Tool sharpness in precision micro cutting process

Jourmal of Harbin Instiute of Technology (New Series)， Vol. 17, No. 1, 2010Tool sharpness in precision micro cutting processL Xiao-zhou, YU Hua -dong, ZHAO Ping, XU Jin-kai李晓舟，于化东，赵平，许金凯( Electomechanical Engineering College , Changchun University of Science and Technology , Changchun 130022 , China, lixiaozhoulgdx@ sina. com)Abstract: As to probe the factors alcting the roughness and surface properties of work piece in mirco-cuttingmachining process, according to the principle of energy balance, using the method of experiments combiningwith theoretical analysis, this paper investigates the effeet of cutting edge radius on the unit cutting force, thecutting component forces ratio F,/Fg, as well as the roughness and surface properties of the work piece. Experi-mental results show that the value of tool cutting edge arc p has a significant impact on elastic-plastic deformationof the cutting area, and its inluence on the surface quality of processing and precision is greater than commonctting. The method of calculating the theoretical limits of the diamond tool cutting edge radius is feasible. Thvalue of 0. 0001 μm has some guiding significance for the developement of suitable cutting thickness to ensurethe normal cutting.Key words: cutting tools; sharpness; micro-cuttingCLC number: TC501. 1 ;TG702Document code: AArticle ID: 1005-9113(2010)01 0123-04Nowadays, the precision of the micro-cutting re-thickness is so thin that the metal deformation mecha-quires the order of0.1 nm - 0.01 nm. In order to real-nism is greatly different from the general cuttingize such high precision cutting, some special methodsprocess, and the metal deformation is more complex.must be adopted involving the machine tool, cuttingThe cutting edge radius ρ has the significant effects ontools, process, environment， controlling methods andthe cutting process as the following aspects.so on. Also, the cutting tool is grinded very sharp.1.1 Effects on the Cutting Force Per Unit AreaIn the precision micro-cutting process, a signifi-In fact, the edge radius of the tools is often largercant feature is that the cutting thickness is quite thin.than the chip thickness and the tools rake angle is neg-As to cut such thin layer of metal at the normal cuttingative in precision micro-cutting process. In this case ,status,，the tools with enough sharpness are re-he cutting force per unit area will certainly increasequired-3. Thus, the cutting tools should have theand can be represented as follows:following characteristics :p = C,/q3(kgf/mm2 )(1)1) High resolution of the cutting thickness, i. e.，Where C, is the coefficient and t is the cutting thickness.the ability of removing the materials with a micro-cut-In the metal cutting process, the rake face dividedting thickness*.into two different regions, the planar surfaces and the2) The ability of making the smooth work-piececylindrical surfaces, and the rake angle of cylindricalsurface with thin denatured layer.part is negative( as shown in Fig. 1). Due to the difer-3) Long life and high durability.ent cutting depth , the functions and the contact areas ofAs we know , the deformation of the work-piece ma-the two parts are also different. Cutting thickness is soterial beneath the cutting edge, the cuting deformationthin and close to the order of p that the tool cutting edgecoefficient and the cutting forces are related to the cut-( cylindrical part in Fig. 1) plays a major role on preci-ting edge radius ρ in precision micro-cutting process'sion micro-cutting process, which leads to larger cut-Also,p is an important factor, which determine the ating force per unit area. When the thin layer of metal isbove three aspects. Therefore, as follows, we mainly an-removed, the thickness of metal remained on the work-alyze and study the cutting edge radiusp in precision mi-piece surface will be larger after cutting as the tool cut-cro cutting, that is, the problem of the tool sharpness.ting edge radius is bigger, and the cutting force per u-nit area will increase. The resistance of metal layer1 The Effects of the Cutting Edge Radiusρ on themust中国煤化工plstic dedomationCutting Processwhen_be thickness of themetalYHCNMHGcewillbelarges0In precision micro-cuting process, the