With the popularization of high-power CO2 laser cutting machines in China, more and more foreign laser cutting machines are entering the country. For the maintenance of such equipment is a challenge, because of the special characteristics of its equipment, compared with the maintenance of ordinary CNC equipment, there are great difficulties, such as its own independent laser generation system, laser optical path, laser cutting system. A laser generator alone can become a discipline independently. Therefore, maintenance personnel need to have electricity, gas, light and other aspects of comprehensive knowledge. This article describes the author has repaired some examples of laser cutting machine, share with peers.

1 CO2 laser working principle

in the CO2 laser discharge tube, filled with CO2, N2, He and other mixed gases, the ratio and the total gas pressure can be changed within a certain range (generally CO2: N2 : He = 1 :0.5:2.5, the total gas pressure of 1 066.58 Pa). Any molecule has three different forms of motion: first, the movement of electrons in the molecule, which determines the electronic energy state; second, the vibration of atoms in the molecule, i.e., the atoms vibrate periodically around their equilibrium position, which determines the vibrational energy state of the molecule; and third, the rotation of the molecule, which determines the rotational energy state of the molecule. CO2 lasers utilize the jump between the vibrational and rotational energy levels of CO2 molecules to generate laser light at a wavelength of 10.6 μm. The CO2 molecules in the discharge tube are inverted by injecting energy into the molecules of the CO2 gas using a gas-discharge pumping method: the two outputs of the DC voltage are connected to the two electrodes of the discharge tube, and the gas between the two electrodes is completely insulated and has an internal resistance of infinity when no voltage is applied or when the voltage is very low. When no voltage is applied or the voltage is very low, the gas between the two electrodes is completely insulated, the internal resistance is infinite, and no current flows; as the voltage rises, charged particles begin to move in the gas, and the internal resistance of the gas begins to decrease, and when a certain voltage value is reached, the internal resistance decreases sharply, the current increases rapidly, the gas is pierced, and the discharge begins, and the value of this voltage is called the ignition voltage; after the gas in the discharge tube is pierced and discharged, the current grows, carriers in the gas increase, and the internal resistance of the laser discharge tube decreases, and further causes the laser tube to be discharged, which is called the ignition voltage. After the gas in the discharge tube is punctured and discharged, the current grows, the carriers in the gas increase, the internal resistance of the laser discharge tube decreases, which further causes the current to increase, this process is repeated, and the discharge tube shows a negative resistance effect. In order to make the discharge work stably at a certain point of the current-voltage characteristic curve of the discharge tube, current-limiting measures have been taken in the power supply circuit of the discharge tube. When the discharge tube discharges, in the gas mixture, the N2 molecule is excited by the electron energy gained from the collision with the electron, and when the N2 molecule collides with the CO2 molecule, the energy gained from the electron is transferred to the CO2 molecule, so that the CO2 molecule is excited, which is conducive to the generation of laser light; the He gas in the tube has a cooling effect, which can prevent the temperature of the CO2 gas from rising, and at the same time, it can make the laser energy level decrease, and improve the efficiency of the laser. The He gas in the tube has a cooling effect and prevents the temperature of the CO2 gas from rising.

2 Failure Example Analysis

2.1 Light Path Shift and Laser Head Nozzle Damage During laser processing

The laser light path of a foreign 3D laser cutting machine is shifted and the laser head nozzle is damaged by the laser. This required adjustment of the laser path from the laser generator. The laser is divided into internal and external optical paths from the laser to the workpiece. The laser oscillates in the laser to produce laser light, which is called the internal optical path. The laser oscillates in the laser to produce laser light, which is called the internal optical path. The optical path after the laser is emitted is called the external optical path. The adjustment of the external optical path of this laser cutting machine is made by two mirrors at the back of the machine, which are for X-axis transverse adjustment and Y-axis longitudinal adjustment respectively. After adjusting the X-axis and Y-axis, the Z-axis needs to be adjusted vertically. On the laser thermal paper, the laser spot needs to be the same area in order to meet the processing requirements, and after the adjustment, the laser nozzle has to be installed and the lens has to be adjusted, so that the laser will not deviate when cutting three-dimensional three-dimensional workpieces.

