Investigation of Modal Characteristics and Milling Dynamics for a Machine-Tool System

工具機結構模態特性與銑削動態之研究

林盛勇(Shen-Yung Lin) ； 宋孟儒(Meng-Ru Song)

《中國機械工程學刊》 42卷6期 (2021/12) Pp. 561-569

摘要

The vibrations easily deduce the resonance or fatigue for a structure system and resulting in structure damage. Therefore, the modal characteristics of the structure itself should be investigated in detail, and make efforts to avoid the occurrence of resonance and unnecessary dissipation of loss. In addition to structure modal characteristics, process dynamics is another important factor that strongly related to cutting stability for machining accuracies of the machine-tool. An integrated execution methodology is proposed in this study for this purpose, in which the modal test, dynamic stiffness test and cutting experiments are performed step-by-step in a sequential manner. To this end, hammer, spectrum analyzer, accelerometer, ME'scope software and dynamometer are utilized together to conduct the tests and experiments on a CNC milling machine-tool, and the modal-related parameters and cutting process dynamics can systematically be investigated. From the synthesized results and investigations, it shows that the mode shapes, Mode 1 to Mode 3, mainly regarded to the deformation in head-stock and column structure components of a machine tool. While the main deformation parts corresponding to mode shapes of Mode 4 and Mode 5 are worktable and saddle, and these two modes exhibit a bending deformation behavior which will possibly make the joint interface to separate. The results from dynamic stiffness test show that when the head-stock moved along z-axis from -100mm height position down to -300mm location, the dynamic stiffness of the structure in xaxis direction may be changed from 217N/μm to 106N/μm, which have some impact on milling stability. The results from the milling experiment show that vibration marks left on the machined surface obviously under the conditions of cutting speed of 143.3m/min, feed rate of 400mm/ min and axial depth of cut of 0.5mm during up-milling process, and this phenomenon is judged as the chatter occurrence when comparing with those cases of stable milling state. The results obtained in this study can be utilized as a reference for milling parameter planning in industry.

關鍵字

Modal characteristics ； Dynamic stiffness ； Cutting stability ； Spectrum analysis

並列摘要

振動易造成結構的共振或疲勞從而破壞結構，因此必須詳細研究結構本身的模態特性，盡力去避免共振的發生以及不必要的損失消耗。切削製程動態是另外一個影響切削穩定性的重要因子，它與切削穩定性強烈相關，關係到工具機加工的精度。為此，本文提出一整合性的執行方法，以逐步依序之方式進行模態測試、動剛性測試及銑削實驗。藉由結合使用敲擊錘、頻譜分析儀、加速規、ME'scope軟體及動力計，在一銑床上完成上述之測試與實驗，模態相關參數與切削過程動態則能夠有系統地被檢視。綜合以上之結果與探討顯示，Mode 1至Mode3其模態振型影響之結構部件為頭座與立柱。而Mode 4及Mode 5所對應的模態振型，主要變形部件為工作台與鞍座，這兩個模態的彎曲變形將造成接合面可能的分離。動剛性實驗結果顯示，當頭座沿Z軸高度從-100mm降至-300mm時，X軸向結構動剛性從217 N/μm變化至106 N/μm，對切削穩定性將有所影響。銑削實驗結果顯示，逆銑過程當切削速度143.3 m/min、進給率400 mm/min及切削深度0.5mm時，加工表面出現顫紋，與穩定切削之結果比對研判該銑削條件組合已發生切削顫振。以上這些結果可作為日後銑削製程參數規劃之參考。