Need to pay attention to the maintenance of CNC machine tools - Database & Sql Blog Articles

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Most traditional machine tools or old machine tools of more than 10 years have the disadvantages of poor quality, low variety, high cost and long delivery period. Some equipments only pay attention to the introduction of equipment, instruments, production lines, neglecting software, technology, management, etc., resulting in incomplete projects, equipment potential can not be played; some can not even start running, did not play its due role; some production lines The products are sold well, but they can't reach the standard because of equipment failure; some suffer losses due to high energy consumption and low product qualification rate; some have been introduced for a long time, and need to be updated with technology. The role it should have.
As long as the main technical difficulties are solved, the key technical problems are solved, and the repair and transformation can be carried out, the largest stock assets can be revitalized with the minimum investment, and the maximum economic and social benefits can be obtained. Especially for large machine tools and special machine tools, the savings are even more impressive. This is a huge transformation market.
1 The most basic requirements for the transformation of CNC machine tools
1.1 The CNC machine tool that is designated as the object of transformation must have the mechanical precision to reach the corresponding level, and the transformation has value and practical significance. Otherwise, the level of the CNC system and the servo part will not be effective. First of all, you should understand and master the user's use of the equipment, the current problems of the equipment, what needs to be modified, what goals are required, and so on. Strive to fully grasp the parameters of the equipment to determine the selection and grade, such as I / O points, storage capacity, according to actual needs to determine the transformation grade. The modified CNC machine tool should reach: high static and dynamic stiffness; the friction coefficient between the motion pairs is small; the transmission has no gap; the power is large; it is easy to operate and maintain. The above requirements should be met as much as possible in the transformation of CNC machine tools in order to obtain the intended transformation purpose.
1.2 Data collection, editing and preservation should be carried out throughout the transformation process. The original data of the machine tool is the most basic information to understand the factory condition of the machine tool. The electrical schematic diagram, electrical maintenance manual and control program need to be carefully studied. It is important to note that the information provided in the original material is not completely correct. Some foreign machine tool factories provide general drawings of series models. Sometimes the electrical and hardware changes before the machine leaves the factory are not reflected in the electrical data. This requires us to compare the details of the key links with the actual machine tools. Implement and avoid making mistakes. Some large and heavy-duty CNC equipments are designed and manufactured according to the needs of users. Therefore, there are some special features in the system and structure. For example, the manufacturer designs and manufactures the operation panel that is more suitable for the specific machine tool, which is convenient for the user, including Among them, the arrangement of letter keys, number keys and function keys is a series of personalized considerations related to the size of the window display capability of CNC and plc.
Other manufacturers add a number of pages to the diagnosis page that will help with future repairs and troubleshooting according to user needs. To renovate such equipment, first of all, technical data collection should be strengthened. Secondly, it is necessary to abide by the prior drawings. According to the construction principle of the map, the modification should be recorded in time. The entire project line is very complicated. If all the debugging is finished, it is easy to omit the finishing. The user should be left with complete and accurate modification information. At least the user manual, programming manual, PLC ladder diagram, hardware wiring diagram, etc.
1.3 In the transformation of the professional team, at least one of the experts who are familiar with the entire CNC machine tool including the control machine, PLC system, servo system and mechanical system will make an overall plan. From the modeling, ordering, material preparation, wiring, plc programming, mechanical modification, commissioning, acceptance and other important links to make appropriate arrangements; coordination of all aspects of work, this can often shorten the transformation cycle, and ensure the quality of the transformation.
1.4 Measurement, calculation, design, drawing, and parts production for the mechanical modification of the electrical transformation shall be completed in advance. At the same time, the parts that need to be dismantled, modified and processed after the shutdown should be planned in advance, and clear requirements should be put forward, and the whole transformation work should be properly connected. In principle, imported CNC equipment is still used in the transformation of imported equipment; domestically produced equipment is still using domestic CNC system.
1.5 When the main aspects of mechanical transformation, electrical wiring, PLC programming, etc. are involved in the NC transformation, it is necessary to cooperate well with the users. For the NC reformers, the most concerned about the original machine tool interface software (including PLC, PAL, PMC, etc.) and electrical connection diagram. The software and hardware connections are related to the matching of the CNC system to a specific machine. The connection between the servo system and the machine tool is relatively straightforward and clear.
