Mechanical Micro-machining · 2017-08-29 · Mechanical Micro-machining Kushendarsyah Saptaji*...

Mechanical Micro-machining

Kushendarsyah Saptaji*School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore, Singapore

Abstract

As a result of the current trend toward products miniaturization, there is a demand for developmentin micro-manufacturing technologies in order to have better quality products, and cheaper, moreefficient, and effective processes. Miniaturized products and components in the range from a fewhundred micrometers to a few micrometers size are becoming common and widely used in dailyhuman life. The micro-products and micro-components are used in many industries especiallyrelated with micro-electromechanical, aerospace, medical, environment, biomedical and biochem-ical industries, and also in the field of chemistry. Many manufacturing methods have been developedto produce these micro-sized products, namely micro electro mechanical system (MEMS) basedprocesses such as dry etching, lithography, electroplating, ultraviolet - lithographie galvanoformungabformung (UV-LiGA), non-conventional based micro-machining such as microelectron dischargemachining (EDM), and mechanical micro-machining. This chapter discusses mechanical micro-machining especially challenges related with this process. Some main challenges are discussed inthis chapter such as size effect, microstructures of the materials, surface quality, burr formation, andmicro-tools performance. Some solutions are offered in order to solve these challenges.

Introduction

Various researchers have looked to alternative processes such as mechanical micro-machiningbecause of its advantages. The mechanical micro-machining process can increase the efficiency,reduce cost, save energy, produce high aspect ratio, have wider range of materials, have flexibility,and simplify the production of micro-parts and micro-components.

A mechanical micro-machining process such as micro-milling is more convenient than a MEMSbased process to produce high aspect ratio structures with complex geometry (Bissacco et al. 2005).Furthermore, MEMS based processes are usually limited to silicon while mechanical micro-machining can be applied to a wider range of materials (Kussul et al. 1996). Meanwhile, lithographycan be applied only to metals that can be easily electroplated (Hupert et al. 2007; Becker et al. 1986).Mechanical micro-machining is a flexible process that can also be applied to silicon based substratematerials. Rusnaldy and Kim (2008) have conducted micro-milling experiments on silicon in orderto obtain ductile regime machining. Morgan et al. (2004) micro-machined glass and soda-lime glassmaterials using polycrystalline diamond (PCD) tools made by micro-EDM. Micro-milling has alsobeen used to produce 3D features on polycarbonate materials (Lee et al. 2006).

*Email: [email protected]

Handbook of Manufacturing Engineering and TechnologyDOI 10.1007/978-1-4471-4976-7_12-1# Springer-Verlag London 2014

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Micro-milling can be used to produce micro-features because it is simpler and less time consum-ing. Hyuk-Jin and Sung-Hoon (2007) have shown, by cost estimations, that the micro-millingprocess produces cheaper aluminum embossing mold injection molding than lithography basedprocesses. Nevertheless, besides its advantages micro-milling is limited in its ability to create sharpinterior corners because of the finite radius of the milling tool (Friedrich and Kikkeri 1995).

Recently, mechanical micro-machining especially micro-milling is applied in the production ofembossing mold used for hot embossing process. By using the micro-milling process, the micro-features in the embossing mold are produced directly by removing the materials to form the positivefeatures on the workpiece; an additional finishing process might be needed to make the featuressmoother. Guber et al. (2004) produced a brass embossing mold with channel width and depth of50 mm each and sidewall roughness of about 200 nm using micro milling. Hupert et al. (2007) alsodeveloped embossing mold for hot embossing using micro milling on a 6.3 mm thick brass plate,with tool diameter as small as 50 mm (Fig. 1). The mold made by micro-milling requires fewer stepsand the result is comparable to that developed with LiGA (Hupert et al. 2007). Mecomberet al. (2005) also made low cost micro-embossing molds from aluminum, milled with a 51 mmdiameter high density micro-grain carbon tool using conventional CNC milling. Moreover Bissacoet al. (2005) applied milling tool diameter of 200 mm to produce microinjection molds made fromhardened steel.

