Effects of tool flank wear on orthogonal cutting process of aluminum alloy

Available online at www.sciencedirect.comOCIENCE @o)DIRECT.Transactions ofNonferrous Metals骂RSociety of ChinaScienceTrans. Nonferrous Met. Soc. China 16(2006) s1562 -s1569Presswww.csu.cdu.cn/ysxb/Effects of tool flank wear on orthogonal cutting process of aluminum alloyYUAN Ping(袁平), KE Ying-lin(柯映林)Collge of Mechanical and Energy Engineering, Zhejiang University, Hangzhou 310027, ChinaReceived 28 July 2006; accepted 15 September 2006Abstract: The tool flank begins to wear out as soon as cutting process proceeds. Cutting parameters such as cutting forces andcutting temperature will vary with increasing degree of flank wear. In order to reveal the relationship between them, the theoreticalsituations of cutting process were analyzed considering the tool flank wear effect. The variation rules of cutting force, residual stressand temperature dsributions along with the tool flank wear were analyzed comparing with the sharp tool tip. Through FEMsimulation method, ffections of the tool flank wear value VB on cutting forces, residual stress and temperature distributions wereanalyzed. A special result in this simulation is that the thrust force is more sensitive to tool flank wear, which can be used as arecognition method of tool condition monitoring. The FEM simulation analysis result agrees well with the experimental measuringdata in public literatures and some experiments made also by the authors.Key words: aluminum aloy; orthogonal metal cuting; finite element simulation; tool flank wear; cutting forces, cutting temperature;residual stressyield stresses of the work material. Recently, through a1 Introductioncutting experiment in a lathe, the model of tool forces forwom tools considering flank effects was constructed [5].It is well known that during the metal cuttingFurther experimental researches on the cutting ofprocess, a sharp cutting tool will soon get worn and thisaluminum, steel and cast iron considering the tool flanlsituation will bring some influences on the whole cuttingwear has got some development so far[6- -10].process. As a cutting tool gets worn, cutting parametersFinite element method (FEM) is a powerful tool tosuch as cutting temperature and cutting forces vary morepredict cutting process variables, which are dificult toor less in some degree. Early in 1959, the effects of tool !obtain by experimental methods. XIE et al[1] discussedflank wear on interface temperature using the tool-workmodeling techniques on continuous chip formation bythermocouple technique was studied[1]. Then in 1961,using the commercial FEM code ABAQUS anthe forces on a single-point cutting tool under conditionsestimated the 2D tool wear in orthogonal cutting 0of progressive flank wear were analyzed under theturming operation. In order to reveal the relationshipassumptions of constant rake, friction and shear angle[2].between the cutting process variables and tool flankSome researchers studied the influence of flank wearwear further, in this paper, through a FEM simulationupon the contact and internal stresses in a single-pointbase on commercial software DEFORM, the variationtool, proposed a comprehension model for therule of cutting forces and temperature along with tooldistribution of contact stresses along the wear land andflank wear was studied, and the varying tendency was ,compared with the experimental data obtained frompointed out.photo elastic investigations and actual cutting test[3].CHEN and PUN[4] made an investigation into stresses at2 Construction of FEM modelthe cutting tool wear land, the result of which indicatedthat the coefficient of friction at the tool/work interfaceIn this ex中国煤化工:ing test waswas constant, and the interface stresses were below theapplied. In this:MYHC N M H Guof machineFoundation item: Projec(S0435020) supported by the National Natural Science Foundation of ChinaCorresponding author: YUAN Ping; Tel: +86-571-87951061; E-mail: pingxp@zj.comYUAN Ping, et