柴油機(jī)氣缸體三面鉆削組合機(jī)床總體及左主軸箱設(shè)計(jì)

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In the present analysisorthogonal cutting models are proposed, taking severalprocessing parameters into account; the models are validat-ed with experimental results from the relevant literature anddiscussed. Additionally, oblique cutting models of highspeed hard turning are constructed and discussed. From thereported results useful conclusions may be drawn and it canbe stated that the proposed models can be used forindustrial application.Keywords Machining.Finite element method.Hard turning1 IntroductionHard turning, a machining operation used for the process-ing of hard materials such as hardened steels, has beenbrought into the forefront of modern metal cuttingoperations with the increasing demand for manufacturinghigh quality components, e.g., gears, shafts, bearings, diesand tools, from these kinds of materials. Cutting toolsemployed in hard turning are made of specialized toolmaterials, such as cubic boron nitrite (CBN), that are able toovercome the problems experienced during the process 1.These cutting tools are ideal for machining iron-basedmaterials at the severe cutting conditions associated withhard turning; they possess exquisite properties, even atelevated temperatures, allowing for their application at highcutting speeds and without the use of any cutting fluids 2;by dry cutting not only environmentally-friendly character-istics are attributed to the process, but also cost reductioncan be attained by omitting buying and disposal costs of thecutting fluids 35. In addition the combination of hardturning and high speed machining is proved to be veryadvantageous since a great reduction in processing time canbe achieved 1.Hard turning is very advantageous for a wide spectrumof applications and is also considered as an alternative for avariety of processes, since the single-step superfinish hardturning can replace the abrasive processes, traditionallyused as finishing operations, or non-traditional processes,such as electrical discharge machining (EDM), in machin-ing hard parts, offering accuracy equal to or better than thatprovided so far, flexibility and considerable machining timeand cost reduction 47.Note, however, that hard turning has not been introducedinto modern industry as much as it should be, mainlybecause of phenomena such as rapid tool wear or crackingInt J Adv Manuf Technol (2008) 38:441446DOI 10.1007/s00170-007-1114-9A. G. Mamalis (*):A. Markopoulos:D. E. ManolakosManufacturing Technology Division,National Technical University of Athens,Athens, Greecee-mail: mamaliscentral.ntua.grJ. KundrkDepartment of Production Engineering, University of Miskolc,Miskolc, Hungaryand chipping of the cutting edge due to extreme pressureand temperature imposed on the cutting tool, which lead topoor machining results 8. Furthermore, as a novelmachining process, it needs to be further studied so that itmay be optimized. Most research work is limited toexperimental results, but, also, the modelling of hardturning can provide useful data to better understanding theprocess. Numerical modelling and, especially, the finiteelement method (FEM) have been widely used in the pastfor the analysis and the prediction of the cutting perfor-mance in machining operations. FEM has been a verypowerful tool in the cutting technology and can be appliedto high speed hard turning as well.In the present paper FEM is employed in order tosimulate dry high speed hard turning, investigating theinfluence of the cutting speed on the performance of thecutting operation and predicting the crucial processingparameters, some of them being sometimes very difficultto be measured or calculated otherwise, e.g., temperaturefields within the workpiece and the tool during the process.However, hard turning is a rather complex process, withcutting conditions that are different from conventionalturning and, therefore it is desirable to take into accountsome special characteristics; for this purpose, the FEMprogramme Third Wave AdvantEdge, which is speciallydesigned to simulate cutting operations, is used. For thesimulation of hard turning both an orthogonal and anoblique cutting model are proposed.2 Finite element modellingSimulations of various machining operations using thefinite element method have been reported over the last threedecades; in References 9, 10 a collection of such paperscan be found. The first models that appeared in the 1970sused the Eulerian formulation for modelling orthogonalcutting. In this approach the finite element mesh is