Tool status detection methods can be divided into direct measurement methods and indirect measurement methods.
1. Direct measurement method
The direct measurement method can identify changes in the appearance, surface quality or geometry of the blade. It can generally only be performed when not cutting. It has two obvious Disadvantages: First, it requires shutdown inspection; second, it cannot detect sudden tool breakage during processing. Direct measurement methods for tool wear used at home and abroad include: resistance measurement method, tool-workpiece distance measurement method, optical measurement method, discharge current measurement method, radiation measurement method, microstructure coating method and computer image processing method.
(1) Resistance measurement method
This method uses the electrical signal pulse generated by the contact between the cutting edge to be tested and the sensor to measure the actual wear status of the tool to be tested. The advantage of this method is that the sensor is cheap. The disadvantage is that great attention must be paid to the material selection of the sensor. It must have good machinability and have no obvious impact on the tool life. Moreover, the work is not very reliable because of the accumulation of chips and tools. Debris may cause a short circuit in the sensor contact area, thus affecting accuracy.
(2) Tool-workpiece distance measurement method
As the tool wears during the cutting process, the distance between the tool and the workpiece decreases. This distance can be measured with an electronic micrometer, ultrasonic measuring instrument, Pneumatic measuring instruments, inductive displacement sensors, etc. are used for measurement. However, the sensitivity of this method is easily affected by factors such as workpiece surface temperature, surface quality, coolant, and workpiece size, which limits its application.
(3) Optical measurement method
The principle of optical measurement method is that the worn area has stronger light reflection ability than the unworn area. The greater the tool wear, the greater the reflective area of ??the blade. The larger the sensor, the greater the light flux detected by the sensor. Since deformation caused by thermal stress and tool displacement caused by cutting force all affect the detection results, the result measured by this method is not the true wear amount, but a relative value including the above factors. The effect is better when it is larger.
(4) Discharge current measurement method
Add high-voltage electricity between the cutting force tool and the sensor, and the size of the (arc discharge) current flowing in the measurement circuit depends on The geometry of the blade (i.e. the distance from the tip of the blade to the discharge electrode). The advantage of this method is that it can perform online detection and detect changes in the geometric dimensions of the tool such as chipping and broken blades, but it cannot accurately measure the geometric dimensions of the blade.
(5) Ray measurement method
Incorporate radioactive substances into the tool material. When the tool wears, the radioactive material particles will pass through a pre-designed ray measuring instrument. The amount measured by the radiation measuring instrument is closely related to the tool wear, and the radiation dose reflects the amount of tool wear. The biggest weakness of this law is that radioactive materials pollute the environment and are very harmful to human health. In addition, although this method can measure the wear amount of the tool, it cannot accurately determine the condition of the cutting edge of the tool. Therefore, this law is only applicable to certain special occasions and should not be widely adopted.
(6) Microstructure coating method
The microstructure conductive coating is combined with the wear-resistant protective layer of the tool. The resistance of the microstructured conductive coating changes with the wear state of the tool. The greater the amount of wear, the smaller the resistance. When the tool chips, breaks, or is excessively worn, the resistance tends to zero. The advantage of this method is that the detection circuit is simple, the detection accuracy is high, and online detection can be realized. The disadvantage is that the requirements for the microstructure conductive coating are very high: it must have good wear resistance, high temperature resistance and impact resistance.
(7) Computer image processing method
Computer image processing method is a fast, non-contact, non-wear detection method, which can accurately detect different forms on each blade state of wear. This detection system usually consists of a CCD camera, light source and computer. However, because optical equipment has high environmental requirements, and the working environment of cutting tools in actual production is very harsh (such as cooling medium, chips, etc.), this method is currently only suitable for laboratory automatic inspection.
