｜立式加工機刀具之全角度切削穩定性分析

摘要：銑削加工過程中刀具產生顫振將嚴重影響工件表面精度，並危及刀具與工具機的壽命。為了避免顫振的發生，通常會選擇較為保守之加工條件，但卻降低生產效率。如何在生產效率及減少顫振發生間取得平衡，則需建立一套避免顫振的加工準則。根據切削力學，切削顫振的發生與切削力大小及工具機結構剛性有關，而刀具切削穩定性範圍與其動態響應特性有關並可應用穩定性圖來界定。基於此觀點，本研究建構全方位切削穩定性分析法則，計算主軸刀具系統切削穩定性範圍。研究中，首先透過機台動態特性實驗，擷取主軸刀具端之頻率響應函數，再導入刀具與加工件材質之切削負荷係數，預測刀具之全方位切削穩定性範圍。根據本研究實驗分析結果，主軸刀具之切削穩定性與臨界切深會隨切削進給方向以及主軸在Z軸向定位高度之變換而改變。此種現象說明主軸刀具系統切削穩定性之變動性。應用本研究發展全方位切削穩定性分析法則，將可建立特定刀具統在規劃加工行程內之最佳切削穩定性條件，提供選用切削條件之決策依據。

Abstract: Chatter vibration induced by self excitation during chip generation process may produce poor surface quality and cause damage to the cutter and machine tool. To avoid the occurrence of chattering, machining operations are usually performed under conditions with more conservative consideration for tooling selections, but the productivity and efficiency of material removal is thus reduced. It is therefore of importance to establish machining criteria for achieving optimum material removal rates with highest machining stability. On the other hand, according to machining mechanics, chattering is caused by the dynamic interaction between the spindle tool system coupled with the machine frame structure and the cutting process itself. Besides, machining behavior can be characterized in terms of the stability lobes diagram. Therefore, the prediction of machining stability is not only of great importance for the design of a machine tool towards high-precision and high-speed machining, but can also provide information for selecting adequate cutting conditions to achieve stable machining without chattering.

This study was aimed to present the criteria for the evaluation of the machining stabilities of a milling machine tool. For this purpose, we first conducted vibration tests on the spindle tool to assess the tool tip frequency response functions along the principal modal axis. Then, basing on the orientation dependent stability analysis model proposed in this study, we evaluated the variation of the dynamic characteristics of the spindle tool and the corresponding machining stabilities at a specific feeding direction. Following the stability analysis model, the limited axial cutting depths for stable machining within the whole range of interested feed directions were obtained. Current results demonstrate that the stability boundaries and limited axial cutting depth of a specific cutter were affected to vary with the changing of the feeding direction and the feeding height of the spindle head. It is believed that realizations on the variations of machining stabilities of a spindle tooling system within the entire range of feed directions can provide a valuable reference for the selection of the machining conditions in tool path planning.

關鍵詞：切削顫振、切削穩定性、頻率響應函數

Keywords：Machining chatter, Machining stability, Frequency response function

前言
銑削作業是金屬加工的重要製程之一。因銑削方式屬於多刀刃不連續切削，故在銑削過程中會產生週期性的銑削力。此一週期性銑削力會激發工具機主軸、刀具及工件夾具結構產生強迫性振動 （Forced vibration）。當切削速度較快時，切削力的頻率也跟著提高，有可能會激發到工具機結構的前幾項低階自然振頻而產生嚴重的共振現象。此時，將影響刀具切削加工動作而在工件表面產生切削波紋。當刀具再次經過此切削面時，此表面波紋將使切削力變動量增加，導致切削振動擴大，產生所謂顫振現象（Chatter）。切削顫振現象被認為是源自於工具機結構的動態特性（Dynamics of machine tool structure）以及切削動態特性（Cutting process dynamics）所耦合而成的振動現象[1,2]，顫振對加工工件的表面精度及尺寸準確度產生不良之影響，亦會產生噪音，加速刀具磨損，減短壽命。為了避免顫振的發生，通常會選擇較為保守之加工條件，但卻降低生產效率。如何在生產效率及減少顫振發生間取得平衡，則需建立可以避免顫振的加工準則。