spatiallyfixed and the material flows through it, in order to simulatethe chip formation. The computational time in such modelsis reduced, due to the few elements required for modellingthe workpiece and the chip, and it is mainly used forsimulating the steady state condition of the cutting process.The elements do not undergo severe distortion, since themesh is a priori known, but this formulation requirescomplex programming. Furthermore, experimental datamust be on hand prior to the construction of the model inorder to determine the chip geometry.Although this formulation is still utilized by someresearchers, the updated Lagrangian formulation has beenproposed and is more widely used today. In this approach,the elements are attached to the material and the unde-formed tool is advanced towards the workpiece. For theformation of the chip, a chip separation criterion in front ofthe tool edge is applied. There are many criteria proposedso far which can be geometric or physical and may involvefor example a critical distance between the tool and theworkpiece; when the tool reaches this critical distance fromthe workpiece the elements ahead of the tool edge are dividedandthusthechipisformed.Otherseparationcriteriapertaintocritical values of e.g., stress or strain in order to initiate thechip formation and even crack propagation criteria have beenreported for this procedure. A disadvantage of this method isconnected to the large mesh deformation observed during thesimulation; due to the attachment of the mesh on the work-piece material, the mesh is distorted because of the plasticdeformation in the cutting zone. In order to overcome thisdisadvantagecontinuous remeshing and adaptive meshing areusually applied, increasing considerably the required calcula-tiontime.Nevertheless,the advancesincomputers havemadeit possible to reduce the time needed for such an analysis toacceptable levels. Note that an arbitrary Lagrangian-Eulerianformulation (ALE) has also been proposed with the aim ofcombining the advantages of the two methods, but it is not aswidely used.Most of the modelling work published so far pertains to2D models of orthogonal cutting, while 3D models are ratherrare in the relevant literature. That is mainly because, eventhough 3D cutting is more realistic, since cutting is 3D innature, it requires a much more complex consideration ofworkpiece and cutting tool geometry, contact properties and,of course additional computational time. In particular, thework dedicated to hard turning is even more limited 1115.The models provided below are developed employingthe Third Wave AdvantEdge software, which integratesspecial features appropriate for machining simulation. Theprogramme menus are designed in such a way that theyallow the user to minimize the model preparation time.Furthermore, it includes a wide database of workpiece andtool materials commonly used in cutting operations, offeringall the required data for effective material modelling. TheAdvantEdge code is a Lagrangian, explicit, dynamic code,which can perform coupled thermo-mechanical transientanalysis. The program applies adaptive meshing andFig. 1 Orthogonal cutting model schematic diagram442Int J Adv Manuf Technol (2008) 38:441446continuous remeshing for chip and workpiece, allowing foraccurate results. For an analytical discussion on thenumerical techniques used in the programme and a compre-hensive presentation of its functions see Reference 16.3 Results and discussion3.1 Orthogonal cutting modelsThe orthogonal cutting schematic diagram used in theprogramme is shown in Fig. 1. The depth of cut is perpen-dicular to the plane shown in the figure and in the planestrain case, it is considered to be large in comparison to thefeed.In the present analysis the workpiece material is the AISIH-13 hot work tool steel and its length is taken equal to l=3 mm. The tool material is CBN and the modelling of tool-chip interface friction is based on Coulombs friction law,with friction coefficient set constant at the value =0.5. Acutting tool, with 5 rake angle, 5 clearance angle and0.02 mm cutting edge radius, is used for the analysis.Furthermore, the feed is taken equal to f=0.05 mm/rev,while three different cutting speeds, namely vc=200, 250and 300 m/min, are considered. In Fig. 2(a) and (b) theinitial mesh and a typical mesh created after the tool has cuthalf of the workpiece length (l=1.5 mm for time t=3104s), for vc=300 m/min respectively, are shown. In thisfigure, the continuous meshing and the adaptive remeshingprocedures can be observed. Note, that, in Fig. 2(a) themesh is