Nine points for attention in preventive maintenance of CNC machine tools

If any CNC machine tool wants to work continuously and reliably for a long time, in addition to the quality factors of the machine tool itself, it is also closely related to the correct maintenance, timely troubleshooting and timely repair in the process of use. From the perspective of improving the effectiveness of CNC machine tools, maintenance should include two aspects: one is daily maintenance (preventive maintenance), which is to extend the MTBF of machine tools; On the other hand, it is fault maintenance, which aims to minimize the MTTR. Doing these two tasks well is the basic guarantee to give full play to the efficiency of the equipment.

In order to give full play to the benefits of CNC machine tools, it is important to do a good job of preventive maintenance to reduce the number of failures of the CNC system and improve the mean time between failures of the system. In addition, it is also necessary to prepare for maintenance at any time, and repair the system in time in case of failure, so as to minimize downtime and repair time. This requires that you must be familiar with the structure and performance of the equipment, the composition and basic operation of the numerical control system, the detection points available for maintenance on the printed circuit board used by the system, and the normal level and waveform, so as to compare and analyze the faults during maintenance. In addition, the technical data and original setting parameters of the numerical control system and programmable logic controller (PLC), as well as the commonly used typical part programs shall be properly stored. According to the actual use, some vulnerable spare parts can be properly equipped, such as fuse, brush, transistor module and printed circuit board which are prone to failure. For standby circuit boards, they should be installed on the CNC system regularly for power-on operation to avoid failure due to long-term use.

The key to preventive maintenance is to strengthen the daily maintenance, which should generally be as follows:

1. The NC system programming, operation and maintenance personnel equipped for the NC machine tool shall be familiar with the characteristics of the mechanical structure, NC device, strong current equipment, hydraulic system, gas circuit and other parts of the equipment used, as well as the specified use environment and processing conditions. The machine tool shall be used correctly and reasonably in strict accordance with the requirements of the machine tool and numerical control device operation manual to avoid faults caused by improper operation.

2. Many systems use the paper tape reader as the input device of the program. System parameters, parts processing program and other paper tape information must be input into the CNC system through the paper tape reader. If there is dirt on the tape reading part of the paper tape reader (i.e., the luminous and light-receiving parts of the reading head), it will cause errors in the read tape information. Therefore, the surface of the reading head, the paper tape pressing plate and the paper tape channel surface should be checked frequently to remove the dirt in time. The moving parts of the paper tape reader, such as the driving wheel roller, the guide roller pressing roller, the tensioning arm roller, etc., should be regularly cleaned and well lubricated.

3. Clean the air filter regularly. When the air filter installed on the door of the numerical control cabinet and electrical cabinet is dusty, the cooling air in the cabinet will not circulate smoothly. If it does so for a long time, the temperature in the cabinet will rise and the system will not work reliably. Therefore, it should be checked regularly according to the use environment, and removed and cleaned at least once every six months. The specific method is: first remove the fastening screws, take out the inner core of the air filter, and blow the dust on the filter element from the inside to the outside with compressed air. If the filter is dirty, you can also gently vibrate the filter at the same time. If the above method cannot work, you can use a neutral detergent (the proportion of detergent is 5%) to wash, but do not rub, and then put the filter element in a cool and ventilated place to dry.

4. Maintain the battery regularly. For numerical control devices that use CMOS memory to store system parameters, in order to avoid parameter loss during shutdown and power failure, battery power supply is used to maintain. When the battery voltage is lower than the CMOS holding voltage, the battery can be automatically charged when the machine tool is started. Generally, the battery can ensure that the information is saved for more than 1000 hours after power failure. When the machine tool is shut down for a long time, it should also be powered on regularly according to the requirements of the manual to make the battery supplement power. If the battery failure alarm is displayed on the CRT or by the indicator light of this type of numerical control device, it means that the voltage is too low, the battery has failed, and the battery needs to be replaced. In order to save the original data, the battery should be replaced when the power is turned on, and the polarity of the battery should not be reversed.

5. The DC servo motor shall be inspected and cleaned regularly. DC servo motor is equipped with electric brush, which is gradually worn due to contact and friction with commutator during operation. After excessive wear of the brush, it will affect the working performance of the motor and even cause damage to the motor, so it must be checked and replaced regularly. For general machine tools such as CNC lathes, CNC milling machines and machining center machines, they can be inspected once a year; However, for machine tools such as punch machines that frequently perform acceleration and deceleration work, they should be checked every two months. The inspection shall be carried out when the power supply of the CNC system is disconnected and the motor has been completely cooled. First, remove the brush cover, take out the brush, and measure its length. Generally. When the brush is worn to half of the original length, it should not be used again. It must be replaced with a new brush of the same model. The second step is to carefully check whether the arc contact surface between the brush and the commutator has deep grooves or cracks, and whether the brush spring has spark marks. If there are any of the above phenomena, the surface of the commutator must be carefully checked. If the commutator is normal, replace it with a new brush, and check again after one month; If the above phenomenon also occurs, it is necessary to consider whether the working conditions of the motor are too bad to cause the fault of the motor itself. Before installing a new brush, clean the brush hole with clean compressed air free of metal powder and moisture. Be sure to blow the brush powder stuck on the hole wall. If it is difficult to blow clean, use screwdriver and other tools to assist in cleaning until the hole wall is completely clean. However, attention should be paid to avoid damaging the commutator surface and hole wall with the screwdriver tip. Finally, install a new brush, tighten the brush cover, and make the servo motor run in idle for a period of time to make the surface of the new brush coincide with the commutator.

6. Pay attention to the closed CNC cabinet door. Generally, it is necessary to avoid opening the NC cabinet door at will, especially for long-term open operation. Clean the air filter in time. And never use the method of opening the cabinet door to dissipate heat, otherwise it will not be worth the loss. Because there is a large amount of dust, oil mist and metal powder floating in the air in the workshop, these sundries fall on the printed circuit board and electronic components, which is easy to cause the insulation resistance of components to drop and cause failures, and even cause the components and printed circuit board to be damaged and scrapped. Especially for CNC machine tools that install the spindle control system in the strong current cabinet, if the strong current cabinet door is not closed tightly or the seal is poor, it is also easy to cause damage to the electrical components and make the spindle control failure.

7. Clean the cooling device regularly to enhance the cooling effect. Some servo motors or spindle motors are equipped with forced cooling devices on the casing. If the protective screen or cooling fin of the cooling device is dirty, which affects the air circulation, it will inevitably reduce the cooling capacity and cause failure due to heat loss. Therefore, these cooling devices should be cleaned regularly. The specific method is as follows: if the ventilation is hindered by too much dust on the protective screen, it can be removed for cleaning; When there is a lot of dust in the radiator, compressed air can be used to blow it out, or a thin rod can be used to sweep the dust in the middle of the radiator. However, during operation, be careful not to extrude, deform and overlap the cooling fins, so as not to affect the cooling effect. The above cleaning cycle is generally once every six months, which can also be appropriately shortened according to the specific situation.