The cutting phenomenon of micro-machining is different than conventional macro machining, inwhich the conventional metal cutting principles cannot be directly applied. There is no generaldefinition which exists for the micro-machining process. Liu et al. (2004) defined the micro-machining process occurs when the cutting edge radius becomes comparable to the depth of cut.

Fig. 1 SEM of brass embossing mold produced using 200 mm diameter micro-milling tool (a, b) and the replicatedembossed into PMMA (c, d) (Hupert et al. 2007)


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According to Chae et al. (2006) micro-machining is the machining process to create features thatrange from tens of micrometers to a few millimeters in size. In addition, Min et al. (2006) definedmicro-machining as machining with a tool whose dimension is on the order of the average grain sizeof the workpiece material and/or the specific feature being generated or machining with a tool whosedimension is small enough to lose isotropic homogeneity with respect to the workpiece material.While Uriarte et al. (2007) categorized micro-milling as the process with feed per tooth less than1 mm, depth of cut in the range of 2–15 mm, spindle rotation speeds higher than 50,000 rpm, and tooldiameter less than 0.3 mm.

Moreover, the micro-machining process will be more efficient when it is applied in the desktoprather than computer numerical control (CNC) machine. By using micro-machining desktop, theconsumption of energy saved can even be as large as three magnitudes when compared to CNC(Liow 2009). Most of the energy used when producing micro-parts in conventional CNC is to movethe tables and to rotate the spindle at the highest rpm to compensate for the small chip sizes.

There are some aspects that must be considered during the micro-machining process such as sizeeffect, multiphase microstructure of materials, surface quality, burrs, and micro-tools (Liuet al. 2004; Chae et al. 2006). These aspects are related to each other and simultaneously influencethe process performance and quality of the machined workpiece. These aspects are discussed below.

Size Effect

Size effect in cutting process occurs when the uncut chip thickness (depth of cut) is on the same orderas that of the cutting edge radius. This phenomenon generally occurs when cutting at the micro scaleor cutting using a blunt tool. One of the significant influences of size effect is the increasing of energydissipation. Lucca et al. (1991) observed that energy dissipation increases with the decreasing uncutchip thickness when orthogonal cutting oxygen free high conductivity (OFHC) copper using a singlepoint diamond. Afterwards Lucca et al. (1993) also presented that the specific cutting energyincreases as the depth of cut decreases for sharp tool (new) and blunt tool (worn). They arguedthat rake angle has significant effect on the specific cutting energy when the cutting edge radius is inthe same order of depth of cut. In this condition, the rake angle becomes negative due to the radiusshape at the tip of the tool.

Consequently, as the cutting edge radius increases, the cutting force also increases due to theoccurrence of two mechanisms in the process which are shearing (chip removal) and ploughing(elastic deformation) (Liu et al. 2006). Increasing the cutting force and the dominant effects ofploughing could generate the tool wear faster (Uriarte et al. 2007) due to the increase of heat fromchip-tool rake friction and excessive tool deflection (Lucca et al. 1991; Liu et al. 2006). Ploughing,adhesion, and asperity deformation are believed to be the main effects of the variation of thecoefficient of friction along the tool-chip contact interface in micro-cutting (Venkatachalam andLiang 2007).

In contrast, the increase of cutting edge radius and negative rake angle are favorable in the micro-cutting of brittle materials such as silicon. The negative rake angle produces high hydrostaticpressure that enables plastic deformation to occur in front of the cutting edge resulting in the ductilecutting mode (Yan et al. 2009). Negative rake angle and small depth of cut are the conditions neededto achieve ductile cutting mode where beyond this point brittle cutting conditions occurred resultingin more damage on the surface and sub-surface. In ductile cutting conditions, the amorphous phasewill be produced in the subsurface (Minowa and Sumino 1992), and the amorphous layer thicknessincreases in proportion to depth of cut (Yan et al. 2009).


