沖壓木塊機(jī)的設(shè)計(jì)【說明書+CAD】

沖壓木塊機(jī)的設(shè)計(jì)【說明書+CAD】,說明書+CAD,沖壓木塊機(jī)的設(shè)計(jì)【說明書+CAD】,沖壓,木塊,設(shè)計(jì),說明書,CAD MACHINE TOOLDetermining mechanical characteristics of material resistanceto deformation in machiningValerii KushnerMichael StorchakReceived: 14 April 2014/Accepted: 4 July 2014/Published online: 22 July 2014? German Academic Society for Production Engineering (WGP) 2014AbstractThis paper analyses experimental results anddifferent hypotheses about the resistance of the machinedmaterial to plastic deformation in machining. It is neces-sary to take into account that strain rate and temperatureaffects the mechanical properties of the material. It isuseful to describe the regularities of material resistance toplastic deformation with differential equations, determin-ing a dependence of the specific deformation work ondeformation. For machining processes, the correlationsbetween yield point and deformation or rather flow curvesare analytically deduced from the differentiation of thespecific deformation work. It has been found out that theflow curves are vaulted for the adiabatic conditions ofdeformation in the chip forming area and the accumulationzones near the cutting edge. The yield point here reaches itsmaximum for deformations that are usually lower than thetrue final shear of the material penetrating through the chipforming area. It is suggested to take these maximum valuesof the yield point as mechanical properties of the materialto be machined. The main goal of the theoretical andexperimental investigations presented in this paper is toestablish the analytical dependence of the specific defor-mation work and therefore also of yield point and specifictangential forces on deformation, taking account of theeffect of temperature on yield point. The main advantagesof applying the specific deformation work is not only itsdirect relation to deformation temperature but also thepossibilityofexperimentallydeterminingthisworkthrough specific tangential forces and true final shear. Inthis way it is possible to establish how deformation tem-peratureaffectsyieldpointbymeansofempiricalconstants.KeywordsMachine tool ? Cutting ? Mechanicalproperties ? Flow curve1 IntroductionThe resistance characteristics of the machined material todeformationincuttingareusuallydeterminedbymechanical test methods for tension and pressure and thesubsequent extrapolation to the necessary amount of plasticdeformation occurring during machining. However, thedeformation of the material and hence the specific defor-mation work during machining is usually higher by oneorder of magnitude, and the strain rate is higher by abouteight orders of magnitude than in standard test methods fortension and pressure. In addition, large plastic deformationand an inhomogeneous deformation distribution in theprimary shear zone cause a heterogeneous temperaturedistribution. In turn, this leads to an uneven resistance ofthe machined material to plastic deformation. It has to betaken into account that the conditions differ for the defor-mation of the material in the chip forming area or ratherprimary and secondary shear zones as well as in theaccumulation zones and in the areas of the plastic contactbetween chip and wedge 1. It must also be taken intoconsideration that the resistance of the material to plasticdeformation in the shear and accumulation zones variesvery much.V. KushnerDepartment of Mechanical Engineering and Materials Science,Omsk State Technical University, pr. Mira 11, 644050 Omsk,RussiaM. Storchak (&)Institute for Machine Tools, University of Stuttgart,Holzgartenstrae 17, 70174 Stuttgart, Germanye-mail: michael.storchakifw.uni-stuttgart.de123Prod. Eng. Res. Devel. (2014) 8:679688DOI 10.1007/s11740-014-0573-8Deformation, strain rate and temperature are linked toeach other during the machining process. When deter-mining the mechanical properties of a material with astandard test, an operator establishes these factors irre-spective of such interactions. This leads to substantialerrors when determining the mechanical properties of thematerial to be machined. How the above-mentioned factorsinfluence the dependence of yield point on deformation inmachining is not taken into sufficient account. Such con-siderations are necessary because the conditions of thematerials deformation in machining processes differ con-siderably from standard tests. At present, analysing theregularities of the machined materials resistance to plasticdeformation in cutting is limited to examining only themean values of yield point, which are extended to a widerange of change in deformation. This range includes boththe areas of the hardening of the material to be machinedand the areas of the softening 26. Many investigationsare exclusively restricted to the experimental determinationof specific tangential forces in the chip forming area orprimary shear zone 35, 7, 8. This is, however, insuffi-cient to be able to establish a dependence of yield point ondeformation or rather a flow curve