工具機結構特性與銑削穩定性評估技術之研究

Investigative of Machine-Tool Structure Characteristics and Assessment Technique of Milling Stability

洪正廷(Zheng-Ting Hong)

指導教授 ： 林盛勇

國立虎尾科技大學/工程學院/機械與電腦輔助工程系碩士班/碩士(2012年)

https://doi.org/10.6827/NFU.2012.00116

摘要

近年來高速化與高精密已是工具機發展的主要趨勢，工具機結構必須具備高剛性及高切削穩定性等特性，方能符合高精密度元件快速加工之需求。加工精度及使用壽命的展現乃工具機性能的關鍵性指標，因掌握了機台結構本身的剛性與動態特性，將有助於結構設計製造上的補強及避免結構共振與顫振的發生。而切削穩定性評估技術的建構，可供加工業者作為製程參數的選用參考，將有助於工具機容量的發揮及生產速率的提升並有效地增進其附加價值。因此，發展工具機結構特性與切削穩定性評估技術，以預測合適的製程參數組合，讓使用者在穩態切削條件下進行加工，乃為工具機業者當前努力的方向與課題。
本研究是以實務性的做法來發展工具機結構特性與銑削穩定性評估技術。首先，以整機數值模擬分析與實驗模態分析，探討加工中心機結構的自然頻率、阻尼比與模態振型。實驗模態分析可求得整機系統的結構特性，並用來修正數值模擬分析模型。接著，執行刀具-主軸動剛性實驗，藉以求取此子系統的動態剛性與對應頻率，輔以切削穩定性預測軟體CutPro，建立起由刀具構型、工件材料與切削條件等製程參數組合所對應之銑削穩定性耳垂圖，據以探討銑削過程的動態行為。找出圖中穩定/不穩定邊界各區塊座落點的銑削參數，透過銑削實驗取得切削噪音、動態切削力與刀具振動頻譜等銑削性能參數，完成以多面向切削性能參數交叉判辨顫振之有無。本實驗模態分析結果顯示，在主軸迴轉頻率範圍內共計有七個模態容易引發結構共振頻率，加工條件規劃時必須避免使用這些激振頻率的轉速。銑削穩定性的評估技術方面，切削噪音、切削力及線上刀具振動頻譜三者所呈現的結果型態於顫振的判認大致上是相互吻合的，少部分較不易辨識的模糊情況，亦能以此交叉判認的方式予以克服，成功地建立起切削顫振的判認準則。CutPro預測軟體所建構之穩定性耳垂圖與銑削實驗結果存在一些落差，推判原因為機台主軸內部接觸元件已出現損耗，造成刀具兩個徑向動剛性值不一致，影響預測結果的正確性。

關鍵字

模態參數 ； 頻率響應函數 ； 再生式顫振 ； 穩定性耳垂圖

並列摘要

In recent years, high cutting speed and high precision have become the main development trend of the machine-tool which should possess high rigidity and high cutting stability to fulfill the demands of a rapid high-precision component machining. The machining precision and service life are the key performance indicators of the machine-tool, which can help to reinforce the design and manufacturing of the structure, as well as avoid the occurrences of structure resonance and cutting chatter, by controlling both the rigidity and dynamic characteristics of the machine-tool properly. The evaluation technology of cutting stability provides a reference on process parameter planning for factory workers, and is conducive to utilize the machine-tool capacity into full play. Increasing productivity and added value enhancement are also accompanied naturally. Therefore, developing structural characteristics of the machine-tool and evaluation technology of cutting stability are the basis for predicting the appropriate combinations of process parameters. It allows the users may machine a component under stable cutting conditions. This is the main direction and issue in development of machine-tool.
This study attempts to develop the structural characteristics of machine-tool and evaluation technology of milling stability from a practical perspective. First, the natural frequency, damping ratio and mode shape were investigated based on the numerical simulation analysis and experimental modal analysis for an overall machine-tool unit. The structural characteristics of a machine-tool could be obtained through experimental modal analysis which results were also used to modify the numerical simulation analysis model. Then, dynamic stiffness experiment was conducted on cutting tool-spindle system to determine the dynamic stiffness and corresponding frequency of this subsystem. In addition, the milling stability lobe diagram corresponding to the process parameter combination, such as cutting tool geometrical configuration, workpiece material and cutting condition, was established using CutPro for predicting the cutting stability, so as to investigate the dynamic behavior during the milling processes. The process parameters corresponding to distinct areas distinguished by stable/unstable border-line were located in this lobe diagram and they are further verified by cutting noise, dynamic cutting force, cutting tool vibration, etc obtained from milling experiments to determine whether the chatter occurred or not. The chatter occurrence judgment is completed through the above crossing comparable manner by multiple cutting performance parameters. According to the modal analysis results, there are 7 modes within the spindle rotation frequency range which are easily to excite structure resonant frequency. Hence, the above corresponding rotation speeds of this spindle should be avoided in machining conditions planning. For the evaluation technology of milling stability, the results of cutting noise, cutting force and online cutting-tool vibration were approximately consistent with each other on judging the chatter occurrence aspect. Some few indistinct and vague chatter cases could also be recognized by this crossing comparable judgment manner. Thus, a cutting chatter judgment criterion is successfully established in this study. However, some difference still exists between those obtained from CutPro predictions and milling experiments. This is inferred that some wear and failure defects existed in the contact components inside the spindle system of this machine-tool, which causes the inconsistency values of radial dynamic stiffness along different directions and results in the prediction accuracy.

並列關鍵字

modal parameter ； frequency response function ； regenerative chatter ； stability lobe diagram

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