CNC machine tools are highly efficient automated machines and their place in production is becoming increasingly important. Therefore, the troubleshooting and daily maintenance of equipment is very important, but most of the failures of CNC machine tools are in the form of integrated failures, so the maintenance is more difficult. Combined with the mechanism of CNC machine tools, we have summarized the four main methods, namely: look, smell& hear, ask and touch.

I. Look

To find the cause of the fault through the human hand, eye and other sensory organs, which is also the most simple and direct method.

When the machine tool failure, the first thing you need to understand is the phenomenon of failure and the causes and processes. If the fault can be reappeared, the process of the fault should be observed and the most basic situation should be mastered. At the same time, observe whether there is mechanical damage; and whether there are burn marks, whether the resistance and conductors have been discoloured; whether there are any abnormalities in the running and sealing parts, such as splashes, shedding, spills, oil, smoke, sparks, etc.; whether the circuit breakers, relays, etc. tripped, whether the fuse is melted; whether the machine power supply is out of phase, whether the three phases are seriously unbalanced, whether the machine voltage is normal; whether there are parts on the electrical components are off, broken wires, jammed, loose joints, etc.; whether the switch is suitable; whether the operator's processing procedures are correct, etc.

2. Smell& Hear

Identify whether there is an odour. When the machine tool moving parts have violent friction, the electrical insulation will burn, while the oil, smoke, gas and insulation materials will produce a burnt smell; the machine will produce ozone smell when discharged, you will also hear the discharge sound.

3. Ask

Ask what happens when a machine tool breaks down. When machining, it is advisable to have strict management measures in place so that operators can make detailed notes in the event of a breakdown. This can avoid the occurrence of faults when maintenance personnel are not around and can accurately reflect the specific circumstances of the fault.

When a fault occurs in a CNC machine tool, the first thing to do is to stop the machine and protect the site, the operator to record the fault in as much detail as possible, where the important information is to be observed and recorded in detail. For example: when the fault occurs, the phenomenon of the fault occurs in the part of the machine as well as when the fault occurs in the state of the machine and the control system. If the fault occurs in the automatic machining mode, the machining program number, the program segment number where the fault occurred and the tool number used for machining should be recorded. In the event of a fault such as poor machining accuracy or excessive contour error, the number of the workpiece being machined should be recorded and the non-conforming workpiece retained for specific analysis. In the event of a fault, if the system has an alarm display, the alarm display and alarm code of the system should be recorded. If the fault occurs when machining a part, the probability of the fault occurring when machining a similar part should be recorded.

4. Touch

Diagnosis and repair based on the phenomenon of failure; this step is also the most important one in the maintenance of CNC machine tools.

Which can be divided into the following specific steps:

4.1 System parameters check

Nowadays, the operating system of CNC machine tools is becoming more and more self-diagnostic, and most of the faults of CNC machine tools can be diagnosed. When a fault occurs in the CNC machine tool, sometimes an alarm message will be displayed on the monitor; sometimes the alarm device on the CNC unit, PLC unit and drive unit, such as the alarm light will flash, beep, etc. In this case, the first thing to do is to check the maintenance manual and check the corresponding parameter settings. Missing or incorrectly set system parameters can cause changes or failures in machine performance. For example, in the FANUC system machine tool automatic processing, the machine tool holder stops moving and the screen shows 500, 501 alarm, check the parameter manual to know the corresponding parameters for the storage travel limit positive and negative limit value exceeded, then the machine can be changed to manual state to adjust the tool holder to the correct travel range and correct the parameters, the alarm can be relieved.

4.2 Reset machine tool

During processing, the system alarm caused by instantaneous fault can be cleared by hardware reset or turning on and off the system power supply in turn. In case of system confusion caused by power failure, unplugging and plugging of circuit board or undervoltage of battery in the working storage area of the system, the system must be initialized and cleared. Before clearing, pay attention to copy and record important data. If the fault still cannot be eliminated after initialization, carry out hardware diagnosis.

4.3 Diagnosis by measurement

Measurement method is the basic method to diagnose equipment failure, we can use multimeter, oscilloscope, logic tester and other instruments to measure the electronic circuit. For example, to determine the phase sequence of the three-phase power supply of the CNC system, you can use the phase sequence meter. That is, connect the three-phase power supply line to the phase sequence meter, and when the phase sequence is correct, the phase sequence meter rotates in a clockwise direction, and vice versa. You can also use a dual-channel oscilloscope to measure, if the phase sequence is correct, the waveforms of each two phases differ by 120° in phase.

4.4 Principal analysis method

When other maintenance methods are difficult to solve the fault, you can check step by step from the working principle of the machine tool. For example, we once encountered the phenomenon of disordered teeth when machining threads on a machine tool using FANUC 0itd system. According to the basic principle of position control of NC system, it can be basically determined that the fault is on the rotary encoder, and it is likely that the feedback signal is lost. In this way, once the NC device gives the command position of feed rate, the actual position fed back will always be incorrect, and the position error cannot be eliminated, resulting in problems in thread interpolation. When the pulse encoder was removed for inspection, it was found that the filament in the encoder was broken, resulting in no feedback input signal, which was consistent with the phenomenon of principle analysis. After replacing the encoder, the fault was eliminated.

4.5 Component exchange method

For some faults involving the control system, sometimes it is not easy to confirm which part is the problem, in the case of ensuring that there is no further damage, you can take the suspected faulty parts or components with the same spare parts or the same parts or components on the same type of machine tool or on other parts of this machine tool to replace them to determine whether a fault has occurred. If the fault is lifted after replacing the device, it can be determined that the device is damaged causing the fault; if the fault remains, the device is proven to be intact and can be tested by other methods.

These four methods are not only applicable to the maintenance of CNC machine tool faults, but also to other types of mechanical equipment faults.

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