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數(shù)控磨削加工中砂輪磨損量的檢測方法
數(shù)控磨床在工件磨削過程中，不可避免的會產(chǎn)生砂輪的磨損現(xiàn)象，為了保證加工精度，需要及時地對砂輪的直徑變化或磨損量進行檢測。由于磨削加工是一種高精度的加工方法，限于較高測量精度的要求，目前還難以直接對砂輪的直徑進行精密的在線測量。同樣，由于砂輪表面砂粒的影響，對于砂輪磨損量的高精度檢測也是一件困難的事情。因此，目前在數(shù)控磨床上還難以實現(xiàn)砂輪磨損的自動補償。針對這種情況，本文結(jié)合數(shù)控曲軸磨削加工的研究，提出了一種基于聲發(fā)射檢測技術的砂輪磨損測量及誤差補償?shù)姆椒?。即利用安裝在砂輪架上的聲發(fā)射傳感器，測量砂輪相對于機床某一基準的相對位置，通過測量該相對位置的變化，間接地檢測砂輪的磨損量,并將測量結(jié)果反饋至數(shù)控系統(tǒng)，從而實現(xiàn)砂輪磨損的自動補償。
1 砂輪磨損量的檢測方法
考慮到砂輪表面砂粒的影響以及加工環(huán)境的影響，采用激光測量等方法均難以實現(xiàn)砂輪磨損量的在線檢測。近幾年在磨削狀態(tài)監(jiān)控中，聲發(fā)射技術已成為廣泛采用的一種方法〔1〕。切削過程中的聲發(fā)射是重要的切削現(xiàn)象之一，所謂聲發(fā)射現(xiàn)象是固體材料由于結(jié)構(gòu)變化引起應變能的快速釋放而產(chǎn)生的彈性波，簡稱AE(Acoustic Emission)。在磨削加工中，當磨削加工處于一種穩(wěn)定狀態(tài)時，AE信號并不發(fā)生變化。只有當磨削狀態(tài)發(fā)生變化時，聲發(fā)射信號才隨之發(fā)生變化。當砂輪與工件相靠近時，砂輪會有高頻聲發(fā)射信號產(chǎn)生，安裝在砂輪架上的聲發(fā)射傳感器信號會明顯增強，通過試驗研究可以確定一個閾值，根據(jù)該閾值可以確定砂輪與工件的相對位置?；谠撛?，考慮將聲發(fā)射傳感器安裝在砂輪架上，用于測量砂輪磨削過程中的聲發(fā)射信號。金剛石筆作為砂輪定位的基準，安裝在磨床床身的某一位置，當需要測量砂輪的磨損量時，數(shù)控系統(tǒng)控制砂輪靠近金剛石筆，當砂輪接近金剛石筆時，聲發(fā)射信號增強，通過試驗研究，可以確定一個判定砂輪與金剛石筆相對位置的閾值。當聲發(fā)射信號超過預先設置的閾值時，由聲發(fā)射信號處理裝置向數(shù)控系統(tǒng)的PLC發(fā)出開關量信號，機床數(shù)控系統(tǒng)則記錄下當前砂輪的位置，并將砂輪的當前位置與以前的位置相比較，計算出砂輪磨損量的補償值。進一步通過數(shù)控軟件在數(shù)控系統(tǒng)中實現(xiàn)砂輪磨損的自動補償。上述方法是否可行，主要取決于能否找到一個能夠準確判斷金剛石筆與砂輪相對位置的閾值，當砂輪沒有磨損時，利用該閾值判斷的金剛石筆與砂輪相對位置不應發(fā)生變化或具有很小的位置測量誤差。為了說明這一問題，對砂輪磨損量對加工誤差的影響b(以此確定位置測量的允許誤差)以及所提出方法的位置測量精度進行了試驗研究。
2 試驗研究
為了確定聲發(fā)射技術應用于砂輪磨損量測量的測量精度，我們在瑞士Studer精密萬能外圓磨床上進行了試驗研究。試驗時，將金剛石修整筆固定在磨床尾架上，砂輪的最小進給量1μm，砂輪粒度為80、中軟、紅剛玉、直徑300mm，砂輪線速度為45m/s，頭架轉(zhuǎn)速為50r/min。聲發(fā)射信號監(jiān)測系統(tǒng)選用美國許密特工業(yè)公司的L—4200—4型聲發(fā)射控制機，其聲發(fā)射信號工作頻率范圍為50kHz~950kHz，響應時間小于1ms。試驗時，首先控制砂輪的進給運動，使其靠近金剛石修整筆，并在聲發(fā)射控制機上設置一個閾值。然后將砂輪分別以5μm、10μm、15μm、20μm作為砂輪退刀量退回砂輪，再重新控制砂輪緩慢進給，靠近金剛石修整筆，直到聲發(fā)射控制機監(jiān)控屏幕上顯示達到閾值，立即停止砂輪的進給，記錄各次的進給量。以上試驗過程不斷重復，從而測量出聲發(fā)射傳感器測量金剛石修整筆與砂輪相對位置的重復測量精度。表1給出了試驗結(jié)果。由表1可知，誤差值在-5~+5μm之間，其值小于允許的理論計算誤差值6μm。因此，本文所提出的方法可以用于砂輪磨損量的檢測。
表1 金剛石修整筆與砂輪相對位置的測量誤差
試驗次數(shù)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
退刀量(μm)
5
5
5
5
5
5
5
10
10
10
10
10
10
10
進刀量(μm)
5
6
7
8
5
5
5
10
11
12
10
10
11
10
誤差(μm)
0
1
2
3
0
0
0
0
1
2
0
0
1
0
試驗次數(shù)
15
16
17
18
19
20
21
22
23
24
25
26
27
28
退刀量(μm)
15
15
15
15
15
15
15
20
20
20
20
20
20
20
進刀量(μm)
13
15
14
14
15
13
14
20
19
18
16
16
17
17
誤差(μm)
-2
0
-1
-1
0
-2
-1
0
-1
-2
-4
-4
-3
-3
3 結(jié)論
本文基于聲發(fā)射檢測技術，給出了數(shù)控曲軸磨削砂輪磨損的間接檢測方法，該方法同樣適于平面磨削、外圓磨削等情況，其區(qū)別僅在于要求的檢測精度不同。實際試驗表明，所提出的方法檢測誤差在-5~+5μm之間，可以滿足高精度磨削加工的需要。
Test Method of Grinding Wheel Wear in CNC Grinding
CNC grinding machine in grinding process of workpiece, which is caused by abrasion wheel, in order to ensure the precision needed in a timely manner the diameter of the wheel to detect changes or wear. Since grinding is a precision machining method, limited to the requirements of high precision, it is hard to direct the diameter of the wheel-line measurement precision. Similarly, the impact of sand surface grinding wheel, grinding wheel wear for high-precision detection is a difficult task. Therefore, it is still difficult in the CNC grinding machine to achieve automatic compensation for wheel wear. In response, this paper studies CNC Crankshaft Grinding, a technique based on acoustic emission measurement of grinding wheel wear and error compensation method. That is installed on the wheel rack using acoustic emission sensors, measuring wheel with a benchmark for the relative position of machine tools by measuring changes in the relative position of the indirect detection of grinding wheel wear, and measuring the results fed back to the control system, which achieve automatic compensation for wheel wear.
1. A grinding wheel wear detection methods.
