1. Direct measurement method
Direct measurement can identify the change of the appearance, surface quality or geometric shape of the blade, and it can only be done when it is not cutting. It has two obvious shortcomings: first, it needs to stop and check; Secondly, it is impossible to detect the sudden fracture of the tool during machining. The direct measurement methods of tool wear at home and abroad include: resistance measurement, tool workpiece spacing measurement, optical measurement, discharge current measurement, ray measurement, microstructure coating method and computer image processing method.
(1) resistance measurement method
This method is to measure the actual wear state of the measured tool by using the electrical signal pulse generated by the contact between the measured cutting edge and the sensor. The advantage of this method is that the sensor is cheap, but the disadvantage is that the sensor must be carefully selected, which not only has good machinability, but also has no obvious influence on the tool life and is unreliable, because the chips on the tool and the accumulated chips may cause the contact part of the sensor to be short-circuited, thus affecting the accuracy.
(2) Measuring method of tool workpiece spacing
With the wear of the tool in the cutting process, the distance between the tool and the workpiece decreases, which can be measured by electronic micrometer, ultrasonic measuring instrument, pneumatic measuring instrument and inductive displacement sensor. However, the sensitivity of this method is easily affected by surface temperature, surface quality, coolant and workpiece size, which limits its application.
(3) Optical measurement method
The principle of optical measurement is that the worn area has stronger light reflection ability than the non-worn area. The greater the tool wear, the greater the reflection area of the blade and the greater the luminous flux detected by the sensor. Because the deformation caused by thermal stress and the tool displacement caused by cutting force will affect the detection result, the result measured by this method is not the real wear amount, but a relative value including the above factors. This method works better when the tool diameter is larger.
(4) Measurement method of discharge current
When a high voltage is applied between the cutting tool and the sensor, the current flowing in the measuring loop (arc discharge) depends on the geometry of the blade (i.e. the distance between the cutting tool tip and the discharge electrode). The advantage of this method is that it can be used for on-line detection, and it can detect the change of geometric dimensions of tools such as broken teeth and broken knives, but it can't accurately measure the geometric dimensions of blades.
(5) Radiometric method
When radioactive material is mixed into the tool material, when the tool is worn, radioactive material particles will pass through a pre-designed ray detector with a chip. The amount measured by X-ray measuring instrument is closely related to tool wear, and the radiation dose reflects the size of tool wear. The biggest weakness of this law is that radioactive substances pollute the environment, which is very unfavorable to human health. In addition, although this method can measure the tool wear, it can not accurately measure the state of the cutting edge of the tool. Therefore, this rule only applies to some special occasions and should not be widely used.
(6) Microstructure electroplating method
The microstructure conductive coating is combined with the wear-resistant protective layer of the tool. The resistance of microstructure conductive coating changes with the change of tool wear state, and the greater the wear amount, the smaller the resistance. When the tool appears tooth collapse, fracture and excessive wear, the resistance tends to zero. The advantages of this method are simple detection circuit, high detection accuracy and online detection. The disadvantage is that the requirements for microstructure conductive coating are very high: it should have good wear resistance, high temperature resistance and impact resistance.
(7) Computer image processing method
Computer image processing is a fast, non-contact and non-wear detection method, which can accurately detect different forms of wear on each blade. This detection system usually consists of CCD camera, light source and computer. However, due to the high environmental requirements of optical equipment and the harsh working environment of tools (such as cooling media, chips, etc.) in actual production. ), this method is only suitable for laboratory automatic detection at present.
2. Indirect measurement method
Indirect measurement method is to measure the parameters that reflect the influence degree of tool wear and damage by using the influence effect of the state of tool when it is worn or about to be damaged on different working parameters. These parameters can be detected when the tool is cutting without affecting the cutting process. Its disadvantage is that the detected process signal contains a lot of interference factors. Nevertheless, with the development of signal analysis and processing technology and pattern recognition technology, this method has become a mainstream method and achieved good results. Indirect measurement methods of tool wear used at home and abroad include cutting force measurement, mechanical power measurement, acoustic emission, thermal voltage measurement, vibration signal and multi-information fusion detection.