al/Trans. Nonferrous Met. Soc. China 16(2006)s1563tool spindle is 9 000 r/min and the feed speed is 0.3 mm Table 1 Material properties of tool and workpiece _per rotation (i.e. 0.15 mm/r). The diameter of two flutesElasticPoisson's Density/cylindrical milling tool was 20 mm. For the cutter tool,Materialmodulusratio (kg:m 3)(20"C)(GPathe rake angle was 3", the clearance angle was 15*. TheToolWC6500.2514 900material of cutting tool was a kind of WC cementedcarbide alloy. The material of workpiece was aluminumWorkpiece705070.302 850alloy 7050. Only. dry cutting was considered, and noThermalhydrant was used here.Tm 2 0-1 conductivity/Heat capacity/Cofficient of thermal(N:mm -.C71)(W:mlK)expansionBased on the Thermo-Elastic and Plastic theory, theplane strain finite element model of cutting aluminumool2.43595.0X 10-6alloy 7050 was constructed. A kind of commercial15114.821.6X 10software DEFROM 2D M was used in this work.In this FEM simulation model, the dimension ofTable 2 Cutting conditionsworkpiece was set as 3.1 mm in length and 1.1 mm inCutting speed/Depth of cut/mmInitial temperature/height, while the cutting tool tooth was set as about 0.5(m:min )mm in width and 0.8 mm in height. The length of tool565.50.120flank wear was chosen as 0, 0.05, 0.1, 0.2 and 0.3 mm inactual FEM simulation, but in this study, among themonly three situations were analyzed in detail, i.e. 0, 0.1Based on the effective cutting chip area per rotate,and 0.2 mm.the curved chip was converted into uniform chip, justIt should be noticed that the workpiece and tool areike plane cutting. And the equivalent chip thickness wasdistributed with non- uniform mesh, as ilustrated in Fig.1.calculated out as 0.1 mm according to the feed per toothIn the contact area between the tool and workpiece, thevalue (here the feed per tooth value is 0.15 mm),very small size elements were used. While in other area,according to a method listed in Ref.[12].larger size elements were tolerable. The total number ofelements was kept at a not very high level while the3 Results and analysissimulation precision was rather high enough. Throughthis method, the efficiency has been greatly improved in3.1 Chip formation and shear angleFEM simulation.Traditional theoretical analysis shows that, from theformula below, the deform factor ζ can be calculated out.5=-(1)Step-Iwhere ach is chip thickness, ac is un-deformed chipthickness.Then from the formula below, the shear angle φ canbe calculated out astanφ=-COSY(2ξ- sinyowhere Yo is the rake angle.In this study, chip shapes obtained from simulationsFig.1 Orthogonal cutting model with tool flank wearwith different tool flank wear values are compared. FromThe boundary condition was that constraints inthe simulation results, it can be seen that, when a cuttinghorizontal and vertical directions were applied to thtool gets worm, the tool-chip contact length and the curvebottom of workpiece. Cutting tool was set as rigid, whileradius of the continuous chip do not change in general.workpiece was set as plastic. Shear friction cofficientThe thicknesslue too, whichwas set as 0.3, which was a suitable value in high speedis kept at abolYH中国煤化工Con here. As thecutting of aluminum. The initial temperature was 20 C .chip thicknessCNMHGa mm, and theThe material properties of the tool and workpiece and theun- deformed chip thickness ac is 0.1 mm, so the deformcutting conditions used in FEM simulations are listed infactor ζ can be calculated as 1.4 here from Eqn.(1). ThisTables I and 2.means that the deform factor does not change with thes1564YUAN Ping, et al/Trans. Nonferrous Met. Soc. China 16(2006)degree of flank wear.As the rake angle y is 3° here, so the shear angle φ700 |can be calculated out to be about 36.54° from Eqn.