2.2 No power output and frequency converter overload faults when applying high voltage

A foreign laser cutting machine has no power output and frequency converter overload faults when applying high voltage. This 890 laser adopts CO2 laser generator, and its working principle has been introduced earlier in this paper. First, the work of the vacuum; second, when the vacuum level reaches 8 Torror, He, CO2, N2, air gas mixture back to the resonant cavity, and then the frequency converter began to make the turbine rotation acceleration. After the gas is filled, the turbine finishes accelerating the gas; third, after the accelerated gas flows into the resonance chamber, the high voltage power supply starts to work. Third, after the accelerated gas flows into the resonant cavity, the high-voltage power supply starts to operate, causing the laser to oscillate above the intracavity mirrors at both ends of the cavity. After repeated oscillation, the laser level rises continuously, and when it reaches level 4 laser, the laser is emitted at the output window. The machine fault occurs in the second stage, in the turbine acceleration gas stage, about to start the high-voltage power supply at the same time, the system prompts inverter fault, the machine high-voltage power supply can not be turned on. The inverter parameters were checked and there was no problem. Since the system prompts inverter error, continue to search for inverter problems, and suspect that the inverter power board is damaged. After replacement, the fault remains. However, during the replacement, it is found that the load current of the frequency converter has exceeded the set value, and the current is not very stable. This indicates that the turbine is running with a large load. There are two possibilities: the first is that the ceramic bearing of the 18,000 rpm turbine was damaged, which increased the load on itself; the second is that the gas mixture was impure, which increased the load on the turbine during the acceleration process. Considering the first possibility, the speed of the turbine rises gradually during the start-up phase of the machine inverter, and the sound produced by the rotation is not particularly harsh, which means that it is rotating evenly. This possibility is ruled out. Considering the second possibility, look for gas leakage in the resonance chamber, the result is not very obvious, but after replacing a bottle of N2 gas, there is no alarm in the frequency converter, which means that the gas mixture is not pure, resulting in excessive load on the turbine, and eventually leading to overcurrent failure of the frequency converter.

2.3 Unstable power and no power output after restart

A foreign 2D laser cutting machine has unstable power and no power output after restart. The laser generator is a CO2 laser from a foreign company with a power of 3 kW, and there is no alarm on the machine itself. Remove the baffle plate of the laser generator and observe the situation of laser pressure rise, while the pressure is rising, there is violet light in the resonance cavity, and with the enhancement of the violet light, the current and excitation power are also rising. It was found that when the actual machine tool was pressurized, there was only a weak violet light in the resonant cavity, and there was no current or laser power, so the laser was not built up in the resonant cavity. After checking, it was found that the efficiency of the machine was very low in the vacuum stage, so the F3 function key was pressed to check the vacuum leakage rate of the laser, which is generally below 0.01, and the result was 4,095, which indicates that the leakage rate in the vacuum cavity is very large. It is necessary to find out the leakage points in the vacuum cavity related to the resonance cavity, turbine, cooler, and so on. However, analyzing the phenomenon, even if the laser is leaking, it should have some power, so we focused on checking the resonance cavity, and when we checked a mirror near the feedback box, we found that the mirror was seriously damaged, and we checked other parts without any problem. This means that the damaged mirror, resulting in vacuum cavity leakage. After replacement, the laser is normal.

3 Conclusion

The above failure shows that the laser cutting machine is not a general CNC processing equipment, it has its own specificity, the adjustment of the laser optical path (especially three-dimensional laser machine) to determine the accuracy of the machine tool processing workpiece. Its adjustment must know the optical knowledge, but also master the electrical knowledge of CNC, is a comprehensive application of photoelectricity, but also test the maintenance experience of maintenance personnel, not only this, but also master the working principle of the laser generator. The cause of the alarm is obviously the internal failure of the laser generator. The structure and function of the laser generator were mentioned earlier, and the frequency converter is used to control the work of the turbine. The overcurrent alarm displayed by the laser generator is not only caused by itself, but also by the problem of its load (the turbine it controls). If the turbine is not working properly, it is necessary to check the ratio of the gas mixture flowing into the turbine at the time of operation, as well as to have a general knowledge and understanding of the laser generator. The power output of a CO2 laser can be affected by a number of factors, some of which are: firstly, impurity of the gas, which results in incorrect power; secondly, prolonged periods of inactivity, which can lead to unstable power (especially with PRC lasers), which requires sufficient time for warming up; and thirdly, inefficient vacuum pumps, which can lead to unstable power, and so on. Some causes of failure must be judged in conjunction with the actual laser failure phenomenon.