Research On Stability And Nonlinear Characteristic In Cutting Processing

Cutting cutter is a very detrimental and instable dynamic phenomenon that occurs between the tool and the workpiece(or medium).Since the cutter can cause vibration marks on the machined surface,it seriously affects the performance of the workpiece or machine tool.When the cutter is too large,the various components of the process system have to subject to dynamic loads.In some severe cases,it can make the cutting process unable to continue,and causes grievous noise pollution which is bad to the operator's physical and mental health.In order to avoid the cutter phenomenon,the operation of reducing the amount of cutting has to be adopted,yet this operation often limits the improvement in cutting efficiency.The typical cutting process system of machine tool and the drilling rig system of coal seam in the engineering field are taken as the research objects in this research.The time-domain prediction method for milling stability and prediction method for milling stability of spiral milling cutter are firstly studied.Besides,the improvement effect of the stability of the milling process after using pitch-variable milling cutters and speed-variable milling method,the axis and torsion-coupling self-excited vibration characteristics,linear stability and Hopf bifurcation for the drilling system of coal seam are also investigated.The detailed research work is outlined as follow(1)A milling stability prediction model for equidistant spiral milling cutters is established,and a semi-discrete method,a full discrete method and an improved full discrete method are proposed.It has been found that the position of the lobes is completely consistent,yet the peaks of the leaflets are slightly different.When the radial cutting depth is small,the stability limit obtained by the improved full-discretization method is slightly lower than the other two methods,thus the stability prediction results obtained are also more conservative than the other two methods.However,whether the radial cutting depth is large or small,the computational efficiency of the improved full-discretization method is significantly higher than the other two methods.(2)The effects of different parameters on the stability of the milling process of equidistant spiral milling cutters are studied.It has been found that the as the cutting force coefficient increases,the peaks and troughs of the stable lobes decrease,the critical axial depth reduces,and the stable cutting area shrinks.The reason is that the larger the cutting force coefficient,the more difficult it is to process the materials.Therefore,the chattering is more likely to occur during milling processing.Besides,when the damping ratio is increased,the peaks and troughs of the stability leaflet map have a certain degree of improvement.As the critical axial depth of cut increases,the milling stability area also increases.What is more,when the radial immersion ratio is increased,the stability of the leaflet diagram decreases overall and the critical axial depth also reduces.So,the large radial cutting depth is more susceptible to chattering rather than the small radial cutting depth.(3)The stability of the variable pitch milling process,the variable speed milling process and the variable milling process of the variable pitch milling cutter are investigated.It has been found that under the conditions of down-milling with large radial cutting depth,the three kinds of chatter control methods can all improve the stability of milling chatter.The stability improvement effects of variable pitch milling cutter and variable pitch milling cutter under variable speed milling are related to the pitch transformation form,yet the stability improvement effects of variable speed milling and variable pitch milling cutters under variable speed milling are related to the spindle speed modulation parameters.Besides,in the reverse milling process,the stability of the variable pitch milling process,the variable speed milling process and variable pitch milling cutters under variable speed milling process are better than that in the down milling process under the same conditions.Moreover,when the radial cutting depth is small,the stability of the variable pitch milling process,the variable speed milling process and the variable pitch milling cutters under variable speed milling process are better than that in the large radial cutting depth under the same conditions.(4)An axial-torsional vibration model of the drilling system with constant delay(CD)and state-dependent delay(SDD)is established.According to the SDD model,the system exhibits a typical stick-slip vibration behavior at low input speeds,where the torque accumulation during the viscous period and a sudden change in the speed of the sliding period can be discovered.During the sliding phase,the input torque in the system is insufficient to overcome the friction torque applied on the drill bit.Therefore,the drill pipe will continue to reverse the accumulated energy under the drive of external input until the slip occurs.During the sliding phase,the energy accumulated in the drilling system is sufficient to break the coal seam,so the drill bit slips off and releases energy instantly.However,firstly the drill bit reverses briefly,and then the speed continues to be increased until it reaches to a certain value to keep the decay state.Thus,it can be concluded that the output characteristics of the drilling system in the SDD model are more accurate than that in the CD model.(5)Through the analysis of the linear stability of the drilling system,it has been found that the stability interval of the system increases with the rise of the rotational speed.And in the case of small control parameters,there is little difference between the constant delay and the state dependent delay in the concerned stability.However,as the control parameters increase,the stability interval of the SDD model goes up gradually,and it is also significantly higher than that of the CD model.(6)Through the study of the characteristics of Hopf bifurcation of drilling system,it has been found that in the CD model,only the subcritical Hopf bifurcation region exists in the system.And in the SDD model,when the drill string system is in the low speed zone,there is only a supercritical Hopf bifurcation point.Moreover,the Hopf bifurcation phenomenon of the system increases with the rise of the control parameters,and the subcritical Hopf bifurcation point on the right side of the stability lobe map is gradually transformed into the supercritical Hopf bifurcation point.What's more,when the drilling system is in the middle and high speed zone,it can be discovered that there are subcritical and supercritical Hopf bifurcations in the system.As the rotational speed rises up,the depth of the system is gradually increased.Which means that by employing the same parameters,the instability in high-speed drilling system requires a greater cutting depth.