(2).600F234Meanwhile, the average shear angle in the steady statefrom FEM simulations is about 35', which can be seen500from Fig.2 (where the sign bvB means the flank wear! 400bvB=0width of the flank wear land in the central portion of the2一bvB=0.05 mmactive cutting edge). These two values are very close. So,300bvB-0.10 mm4一bvB=0.20 mmthe simulation value of shear angle agrees well with the200theoretical analysis value.1005厂0.040.080.120.16江强50Time/msbvp=0-bvB-0.05 mm2.0 |-bvB-0.10 mm4- bvB=0.20 mm401.52- bvp=0方35-冒，- bvB=0.05 mm品1.0vB=0.10 mm0vB=0.20 mm5 - bvB=0.30 mm25L0.520140180Fig.2 Shear angle graph of dfferent flank wear length in0.08 0.12 0.16simulations3.2 Influences on stress, strain and strain rate800(@)The simulation results show that the degree of flank700wear has no influence on the effective stress, and does，600not affect the strain and strain rate much. Fig.3 shows agraph that shows stress, strain and strain rate may varywith cutting time at different flank wear values. From目400this graph, it can be seen that, during the steady cuttingA 300stage, all effective stress values keep around 655 MPa.2- Bvp=0.05 mmAnd the values of strain maintain between 1.8 and 2.0,- bvg=0.10mmwhich is increased only about 10% when the tool gets=0.20 mmworm. The strain rate keeps also between 3.5X10* s~'and 4.0X10's'.Fig.3 Stress, strain and strain rate variation graph with time in3.3 Infuences on variation rules of cutting forcedifferent flank wear values: (a) Stress; (b) Strain; (C) Strain rate3.3.1 Theoretical analysis of cutting forceFrom the previous researches[2, 8], we know thatforce component which is parallel to the shear plane, Fwthere have cuting force relationships asis the wearland force component in the thrust direction.F.=FEs+Fcw(3)The relationship is also revealed in Fig4. From abovehe formulae above, both force_ in cutting and thrustF=Fs+Fw(4)directions will in中国煤化工ar increases.where F. is the total cutting force component, Fcs is theCorrect FEM sin:YHCNMHGfllwthiscutting force component required for chip formation, Fewtendency.is the wearland force component in the cutting direction,3.3.2 FEM analysis of cutting forceFi is the total thrust force component, Fis is the thrustThere is a slight increase of the horizontal cuttingYUAN Ping, ct al/Trans. Nonferrous Met. Soc. China 16(2006)s1565force with the increase of tool flank wear, which can becuts into the workpiece a lttle; but an ideal absoluteseen from Fig.5 (a). The increasing amplitude of average sharp tool keeps only an average value of 7 N. Thisforce value in horizontal direction is about 5 N (from 90phenomenon is very typical for the flank worm tool tip,N to 95 N), although the maximum peak value gets 100because another simulation also made by us shows that aN. And as can be seen from Fig.5(b), when a sharprounded tool tip without flank wear only increases theool gets worn, the vertical force has a typicalaverage value a little, but don't have this typicalchange--- a shocking phenomenon happens. Oushocking phenomenon.simulation results shows that, for a worn tool, verticalAnother experiment also pointed out the increase ofload gets a peak value of 42 N when the cutting tool justthrust force with the increase of flank wear length byLIANG[9].Let Fu means the X load, Fv means the Y load, sotheir composite force can be got from the formula:ChipTool_VF=|E+FE(5)When a sharp tool cuts at first, X load is near 90 N,FRsFt'csFsyARFnY load is near 35 N, and the composite force is just 96.6中2Fes RwN. When tool gets worm, X load is near 95 N, Y load isnear 45 N, and the composite force is just 105.1 N. Thecomposite force increases up to at least 8% when cuttingF'ewtool gets worn.Fig.4 Orthogonal cutting forces model considering tool flankAs can been seen from Fig.6, the position of toolwear effectrelative to workpiece is shown here just when the thrustforce gets the maximum value. This is just the time whena)i 4.the tool cuts into the workpiece in a very short distance10and the chip has just formed a lttle. This phenomenon is .very special. It is more important for high speed mill30 f1/ 2machining rather than turn machining, because millingcutter flutes cut into workpiece frequently in a very short0}time. And this phenomenon can be set as a recognition-bvB=0.10 mmmethod of tool flank wear monitoring in high speed mill40-bvp=0.20 mmmachining.5 - - bvB=0.30 mm20卜LandPrdeten0.03 0.06 0.09 0.12 0.150.18? Stp 24Time/ms225-是20 1bvp=015bvB-0.05 mm-0.10 mmvp=0.20 mm0卜...