for machining processesand to evaluate maximum values of yield point.For different machining conditions, deformation partlyoccurs in a relatively large region of the chip forming area.This deformation is partly located in a narrow regionimmediately at the boundary of the chip forming area 5.The localisation of deformation in this narrow area has aconsiderable effect on the softening of the material to bemachined. Such an effect must absolutely be taken intoconsideration when determining the flow curve.Itisimpossibletodetermineaflowcurveofthemachinedmaterial by experiment under cutting conditions due to theuneven distribution of deformation in the shear zones.Moreover, the yield point of the material to be machineddoes not only depend on the size of deformation but also onthe change in temperature, which involves changes indeformation and yield point. The parameters mentionedhave a certain interaction with each other, which cannot beestablished by experiment. That is why not only experi-mental but also theoretical or analytical examinations haveto be carried out to determine flow curves.The main goal of the theoretical and experimentalinvestigations presented in this paper is to establish theanalytical dependence of the specific deformation work andtherefore also of yield point and specific tangential forceson deformation, taking account of the effect of temperatureon yield point.2 Analysis of hypotheses about the regularitiesof material resistance to deformation in machining2.1 Effect of strain, strain rate and temperatureon the yield pointMany researchers assumed that there are uniform regular-ities, applying to the resistance of the material to bedeformed to plastic deformation, for different cutting lay-outs of a material, including tension, forming and materialremoval, see, e.g., 35. A large number of investigationson tensile tests assume that stress intensity depends onstrain ei, strain rate _ eiand an increase in homologoustemperature DT0913:ri f ei;DT0; _ ei1Among other things, the equation (called conditionalequation in the following) representing a dependence ofshear yield point spon deformation ep, strain rate _ e and anincrease in homologous temperature DT0is defined asfollows 15:spSbffiffiffi3p ?epffiffiffi3p?ln1d=100 !m?_ e_ e0?k?DT0?exp?B?DT0;DT0DTTmT?T0Tm;2where Sbis true tensile strength, epis current value of thetrue shear, e0ffiffiffi3p? ln 1 d=100 is true shear defor-mation at a specific strain d, is strain shear rate in tension,T is the temperature of a deformable material, m, n, k, Bare empirical constants representing an effect of deforma-tion, strain rate and temperature on a current value of theyield point sp, D?T0is the mean value of increase in thehomologous temperature in the chip forming area, DT isthe change in absolute temperature, T is the current tem-perature, T0is room temperature, Tmis the melting tem-perature of the material.The conditional equation in the form of (2) cannot bedirectly applied as material model for machining processes,as an increase in temperature during cutting is not anindependent variable and depends on deformation epaswell as the current yield point sp. Hence, the conditionalequationformeetingthedeformationconditionsinmachining has to be searched for in the form of a flowcurve sp(ep) containing empirical constants. These con-stants represent effects of deformation, strain rate andtemperature.680Prod. Eng. Res. Devel. (2014) 8:6796881232.2 Hypothesis about an application of the simple formof loading by cuttingThe dependences of specific tangential forces stin theconditional shear plane on true final shear ewwhen turningdifferent steels were compared with the dependences ofshear yield point on shear deformation in tension sp(ep) 3.The specific tangential forces stwere interpreted here asmaximum values of the yield point in the chip formingarea. It is, however, more correct to interpret the specifictangential forces st, obtained by experiment, as quotient ofthe specific deformation work in relation to true final shearewor rather as average of the yield point. This follows froma definition of the specific tangential force stin the chipforming area by the projection Fsof the forces Rnand Rmon the rake face of the wedge onto the shear plane A andthe deformation capacity in the chip forming area throughthis force Fsand the specific deformation work AwFig. 1.The following dependences can be deduced from theabove-mentioned and Fig. 1:Fs? v2 ss?a ? bsin uy? v2 Aw? Sb? a ? b ? v3where b is width of cut.AwZew0spSbdep4The following can be concluded from (3):ssSb1ew? Aw;ssv ? sinuyv2? Aw? Sb;v2 v ?cos ccos uy? c?;ewv2v ? sinuyK 1=K ? 