Taking into account the impact of grinding wheel surface grit and processing environment, using laser measurement methods are difficult to achieve on-line detection of grinding wheel wear. In recent years, the grinding condition monitoring, acoustic emission technology has become a widely used method 〔1〕. Cutting process of cutting acoustic emission is an important phenomenon of the so-called acoustic emission phenomenon is caused by structural changes of solid materials due to the rapid release of strain caused by elastic waves, referred to as AE (Acoustic Emission). In the grinding process, when the grinding process in a steady state, AE signal does not change. Only when the grinding conditions change, the acoustic emission signal was changed with it. When the grinding wheel and workpiece with close, the wheel will produce high-frequency acoustic emission signals, install the shelves in the wheel acoustic emission sensor signals will be significantly enhanced through the pilot study determine a threshold, according to the threshold value can determine the grinding wheel and workpiece relative position. Based on this principle, consider the acoustic emission sensors installed in the wheel rack, used to measure the grinding wheel in the process of acoustic emission signals. Diamond grinding wheel orientation as a benchmark document, installed in the bed of a grinding position, when the need to measure wheel wear, the CNC control wheel near the diamond pen, when the wheel close to the diamond pen, acoustic emission signal enhancement, the experimental study , a judge can determine the relative position of grinding wheel with diamond document threshold. When the acoustic emission signal exceeds a pre-set threshold, the acoustic emission signal processing device to send digital control system, PLC binary signals, machine tool numerical control system is to record the current wheel position and wheel position of the current position compared with the previous comparison, calculate the value of grinding wheel wear compensation. Further through the digital software in the CNC system to achieve automatic compensation for wheel wear. The feasibility of this method depends largely on the ability to find a can accurately determine the relative position of grinding wheel diamond pen and the threshold, when the wheel is not worn, the use of the threshold to judge the relative position of grinding wheel diamond pen and should not change or have a very the location of a small measurement error. To illustrate this problem, on the grinding wheel wear on the machining error of b (position measurement in order to determine the allowable error), and the location of the proposed method accuracy were studied.
2. Study
To determine the acoustic emission technology for grinding wheel wear measurement accuracy, we in Switzerland Precision Universal Cylindrical Grinding Machine Studer were studied. Tests will be fixed in the grinding diamond dressing document the end of the shelf, the smallest wheel feed 1μm, wheel size is 80, in the soft, red corundum, diameter 300mm, wheel speed is 45m / s, the first frame speed 50r / min. AE signal monitoring system used the United States Xu Schmitt Industries, Inc., L-4200-4-type acoustic emission control machine, the acoustic emission signal frequency range of 50kHz ~ 950kHz, response time of less than 1ms. Test, the first feed control wheel movement, so close to the diamond dressing pen, and acoustic emission control plane to set a threshold value. Then the wheel respectively 5μm, 10μm, 15μm, 20μm, as the amount refunded back the knife wheel grinding wheel, and then slowly regained control of the feed wheel, near the diamond dressing pens, launch control until the sound of the monitoring and controlling the screen to reach the threshold value, stop wheel feed, record the times of the feed. Repeat the above test procedure to measure out loud emission sensor diamond grinding wheel dressing pen and the relative position of the repeated measurement accuracy. Table 1 shows the test results. Can see from Table 1, the error values between -5 ~ +5 μm, the calculated value is less than allowable error 6μm. Therefore, the proposed method can be used for grinding wheel wear detection.
3 Conclusion
Based on acoustic emission detection technology, is given CNC crankshaft grinding wheel wear of indirect detection methods, the method is also suitable for grinding, cylindrical grinding, etc., the only difference is that the detection accuracy of different requirements. Practical experiments show that the proposed method detected the error between -5 ~ +5 μm to meet the needs of high-precision grinding.
4