=0.30 mm中国煤化工Fig.6 Relative.MYHCNMH Gust force0.030.060.090.120.150.18The normal pressure distribution affected by flankFig.5 Force load graph with different flank wear length inwear can be seen in Fig.7. The effective stress named insimulation: (a) X-load; (b) Y-loadthe software DEFORM is in fact the residual stress. Its1566YUAN Ping, et al/Trans. Nonferrous Met. Soc. China 16(2006)can be seen that lttle influence on the effective stressmm. And the rake angle is 3", the clearance angle is 15.produces in the whole field of workpiece, but the normalThe type of machine tool is Deckel Maho DMU-70Vpressure distribution around the interface of tool andfive axes HMC. A force measurement of Kistler 9257workpiece changes a lttle. There exists normal pressurewas used.force distributed around the flank wear area.MillWorkpiece oftool3 mm in heightStep 116; NormalWorkpiecepressure中eB 0.111C 0.222e包|D 0.333E 0.444F 0.5561H88aG 0.667K istlerH0.778.dynameterFyFzI 0.889.J 1.000Fig.8 Orthogonal high speed milling force measuringLx▲0experimcnta)口1820Step 117， NormalI DMU-7OVI pressure|B 0.111 .E 0.444 .H 0.778I 0.889J J 1.000(b)。1090Step 1031 pressureFig.9 Photo of force measuring equipmentFig.10 shows the cutting force measuring valuesalong with cutting time measured in channel 1 and0.889channel 2 (in X and Y direction separately, i.e. feedI 1.000direction and normal direction). The time field is onlychosen for 10 ms. The reason for containing plus and0o口2290minus force values is that there is a half circle cuttingjust in one cutting for a single cutting tooth step. From it,we can get one field of cutting forces. When cutting withc)a sharp tool at first, the maximum value in one channel isFig.7 Normal pressure comparison for different tools (tool350 N, in another channel is 400 N. Then after thecutting tool gets worn to somne degree, the maximumflank wear value bvg=0, 0.1, 0.2 mm)value in one channel is 370 N, in another channel is 4203.3.3 Some verifying experiment data of cutting forcesN. So, it shows cutting forces increase in some degreeIn order to get the correct information in cuttingwhen the cutting tool gets worn. Cutting forces measuredaluminum alloy 7050, a full immersed slot millingin these two channels increase by about 5% to 6%.experiment was made by us. Fig.8 shows the draft of slotAs is listed中国煤化工npostion ofmill cutting experiment equipment, and Fig.9 shows theforces in FEMwhich justphoto of experiment equipment. The workpiece is a thinmeans the magniYHC N M H Gworkpiece ofplate with a thickness of 3 mm, the material of which isone millimeter thick. Since in this experiment thaluminum alloy 7050-T7451. The cutting tool is a twoworkpiece plate thickness is 3 mm, the force aboveflutes carbide cylindrical cutter with a diameter of 20should be amplified with the thickness of 3 mm, so theYUAN Ping, et al/Trans. Nonferrous Met. Soc. China 16(2006)s1567result force is about 330 N. There still have some smallincrease of a thermal conduction increasing effect. Thegaps between the simulation values and experimentalcontact area between cutting tool and workpiece is alsoones, which may be shortened by the exactness inincreased when tool flank area gets increased. Thereforemeasuring the degree of tool flank wear, and may also bethe increased thermal conduction limits the increment ofshortened by the accuracy limit in force measuring.temperature. This idea agrees with some conclusions inComparing simulation values before, the differencethe previous researches[13- 14]. And the temperaturebetween simulation values and experiment values isdifference led by various tool flank wear keeps within anrather small considering difficulties in measuring theamplitude of 10 C. It means that, in the procedure ofdegree of tool flank wear. It means that our FEMhigh speed cutting, the heat