Research On The Mechanistic Modeling Of Milling Process Damping

Process damping significantly improves the machining stability in the cutting processes with relatively low speeds.Just because of this,it plays an important role in the selection of optimal stable cutting region with higher material removal rates for difficult-to-machine materials such as titanium alloy.Although many research efforts have been focusing on process damping,there are still many shortcomings in the existing models.Many famous scholars even listed process damping as the most challenging problem needed to be solved.To this end,studying the generation mechanism of process damping and establishing accurate prediction model are essential for revealing the dynamic behavior of difficult-to-machine materials.This thesis investigates the mechanism of milling process damping to form a complete set of mechanistic models based on the metal cutting and mechanical theories.First of all,the ploughing-based damping model suitable for the rigid milling process and the velocity-based damping model applicable to the thin-wall milling process are put forward.Then,a unified process damping model is established by comprehensively considering multi-factors such as the cutting velocity,the ploughing indentation and the stiffness of the milling system.The correctness of the proposed models is verified by carrying out a series of cutting experiments with titanium alloy.Finally,based on the above achievements,a milling process damping simulation software is developed.The main research works and contributions are as follows:(1)Ploughing-based damping model suitable for the rigid milling process is established,two difficulties in which are solved.First,an analytical method for calculating the indented area considering the actual geometry of the tool tip is proposed.It expresses the indented area as a unified formula,i.e.the summation of statically and dynamically indented components,in the case of whether the tool vibrates toward or deflects from the workpiece,which greatly facilitates the analyzing of chatter stability.Second,an identification method for calibrating the milling process damping is derived based on the operational modal analysis.Besides,the experimental setup suitable for measuring the cutter’s upper and lower deflections is designed.By doing so,the tangential and radial ploughing force coefficients can be simultaneously obtained with only a few chatter-free milling tests.(2)Velocity-based damping model applicable to the thin-wall milling process is put forward.This model calculates the actual cutting velocity by considering the relative vibrations of cutter-workpiece system in both feed and normal directions.Then,the instantaneous uncut chip thickness and dynamic shearing force are modified based on the definition that the chip thickness should be measured in the direction vertical to the actual cutting veloc-ity.For the convenience of stability analyzing,the dynamic shearing force represented by actual cutting angle is converted to a linearized expression with nominal cutting angle by means of Taylor series expansions around the harmonic response.By doing so,the process damping force component related to the vibration velocities can be easily extracted from the modified shearing force,which just clarifies the mechanism of velocity-based damping model.(3)A unified process damping model considering the varying stiffness of the milling system is established.First,a proportion factor is introduced to modify the indented area influenced by the milling system’s deflections.And,the contributions of both ploughing indentation and velocity change to process damping are theoretically formulated in a unified way.By doing so,a unified process damping model suitable for the milling systems with arbitrary stiffness is established.Second,although the proportion factor is included in the indented area,it is difficult to directly formulate the boundaries of the actual indented zone.With the aid of inverse cutting force modeling,the modification of the indented area is transformed into the correction of the ploughing force coefficients,and an efficient calibration algorithm is proposed to determine the weighting of ploughing indentation and velocity change on process damping.The stability lobes diagram can be reasonably solved only by substituting the corrected cutting force coefficient into the established unified process damping model.(4)A milling process damping simulation software is developed.With the help of MATLAB GUI development platform,the key technologies of this thesis are integrated,and a prototype system of milling process damping simulation software is developed.The prototype system includes model selection module,parameter input module,stability calculation module and result saving module,and can provide stability predictions for the rigid milling process,thin-wall milling process and transitional stage between the above two milling processes.The research works in this thesis enrich the dynamics of metal cutting processes and help to select proper cutting parameters for difficult-to-machine materials,which can provide theoretical basis and technical support for the manufacturing of aerospace high-end equipment,such as titanium alloy ribs,frames,engine blades,etc.