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Technical methods for cutter tool wear and breakage

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Summary:Technical methods for cutter tool wear and breakage...

There are many methods to monitor tool wear and breakage, which can be divided into direct measurement method and indirect measurement method. Direct measurement methods include optical method, contact resistance method, radioactive method and so on. Indirect measurement methods mainly include: cutting force or power measurement method, tool and workpiece measurement method, temperature measurement method, vibration analysis method, AE method, motor current or power measurement method.

Comparing the existing monitoring methods of tool wear and breakage, we choose acoustic emission (AE) and electrical current signal as monitoring parameters. This is because AE signal can avoid the low frequency region which is the most seriously affected by noise in machining, is less affected by vibration and acoustic noise, and has a high signal-to-noise ratio in the region of interest, so it is easy to process the signal. The response speed is fast and the sensitivity is high, but it is easy to be disturbed when heavy load occurs. The motor current signal is easy to extract, can adapt to all machining processes, no impact on normal cutting, but susceptible to interference, slow time response, light load, low sensitivity. In this way, both AE and electromechanical current are selected as monitoring signals, which can make use of their respective strengths and weaknesses, widen the monitoring range, improve the monitoring accuracy and the success rate of discrimination.

In the cutting process, when the tool is worn and damaged, the cutting force changes correspondingly. The change of cutting force causes the output torque of the motor to change, and then causes the corresponding change of the motor current. The current method realizes the indirect on-line real-time judgment of tool wear and damage by monitoring the change of motor current. AE is a phenomenon in which strain energy is released in the form of elastic waveform when material or structure is deformed or broken by external force or internal force. It has the characteristics of low amplitude and wide frequency range. The AE signal produced by normal cutting is mainly plastic deformation of workpiece material. The power spectrum distribution of AE signal is very large below 100kHz and small above 100kHz.

When the tool wears and breaks, the AE signal of frequency components above 100 kHz is much larger than that of normal cutting, especially the frequency components between 100 kHz and 300 kHz. Therefore, the change of AE signal with frequency component of 100-300 kHz should be monitored by band-pass filter to monitor tool wear and breakage. The principle of judging tool wear and breakage by using AE and motor current signals is as follows: in light load area, judging tool wear and breakage by means of threshold value depending on AE envelope signal; in medium load area, motor current and AE signal play an important role, judging tool wear and breakage by combining the two methods, and improving the success rate of judgment. Yes: if the AE signal exceeds the AE threshold value, the set delay constant is DS (the value of D depends on the composition of the system). If the current signal exceeds the threshold value of the current signal within the time of ds, then the tool limit wear or breakage can be judged. If the current signal does not exceed the threshold value of the current signal in the time of ds, the alarm will not be given and the delay constant will continue to monitor. With AE as the guide, AE signal and electric current signal are used to distinguish "and" mode, which not only makes use of the real-time and sensitive characteristics of AE signal, but also takes into account the hysteresis nature of motor current signal, so it has strong anti-interference ability and improves the success rate of discrimination. In the heavy load area, the motor current signal is the main and the AE signal is subsidiary. The following is a block diagram of the monitoring system. The middle circuit is the power grid voltage monitoring line. Through monitoring the fluctuation of the power supply, the influence of the fluctuation of the power grid voltage on the monitoring is eliminated, and the anti-interference ability of the system is improved. The dotted line in the diagram is to automatically subtract the first-cut current line. The purpose is to automatically subtract the first-cut current and improve the sensitivity of current signal monitoring by judging the change of current.

Wiring diagram of tool wear and breakage monitoring

The actual discriminant formula of current signal is as follows: I = (Ia-Ib) -F (Va-Vb) formula: I is the discriminant current value; F is the ratio of current change caused by voltage change, mainly to eliminate the inconsistency of current and voltage amplification multiples of hardware circuits; Va and Va are the current and voltage values sampled in the monitoring process; Ib and Vb are the value of starting cutting. Current and voltage values. The on-line real-time monitoring of tool wear and breakage is a difficult and important problem in the automatic machining production line. Although there are many methods to monitor tool wear and breakage, there are many factors in the actual cutting process, such as bad environment, the diversity of tools and workpieces, and the great discreteness of data collected. The actual monitoring is very difficult.

If a single monitoring method is used, whether direct or indirect, the effectiveness and success rate of monitoring are questioned because of the influence of such factors as chip, cutting fluid and vibration, poor time response and testing sensitivity, changes in material physical properties, and the influence of cutting conditions. Therefore, on the basis of summarizing the advantages and disadvantages of the existing monitoring methods, the multi-parameter comprehensive monitoring method can be adopted to make full use of the advantages of each monitoring quantity, complement each other, widen the monitoring range and improve the success rate of discrimination.

Two parameters, AE and electromechanical current signal, are used to monitor the tool breakage. The results show that the tool breakage monitoring is effective. The drilling experiment on the CA6410 lathe on the Z512-D bench is to simulate the drilling on the machining center. The drill bit is clamped on the lathe spindle chuck, the workpiece is clamped on the tool holder. During the drilling experiment, the drill bit rotates, the workpiece feeds automatically, and the AE sensor is mounted on the small tool holder. The experimental drilling parameters are as follows: drill speed n=900-1120r/min.

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