cuttinghat the cutting torce per unut area Is large.Received 2008 -09 -01..123●Journal of Harbin Instiute of Technology (New Series), Vol. 17, No. 1, 2010- cos(β - ro)])/(sin φcos(φ +β - ro))(4)According to the cutting equation, it is obtained, ChipToolthatβ-yo < π/2. Further, one can get the relation-ships of sin φcos(φ +β-yo) > 0 and sin(β→γo) -cos(β-yo) > 0.Fig. 1 The cutting tools in micro-cutting processSo, from Eq. (4) it can be concluded that whenAs shown in Fig. 1, the roles played by the4 F: This showsstraight section and arc section, which consist of thethat while p is fixed and the cutting thickness i is belowcutting edge, vary with the values of the cutting edgep[1 -sin(β-ro)], thenF, > F, Smallerp is advan-radiusρ. Whenρ is reduced ( shown in Fig. 1 withtageous to reducing the value of F,/F,dashed line)，the straight section of the cutting edge1.3 Efects on the Surface Roughness of thewill be longer and its efect will be strengthened. OrWork-Piecethe contrast, the effect of the are section will decreaseThe roughness of the work-piece surface is not onlyand the component cutting force per unit area will re-related to the feed , tools cutting edge angle, minor cut-duce. Therefore, ifp is reduced, the sharpness of theting edge angle , bake-rake angle, tools cutting edge in-cutting edge will be improved and it can reduce theclination but also to the tool cutting edge radius p. Thiscutting force per unit area in cutting process.is because the value of p directly influences cutting sta-1.2 Effects on the Cutting Component Force Ra-tus of the cutting region. Generally, the friction and ex-tio F,/F:trusion between the clearance face and the machinedThe tool cutting edge plays a major role in preci-surface increases with the increasing of the p value. Andsion micro-cutting process where friction and extrusionthe roughness of the work -piece surface becomes worse.of rake face and clearance face is large. It makes theThe test of the aluminum alloy micro-cutting shows:cutting component force ratio F,/F, much diferent fromWhenρ = 3 μm, roughnessR。= 0. 032 μmthat in conventional cutting process.ρ = 4 μm, roughnessR。 = 0. 035 μmWhile the tool cutting edge radiusρ is fixed, theρ =5 μm, roughnessR. = 0. 050 μmproportion of the section of cutting edge involved inTherefore, the roughness of the work -piece sur-cutting increases as the cutting depth reducing. Theface R。reduces as ρ decreases. It should be pointed outfriction and extrusion between the section of cuttingthat if the value of ρ increases slowly in cultingedge and the surface of the work piece increase. Thus ，process, the roughness of the work-piece surface re-the value of F,/F: changes， and when the cuttingduces. This is because that the arc radius of the toolsthickness is below a certain value, F, > F. The valuecutting edge has the effcts of extrusion and finishingofF,/F: increases as the cutting thickness continues toon the work-piece surface.decrease.1.4 Effects on the Properties of the MachinedWhen the cutting thickness is fixed, the propor-Surfacetion of the section of cutting edge participating in cut-The cold hardening, characteristics and values ofting decreases as the p reducing，hence, the frictionthe residual stresses are greatly related to the value ofp.and extrusion between the section of cutting edge andWork-piece bears tensile sresses in a large area ac-the surface of the work piece decreases, and the valueted by the tool tip while using the rake angle to cut met-of F,/F. reduces.al. The compressive region increases and the tensile re-The smaller value ofF,/F, is better in order to ensuregion decreases as the rake angle becomes smaller. Whilethat the cutting process can be caried out in the normalthe rake angle becomes negative ,the work-piece materialsconditions. Thus ,p should be selected as small as possible.near the tool tip bears larger-area compressive stress.The cutting component forces F, and F. can be cal-The cutting edge( negative rake angle) plays a ma-culated byjor role in precision micro cutting, so the metal withr二 2r,b[L -ρ-psin(β - yo)]cos(β - yo) .(2)the compression stress occupies the most cutting area.In the case of the other conditions unchanged, if thesinφcos(φ -β -y)cutting edge radius increases , the proportion of the cut-。