2. Indirect measurement method
The indirect measurement method uses the impact of tool wear or the state when it is about to be damaged on different working parameters, and measures the various effects of tool wear and damage. The degree of parameters can be detected when the tool is cutting and does not affect the cutting process. Its shortcoming is that the various process signals detected contain a large number of interference factors. Nonetheless, with the development of signal analysis and processing technology and pattern recognition technology, this method has become a mainstream method and has achieved good results. The indirect measurement methods of tool wear used at home and abroad include: cutting force measurement method, mechanical power measurement method, acoustic emission, thermal voltage measurement method, vibration signal and multi-information fusion detection.
(1) Acoustic emission signal measurement method
Acoustic emission technology is used to monitor the wear and damage of tools. This is a new application field of acoustic emission in the field of non-destructive testing in recent years. The principle is that when solid materials undergo deformation, fracture and phase change, strain energy will be rapidly released, and acoustic emission is the resulting elastic stress wave. When the tool is broken, a higher amplitude AE ??signal can be detected. Acoustic emission tool monitoring technology is recognized as a new monitoring technology with the most potential. Since the 1980s, domestic and foreign efforts have been devoted to the development and application of this technology, and great results have been achieved.
As early as 1977, Iwatak and Moriwaki proposed the use of acoustic emission technology for online detection of tool wear. On this basis, Moriwaki proposed an acoustic emission tool damage detection method. Kannatey-Asibu and Dornfeld theoretically studied the spectral characteristics of acoustic emission signals, and combined with pattern recognition methods to achieve online monitoring of tool damage. Although my country's research on acoustic emission monitoring technology started late, it has developed rapidly. Huang Weigong used the envelope analysis method to obtain the envelope of the acoustic emission signal in tool wear, used the parameters of the time series model as eigenvalues, and identified the tool wear equation through the neural network. The experiment proved that the effect is good; Li Xiaoli analyzed the parameters of the acoustic emission signal in the boring process. FFT analysis of typical AE signals is performed to reflect the tool wear status through changes in the amplitude of the AE signals in the frequency domain; Yuan Zhejun conducts wavelet packet decomposition of the acoustic emission signals during the cutting process to obtain the energy distribution of each frequency band of the signal, which is used as a signal characteristics, and establish a fast neural network model based on fuzzy reasoning to identify tool wear status. The chip-55A tool breakage monitor developed by Japan's Murakami Giken Company uses acoustic emission monitoring technology to monitor the status of tools during processing. This product is used in conjunction with the CNC milling machines produced by the company and has good results.
(2) Cutting force signal measurement method
Cutting force changes are a physical phenomenon most closely related to tool grinding and damage during the cutting process. Using cutting force as a detection signal has the advantages of easy picking up, quick response, and sensitivity. It is a method that has been studied more and has great hope for breakthroughs in online methods. Therefore, it is a common method for measuring tool damage in machining centers and FMS.
Based on the cutting force monitoring method, the monitoring data used mainly include cutting component force, cutting component force ratio, dynamic cutting force spectrum and related functions, etc. When the tool is broken, the cutting force changes sensitively. When the tool is less damaged, the cutting edge of the tool is not sharp, which increases the cutting force; when chipping or broken tool occurs, the depth of cutting is reduced or not, which greatly reduces the cutting force. When monitoring the cutting force, the three components of Fx, Fy, and Fz are measured simultaneously in the three directions of X, Y, and Z, and the current of the feed motor and spindle motor is measured by relying on the servo amplifier installed on each motor. The current changes are transmitted to the force valve, and the measured force is read on the display to determine whether the tool is damaged. In 1977, Yukitatsu Mura of Tokyo Denki University in Japan conducted an in-depth study of the changing rules of cutting forces under different processing conditions and tool wear conditions from both theoretical and experimental aspects, and found that under certain conditions, the cutting force ratio is a sensitive factor that can reflect tool wear. Accordingly, he proposed the cutting force ratio monitoring method; in 1984, Lan and Dornfeld's research showed that tangential force and feed force are highly sensitive to tool breakage; Shiraishi et al. measured the machining process A comparative study of detection and control technologies pointed out that the force monitoring method for tool failure is the most promising method and has broad industrial application prospects. Torque monitoring has the same research value as the cutting force method; Cheng Ganghu used the frequency band mean square The value method monitors the wear status of the tool through cutting force; Wan Jun uses the cutting force model and the least squares method to realize the model to automatically track the changes in machining process characteristics to obtain the tool wear amount. A representative achievement in cutting force monitoring technology is the TM-BU-1001 tool monitor launched by the Swedish Sandvik Coromant Company. The force sensor used in this system can be installed on the spindle bearing and feed screw, and three thresholds can be set. , it will automatically alarm once the limit is exceeded.
