High-chromium white cast iron is a kind of anti-corrosion and wear-resistant material with high hardness and strength. Its tensile strength is 650-850 MPa, its as-cast hardness is HRC 48-55, and its quenched hardness is HRC 55-62. However, this kind of material is hard and brittle, and the cutting state is extremely unstable. If it cannot solve the cutting processing problem well, its application will be greatly limited. Tests show that the use of carbide tools for processing high-chromium cast iron, the tool wear is very fast, the cutting is easy to form a chipping cutting, the surface roughness, can only be used for general roughing. For high-chromium cast iron finishing, lower roughness can be achieved with ceramic tools. However, the impact resistance of ceramic tools is relatively weak, and the brittle materials used for cutting high-chromium cast iron are more vulnerable to impacts and cause tool failure such as chipping. Because ceramic tools are more expensive, they should be avoided as far as possible. Therefore, the correct and reasonable choice of cutting amount is an effective way to avoid the impact damage of the tool. In this paper, under the condition of considering the economical efficiency of tool dynamics, the cutting amount of the outer circle of high-chromium cast iron slurry valve valve plate parts with composite ceramic tool was optimized. 1 Influence of cutting temperature on the dynamic performance of composite ceramic tools The influence of cutting temperature on tool dynamic performance is mainly reflected in the mechanical properties of cutting tool materials and the additional thermal stress generated by the cutting temperature field. The effect of temperature on the mechanical properties of composite ceramic tools The physical and mechanical properties of composite ceramics are generally reduced as the cutting temperature increases. Tests show that within a certain range, when machining steel parts, the mechanical performance indicators and temperatures of composite ceramic tools have the following empirical formula: sbt=sbo(1-bt) (1) sst=ssoexp(1-ct) (2) Et=Eoexp(-dt) (3) Ht=Ho(1-kt) (4) Where: sbt, sst, Et, Ht are the tensile strength, yield limit, and elasticity, respectively, of the composite ceramic tool at temperature t. Modulus and hardness: sbo, sso, Eo, Ho are the tensile strength, yield limit, elastic modulus, and hardness at room temperature, respectively: b, c, k, and d are constants determined by the test. Additional thermal stress generated by the cutting temperature field In continuous turning, when the cutting amount and the external environmental conditions are constant, it can be considered that there is a steady-state heat source on the front and back surfaces of the cutter, and the cutting temperature field on the cutter can adopt three-dimensional thermal conduction. Finite element method calculation. Because the tool is working in the elastic range, the additional thermal stress caused by temperature is st = at (t-t0) E (9) where: t0 is the temperature without stress: at is the coefficient of linear expansion of the composite ceramic. 2 Dynamic stress and dynamic strength of composite ceramics The dynamic stress of the composite ceramic tool is calculated using three-dimensional finite element method. The model is shown in Fig. 1. The cutting force Fz acts in the middle of the cutting edge. The unit adopts an eight-node three-dimensional block element. The contact edge between the tool and the tool body is elastically restrained, and the rest are free edges. Calculating the maximum stress under this condition is exactly the same as solving the general solid mechanics, but considering the effect of temperature, there are
In the above two formulas: CV is a constant: ap is the margin. Substituting Equation (13) and Equation (14) into Equation (12) gives Cw=Cc+tzCm+kvCm/(Vfap)+kvCd/[CVf(-1/n)+V(1-1/m)ap(1) -1/p)] (15) Where: Cm=Cj+Cg. Formula (15) is the cost objective function of the tool cutting optimization calculation. Constraint functions The optimization functions for the cutting amount calculation mainly include the following aspects: Cutting consumption constraints. If the machine's cutting speed and feed rate are in the range of (Vmax, Vmin), (fmax, fmin), then the following constraint function G1(f,V)=V-Vmax≤0 (16) G2(f) V)=Vmin-f≤0 (17) G3(f,V)=fmax-f≤0 (18) G4(f,V)=f-fmax≤0 (19) Accuracy constraint. If the given surface roughness is Ra, then there is a constraint function G5(f,V)=f−(8Rar)1â„2≤0 (20) Machine power constraints. If the allowable power of the machine tool is p, the following constraint G6(f,V)=VFz1+zfFzy-p/CFzaKFz≤0 (21) Tool life constraint G7(f,V)=V1/mf1/nap1/ p-CV/T0 ≤ 0 (22) In the above formulae, XFz, yFz, KFz, CFz are experimental constants. Tool dynamic strength constraint function G8(f,V)=smax-st≤0 smax is the maximum dynamic stress calculated by the finite element method for the cutting amount. Optimized calculations Because the tool's maximum dynamic stress is calculated using finite elements, there is no quantitative analytical function. In the optimization calculation, the constraint function G8 cannot be introduced. Therefore, we do not consider G8 in the calculation and optimize a group of cutting amounts. Then we optimize the cutting amount according to this group and calculate the maximum dynamic stress of the tool by the finite element method. If the dynamic strength is satisfied, the cutting amount is optimal. If it is not satisfied, the cutting amount is reduced until the dynamic strength is satisfied. The cutting amount parameter obtained at this time is the most reasonable cutting amount. The calculation diagram is shown in Figure 2.
Figure 1 Stress contour map
Figure 2 Calculation block diagram
In the above two formulas: CV is a constant: ap is the margin. Substituting Equation (13) and Equation (14) into Equation (12) gives Cw=Cc+tzCm+kvCm/(Vfap)+kvCd/[CVf(-1/n)+V(1-1/m)ap(1) -1/p)] (15) Where: Cm=Cj+Cg. Formula (15) is the cost objective function of the tool cutting optimization calculation. Constraint functions The optimization functions for the cutting amount calculation mainly include the following aspects: Cutting consumption constraints. If the machine's cutting speed and feed rate are in the range of (Vmax, Vmin), (fmax, fmin), then the following constraint function G1(f,V)=V-Vmax≤0 (16) G2(f) V)=Vmin-f≤0 (17) G3(f,V)=fmax-f≤0 (18) G4(f,V)=f-fmax≤0 (19) Accuracy constraint. If the given surface roughness is Ra, then there is a constraint function G5(f,V)=f−(8Rar)1â„2≤0 (20) Machine power constraints. If the allowable power of the machine tool is p, the following constraint G6(f,V)=VFz1+zfFzy-p/CFzaKFz≤0 (21) Tool life constraint G7(f,V)=V1/mf1/nap1/ p-CV/T0 ≤ 0 (22) In the above formulae, XFz, yFz, KFz, CFz are experimental constants. Tool dynamic strength constraint function G8(f,V)=smax-st≤0 smax is the maximum dynamic stress calculated by the finite element method for the cutting amount. Optimized calculations Because the tool's maximum dynamic stress is calculated using finite elements, there is no quantitative analytical function. In the optimization calculation, the constraint function G8 cannot be introduced. Therefore, we do not consider G8 in the calculation and optimize a group of cutting amounts. Then we optimize the cutting amount according to this group and calculate the maximum dynamic stress of the tool by the finite element method. If the dynamic strength is satisfied, the cutting amount is optimal. If it is not satisfied, the cutting amount is reduced until the dynamic strength is satisfied. The cutting amount parameter obtained at this time is the most reasonable cutting amount. The calculation diagram is shown in Figure 2.
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