工具機切削動態特性系統鑑別

Identification of Dynamic Characteristics of Machine Tools Using Operational Modal Analysis

鄭錫勳(Hsi-Hsun Cheng)

指導教授 ： 鄭志鈞

國立中正大學/工學院/機械工程學系/碩士(2015年)

摘要

銑削過程中，材料移除率(Material removal rate, MRR)反應加工時間的長短，本研究應用操作模態分析法(Operational modal analysis, OMA)決定最佳切削轉速，只需於工具機之主軸與平台黏貼多顆加速規，便可透過操作模態分析法與PolyMAX鑑別工具機主軸與平台之自然頻率與相應阻尼比。操作模態分析法以位移傳遞率為基礎進行鑑別，但由於切削過程中的頻率組成相當複雜，為避免錯誤，因此藉由奇異值分解找出位移傳遞率的一致性判定自然頻率，且位移傳遞率差值之倒數具有頻率響應函數之特性，當獲得正確自然頻率後，即可使用PolyMAX擬合頻率響應函數計算相應阻尼比，若將操作模態分析法鑑別工具機加工時之特性與傳統工具機靜態時敲擊測試之結果進行比較，其兩者最大差異在於切削阻抗，因此鑑別之自然頻率將不同。當獲得自然頻率與阻尼比後，即可依顫振數學模型繪製描述切削轉速與深度關係之穩定界線，根據此顫振穩定界線提供之最佳轉速進行切削驗證，其實驗結果顯示此轉速不僅能高效率的移除材料且同時兼顧良好的切削表面，提高加工之效率。

關鍵字

材料移除率 ； 操作模態分析 ； 顫振 ； 顫振穩定界線圖

並列摘要

A technique based on Operational modal analysis (OMA) in determining the optimal spindle speed to achieve higher material removing rate (MRR) for machine tools in machining is proposed. With several accelerometers attached respectively on the spindle and the work table of a machine tool, the natural frequency and its corresponding damping ratio of this machine tool during machining are identified using OMA and PolyMAX. In particular, the OMA technique is conducted based on displacement transmissibility measurement and singular value decomposition (SVD), which reduces the possibility in wrongly identifying natural frequencies due to presence of harmonics in machining. After identifying the natural frequencies, the corresponding damping ratio is then determined using traditional PolyMAX estimation algorithm. The identified results are compared with those from traditional impact testing technique. The differences in the identified results between two methods, i.e. OMA and impact testing, are attributed to the cutting impedance involved only in the OMA. With the identified natural frequency and damping ratio, the stability diagram which depicts the machining stability in terms of spindle speed and stiffness ratio can then be created. Experiments are presented to validate the optimal spindle speed chosen from the stability diagram. Results show that the MRR increases with a better surface roughness using the optimal spindle speed.

並列關鍵字

Stability lobe diagram ； Material removing rate ； Operational modal analysis ； Chatter ； PolyMAX

參考文獻

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