(3) Power measurement method
Power measurement method is also a method with great application potential in industrial production. This method determines whether the tool is damaged during the cutting process by measuring the spindle load power or current and voltage phase difference and current waveform changes. This method has convenient signal detection, can avoid interference from chips, oil, smoke, vibration and other factors in the cutting environment, and is easy to install. Based on the analysis of the power signal during the drilling process of the machining center, Pan Jianyue proposed and adopted the original processing method of power data to establish an online monitoring system for drill bit wear; Liu Xiaosheng combined regression analysis technology and fuzzy classification to establish a boring boring system. The mathematical model between cutting parameters and current indirectly reflects the intrinsic relationship between tool wear and boring cutting parameters, and uses power signals to identify tool wear; Guo Xing proposed a milling cutter breakage power monitoring based on artificial neural networks Method: A milling cutter breakage power monitoring system was established. Experiments show that the system can sensitively detect cutter breakage and implement monitoring. Yuan Zhejun systematically studied the influence of tool abnormality on the main motor power during the cutting process, and proposed to use multiple parameters such as the instantaneous value, derivative value, static average value and dynamic mean square value of the main motor power to comprehensively monitor the cutting tool during the drilling process. Abnormal state; Wan Jun uses the discrete autoregressive AR model to process the power signal. The model parameters are recursively modified at each signal sampling moment through an adaptation algorithm to adapt to the cutting conditions. At the same time, in order to distinguish the power caused by tool wear and changes in cutting conditions. Signal changes, the article introduces normalized deviation processing. When the tool cuts out the workpiece, the normalized deviation is obviously smaller than the change in the normalized deviation when the tool is worn. An alarm threshold is set during monitoring. When the normalized deviation exceeds the limit, Immediate alarm, with good effect.
The representative manufacturer that has successfully applied motor power monitoring technology is the American Cincinnati Milacron Company. The tool monitoring system developed by this company is used in conjunction with the Saber series vertical machining centers produced by the company.
(4) Workpiece size measurement method
The wear or damage of the tool tip during processing will inevitably cause changes in the size of the workpiece. By measuring the dimensional change of the machined surface of the workpiece, it can be indirectly judged Check the wear and damage of the tool. From the perspective of measurement methods, there are two types: contact type that measures the workpiece and non-contact type that measures the gap between the tool and workpiece. The advantage of the method of measuring workpiece dimensions is that it can directly and quantitatively give the tool radial wear or damage value, and it can be combined with online and real-time compensation of processing accuracy to ensure processing quality and achieve the ultimate goal of tool wear and damage monitoring during finishing. The disadvantage is that real-time measurement is susceptible to interference from the test environment, and coolant, chips, etc. affect the measurement results; factors such as thermal expansion and force deformation of the workpiece and tool during processing, spindle rotation accuracy, feed motion accuracy, vibration and other factors will also directly affect the measurement. accuracy. In addition, when processing variable-section workpieces, the sensor is required to accurately track and position, which will also cause positioning errors and increase the difficulty of implementation.