2 ? sin ccos c5where m2is the velocity of the chip shear relative to theworkpiece in the direction of the shear plane A, ewis truefinal shear, K is chip compression ratio.To extrapolate the dependences sp(ep), a law of singleload was used which does not take account of an effect ofstrain rate and temperature on yield point 15:spSbffiffiffi3p ?epe0?m62.3 Hypothesis about the stability of specific tangentialforces in the chip formation zone and their relationto material strength by tensile testIt was detected that specific tangential forces stinmachining were close to the shear yield point in tensionin several cases 3. This yield point was established hereby extrapolating the dependence (3) on the size ofdeformation of the true shear during machining. For thisreason an empirical correlation was suggested to estimatespecific tangential forces stin the shear plane duringmachining 3:st? A ? 2:5m A2:5;7where A is the empirical coefficient; A2.5is the shear yieldpoint in tensile tests, which is extrapolated to a deformationof e = 2.5.Regarding the majority of several examined steels, theforces stdid nevertheless not increase with growingdeformation as required by the law of single load (see 6).They rather remained constant or even decreased.Apart from the dependence (7), other correlationsbetween forces stand strength characteristics in tensiletests were suggested as well. Among other things, thefollowing empirical correlations were obtained for estab-lishing specific tangential forces stin the chip forming areaand on the rake face qFwhen turning different steels usingtools with shortened rake faces 1, 16:Fig. 1 Layout for establishing the specific tangential force ssin thechip forming area (a), velocity diagram and cutting scheme (b)Prod. Eng. Res. Devel. (2014) 8:679688681123st 0:8 ? Sb;qF 0:6 ? Sb8These correlations obtained by experiment are presentedin Fig. 2.These constant and decreasing dependences st(ew) showthat the flow curves sp(ep) for tensile tests and machiningdo not agree with each other. This corresponds with theexperimental investigations on machining and pressure ofaluminium in the range of changes in shear deformationbetween 0.6 and 1.5 9. The specific tangential forces inthe shear plane during machining were considerablygreater here than the compression strength.That the flow curves for pressure and machining in therange of smaller deformations do not agree can beexplained by the effect of strain rate on the stresses in theshear plane during machining. The effect of temperature onthe yield point and the specific tangential forces inmachining are assessed differently as well. One side con-tests that temperature affects the specific tangential forcesin the chip forming area and on the rake face of the wedge3, 4, 14. This is confirmed by the fact that the temperaturein the chip forming area does not exceed a limit of 400 ?Cas a rule 15. In addition, it was assumed that a decrease inyield point during machining is completely compensatedby its increase on account of a major effect of strain rate athigher temperatures 16. Assuming that temperature hasno substantial effect was accepted regarding both the spe-cific tangential forces in the chip forming area and thespecific tangential forces on the rake face of the wedge 8,13, 17. Another side confirms that strain rate and tem-perature have a considerable effect on the yield point inmachining 1, 15.The following analysis of the experimental data is pre-sented to justify the latter point of view.3 Effect of strain rate and temperature on specifictangential forces in the chip forming area3.1 Effect of strain rateOwing to the analysis of results obtained by experiment, itis detected in 3 that the ratio of strain rates in tension andmachining affects the mean value of yield point. Specifictangential forces stare examined when machining differentsteels at a depth of cut of a = 0.22 mm and a toolorthogonal rake angle of the wedge of c = 20?. Experi-mental analyses were carried out at extremely low cuttingspeeds of m = 0.2 m/min to rule out an effect of temper-ature on specific tangential forces st3. In addition, tensiletests were simultaneously conducted at the same strainrates. A quotient of the strain rates in machining and tensiletests was approximately two powers smaller than withconventional cutting parameters. This quotient is, however,great enough and reaches about 106.In view of the fact that specific tangential forces stin thechip forming area are characterised by mean values of yieldpoint, they are compared with extrapolated mean values oftensile strength:st;m1ew?Zew0Sbffiffiffi3p ?epe0?mdep1ew? Aw;t? SbA ? Sbm 1? emw;9Aw;tZew0spSbdep;10where Aw,tis the dimensionless specific deformation workin tensile tests, which is extrapolated to deformations of thetrue final shear during machining ew.If the strain rate is taken into account and an effect oftemperature is ruled out, a dependence of specific tangen-tial forces stin the chip forming area at mean values ofFig. 