(1) Acoustic emission signal measurement method
Acoustic emission technology is a new application field of acoustic emission in nondestructive testing in recent years. The principle is that when solid materials are deformed, broken and transformed, strain energy will be released quickly, and acoustic emission is the elastic stress wave that follows. When the tool breaks, high-amplitude acoustic emission signals can be detected. Acoustic emission tool monitoring technology is recognized as the most potential new monitoring technology. Since 1980s, domestic and foreign countries have devoted themselves to the development and application of this technology, and made great achievements. As early as 1977, Iwatak and Moriwaki proposed to use acoustic emission technology to detect tool wear online. On this basis, Moriwaki proposed an acoustic emission tool breakage detection method. Kannatey-Asibu and Dornfeld theoretically studied the spectral characteristics of acoustic emission signals, and realized on-line monitoring of tool breakage by combining pattern recognition method. Although the research of acoustic emission monitoring technology in China started late, it developed rapidly. Huang uses the envelope analysis method to obtain the envelope of acoustic emission signal of tool wear, takes the parameters of time series model as eigenvalues, and identifies the tool wear equation through neural network. The experiment proves that the effect is good. Lee Hyo Ri analyzes the typical acoustic emission signal in boring process by FFT, and reflects the tool wear state through the change of acoustic emission signal amplitude in frequency domain. Yuan Zhejun decomposed the acoustic emission signal in the cutting process by wavelet packet, obtained the energy distribution of each frequency band of the signal as the signal characteristics, and established a fast neural network model based on fuzzy reasoning to identify the tool wear state. Chip-55A tool breakage monitor developed by Japan Murakami Jiken Company uses acoustic emission monitoring technology to monitor the tool state during machining. This product is matched with the CNC milling machine produced by the company, and the effect is good.
(2) Measuring method of cutting force signal
The change of cutting force is a physical phenomenon that is most closely related to the grinding and damage state of the tool during cutting. Using cutting force as detection signal has the advantages of easy picking, quick response and sensitivity, and it is a promising breakthrough in online method, so it is a common method to measure tool breakage in machining centers and FMS.
Based on the cutting force monitoring method, the monitoring data mainly include cutting component, cutting component ratio, dynamic cutting force spectrum and correlation function. When the tool is broken, the cutting force changes sensitively. When the tool damage is small, the cutting edge of the tool is not sharp, which increases the cutting force; When the cutting edge collapses or the tool breaks, the cutting depth decreases or does not decrease, which greatly reduces the cutting force. When monitoring cutting force, three components of FX, Y, Z FY and FZ are measured simultaneously in X, Y and Z directions. The current changes of the feed motor and spindle motor are measured by servo amplifiers installed on each motor, and the current changes are transmitted to the force valve, and the measured force is read on the display, so as to judge whether the tool is damaged. During the period of 1977, Yuki Murakami of tokyo denki university, Japan, made an in-depth study on the variation of cutting force under different machining conditions and tool wear conditions from both theoretical and experimental aspects, and found that the cutting component ratio is a characteristic quantity that can sensitively reflect the change of tool wear under certain conditions. Accordingly, he proposed a monitoring method of cutting force ratio. From 65438 to 0984, the research of Lan and Dornfeld shows that tangential force and feed force are highly sensitive to tool breakage. Shiraishi and others pointed out that the force monitoring method of tool failure is the most potential method and has broad industrial application prospects through the comparative study of measurement, detection and control technology in machining process, and torque monitoring and cutting force method have the same research value. The frequency band average method is used to monitor the tool wear state through cutting force. Wanjun uses cutting force model and least square method to realize the model to automatically track the change of machining process characteristics, so as to obtain the tool wear. The representative achievement of cutting force monitoring technology is TM-BU- 100 1 Tool Monitor introduced by Sandvik Coromant Company in Sweden. The force sensor used in this system can be installed on the spindle bearing and feed screw, and three thresholds can be set. If the thresholds are exceeded, it will automatically give an alarm.
(3) Power measurement method
Power measurement is also a potential method in industrial production. This method is to determine whether the tool is damaged in the cutting process by measuring the spindle load power or current-voltage phase difference and current waveform change. This method has the advantages of convenient signal detection, avoiding the interference of chip, oil, smoke, vibration and other factors in cutting environment, and simple installation. On the basis of analyzing the power signal of drilling process in machining center, Pan proposed and adopted the method of recovering power data, and established an on-line monitoring system for bit wear. Liu combines regression analysis technology with fuzzy classification to establish a mathematical model between boring parameters and current, which indirectly reflects the internal relationship between tool wear and boring parameters, and uses power signals to identify tool wear; Guo Xing proposed a monitoring method of milling cutter breakage power based on artificial neural network, and established a monitoring system of milling cutter breakage power. Experiments show that the system can sensitively detect and monitor tool breakage. Yuan Zhejun systematically studied the influence of tool abnormality on the power of the main motor in the cutting process, and proposed that the instantaneous value, derivative value, static average value and dynamic mean square value of the main motor power should be used to comprehensively monitor the abnormal state of the tool in the drilling process. Wanjun uses discrete autoregressive AR model to process the power signal, and recursively corrects the model parameters at each signal sampling time through adaptive algorithm to adapt to the cutting conditions. At the same time, in order to distinguish the change of power signal caused by tool wear and the change of cutting conditions, the paper introduces normalized deviation processing, which is obviously less when the tool cuts the workpiece than when the tool wears. When monitoring, the alarm threshold is set. When the normalized deviation exceeds the limit, the alarm will be given immediately, and the effect is very good. The representative manufacturer of successful application of motor power monitoring technology is Cincinnati Mielack Dragon Company. The tool monitoring system developed by our company is used in conjunction with sabre series vertical machining centers produced by our company.