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Ploughing occurs when the chip thickness (h) is below the minimum chip thickness (hm) or whencutting using large edge radius and small uncut chip thickness, as shown in Fig. 2. Aside fromyielding higher forces, a significant amount of ploughing also generates more burrs.

The cause of ploughing and size effect in micro-machining is still not well understood. Waldorfet al. (1998) proposed a slip-line model by considering the ploughing effect to predict the forces inorthogonal cutting. Liu et al. (2006) developed a model for predicting the minimum chip thicknessvalues that occur for ploughing, especially for 1040 steel and aluminum alloys Al6082-T6. Theyfound that the normalized minimum chip thickness (ratio of the minimum chip thickness to thecutting edge radius) was increased as the cutting velocity and tool edge radius increased whenmachining carbon steels due to the more dominant effect of thermal softening than the strainhardening for carbon steels. The normalized minimum chip thickness was found to almost stayconstant over a range of cutting velocities and tool edge radii when micro-machining Al6082-T6.

Joshi and Melkote (2004) developed a strain gradient plasticity model in the primary deformationzone to explain the size effect. They proposed a model to calculate the specific energy. Laiet al. (2008) argued that material strengthening behavior is the main cause of the size effect inmicro scale cutting. The strain gradient plasticity model is applied to model the material strength-ening behaviors. Liu andMelkote (2006) also observed the size effect using strain gradient plasticitymodel and found that the material strengthens due to the strain gradient as the uncut chip thicknessreduces to a few microns. Subbiah and Melkote (2006) observed the constant cutting force occurredat high rake angles (>70�). High rake angles minimized the energy in shear and friction due to theminimum shear in the chip. They found that there was a constant force component that will notchange with a change in uncut chip thickness.

Aramcharoen and Mativenga (2009) investigated the size effect when micro-milling H13 hard-ened tool steel. They found that the specific cutting force increases non-linearly when the ratio ofdepth of cut to the cutting edge radius is less than 1 (Fig. 3). This phenomenon occurred due to thesize effect where the ploughing and elastic deformations are dominant. The specific cutting force iscalculated by dividing the feed force by product of chip load and axial depth of cut. The ratio of thedepth of cut to the cutting edge radius for different materials is mainly affected by the microstructure,hardness, material phase, and tool geometry. In certain cases of micro-machining, the cutting edge ofthe tool can also be assumed to be perfectly sharp when the minimum chip thickness is 40 % of thecutting edge radius for aluminum (Bourne et al. 2011). The use of diamond tool can minimize thesize effect or cutting edge radius effect. However the diamond cannot be used to cut ferrous materialsdue to the excessive wear of the tool.

Several works have also been conducted to investigate the size effect in micro-cutting usingcommercial finite element software. Weber et al. (2007) found that the non-linear increase of thespecific cutting force with decreasing depth of cut is strongly influenced by the rate dependentmaterial model when investigating the contribution of rounded cutting edge to the size effect in

Fig. 2 Relation of minimum chip thickness with cutting edge radius (Chae et al. 2006)


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cutting and the characteristics of machined surface. It is observed that the increase of cutting speed isfollowed by the increase of specific cutting force and the depth of plastic zone increases almostlinearly with cutting edge radius.

Multiphase Microstructure of Materials

Another important phenomenon that distinguishes micro-cutting from conventional macro-cuttingis the assumption of a non-homogeneous workpiece as the depth of cut is on the same order as thesize of the grains (microstructure) and the cutting edge radius (Chae et al. 2006). In the conventionalcutting process the depth of cut is relatively larger than the grain size; hence the workpiece isconsidered homogeneous and the cutting edge is sharp. In contrast, in micro-cutting the depth of cutis relatively comparable with the grain size and cutting edge radius; hence the workpiece materialscannot be considered as homogeneous and the cutting edge radius has a value. Figure 4 showsworkpiece has homogeneous and isotropic properties in macro-cutting while in micro scale the grainsize is relatively comparable with the depth of cut.