denser near the tool tip, where deformation is aboutto take place, while in Fig. 2(b) new elements are created inthe shear zone where the strain rate is expected to be high.Note, also, that the mesh density in the chip, especially inits inner and outer surfaces, is also high because of thedeformation of the material in this area; finer mesh canfollow the curve of the curling material more closely and,furthermore, provide more accurate results.For the validation of the proposed hard turning modelexperimental results from the relevant literature are used,where the high speed turning of hard steel tubes (55 HRC),in order to achieve orthogonal cutting conditions, isperformed 17. In Fig. 3 the experimental values of thethrust force Ftand the cutting force Fcare compared to theones predicted from the models. From this figure it can beseen that the experimental and the numerical results are in avery good agreement and, generally, they follow the sametrends; thrust force, which is the largest force component,decreases for increasing cutting speed, while cutting forceincreases slightly. Nevertheless, in almost all the cases, theFig. 2 (a) Initial mesh and (b) mesh at l=1.5 mmFig. 3 Numerical and experimental results of thrust and cutting forcesfor three different cutting speedsInt J Adv Manuf Technol (2008) 38:441446443numerical values seem to overestimate the experimentalones while the discrepancies are larger for higher cuttingspeeds; this may be attributed to the large strain-ratesdeveloped during the process that alters the materialbehaviour in such a way that they cannot be taken intoaccount by the model or to inadequate friction modelling,which means that a more advanced friction theory needs tobe modelled.Note, also, that it is possible, besides the cutting andthrust forces, to extract from the proposed model predic-tions for values that it would be very laborious or evenimpossible to obtain otherwise. Examples of such cases are:the temperature distribution in the workpiece and tool in theform of isothermal bands and the von Mises stressesdeveloped during cutting. In Fig. 4(a) and (b) thetemperature fields and the von Mises stresses, respectively,for cutting with vc=300 m/min, are shown. These figuresdemonstrate the model at a step of the analysis, specificallyfor length of cut l=1.5 mm, where cutting is well into thesteady-state region.The form of the results is similar for all conditions, exceptof course the magnitude. From the results obtained, it may beconcluded that the maximum temperature increases withincreasing cutting speed, being 620C, 690C and 730C forthe three different cutting speeds considered. This mayexplain that the thrust force decreases for higher cuttingspeed, since softening of the material for higher temperaturetakes place. The regions that are mostly thermally loaded arethe chip and the rake face of the tool, in the chip-toolinterface close to tool tip, due to the plastic deformation ofthe chip and the frictional forces; the part of the chip that iscurledawayfromtherakefaceisprogressively cooleddown.The stress has an almost constant value along the centre ofthe shear zone, while near the tool tip ithas lower values; thiscan be explained due to the temperature rise of this areawhich softens the material.The knowledge of the maximum temperature and of thedistribution of the temperature fields in the rake face of thetool is of great interest because high temperatures in CBNtools are connected to wear mechanisms that reduce the toollife. With the numerical results provided by the model it ispossible to minimize unwanted effects and to choosesuitable cutting conditions in order to optimize the process.Another option of the proposed orthogonal cuttingmodel is to simulate the burr formation developed whenthe cutting tool exerts the full length of the workpiece; thiscan be achieved when the analysis is appropriatelyextended, so that the cutting path of the tool is longer thanFig. 4 (a) Temperature distribution and (b) von Mises stresses in theworkpiece, the chip and the cutting toolFig. 