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A Brief Review on Micromachining of Materials

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  98 P. Cardoso and J.P. Davim k % 2PbMZOP CaPe 4Z 4[ P   Rev. Adv. Mater. Sci. 30 (2012) 98-102Corresponding author: J. Paulo Davim, e-mail: pdavim@ua.pt A BRIEF REVIEW ON MICROMACHINING OF MATERIALS P. Cardoso and J.P. Davim Department of Mechanical Engineering, University of Aveiro, Campus Santiago, 3810-193 Aveiro, Portugal Received: December 05, 2011 Abstract. The demand for miniaturized devices with high aspect ratios and superior surfaceshas been rapidly increasing in advanced industries. There is a growing need for fast, direct, andmass manufacturing of miniaturized functional products from metals, polymers, compositesand ceramics. The current article presents a brief review on micromachining with special emphasisin micromilling. 1. INTRODUCTION The miniaturization of devices is today demandingthe production of mechanical components withmanufactured features in the range of a few to a fewhundred microns in fields that include optics,electronics, medicine, biotechnology,communications, and avionics, to name a few.Specific applications include microscale fuel cells,fluidic microchemical reactors requiring microscalepumps, valves and mixing devices, microfluidicsystems, microholes for fiber optics, micronozzlesfor high-temperature jets, micromolds, deep X-raylithography masks, and many more [1] .  As a response to this demand, various micro-manufacturing techniques have recently emerged,such as X-ray lithography electrodeposition molding(LIGA), deep reactive ion etching, deep UVlithography, electrical discharge machining, laser machining and computer numerical controlled (CNC)micromachining. Most of these techniques requireinaccessible, expensive, or time-consumingequipment [2] , so one of the viable micro-manufacturing techniques for creating three-dimensional (3D) features on metals, polymers,ceramics, and composites is mechanicalmicromachining. Micromachining utilizes miniaturemilling, drilling and turning toolsas small as 10  min diameter to produce micro-scale features. Al-though geometric and material capabilities of micromachining have been demonstrated by [3]industrial application of micromachining has beenhindered by the lack of experience and knowledgeon the micro-machinability of materials [4]. 2. CHARACTERISTICS OFMICROMACHINING -MICROMILLING Micromilling, one of the mechanical micromachiningmethods, is a process that utilizes end mills thattypically vary in diameter from 100 to 500  m andhave edge radii that vary from 1 to 10  m. Additionally, the micromilling process has severalsalient features that differentiate it from the macro-endmilling process. As the endmilling process isscaled down from conventional sizes (100  m/toothfeed rates, 1 mm depths of cut) to micro-endmillingsizes (1  m/tooth feed rates, 100  m depths of cut),different phenomena dominate the micro-endmillingprocess compared to those typically observed inconventional milling [5] .  5TMZ[W MZP gfX KL stated that the fundamental difference betweenmicromilling and conventional milling arises due toscale of the operation, in spite of being kinematicallythe same. However, the ratio of feed per tooth to  99  A brief review on micromachining of materials radius of the cutter is much greater in micromillingthan conventional milling, which often leads to anerror in predicting cutting forces. Also, the runout of the tool tip, even within microns, greatly affects theaccuracy of micromilling as opposed to theconventional milling.The chip formation in micromilling depends upona minimum chip thickness and hence the chip isnot always formed whenever tool and workpiece isengaged as opposed to conventional milling. Thetool deflection in the micromilling greatly affects thechip formation and accuracy of the desired surfaceas compared to conventional milling. The tool edge MPUa eUOMXXe NcZ q  m) and its uniformityalong the cutting edge are highly important as thechip thickness becomes a comparable in size tothe cutting edge radius [7]. Since the chip load issmall compared to the cutting edge radius, the sizeeffect and ploughing forces become significant onboth surface and force generation in micromilling.Micromilling may result in surface generation withburrs and increased roughness due to the ploughing-dominated cutting and side flow of the deformedmaterial when the cutting edge becomes worn andblunter.There are several phenomena in micromilling thatprevent the results of conventional milling from beingapplied to it directly. First, it cannot be assumedthat the microstructure of the workpiece material ishomogeneous [5]. As tool size becomes smaller,its effect becomes more important. In this work it cM aP MZ n %- YY [[X MZP R[ UYXUOUe TU effect was not assumed. Second, the effect of thecutting edge radius is not negligible: it affects thechip forming mechanism. Minimum chip thicknessis a function of this parameter, and determines the Fig. 1.  Inputs and influences in micromilling.transition between two cutting conditions; wherechips are produced and where ploughing takes place[6].  996 P .3.044. 8 MATERIALS Precision cutting tools and machine tools are criticalto micro-mechanical cutting processes, since thesurface quality and feature size of the micro-structures are dependent on them. Nowadays, thegeometries of micromilling tools are created byscaling down macro tools but due to the increasingminiaturization of components, it is becoming ever more complex to produce the required tools. Inaddition, several researchers [8-9] have shown thatmicro tools respond to influences in a very differentway than macro tools do.Conventional milling tools vary widely in size anddesign for different applications. In end milling, thecommon issues are tool deflection and unevendistribution of cutting force among the cutting edges.The forces are concentrated on the side of the tooland cause the tool to bend in the direction of theworkpiece feed. The extent of deflection alsodepends greatly on the rigidity of the tool and thedistance extended from the spindle. In fact, thedeflection is directly proportional to the cube of theextension [10] .  