obtainedbefore entering the next boundary path.Based on the above analysis, it is expected that the tool shapecan be maintained unchanged in micro USM when the tool pathsare designed based on the Uniform Wear Method.Tool Wear Compensation. The tool shape similarly can bekept unchanged in micro-USM. The ultrasonic machining using asingle tool can be considered as one similar to the milling process.However, it is necessary to compensate the tool wear length tomachine 3D microshapes correctly with a simple-shaped tool. Thecompensation equation can be derived from the definition of toolrelative volume wear ratio and the assumption that the tool feed ofeach layer h consists of two parts, the wear length htand theremaining length hw, which equals the average machined depth ofthe layer. The relationship shown in Fig. 9 can be written ash5hwSySwSt11D(1)where the tool wear ratio y can be measured and calculated in slotmachining as mentioned in the experimental section below. Thecross-sectional area of the tool Stis measured after the tool isfabricated. The area of each layer Swcan be obtained by slicingthe 3D microshape in layers along the z-axis. When the depth of asliced layer hwis known, only by adjusting the depth of cut hneeds to be adjusted to generate accurate 3D shapes.Integration of Uniform Wear Method With CAD CAM Sys-tem. There are many CAD/CAM systems available to generatetool paths for many machining operations, such as turning, mill-ing, and wire EDM. Howeve