CNC立式銑削再生式顫振預測之研究

Abstract

銑削製程由於能製作複雜形狀而被廣泛用於機械製造，然而不適當的切削條件之選用會容易產生加工時的顫振。再生式顫振屬於加工顫振中最常見的一種，再生式顫振發生時會造成刀具的損傷與降低工件的表面品質並且產生大量的噪音。由於再生式效應與切削過程的動態力有關，而此動態力在特定的頻率下會造成系統不穩定，引發自激式振動的再生式顫振。藉由區分刀具通過頻率與再生式顫振特有的頻率，即能偵測到顫振的形成。然而純粹的頻域分析無法對時間序列做分析，因而無法達到即時預測的效果。 本文提出一種線上再生式顫振的預測法則，藉由量測線上切削訊號頻率的轉變，達到預測顫振發生之目的。藉由計算連續短暫時間區間的頻率，再由本文提出的公式來描述每時刻切削訊號的主要頻率，以觀察頻率隨切削進給的轉變。並定義一套再生式顫振的預測法則，找出頻率從刀具通過頻率趨向再生式顫振頻率之轉變，讓顫振在未完全成型之前，預測再生式顫振的發生。本文設計三部分實驗：固定軸向切深、變動軸向切深與曲線加工的實驗，並以不同的時頻分析，分別分析訊號的變化，並比較預測再生式顫振的效果。實驗分析發現，透過動力計量測切削力訊號，並藉由觀察頻率隨時間的轉變，能提早發現再生式顫振；而利用麥克風量測到的空氣壓力變化也能有效發揮判斷效果，兩者皆能在切削力還未因為顫振大幅增長時，便即時找出可能會發生再生式顫振的情況。

Milling process is widely used in manufacturing due to the ability to produce complicate form of workpieces. However the cause of inappropriate cutting conditions may induce chatter. Regenerative chatter, one of the most common types of chatter in milling, damages the cutting tool as well as surface quality and makes a lot of noise. Because the regenerative effect is caused by cutting process and dynamic force, which makes system unstable and induces self-excited vibration in specific chatter frequency. By means of distinguishing the chatter frequency and tool passing frequency, chatter can be online detected. But only frequency domain analysis can’t analyze the time sequence and achieve chatter prediction. To achieve chatter prediction, this work proposes an online chatter prediction rule by the method of monitoring the trend of frequency change. Dynamometer and microphone are used to record signal and then continuously calculate the frequency of cutting signal as well as their dominated frequency. The frequency, calculated from new formula proposed by this work, can represent the frequency trend in each time period. And the chatter prediction rule warns when discover the change of frequency. This essay includes three part of experiment: fixed depth of cut, varying depth of cut and cutting along a curve. Different time-frequency analysis are used to analyze the signal and compare their ability to predict chatter in experiment. The experiment result shows that, through monitoring frequency varying with time, both force and sound can judge the chatter occur when the chatter amplitude is still small.