5 Burr formation and plastic strain of the workpiece and the chip444Int J Adv Manuf Technol (2008) 38:441446the workpiece length. In Fig. 5 the burr formation in theworkpiece and the plastic strain of the workpiece and thechip, for vc=300 m/min, can be seen.3.2 Oblique cutting modelsThe 2D models presented so far are more popular in themodelling of cutting operations since they are relativelysimple and they can offer acceptable accuracy. Neverthe-less, orthogonal machining is an ideal representation ofcutting, where the chip deforms in a plane; in reality, chipdeformation in turning takes place in all the threedimensions. The 3D models proposed so far in the literatureare different from the classical oblique cutting modelapproach 14, 15. However, it is this approach that willbe presented in this work since much experience hasalready been accumulated from its application to similarproblems, adequately handling 3D modelling with the finiteelement method. For the simulation the same software, aspreviously, is used. Furthermore, all the parameters of theworkpiece and the cutting tool, as well as the cuttingconditions are the same as in the models of the previousparagraph, for cutting speed vc=300 m/min. Additionally, aback rake angle of 20ois given to the tool.The proposed oblique cutting model for dry high speedhard turning can provide, as in the case of 2D modelling,much useful data. In Fig. 6 the temperature fields in theworkpiece, thechip and thetool can be observed.In thesamefigure, the position of the cutting tool, relative to theworkpiece, can also be seen, explaining the form of the chipcreated. The curling of the chip can be better observed inFig. 7, where a rear view of the workpiece in the same timestep can be seen. Additionally, in the same figure, thecutting tool is omitted so that the temperatures at thelocation where cutting takes place can be observed.Note that in these models more elements than the 2Dmodels described so far are required, with computationaltime being approximately 10 times more; the computationaltime required for a 3D model on a moderate PC is over 100hours. Even though 3D models provide more informationbeing more realistic, they present this drawback that makesthem less practical than 2D models.4 ConclusionsSummarizing the results reported above, one may concludethat hard turning is considered to be a new machiningprocess that has many advantages in comparison to otherprocesses in the machining of hardened steels. Especially,when hard turning is performed at high speeds, it is evenmore advantageous, offering reduced machining time andcost characteristics.The finite element method has been extensively used formodelling machining operations in the past. This method isalso used in the present paper and the commercial FEMprogramme Third Wave AdvantEdge programme isemployed. With the aid of this specialised software,simulation of 2D orthogonal cutting and 3D oblique cuttingmodels are provided.The orthogonal cutting models provide results such ascutting and thrust forces which were compared to experi-mental results from the relevant literature. However, otherresults such as the workpiece and tool temperatures, stressand strain can be predicted; these results would be verylaborious and time consuming to be obtained otherwise.The latter results can be used for the theoretical study of theprocess as well as to be connected with certain phenomenaappearing in this kind of machining such as tool wearwhich in turn affects workpiece surface integrity. Addition-Fig. 6 Temperatures predicted by the oblique cutting model for hardturningFig. 7 Rear view of the modelInt J Adv Manuf Technol (2008) 38:441446445ally, the burr formation in orthogonal cutting was modelledand presented.The 3D oblique cutting models represent a situationwere the chip deforms not in plane as in the ideal case oforthogonal cutting but in all three dimensions; a morerealistic approach is, thus, provided. The proposed obliquecutting models are in a position of providing the same dataas the 2D models, but also some additional information,e.g., for the 3D formation of the chip. Nevertheless, thesemodels are more complicated and require the use of muchmore elements increasing this way the effort and thecomputational time required for the analysis.From the analysis it can be concluded that the proposedmodels are practical, since only a minimum amount ofexperimental work is needed, and produce reliable results,allowing for industrial use in pursue of optimal production.References1. Trent EM, Wright PK (2000) Metal cutting, 4th edn. Butterworth-Heinemann, Woburn, MA, USA2. Tnshoff HK, Arendt C, Ben Amor R (2000) Cutting of hardenedsteel. Annals of the CIRP 49/2:5475663. Byrne G, Scholta E (1993) Environmentally clean machiningprocesses - a strategic approach. Annals of the CIRP 42/1:4714744. Klocke F, Eisenbltter G (1997) Dry cutting. Annals of the CIRP46/2:5195265. Kundrk J, Mamalis AG, Gyni K, Markopoulos A (2006)Environmentally friendly precision machining. Mater ManufProcess 21:29376. 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