_ 2r,b[L -ρ + psin(β -yo)]sin(β -x0)(3)ting component F, in the resultant force and the extru-sinφcos(φ -β - yo)siorrdening also increa-Where T, is shear stress in shear plane, b is cuttingses中国煤化工In that case, thewidth , 1 is culting thickness, β is friction angle in rakelargerCNMHG:urs in the surfaceface, and p is cutting edge radius.Subtract Eq. (2) from Eq. (3)，we can get:the work -piece surface improves.F,-F. = (2r,b[4 -ρ + psin(β - ry)][sin(β-yo)In the work-piece surface layer the cold hardening●124●Joumal of Harbin Institue of Technology (New Series), Vol. 17, No. 1, 2010can be controlled by changing the tool cutting edge ra-curves in Fig.2 and Eq, (6) the cutting edge radius pdius ρ. In the high precision machining, the sharp toolshould increase ( see the straight lines dh,以...(smallp) should be used, when the variation of thein Fig. 2). Obviously, if the angle between the straightwork-piece surface layer is required small, and thlinesd, dr.d. - l, 2- and horizontal axis is ar-blunt tool (largep) can be used if cold hardening isbitrary negative, the variation regulation of ρ is therequired in the surface layer.same as the previous and only the increasing degree ofp changes when Po increases ( see the lines kym; ,2 Mechanism of the Cutting Edge Formationfrym**-k.m, in Fig.2).The last case is that p will decrease when Bo in-The tool cutting edge is formed by the microcreases and the breakage width decreases. In the fig-breakage around the tip in tool grinding machiningure, the straight lines fig1, f282.,"", fngn are of thisprocess. The micro breakage directly influences thecondition and the positive angles are formed betweenroughness and the cutting edge radius. For the toolsthese lines and abscissa axis.used in the precision micro-cutting, it is very importantThe above analyses show that the variation of ρto gain smaller edge arc radius through reducing thecompletely depends ona and Bo ( as shown in Fig.2).micro breakage on the cutting edge.Whenh = 1. 43 and β。changes from 70° to 100° ,Fig. 2 shows the relationship between the breakageρ unchanged.width and the cutting edge radiusp. The tools tip is aWhenk > 1. 43 and B。changes from 70° to 100°，straight line perpendicular to the angular bisector of Po.βo, ρ and a have the following geometric relationship:ρ reduced.Whenk < 1. 43 and B。changes from 70° to 100° ,p=a/2tan(45° -B)(5)ρ increased.There are only the above three cases in practical18b2-900pdtools grinding process.The sharpness and smoothnessof the tool are de-b。=80termined by the cutting breakage occurring in tool12grinding. The smaller the value of breakage is, the县10年sharper the tool is. .s北Moreover, the smoothness of the tool edge is formneddue to stacking of the breakages which are caused by ab-rasive particles. Thus, the effects on the smoothness ofthe cutting edge and sharpness variation law will be dif-ferent for changing the parameters in egrinding process.02468101214a/u m3 Determining the Tool Cutting Edge Radius pFig.2 The relationship between the breakage width andthe cutting edge radius ρIn Section 2, the importance of cutting edgesharpness is analyzed. The sharpness is directly repre-From Eq. (5), it can be seen that ρ increases as a andsented by the value of cutting edge radius p. The aboveB are increasing. But if Bo increases, the tool tip strength-analysis demonstrates that small p is advantageous to re-ens and the breakage reduces as well as p is changed.ducing the cutting force per unit area in cutting edge,In Fig. 2, any horizontal line corresponds to thethe value of F,/F, and the roughness of the work-piececondition ofp = Constant when β。changes from 70° tosurface as well as improving the properties of the work-100° (brc, b2Cr ... becn,). While p is constant, thepiece surface. Therefore, making tools with smallp isbreakage width reduces with Bo increasing from 70° tthe only approach to improve the tools resolution of the100° as follows:micro cutting thickness.tan(45° +0)There should be a limit value ofp. The tip of thek = C20°== 1.43 (6) .sharp tool may shrink owing to the surface tension and0x10tan(45°+4 )surface energy. This shrinkage can be balanced by thearc ancltin. It can, he sumed that the elasticIhis means that the breakage widh a should bestrait中国煤化工ue to the diferencereduced 1. 