produced by friction betweensimulations are basically credible.0ool flank and machined surface occupies only a verysmall part in the whole quantity of heat.500[(a)ch1360 4g-500p 280bvB=0bvB=0.05 mm500(b) ch2200-1~2bvB= =0.10 mmbvB=0.20 mm5一bvB=0.30 mm12040|(C) chl0.080.120.16Time/msFig.11 Temperature- time curves under different flank wearlengths(d) ch2As shown in Figs.12 and 13, the sharp tool andflank wear tool has some influences on the temperature趸distributions of workpiece and tool. In the steady stage,the maximum cutting temperature at tool-chip interfacehas an increase of about5 C (i.e. from 375 C to 380Fig.10 Experimental data of cutting force signal: (a), (b) Before°C), and temperature in the area just below the wholeobvious tool fank wear happening; (C), (d) After tool flank getsmachined surface also increases. Yet for different toolflank wear values, such as 0.1 mm and 0.2 mm, thewom to some degrcedifference is rather small.The same conclusion was also drawn by LIANG[9].As can be seen from Fig. 13, the distance betweenIn his study, an aluminum workpiece was cut by a freshthe maximum temperature point and the cutting tool tiptool and the worm tools of 0.2, 0.4 and 0.7 mm. From hispoint becomes short after that cutting tool becomes worm.results, it is obvious that cutting forces increase with theAnd the temperatures near the tool tip change a litte tolength of tool flank wear, and at the same time thesome degree. For the limitation of equipments, onlyamplitude of vibration increases too.force test was made by the authors so far, thetemperature experiment will be carried out in the next3.4 Influences on the cutting temperaturestage.As can be seen from Fig.11, with the increase oftool flank wear value (from 0 to 0.2 mm), the maximum4 Conclusionscutting temperature also increases up in the contact area中国煤化工between tool and chip. But when the tool fank wear1) The.MY出CNMH Ges on the stess,value still increases up to 0.3 mm, this increasingand almost does not attect the straln rate. And also thetendency of temperature with flank wear becomes nottool flank wear does not affect the basic cutting quanti-very obvious. This variation can be explained by theties such as the shear angle, chip thickness and curling-sl568YUAN Ping, et al/Trans. Noferous Met Soc. China 16(2006)Step 196Temperature/CStep 158C 99.0138B 27.7D 43.1H 296E 50.8336375_8150375→X(a)Step 137B 59.5C 99.0 .D 178|Step 188? 178， 28.9 .H 29C37.846.7. 37655.764.6s 20.091.3I 100Lxb)|Step 189Step 190 Temperature/C100140F 62.8180 J 380G 71.3220H 79.9I 88.4J 97.020.0380(c)Fig.13 Efeet of tool flank wear on temperature dstrbution atc)tool tip: (a) VB=0; (6) VB=0.1 mm; (c) VB =0.2 mmFig.12 Efect of tool flank wear on temperature distribution atworkpiece: (a) VB=0; (b) VB -0.1 mm; ()VB -0.2 mm3) Such conclusions agree well with theexperimental measurements in public literatures and theradius curvature of continuous chip. But it affects theexperiment carried out by the authors. The result can alsohorizontal load force to some degree, and affects thebe used as an on-line method to detect the tool wear inthrust force more sensitively, which can be taken as athe metal cutting process.method to recognize the tool flank wear in on-linemonitoring during the milling process.References中国煤化工2) The cutting temperature risces with the wearCNMHGdegree of sharp cutting tool, but gets stabilized soon.[1] OLBERTS D，n suuy O1 u Cous sUuI alL wear on tool chipinterface temperature [小. Trans ASME, Jourmal of Enginering forTool flank wear will also increase the temperatureIndustry, 1959, 81: 152- 158.undermeath the cutting tool.[2] MCADAMS HT Rosnthal paul, fres on a wom cuting 1ol [I.YUAN Ping, et al/Trans. Nonferrous Met. Soc. China 16(2006)s1569Trans ASME, Joumal of Eginering for Indutry, 1961, 83:[9] LIANGS Y, KWON Y K, CHIOU R Y. Modelling the elect of505- -512.flank wear on machining thrust stability [U] Intemational of Joumal[3] CHANDRASEKARAN H, NAQARAJAN R. 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