(4) Measuring method of workpiece size
The wear or damage of the tool tip in machining will inevitably cause the change of the workpiece size. By measuring the size change of the machined surface of the workpiece, the wear and damage of the tool can be indirectly judged. In terms of measurement methods, there are two kinds of contact and non-contact methods. The advantage of measuring workpiece size is that it can directly and quantitatively give the radial wear or damage value of the tool, and it can be combined with online real-time compensation of machining accuracy to ensure the machining quality and realize the ultimate goal of monitoring tool wear and damage in finishing. Its disadvantage is that real-time measurement is easily disturbed by the test environment, and the cooling liquid and chip affect the measurement results; Factors such as thermal expansion and stress deformation of workpiece and tool, spindle rotation accuracy, feed motion accuracy and vibration will also directly affect the measurement accuracy. In addition, when machining variable cross-section workpiece, it is required that the sensor can track and locate accurately, which will also bring positioning error and increase the difficulty of realization.
(5) Measuring method of cutting temperature
Cutting heat is also an important physical phenomenon in the process of metal cutting, and tool wear will lead to a sudden increase in cutting temperature. There are three ways to measure cutting temperature: (l) The natural thermocouple composed of cutter and workpiece can measure the average temperature of cutting area, and different cutter and workpiece materials need to be calibrated; (2) Thermocouple is fixed at a certain point in the cutter body and consists of two kinds of wires. It measures the temperature at a certain distance from the blade, which has the problems of slow response when the temperature changes and time-consuming preparation in advance. (3) The infrared camera system can measure the temperature field distribution in the cutting area, which has the characteristics of high sensitivity and short response time. However, the instrument is complex, expensive and difficult to focus, and it is difficult to measure the tool temperature at the cutting hood.
(6) Resistance measurement method at the contact between tool and workpiece
The measuring principle can be divided into two types: one is the effect of increasing the contact area between the tool and the workpiece according to the tool wear, which is greatly influenced by the cutting amount and has insulation requirements; The second is to stick a layer of film conductor on the flank of the cutter, which will be consumed with the wear of the cutter. According to the change of its resistance, we can know the wear of the tool flank. This method has high accuracy, but each tool needs to stick a thin film resistor, which is easy to fall off under high temperature and high pressure. The application of this method in actual working conditions is not realistic at present.
(7) Vibration frequency measurement method
In the process of cutting, the workpiece rubs against the wear surface of the cutting edge, which will produce vibration with different frequencies. There are two ways to monitor this vibration: one is to divide the amplitude into two parts and compare the two parts when cutting; Secondly, the amplitude is divided into several independent frequency bands, which are continuously recorded and analyzed by the microprocessor, so that the wear degree of the tool flank can be monitored. The Institute of Automation of the National Bureau of Standards of the United States has gained successful experience in using vibration information in drilling. The developed system uses the acceleration sensor installed on the workpiece to analyze the vibration information in time, identify the bit wear and judge the bit fracture.
(8) Measuring method of workpiece surface roughness
With the increase of tool wear or damage, the roughness of workpiece surface will increase, so the tool wear or damage can be indirectly evaluated. Methods for measuring the surface roughness of workpieces can also be divided into two categories. One is stylus contact measurement, which can directly obtain the evaluation parameter r of surface roughness. This method is only suitable for static measurement. At present, most of these methods are only suitable for measuring room or laboratory environment. The other is non-contact optical reflection measurement to obtain the relative value of workpiece surface roughness. Two types of automatic monitoring are usually used: optical fiber sensor and laser test system. This method has high testing efficiency and can be used to measure the surface of soft materials without trace, but it needs to be calibrated with samples in advance, which is greatly affected by cutting fluid, chips, workpiece materials and vibration. At present, it can't reach the practical application level.
(9) Current signal measurement method
This method, called MCSA for short, takes the stator current of induction motor as the starting point of signal analysis, and studies the corresponding relationship between its characteristics and faults. Its basic principle is that with the increase of tool wear, the cutting torque increases, and the power or current consumed by the machine tool also increases, so the on-line detection of tool wear can be realized. MCSA has the characteristics of convenient testing, high information integration, direct transmission path, convenient signal extraction, unaffected by processing environment, low price and easy transplantation. In the case that the transmission system of machine tool is closed and it is difficult to install general sensors, it should be a method worth exploring.
(10) Thermal voltage measurement method
Thermal voltage measurement method uses the principle of hot spot effect, that is, when the contact point of two different conductors is heated, a voltage will be generated between the other ends of the two conductors, which depends on the electrical characteristics of the conductors and the temperature difference between the contact point and the free end. When the tool and workpiece are made of different materials, thermal voltage related to cutting temperature will be generated between the tool and workpiece. This voltage can be used as an index to measure tool wear, because with the increase of tool wear, the thermal voltage also increases. This method has the advantages of low price, high precision and simple use, and is especially suitable for the field of high-speed machining. The disadvantage is that it requires high sensor materials and accuracy, and can only carry out interval detection.