(5) Cutting temperature measurement method
Cutting heat is also an important physical phenomenon in the metal cutting process. The wear and damage of tools will lead to a sudden increase in cutting temperature. There are three ways to measure cutting temperature: (l) A natural thermocouple composed of a tool and a workpiece can measure the average temperature of the cutting area. Different tools and workpiece materials need to be calibrated; (2) Fixed at a certain point in the tool body, it consists of two thermocouples. This kind of thermocouple composed of metal wires measures the temperature at a certain point at a certain distance from the blade. It has problems such as 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 and has the characteristics of high sensitivity and short response time. However, the instrument is complex, costly and difficult to focus, making it difficult to measure the tool temperature in the cutting coverage area.
(6) Resistance measurement method at the contact point between the tool and the workpiece
The measurement principle can be divided into two types: one is based on the increase in the contact area between the tool and the workpiece due to tool wear. The effect of resistance reduction, this method is greatly affected by the cutting amount and has insulation requirements; the second is to stick a layer of thin film conductor on the flank surface of the tool, which is consumed as the tool wears. According to the change in resistance, it can be seen that the tool back The amount of wear on the blade surface. This method has high precision, but requires a thin film resistor to be attached to each tool, and the thin film resistor is easy to fall off under high temperature and high pressure. This method is not practical yet when applied to actual working conditions.
(7) Vibration frequency measurement method
During the cutting process of the tool, the friction between the workpiece and the side of the worn blade will produce vibrations of different frequencies. There are two methods for monitoring this vibration: one is to divide the amplitude into high and low parts, and compare the amplitudes of the two parts during the cutting process; the other is to divide the amplitude into several independent amplitude bands, and use a microprocessor to monitor these two parts. The belt is continuously recorded and analyzed to monitor the wear degree of the tool flank surface. The National Bureau of Standards' Automation Institute has had successful experience using vibration information in drilling processes. The system developed uses an acceleration sensor mounted on the workpiece to perform time-lapse analysis of vibration information, identify the wear of the drill bit and determine the breakage of the drill bit.
(8) Workpiece surface roughness measurement method
As the degree of tool wear increases or damage occurs, the roughness of the machined surface of the workpiece will show an increasing trend. According to this The wear or damage of the tool can be directly evaluated. Methods of measuring workpiece surface roughness can also be divided into two categories. One type is the scratch-type contact measurement, which can directly obtain the evaluation parameter R of surface roughness. This type of method is only suitable for static measurements. Currently, the vast majority of such methods are only suitable for use in metrology rooms or laboratory settings. The other type is non-contact optical reflection measurement, which obtains the relative value of the surface roughness of the workpiece. Optical fiber sensors and laser testing systems are usually used in automatic monitoring. This type of method has high testing efficiency and can measure the workpiece surface of soft materials without leaving traces. However, it requires sample calibration in advance and is greatly affected by cutting fluid, chips, workpiece material, vibration, etc. It has not yet reached the level of practical application.
(9) Current signal measurement method
This method, referred to as MCSA, uses the stator current of the induction motor as the entry point for signal analysis to study the correspondence between its characteristics and faults. The basic principle is: as the tool wear increases, the cutting torque increases, the power consumed by the machine tool increases or the current rises, so online detection of tool wear can be achieved. MCSA has the characteristics of convenient testing, high information integration, direct transmission path, convenient signal extraction, not affected by the processing environment, low price, and easy transplantation. In situations such as machine tools where the transmission system is closed and general sensors are difficult to install, it should be is 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, it will be between the other ends of the two conductors. A voltage is generated between them, the magnitude of which depends on the electrical properties of the conductor and the temperature difference between the contact point and the free end. When the tool and the workpiece are made of different materials, a thermal voltage related to the cutting temperature can be generated between the tool and the workpiece.
This voltage can be used as a measure of tool wear, because as the tool wear increases, the thermal voltage also increases. The advantage of this method is that it is cheap, has high accuracy, is easy to use, and is especially suitable for high-speed processing areas. The disadvantage is that it requires high sensor materials and accuracy, and can only perform interval detection.