Fig. 4 Schematic view of relation between grain size and depth of cut in orthogonal cutting for macro (left) and micro(right) (Bissacco et al. 2005)

Fig. 3 The specific cutting force in different ratio of uncut chip thickness (Aramcharoen and Mativenga 2009)


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In micro-cutting, the grain size influences the surface generation or surface integrity of themachined workpiece more significantly than in conventional cutting (Wang et al. 2008; Vogleret al. 2004; Simoneau et al. 2006a). The microstructure of workpiece effects the cutting forces andthe chip formation more prominently if the materials consist of several phases with differenthardness (Dornfeld et al. 2006). The forces fluctuate at different materials phase or state due tothe different plastic deformation behavior. In addition, micro-cutting has a dominant vibrationproblem compared to conventional macro-cutting due to the multiphase microstructure (Vogleret al. 2004). Vogler et al. (2004) found that multiphase materials and size effect influence themechanism of chip formation and surface generation especially in the bottom of the slot along thefeed direction in the micro end-milling process.

The cutting force in the micro-cutting process is also affected by the grain boundary of polycrys-talline materials (Furukawa and Moronuki 1988). Furukawa andMoronuki (1988) observed that themean values of cutting forces are almost the same but the dynamic components of the forces aredifferent under the same cutting conditions for grain size of 160 and 620 mm when micro-cuttingaluminum alloys. The dynamic variation of forces at grain boundaries of polycrystalline materials isaffected mainly by different properties of each grain and different grain orientations. In addition tothe grain effect, the crystallographic orientations and active dislocation slips systems also have aneffect on the surface quality and edge conditions especially burr formation (Min et al. 2006). Minet al. (2006) observed that the cutting mechanism, variation in the surface finish, and edge conditionsuch as burrs are influenced by the crystallographic orientation and active dislocation slip systemswhen machining single crystal and polycrystalline oxygen free high conductivity (OFHC) copperusing fly-cutting operation. It is argued that for polycrystalline materials, ductile plasticity and chipformation in machining are usually accomplished with the movement of dislocations along thepreferred slip planes in the shear zone ahead of the tool. Moreover, there are certain crystallographicorientations more favorable for relatively easy dislocation glide and chip formation in single-crystalmaterials.

In the micro-cutting of steel, Schimdt et al. (2002) suggested that the steel needs to be heat treatedin order to have the finest and uniformly distributed carbides so that constant cutting conditions canbe achieved in the micro-cutting process. In addition, the challenge in the micro-machining of steelis also due to the presence of pearlite and ferrite phases. These phases affect surface quality andforces. The micro-cutting of AISI 1045 steel that consists of pearlite and ferrite phases producedprows, micro-voids, micro-cracks, and surface dimples on the machined surface (Simoneauet al. 2006b) (Fig. 5). In addition, surface dimples also occur at a “hard to soft” grain boundary.

Fig. 5 SEM images of the defects in the machined surface of steel material. The dimpled surface in (a) shows examplesof prows (P) and microvoids (V), microcrack (C) is shown in (b), (c), and (d) show dimples on the machined surface.Cutting direction is indicated by the large arrows (Simoneau et al. 2006b)


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Surface Quality

Producing good quality, appropriate surface finish, and geometry are important for the machinedworkpiece. The surface finish or surface texture based on ASME (1985) is defined as geometricalirregularities of solid materials surface while surface roughness is defined as the finer irregularities ofthe surface texture, usually resulting from the inherent action of the production process, such as feedmarks produced during machining. The surface roughness is commonly indicated by parameterssuch as average roughness (Ra) or root mean square roughness (Rq) and calculated by Eqs. 1 and 2,respectively.