43 times with βo increasing from 70° to 100°of suYHCNMH(urinkage. If the val-whileρ = constant.ue of.he inherent value ofSupposing that the breakage width is unchangedtip material, the tip will be damaged.when B。increasing from 70° to 100° , according to theBased on the above analysis, the limit ofp can be●125●Journal of Harbin Institute of Technology (New Series), Vol. 17, No. 1, 2010considered according to the following principle of ener-From the results in the table, it can be seen that thegy balance.theoretical value of the cutting edge. radius limitation aAs shown in Fig. 3,a wedge PAB with unit thick-bout the diamond could reach to 20 A( Angstron).ness and apex angle of 90 degree deforms into P'AB.There is elastic strain energy storied in shadowed area.4 ConclusionsIn precision and micro cutting process, for themachined work-piece surface below the arc section ofthe cutting edge , the elastic-plastic deformation is more入Ecomplex than the conventional cutting. The size of thecutting edge has more effects on the qualities of theofmachined surface and machining precision than theconventional cutting. Also, the efects reduce with theFig.3 Formation of the edge radiuscutting edge radius p decreasing. While the tools withsmaller p using in micro cutting, the produced force perLet the surface energy difference between PAB andunit area is very small and the roughness of the work-P'AB is equal to the strain energy in shadowed area, itpiece surface is batter as well as the resolution of thecan be obtained that:tip on the cutting thickness is very high.Conclusively, it is advantageous to reducing the val-(20 - ))=(架一空品(7)ue of p for improving the work -picce machining precisionWhere K is the surface energy of material, σ is the ef-and surface finish. The theory and calculation methodfective stress in shadowed area, and E, is the youngproposed in this study for the diamond cutting edge radiusmodulus of material.and the specific calculation value have a certain guidingEq. (7) can be modified as:significance to get sharper cutting edge in tools ginding(4 - π)4kE,process. Meanwhile, there is a certain guiding signifi-(π-2)σ2(8)cance to make adaptive cutting thickness and ensure nor-In Eq. (8), the value ofσ is taken as the ffec-mal cutting according to the size of the tool cutting edgetive mean stress when shadowed area being destroyed.radius in precision micro-cutting process.The limit of cutting edge radius p。can be obtainedOwing to restricted factors such as grinding de-by substituting strength σ。into the above equation in-vice, operating method and so on, it is difficult to ob-steadof σ. And it' s meaning is the minimum radiustain the tool cutting edge radius which is close to thewhile there is no self- damaged for the wedge tip.theoretically limit value (20 A) in tools grinding. Howto get sharper cutting edge is the further research work.Then, we have:Pe=(4 - π)4kE;σ。= E,/10(9)References:(π-2)σ:If the strength σ。is equal to the ideal tensile[1] Zong WJ, Cheng K, Li D, et al. The ulimate sharpness ofsingle-crystal diamond cutting tools- -Part I: Theoreticastrength of material, i.e. ,σ。= E,/10, the theoreticallanalyses and predictions. Intemnational Jourmal of Machinecutting edge radius p。can be expressed as follows :Tools & Manufacture, 2007, 47 :852 - 863.[2] Yuan Z J, Zhou M, Dong s. Effect of diamond tool sharp-(4二π)(p.)ben(π-2).4x10仓10)ness on minimum cutting thickness and cutting surface in-tegrity in ultraprecision machining. Joumal of MaterialsTab.1 shows the theoretical p。for several kinds ofProcessing Technology, 1996, 62 :327 - 330.materials.[3] Park H w. Development of Micro-Grinding Mechanics andMachine Tools. Atlanta: Georgia Institute of Technology ,Tab.1 Theoretical results of the cutting edge radius limi-2008. 26 -31.tations[4] ChaeJ, Park S s, Freiheit T. Ivestigation of micro-cuttingYoung modulusCutting edgeoperations. Intemational Jourmal of Machine Tools & Manu-Material E,/ ( x102dgn .Surface energyradius limiationfacture, 2006, 46:313 - 332.K/(erg' cm - )Peey!/ A[5] Liu Kai, Melkote s N. Finite element analysis of the in-10 Sdiamond5300uence of tool edge radius on size effect in orthogonal micro-BN( Cubic)6.948008TC )3. 2900中国煤化工of Mchanical Ssir-Al2O,.9.615349MHCNMHG153011007. 0.63●126.