Study On Metal Cutting Dynamics Based On Different Tool-Chip Friction Models

Metal cutting is always the main machining means of mechanical manufacturing in the past, present, and future. The Improvement of manufacturing level requires better cutting technology, and especially the appearance of advanced manufacturing technology requests cutting technology to realize high degree of digization and informatization. Metal cutting is a complicated process involving large plastic deformation, high strain rate, high temperature, and complex tool-chip friction, as well as has dynamic and highly non-linear characteristics. In initial cutting process, cutting tool has obvious impact effect on workpiece, but which is neglected and the static or quasi-static balance mehod is adopted in past research. Consequently, the deviation between the theoretical analysis result and the experimental result of cutting force and chip deformation is so large that the complicated cutting mechanism can not be revealed and explained meticulously deep and quantitatively. With the progress of computing technology and the rapid development of numerical analysis theory, the high precision numerical simulation and theoretical analysis methods are becoming important means for cutting mechanism research. In this paper, the mild steel cutting process is studied using a nonlinear explicit finite element code and the dynamic analysis theory for plastic structure; it mainly includes the following aspects:(1) The three-dimensional explicit dynamic model of mild steel cutting process is established using a general purpose non-linear finite element code. The model adopts 8-node 3D explicit solid element based on one-point integration Lagrangian formulation. The flow stress of cutting materials is dependent on strain, strain rate and temperature. The chip separation is simulated by adopting compound geometric physical criterion. The problem of mesh distortion in three dimensional large deformations is solved effectively by means of establishing reasonable geometry model and setting accurate calculating parameters. The simulations of orthogonal cutting and oblique cutting with the inclination angle ranging from 10°to 45°are carried out. The numerical analysis results simulate the whole chip formation process from initial to steady-state cutting. The phenomenon of shear slipping, fibration and flowing of metal grains is clearly displayed by the deformation and flowing of 3D solid elements in cutting simulation. The variation of cutting force, the stress, strain, strain rate, displacement field and velocity field of chip materials, the temperature field in chip and tool, and the distribution of tool-chip contact pressure are discussed systematically. The cutting force of mild steel in steady-state cutting is determined. The comparison of calculated result with experimental result shows a good agreement, which verify the validity and reliability of the cutting model.(2) In cutting process, tool-chip friction interface is composed of sliding friction zone and sticking friction zone. The modified Coulomb friction law is employed to simulate the tool-chip friction situation. The influence tendency and mechanism of tool-chip friction on cutting force, chip deformation, cutting temperature, chip flow characteristics, shear plane shape and so on are investigated systematically by setting the values of friction coefficients from 0.1 to 0.6. The difference of previous opinions about chip flow characteristics, shear plane shape, and tool wear are all given reasonable explanation. This study can provide references for the prediction and control of cutting results parameters, and also can help to determine the value of tool-chip friction coefficient in practical machining. (3) The influence mechanism of cutting velocity on cutting process is studied using numerical simulations and experimental methods. Research results show that, the decrease of material strength caused by thermal -softening is counteracted by the increase of materials deformation resistance caused by high strain rate in high-speed cutting. The decrease of tool-chip friction coefficients raised from high cutting speed is the main reason for cutting force decreasing. Therefore, the tool-chip friction model with the relationship of friction coefficient exponentially decline with cutting speed is established. The simulation results are compared with the experimental values of cutting force and chip deformation coefficient and found to be excellent agreement. This model can accurately describe and explain the influence mechanism of cutting speed on cutting process in condition of forming continuous chip.(4) It can be found that cutting tool has obvious inertia impulse effect on workpiece in both cutting simulations and experiments. In this paper the dynamic response characteristics of cutting process is described and verified in detail. The impact action of cutting tool on workpiece is parsed into the model of elastic-perfectly plastic cantilever beam bearing step load. The dynamic response modality of cantilever beam under impact load is investigated. The computing formula of tool-chip contact length, upper and lower limit value of cutting force are all derived. The calculation results indicate that the experimental and simulation values of cutting force per unit area are all in range of calculational limit values. Moreover, it is found that the value of cutting force approximates to the upper limit value when tool-chip friction coefficient is smaller, and the value of cutting force approximates to the lower limit value when tool-chip friction coefficient is bigger.(5) The computational models of normal shear angle and three-axis cutting force of oblique cutting are established. In these oblique cutting models, it is proposed that the tool-chip friction force in normal plane is the component of total friction force in chip flow plane, so the tool-chip friction angle in normal plane decreases with the increasing of chip flow angle. Three-axis cutting force are calculated using this model at different rake angle, inclination angle and tool-chip friction coefficient, the calculated values are excellent agreement with the experimental results. This shows that these models can explain theoretically the simulation results that normal shear angle increases with the increasing of inclination angle. It is an effective improvement for existing computational models of oblique cutting.