Ra ¼ 1

L

ðL

0

Y xð Þj jdx (1)

Rq ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1

L

ðL

0

Y xð Þð Þ2dx

vuuut (2)

where:

L is the sampling lengthY(x) is the ordinate of the profile curve

Surface finish and surface integrity of the machined workpiece affect wear, fatigue resistance,corrosion resistance, crack initiation and propagation, creep life, and friction during the componentuse. Surface roughness measurement can also be used to predict the criterion of tool wear in micro-milling (Hongtao et al. 2008). Cutting velocity andmaterial removal volume have a significant effecton tool wear, which in the end will affect surface roughness. Geometry of tool, cutting parameters(spindle speed, feed rate, and depth of cut), irregularities in cutting operation (tool wear andlubricant), and material properties of tool and workpiece are several factors influencing the surfacequality especially in the micro-milling process (Dimov et al. 2004). The tool wear especially affectsthe wall surface quality of the slot (Schmidt and Tritschler 2004).

By applying low feed rate, better surface finish can be achieved but it gives lower materialremoval rate (Miao et al. 2007). It is observed that the surface roughness increased as the tool edgeradius increased in micro-end milling (Vogler et al. 2004). In the micro-milling of copper materials,feed rate has a significant influence on surface roughness and burr height, spindle speed has a slightinfluence on the surface roughness and burr formation, whereas axial depth of cut has no significantinfluence on the surface roughness (Huo and Cheng 2010).

In micro-milling a slot, the side wall where the tool edge finished the cut (down-milling) has bettersurface quality and dimensional accuracy than the other wall where the cutting edge enters into thecut (up-milling) (Min et al. 2008). The up-milling process produces uneven and rough surfacesespecially in the wall due to the elastic deformation and recovery. Min et al. (2008) observed thisphenomena when micro-milling annealed austenitic stainless steel 304 and aluminum 6061-T6511with the grain size varied in between 5 and 25 mm using two-fluted WC-Co-Carbide (8 % cobalt)micro-end-mills with 254 mm diameter.


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In order to produce low surface roughness in micro-cutting of steel using tungsten carbide tool,Schmidt et al. (2002) and Weule et al. (2001) recommended using high cutting velocities and hard,homogeneous material (Fig. 6). As can be seen in Fig. 6, in the soft material states (T450), surfaceroughness increases significantly at lower cutting velocities. In the case of micro-milling harder AISIH11 hardness 52 HRC, the up-milling has the largest roughness when the tool is new and turned tohave better surface finish when reaching the stabilized cutting edge conditions. In contrast, in themicro-milling of soft AISI H11 of 42 HRC hardness, the lowest surface roughness was found in themiddle of the milling trace (Schmidt and Tritschler 2004).

The quality of the micro-milled surface is also affected by the microstructure of the workpieces(Wang et al. 2008). In macro-size milling, the workpiece material is regarded as isotropic hence thesurface roughness is affected mainly by the milling parameters (Wang et al. 2008) whereas in micro-milling, the grain sizes of the workpiece are normally between 1 and 100 mm, and the tools’ tip edgeradii are less than 5 mm. When the tool passes through the workpiece along the length of the tooledge, there are only a countable number of grains being cut.

Burr Formation

Another aspect of surface quality that is normally discussed and seen in practice is burr formation.Burr is defined as the material plastic deformation produced at workpiece edges as a result of themachining or shearing process (Gillespie 1999). The existence of burrs on the workpiece may causeproblems related to the dimensional accuracy, surface finish, ease of parts assembly, and furthermoreincrease the cost and time production for deburring (Chern 2006). Schaller et al. (1999) suggestedthat proper selection of machining parameters, quality and sharpness of cutting edge, cutting fluidand also material properties of the workpiece are several factors affecting the burr dimensions.In general, the burr dimensions especially those produced in micro-machining are difficult toquantify mainly due to the limitation of metrology equipment. One method that is commonlyused to measure burr is the stylus method. However this method is only suitable to measure burrheights (Aurich et al. 2009).