Also, the smaller the tool diameter,the more prone it is to deflection and this is evenmore so in micromilling, as the tools diameters areever so small.Tungsten carbide cutting tools are generally usedfor the micro-mechanical cutting process, due totheir hardness over a broad range of temperatures.In the early 1990s, use of coatings to reduce wear   100 P. Cardoso and J.P. Davim Workpiece microstructurePearlite, Ferrite, Ferritic and PearliticCutting edge radius2.0 and 5.0  m Axial depths of cut50 and 100  mFeed rates 0.25, 0.5, 1, 2and 3  m/fluteSpindle speed 120,000 rpm Table 1.  Workpiece and machining parameters [5].and friction became more common and most of thesecoatings are referred to by their chemical composi-tion, such as TiN (Titanium Nitride), TiCN (TitaniumCarboNitride), TiAlN (Titanium Aluminum Nitride) or TiAlCrN (Titanium Aluminum Chromium Nitride),among others. Advances in end mill coatings arebeing made, however, with coatings such as Amor-phous Diamond and nanocomposite physical vapour deposition (PVD) coatings. In 2006, Arumugam, et al.  [11] investigated the performance of polished CVDdiamond tool carbide inserts in comparison withunpolished CVD diamond coated carbide tool in-serts in the dry turning of A390 aluminum, a siliconhypereutectic alloy and concluded that polishedchemical vapour deposition (CVD) diamond tool in-serts improve tool life and reduce the cutting forces.However, the size of micro end mills makes coatingdeposition challenging especially around the cut-ting edges. The requirements on the coatings for micro machining tools are not only the desirableproperties such as high hardness, high toughnessand high chemical/erosive and abrasive wear resis-tance, but they must also be dense, have a finemicrostructure and present a smooth surface to theworkpiece, with a reduced coefficient of friction com-pared to that of the uncoated tool [7]. 4. MICROMACHINING OF MATERIALS  P .482 700 The most important machining parameters inmicromilling are spindle speed, feed rate and feedper tooth. Literature shows that many studies havebeen done to show up to which extent theseparameters influence the quality of the machinedparts and the consequences on the tool. In Fig. 1 adiagram of the inputs and influences in micromillingis shown.In 2008, Filiz et al  . [8] investigated the use of the mechanical micromilling process for fabricationof micro-scale piercing element from biocompatiblematerials. The authors used two custom made,special geometry, tools with cutting diameters 254  m and 101.6  m. To investigate the effects of feed,speed, and axial depth of cut on the performance of the tools, a design of experiments study was con-ducted on polymethyl methacrylate (PMMA). Theinvestigation was done based on two spindle speeds(50000 and 100000 rpm), two feeds (1, 5  m/flute),and two axial depths of cut (10, 20  m). They con-cluded that the spindle speed has the most promi-nent effect for all force components, and increasein spindle speed caused an increase in forces.  2X[ UZ %%- 5TMZ[W MZP gfX KL R[YP experimental and modelling studies on meso/micro-milling of AL 2024-T6 aluminum and AISI 4340 steelto predict chip formation and temperature fields.They also studied size effects and minimum chipthickness. To conduct this study, the authors used2-flute tungsten-carbide on cobalt matrix WC-Co ZP YUXX cUT (%p TXUd MZSX PUMY -, YY and 3.175 mm and a fixed spindle speed of 60000rpm. Cutting speed used was 22.62 m/min and 59.85m/min and feed per tooth varied from 0.265  m to4  m. Large force variations were observed as thediameter of the cutter decreased and the spindlespeed increased.In order to study the influence of the tool edgecondition and the workpiece microstructure, Vogler and his colleagues [5] in 2004 performedexperiments with 508  m diameter end mills onworkpiece materials with different microstructuresover a range of feed rates. Four materials wereselected for the experimentation; two speciallyprepared, single phase materials (pure ferrite andpearlite) and two multi-phase materials with differentcompositions of the two single phase materials.They performed 5 mm long full-slot endmilling cutsunder several conditions in order to study theinteraction between ploughing and process conditioneffects on the surface roughness of the slot floor.The conditions the authors used can be seen inTable 1.In 2007, Filiz et al. [4] used a miniature machinetool to perform micromachining experiments on99.99% purity Copper. This machine tool wasequipped with a 160,000 rpm air-turbine, air-bearingspindle with a 3.125 mm precision collet. Thespindle-axis runout was quoted by the manufacturer to be less than 2  m. The micro end mills usedduring the experimentation were micro-graintungsten carbide (WC) tools, fabricated by diamondgrinding, two-fluted and with a 254  m diameter anda 30 j  helix angle. This experimental study includedfull-immersion (slot) cutting with axial depth of cutof 30  m. Four feed rates (0.75, 1.5, 3, and 6  m/flute) and three cutting speeds (40; 80, and 120  101  A brief review on micromachining of materials m/min) were considered in this experimentation. Therange of feed rates was selected to include theploughing, indentation, and minimum chip thicknesseffects in the data. The spindle speed variedaccording the feed rates: 50,000 rpm for 0.75  m/flute, 100,000 rpm for 3  m/flute and 150,000 rpmfor 6  m/flute. Fig. 2. 4[YMU[Z [R MSU R[ T MY oY[[T RP M/ M O[ZMZ [bXM UMX N MMXXX spiral and (c) parallel zigzag. (a)(b)(c)Fig. 3.  Surface profile comparison between strategies for the same feed rate of 6 mm/tooth: a) constantoverlap spiral, b) parallel spiral and c) parallel zigzag.Recently, Cardoso and Davim [12] in order toperform a comprehensive study on surfaceroughness of the machined surfaces, cuttingparameters such as feed rate as well as machiningstrategies were varied to optimisation micromilling.In this research, Al 2011 aluminium alloy was used.It is an Al-Cu-Bi-Pb age-hardened alloy noted for its
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