2 Key points and techniques in maintenance and renovation
2.1 The spindles of large and medium-sized CNC machine tools generally have gear shifting transmission mode. This is to ensure that a large torque can be transmitted at a low speed while expanding the shift range of the constant power region. There is a "gear" problem with gear shifting. In the case of gear shifting, the motor moment is often used to prevent the top teeth. Therefore, it should be noted that the delay of the large inertia component needs to be detected by the time relay. The gear limit switch should answer the success of the computer's gear. Since the momentary jog of the gear is required to output a short-term motion command to the interface, it is difficult to realize in the PLC because the PAL system cannot handle the motor movement problem. So keep the manual gear button switch on the panel, you need to pay special attention to this problem. In the numerical control transformation of large-scale, special-purpose machine tools, in order to achieve some special functional requirements, parameter macro calls can be used. The purpose is to transfer information between the part program and the PLC program. In addition, in some cases, the user can customize some G code and M code, and actually call a special subroutine to complete a specific function. In addition, the problem of clamping and relaxation is prominent in large-scale special equipment. In order to accurately position under high inertia conditions, the coordinate axis should be clamped immediately when it reaches the destination position, and it should be relaxed immediately during exercise. In order to avoid jitter during clamping, it is sometimes necessary to divide the coordinate axis into two levels: low clip and high clip. The requirements of these functions increase the difficulty of programming interface software.
2.2 For CNC lathes, the guide rails should have good precision and processability of ordinary lathes, but also have good resistance to friction and wear, and reduce dead zones due to frictional resistance. At the same time, there must be sufficient rigidity to reduce the influence of the deformation of the guide rail on the machining accuracy, and reasonable rail protection and lubrication. The gears of general machine tools are mainly concentrated in the headstock and gearbox. In order to ensure the transmission accuracy, the accuracy of the gears used on CNC machine tools is higher than that of ordinary machine tools. In the structure, it is necessary to achieve a gap-free transmission. Therefore, when the transformation is made, the main gear of the machine tool must meet the requirements of the CNC machine tool to ensure the machining accuracy of the machine tool.
2.3 Generally speaking, the imported CNC machine tool electrical cabinet and the components in the cabinet can maintain a fairly stable performance and parameters even after being used for a considerable period of time. Therefore, the general electrical cabinets of the high-grade imported CNC machine tools are not greatly modified. Some electrical machines of second-hand machine tools are in poor condition, or users think that the failure rate of electrical cabinet components is too high, they can adopt a full set of replacement methods. As for the use of international famous brands or domestic joint venture brands, it depends on the input of users. Under the premise of user funds, you can try to use international famous brands with good quality and appearance. The function of the cable as a transmission signal is not to be seen. In our retrofit project, we only select the original cable for the cable directly related to the CNC drive measurement system. However, the power cable of the large spindle motor is too expensive, so the original cable is not considered. cable.
2.4 Screw drive is directly related to the accuracy of the transmission chain. The selection of the lead screw mainly depends on the precision requirements of the workpiece and the drag torque requirement. The sliding screw can be used when the precision of the workpiece is not high, but the original screw wear condition, such as the pitch error and the pitch error and the matching nut clearance, should be checked. In general, the sliding screw should be no less than 6 grades. If the nut clearance is too large, replace the nut. The use of a sliding screw is lower than that of a ball screw, but it is difficult to meet the processing of parts with high precision. The ball screw has low friction loss and high efficiency, and its transmission efficiency can be above 90%; high precision and long service life; the starting torque and the moment when moving are close, which can reduce the starting torque of the motor. Therefore, it can meet the processing requirements of higher precision parts.
2.5 Software maintenance of the oscillation axis. After the mechanical part is worn, the positional belt will not meet the requirements and will cause oscillation of a certain coordinate axis. Generally speaking, it is relatively simple for the new numerical control system. It is only necessary to modify the parameters in place. However, it is more difficult to control the old equipment especially for the CNC system. To find out the relevant parameters in the original NC system, and properly expand the position belt and the clamping belt, the shaft oscillation problem can be solved temporarily. However, in order to completely solve the problem, it is necessary to repair the wear of the mechanical parts.
2.6 After the system is rebuilt, commissioning and determining reasonable acceptance criteria are also an important part of technical preparation. The commissioning work involves mechanical, hydraulic, electrical, optical, control, sensing, etc., so it must be carried out by the project leader and other personnel. The debugging steps can be from simple to complex, from small to large, from outside to inside, or first and then global, first subsystem and then complete system. The acceptance criteria are the assessment of the new system. It must be realistic when it is formulated. The standards that are too high or too low will have a negative impact on the transformation work. Once the standard is determined, it cannot be easily modified because it involves all aspects of the entire transformation work.