Fig. 6 Influence of cutting velocity and material state on the surface roughness in the micro-milling of steel (Weuleet al. 2001)


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According to Gillespie (1999) there are six physical processes which form burrs: lateral flow ofmaterial which occurs whenever a solid is compressed, bending of material, tearing of chip fromworkpiece, redeposition of material, incomplete cut-off, and flow of material into cracks. Hashimuraet al. (1999) categorized burrs in face milling according to burr locations, burr shapes, and burrformation mechanisms. Exit burr is defined as the burr attached to the surface machined by the minoredge of the tool, side burr is the burr attached to the transition surface machined by the major edge,and a top burr is defined as a burr attached to the top surface of the workpiece. While Chern(Chern 2006) classified five types of burrs in face milling operation: knife type, wave type, curl type,edge breakout, and secondary burr. Whereas in the micro-milling process, Chern et al. (2007)categorized burrs as primary burr, needle-like burr, feathery burr, and minor burr. The primaryburr, feathery burr, and needle-like burr are mainly produced on the up-milling side of the micro-slotwall. In general, the burrs in the micro-milling process are formed when the cutting edge leaves thesurface being cut.

As feature and/or part sizes become smaller, burr problems are more difficult to solve becausewhile the burr size also becomes small, the burr to feature size ratio increases. Most prominent burrsin micro-milling occur on the top edge of the side walls of the slots, called top burrs (Fig. 7). Incontrast to the surface quality of the side walls in the micro-slots, up-milling has smaller burrs thandown milling. In the up-milling side the burr is a Poisson burr formed only by side bulging action.On the down milling side, the top burr is formed by the action of the chip material tearing away as itflows as well as side bulging deformation; hence the down milling burrs tend to be larger (Saptajiet al. 2012).

The ductility or brittleness as well as the strain hardening behavior of materials have importantcontributions to the burr size (Gillespie 1979). Burr formation in ductile materials is related to thehigh elastic–plastic deformation that occurs in the machining, larger and more burrs are likely to beformed with increasing ductility (Shafer 1975). In the micro-grooving of aluminum alloys such asAl5083-H116 using a single-crystal diamond tool, top burr mainly occurs ahead of a tool and iscaused by expansion of material compressed after starting to flow around a tool rather than becomingpart of a chip. While exit burr formation is formed when a thin membrane of material forms ahead ofa tool and splits into two side segments and one bottom segment as the tool exits a workpiece(Bourne et al. 2011).

The burr formation in brittle or hard materials is mainly affected by the tool wear (Weuleet al. 2001). A broken tool has a larger negative rake angle, increases ploughing, and in the endincreases exit burr dimensions significantly (Liang et al. 2009). In the micro-milling of hardmaterials such as steel, the down-milling side has more burrs (Schmidt and Tritschler 2004).Schmidt and Tritschler (2004) observed that the harder materials state has fewer burrs, however

Fig. 7 The burrs location in the slot-milling process (Gillespie 1976)


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the harder material (AISI H11, 52 HRC) can have more burrs than the softer material (AISI H11,42 HRC) especially due to stronger tool wear progress.

In general, there are two approaches to overcome the burrs. One way is to reduce the burr duringthe machining process by changing the process parameters and another is to remove the burr(deburring) after the machining. Many researchers have discussed the burr reduction throughdifferent machining strategies. It is found that by decreasing the depth of cut, the burr height isdecreased (Chern 2006; Lee and Dornfeld 2005). Weule et al. (2001) experimentally showed thathigh cutting velocities led to less burr formation in the micro-milling of steel. Coated tools can alsoreduce the burr size when micro-milling hardened steel (Aramcharoen et al. 2008).

Lekkala et al. (2011) argued that in the micro-milling of aluminum Al2124 and stainless steelSS-304 using a solid carbide end milling cutter with diameter 300 and 400 mm, the speed has a lesssignificant effect on the burr thickness and height whereas tool diameter, depth of cut, number offlutes, and the interaction between feed rate and number flutes have significant effect on the burrheight. In addition, they observed that up milling produces more side exit burrs in micro-milling ofaluminum.