2 Mean specific tangential forces stin the chip forming area andon the shortened rake face of the wedge qFdepending on true tensilestrength Sbin machining 16682Prod. Eng. Res. Devel. (2014) 8:679688123tensile strength can be approximated by the followingfunction:st Ke? st;m;11where Keis the coefficient of deformability, determiningthe differences between the deformation conditions of themachined material in the chip forming area for machiningand tensile tests.Table 1 shows the experimental data and results whencomparing specific tangential forces stin the shear planewith the mean values of yield point ss,m.In the machining of the examined steels, the forces stareapproximately 1.3 times greater than the mean values oftensile strength extrapolated to the deformation of true finalshear in machining (see Table 1). Hence, the coefficient Keis 1.3. According to this coefficient, an increase in strainrate by about 106, which corresponds to a transition fromtensile tests to machining at relatively low homologoustemperatures T0= 0,165), is capable of causing a consid-erable rise in the mean value of yield point st,m.To estimate the effect of homologous temperature on thecoefficient of deformability Ke, it is analysed how relativechanges in strain rate affect the yield point in the machiningof different materials 4 such as lead, aluminium and steel.The results of the analysis are shown in Fig. 3.Hence,thecoefficientofdeformabilityKeinmachining and other kinds of deformation such as, e.g.,tensile tests does not only depend on changes in strainrate _ e/_ e0but also on changes in homologous temperatureDT0. For modern machining processes, the difference incutting speed is within one degree of power at most.Contrary to this, the difference between the speed of astandardised tensile or pressure test and the cutting speedin machining is eight degrees of power. A change indeformation speed within in one degree, which impliesthe change in cutting speeds for different machiningprocesses, changes the derived coefficient of deformabi-lity from 1.258 to 1.344. This change in the coefficientof deformability can be ignored. Therefore, a quotient ofstrain rates for machining in the range of conventionalcutting parameters and for tensile tests is about 108andcan be assumed as constant 15. Accordingly, the valuesof the coefficient Kehave to be greater with growinghomologous temperature (see Fig. 3). This coefficient canbe represented as a function of the increase in homolo-gous temperature:Ke 108k?DT0123.2 Effect of the deformation temperatureOwing to the experimental data of specific tangential forcesin the chip forming area and on the rake face of the wedge,it can be inferred that there is a hardening effect of themachined material due to the change in coefficient Keaswell as a softening effect of the temperature in machining1. For example, it can be concluded from the experi-mental data shown in Fig. 2a, that yield point rises roughlyproportionally to the increase in true breaking point as wellas that the quotient st/Sbdecreases with increasing truetensile strength Sbor correspondingly increasing defor-mation temperature hD:hD?st? ewCV;13where CVis the specific volumetric heat capacity of thematerial to be machined.Figure 4 shows how temperature affects the mean yieldpoint when machining different steels. Specific tangentialforces in the area of plastic contact between tool and chipare lower than in the chip forming zone, when machiningsteels with a shortened rake face of the wedge (see 8):qF& 0.75 ? st16. Such a ratio of the values of meanspecific tangential forces in the chip forming area and inthe area of plastic contact can be interpreted in favour ofthe decrease in yield point of the machined material on therake face with growing temperature (see Fig. 4b). Thisresults in the uneven distribution of the specific tangentialforces on the rake face of the wedge, involving an increaseTable 1 Experimental data for machining and tensile tests (rbelastic limit)No.Kind ofsteeld, %rb,MPamss,m,MPaewst,MPa1St00383180.33564.14602C10423620.33803.3490320Cr354800.34773.15804Cr18Ni9Ti636340.56702.51,030Fig. 3 Effect of homologous temperature on the coefficient ofdeformability KeProd. Eng. Res. Devel. (2014) 8:679688683123in temperature. This leads to the effect of temperature onyield point in machining.It is assumed that the maximum value of yield point isreached in the accumulation zone B (see Fig. 1a) near thecutting edge at lower temperatures. Accordingly, themaximum value of yield point in the accumulation zones ofthe rake face q0and the flank face should be considerablygreater than stand qF.Regarding todays state of the measuring technology, itis impossible or very difficult to determine the directexperimental reason why there is such a maximum bymeans of establishing the change in specific tangentialforces in the very small area of the accumulation zone B.However, this can be indirectly accounted for by estab-lishing specific tangential forces in the accumulation zoneG of the flank face (see Fig. 1a), where the deformationconditions are very similar to the corresponding conditionsof the accumulation zone B 16. Such changes in specifictangential forces in the accumulation zone G of the flankface were investigated for the machining of steel C45 1.It was found out that the specific tangential forces in theaccumulation zone G are greater than in the chip