重型板式给料机论文(11)

In the case of metal cutting with a continuous chip, experimental observations reveal that chip formation takes place without a crack extension in front of the cutting tool tip. Furthermore, Fig.3. Pressing: (a) the flow-chart for chip-separation is a continuous process just ahead implementing the geometrical of the tool edge and, therefore, a geometrical separation criterion in simulated separation criterion based on distance is a realistic orthogonal metal cutting; (b) a assumption. Note that comparison of the schematic illustration of the algorithm geometrical separation criterion in modelling the used for the geometrical separation cutting process to other chip-separation criteria, criterion. based on the values of effective plastic strain and strain energy density, is made in.

The value of the critical distance, which in the present work is equal to 3 um and represents 5% of the element length, is taken as small enough to ensure continuous chip formation without causing numerical instability. Note that estimation of a proper value for the critical distance and its effect on the accuracy of the results involves difficulties and, therefore, it can be only validated experimentally.

2.2. Workpiece and tool material modelling

The workpiece material used for the plane-strain orthogonal cutting simulation was mild steel with 0.18% C, modulus of elasticity E=188 G Pa, Poisson's ratio v=0.3 and coefficient of linear thermal expansion c=1.281×10^5mm/mm℃

33

洛阳理工学院毕业设计论文 The material was modelled as isotropic elastic-plastic, with isotropic strain-hardening. In the cutting processes, the deformation of the material in the cutting zone takes place at elevated temperature, and high strains and strain-rates. Therefore, in order to allow for their effect on the material properties, the flow stress of the workpiece is taken as a function of strain, strain-rate and temperature using the constitutive equation taken from Ref.

(1)

(2)

where T (K) is the temperature, e the total strain, e? the total strain-rate, s the flow stress (in M Pa).

Due to the high elastic modulus of the tool material, which is tungsten carbide, the tool is considered as a perfectly rigid body and only a heat-transfer analysis is conducted on it. The physical properties of the mild steel and the carbide are tabulated in Table 1.

2.3. Friction modelling

Experimental observations revealed that the tool/chip interface may be divided into a sticking and a sliding region. Therefore, friction modelling in metal cutting must account for both situations. The friction force is modelled as a distributed tangential force Ft, along the chip/tool inter-face, given by

(3)

where, following the notation, m is the Coulomb friction coefficient, Fn the normal reaction force, Vr the relative sliding velocity between the chip and the tool and t=Vr/∣Vr∣ the tangent unit vector in the direction of the relative velocity. C is a constant representing the relative sliding velocity below which friction force starts dropping considerably to zero: in that way, sticking of the tool rake face is reproduced, by allowing variable very small slips. Table 1

Physical properties of the workpiece and tool material Material Mild steel Density (kg/m3) 7833 Thermal conductivity (W/m ℃) 54 33.5 Specific heat (J/kg ℃) 465 234 Tungsten carbide 12700 2.4. Heat transfer

Knowledge of the temperature distribution in the workpiece, chip and tool, is very important, since it has a great effect on the quality of the surface integrity of the tool wear. The main sources of heating, responsible for the high temperature rise observed in cutting processes, are the plastic work and the friction at the chip/tool interface, which are converted into heat. The rate of specific volumetric flux due to plastic work is given by the equation

（4）

where, following the notation, Wp is the rate of the plastic work, r the density, M the mechanical equivalent of heat to account for a consistent system of units and Wh the percentage of plastic

34

洛阳理工学院毕业设计论文 deformation converted into heat, which usually accounts for about 90%.

The distributed heat flux generated at the interface between the chip and the tool rake face due to friction is described by

（5）

where Ft is the contact friction force and Vr the relative sliding velocity between the chip and the tool rake face. This flux is split into two equal parts, assigned to each of the contacting parts, i.e. the chip and the tool.

Machining is performed at ambient temperature (i.e. the initial temperature of both the workpiece and the tool is 20℃) while the heat losses to the environment from the free surface of the workpiece, due to convection heat transfer, are determined by the distributed heat flux

（6）

where h=17.04 W/㎡℃ is the convection heat-transfer coefficient of the workpiece material, Tw the temperature of the workpiece, To the ambient temperature, taken as 20℃.Heat transfer by radiation is considered insignificant and is not therefore taken into account.

2.5. Process parameters

The tool geometry and the cutting conditions used for the orthogonal metal cutting simulation are presented in Table 2. Table 2

Cutting conditions and tool geometry

Tool rake angle (。) 20 Tool clearance angle (。) 5 Tool edge radius 0 Undeformed chip thickness (mm) 0.27 Width of cut (mm) 3.5 Cu …… 此处隐藏：4617字，全部文档内容请下载后查看。喜欢就下载吧 ……