Deburring is defined as the removal of minute amounts of material from edges after major partfeatures have been produced (Gillespie 1979). The deburring process is difficult to apply on micro-features produced by micro-milling; the process must be carefully conducted to avoid damage to thesmall features. Incorrect selection of deburring techniques or parameters may also introducedimensional errors, damage surface finish, and residual stresses. The additional deburring processand edge finishing in micro-scale and ultra precision machining may also be too expensive orimpractical to implement (Min et al. 2006).

Burrs in brass are removed by applying cyanoacrylate and subsequently cut with diamondmilling(Fig. 8) (Schaller et al. 1999). Whereas for stainless steel, burrs can be removed using electrochem-ical polishing (Schaller et al. 1999). Other methods such as powder blasting using white fusedalumina also removes the burrs when micro-milling brass (Yun et al. 2008). A similar method is alsoapplied by Horsch et al. (2006) for quenched and tempered tool steel. Micro-peening and ultrasonicwet peening are also used for deburring and improving the surface finish (Horsch et al. 2006). Thetop burrs on the micro-slots of aluminum alloys, copper, and stainless steel can be reduced usingadditional micro-EDM (Jeong et al. 2009).

Fig. 8 SEM photo of micro-features before burr removal (upper right) and after burr removal (lower left) by diamondmilling in the micro-features filled with cyanoacrylate in brass (Schaller et al. 1999)


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Burr formation can also be minimized if the material is restricted to deform in force direction dueto workpiece geometry (Shafer 1975). Park and Dornfeld (2000) developed finite element models tostudy the exit burr formation of various workpiece exit edge angles of 60�, 80�, 90�, 100�, and 120�

for the case of orthogonal cutting. It is observed that the use of larger tool rake angles (Park andDornfeld 2000) can also reduce the burrs. The top burrs formed in micro-milling can be reduced byincreasing the side edge angle of the micro-slots where the cutting edge will exit (Saptaji et al. 2012).This method can provide better resistance to the plastic deformation closer to the side edges. Inaddition, the use of a tapered tool is a possible technique to avoid top burrs in micro-milling wherethe higher taper angle produces smaller burrs (Saptaji et al. 2012). The top burr height can bedecreased by making the tool edge radius smaller and the rake angle more positive (Lianget al. 2009).

Micro-tools

In the micro-cutting process especially micro-milling, the downscaling of the cutting mechanismfrom macro to micro-scale does not also automatically mean down scaling the tool shape withoutany modification (Li et al. 2011). The tool shape needs to be modified especially due to the differentcutting parameters between macro and micro-scale machining. The tools used for micro-millingneed to have high stiffness, high strength at the cutting edge corner, simple geometry, run outcompensation, etc. (Li et al. 2011). Fang et al. (2003) argued that the tool shape such as the semicircle-based (D-type) shape for end-mills has higher rigidity than that of the two-flute (commercialtype) end-mills, and produced better surface quality than that of the triangle-based (D-type)end-mills (Fig. 9).

In the milling process, as the tool diameter becomes smaller, the spindle speed has to be increasedin order to compensate the material removal rate as shown in Eq. 3. As a consequence of the smalltool diameter and high spindle speed, the tool experiences more vibration and deflection which in theend influences the surface finish, generates error in the accuracy of the geometrical feature produced

Fig. 9 Various types of micro-milling tool shapes, (a) Two-flute end-mills, (b)D-type end-mills with a straight body, (c)D-type end-mills with a straight body, (d) D-type end-mills with a tapered body and (e) D-type end-mills with a taperedbody (Fang et al. 2003)


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by the process, and increases tool wear (Uriarte et al. 2007). The vibration occurs because of thespindle run out and reduced tool rigidity as a consequence of reducing the tool diameter. Vibrationcan also occur due to the ploughing induced by the rapid increase of the thrust forces, due to thetransition from shearing-dominated cutting to ploughing-dominated cutting.

v ¼ p:D:s1, 000

(3)

where:

v ¼ cutting speed (m/min)p ¼ circular constant (3.14)D ¼ diameter of the milling tool (mm)s ¼ spindle speed (rpm)

The vibration of the tool during the cutting process produces intermittent impacts on the tool,which are strong enough to cause discrete chip formation, tool wear, or even tool breakage (Miaoet al. 2007; Malekian et al. 2009). The discrete chip formation generally occurs at low feed rate orlow axial depth of cut in which the ratio of the depth of cut to the cutting edge radius is less than theminimum depth of cut or uncut chip thickness. Chern et al. (2007) suggested that the amount of toolengagement must be kept to a minimum in order to avoid the undesirable premature tool failureproblem in micro-cutting process.

The tool stiffness, such as tool run-out, tool setting error, tool tilting, and tool deflection, alsoaffects the generation of the surface finish (Chern et al. 2007; Ryu et al. 2006). Tool run-out consistsof radial run-out and axial run-out (Fig. 10a). Meanwhile, tool setting error can be defined aseccentricity and tilting between tool and axis and spindle axis (Fig. 10b). The tool deflection effect iscommonly caused by high cutting forces (Fig. 10c).

In order to perform the micro-cutting process successfully, the micro-tools need to be precise andstrong. Focused ion beam (FIB) technique can be used to produce micro-tools such as tungstencarbide used for micro-milling or micro-grooving (Schmidt and Tritschler 2004; Adams et al. 2000).This method has been shown to successfully produce micro-grooving tools with tool edge radius ofabout 0.4 mm (Adams et al. 2000). In addition, wire electro-discharge grinding (WEDG) can be usedto produce a tungsten carbide micro-milling tool with diameter of about 100 mm (Chern et al. 2007).The accuracy of the machined workpiece produced by micro-milling is also influenced by thespecification of the machine tool. The machine tool is required to have high motion accuracy,stiffness, thermal stability, precise spindle bearings, and high resolution of linear and rotary motions(Dornfeld et al. 2006).

Summary

In this chapter, a brief explanation of the mechanical micro-machining process and its challenges hasbeen presented and discussed. The demands for micro-features and micro-components in manyindustries are increasing with the challenge being especially to manufacture these products using anoptimum process. Mechanical micro-machining is one of the alternative processes due to itsadvantages. However challenges related with this process such as size effect, microstructures ofmaterials, surface quality, burrs, and also tool performance need to be studied and solved so that the


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micro-machining process can have better performance in the production of micro-features. Severalsolutions are offered to encounter these challenges:

1. The size effect can be minimized by using diamond tools; however it may not be applied forferrous materials.

2. The effect of multiphase microstructures of materials can be minimized by preparing finer andhomogenous grains in the materials.

3. Good surface quality can be achieved by using optimum cutting parameters and modifying themicrostructure of the workpiece.

4. The burr problem can be avoided by optimizing cutting parameters and using an additionaldeburring process.

5. The tool modifications can be applied in order to have better performance of the micro-tools.

Fig. 10 Tool stiffness problems: (a) Tool run-out, (b) Tool setting error, (c) Tool deflection (Ryu et al. 2006)


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The solutions of one issue must not cause other issues to appear since the issues are related to eachother and simultaneously influence the process performance and quality of the machined workpiece.More alternative solutions need to be investigated in order to optimize the mechanical micro-machining process.

References

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Index Terms:

Burr 8Cutting edge radius 3Deburring 10Focused ion beam (FIB) technique 12Micro electro mechanical system (MEMS) 1Micro-cutting 5–6

cutting force 6grain size 6steel 6vs. macro-cutting 5

Micro-machining 1Micro-milling 2, 11

See also Micro-machiningMicrostructure 6Micro-tools 11Minimum depth of cut 12Ploughing 4Size effect 3Surface quality 7


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