3 Problems and countermeasures in maintenance and renovation
3.1 Connect the neutral line to the PE. The national standard GB/T 52261 clearly stipulates that “in the electrical equipment, the neutral line and the protective grounding circuit should not be connected. The PE combined terminal should not be used inside the mechanical electric cabinet”. However, in the on-site repair and renovation work, more than one CNC machine tool after repair and modification, due to the addition of the device, AC220 V power supply, directly one phase of the three-phase 380 V and the PE ground of the machine tool constitute AC220 V. Obviously, when the P is disconnected due to mechanical failure, the device becomes an AC220 V voltage charged body to the ground. There is also a device that adds the three-phase power to the PE terminal. This poses a direct electric shock to the operator or anyone close to the machine.
Under normal circumstances, the machine tool manufacturer is required by the national standard, and the center line can only be used when the user agrees. The use of the center line should be clearly indicated on the technical documentation (installation diagram and circuit diagram), and the single-use insulated terminal marked N should be provided for the neutral line. Therefore, when the machine tool is rebuilt, it is impossible to save the trouble to connect the center line to the PE.
3.2 Non-single power supply. Some CNC equipments have been repaired and modified to add some mechanisms. Because of the convenient wiring or the connection with other equipment, the new part of the mechanism is not from the power supply of the equipment to cut off the lead under the switch, and the other leads. This violates the "connecting mechanical and electrical equipment to a single power supply". In an emergency, cutting off the power switch on the device, it is impossible to cut off the entire current of the device, posing a potential hazard to personnel.
3.3 The electrical cabinet ventilation of equipment such as CNC machine tools must adopt the “positive pressure” method. However, in some machine tool transformations, it has not been realized. There is a domestic CNC machine tool that uses negative pressure ventilation, so the cutting fluid on the tool holder flows into the electric cabinet along the snake skin tube; and the moisture in the cable trench and the dust during cutting enter the electric cabinet along each channel and gap. . The machine is only used for one year. There is a layer of dirt and obvious water vapor (condensation) marks on the circuit board and various electrical parts in the electrical cabinet. This machine tool has frequent failures, which seriously affects the normal production. 13.3 of GB/T 5226.1 specifies that the enclosure of the control unit should generally have a degree of protection not less than IP 54.
3.4 Under-protection of electrical equipment. In the national standard GB/T 5226.1, thermal protection is required for motors with continuous operation of ≥ 0.5 kW. The heat accumulation of the motor is an important cause of motor burnout. In the process of machine tool equipment modification, a "time-current" protection device that does not match the motor is used, that is, when the time constant of the protection device and the protected device winding is large, an effective protection effect is often not obtained.
3.5 Control the line to bury hidden dangers. For such signals involving personnel and equipment safety, such as emergency stop signals, overtravel signals, alarm signals, etc., it shall be “parking in the form of power failure” and “cutting off all energized wires of the controlled device” (GB/T 5226.1) Medium 9.4.2.1). However, some enterprises design modified CNC machine tools, the travel switch uses normally open contacts, when the machine does not overtravel, the line is "off" (low level) during normal operation, when the moving parts overtravel the limit switch When the line is closed (high level). During the debugging of the machine tool, the maintenance personnel is the measuring line. After the plug is unplugged, it cannot be plugged in. As a result, the line is always in the normally open state during the test run. The controller confirms that the machine is “normal”, so that the screw bearing housing Crashed.
The grounding of the relay coil side in the control circuit has long been considered as an effective safety measure to avoid malfunction and malfunction. However, in some modified machine tools, it is common to have control contacts on either side of the relay coil or to connect the power supply at one end and the contact point at the other end. As the line is put on and off on the machine tool, if the mechanical wear, insulation failure, short circuit to the ground, the relay malfunctions, the consequences are unimaginable. It is worth mentioning that some designers of electrical transformation often design the control output to open the relay coil directly in the form of open collector. Although it is convenient at the moment, it also buryes hidden dangers.
3.6 The protection function of safe operation is incomplete. For equipment with a splash hazard, there is no protective door designed as an interlocking function; and when it is necessary to confirm that the operator's hands have left the dangerous action zone, the part that can be operated does not have a two-hand control prevention device; when the phase is missing, the electrical appliance is burned. Some of the three-way breakers are not provided, and three-way insurance is used separately. The ball screw pair is a precision component. Some machine tools have a lack of protection function after the modification. Dust, especially chips and hard sand, will enter the raceway during work. Some do not have an integral iron shield on the longitudinal lead screw. The two ends of the large carriage that are in contact with the sliding guide rail are not sealed, and as a result, the hard granular foreign matter enters the sliding surface to damage the guide rail.
3.7 The awareness of standardization is weak and the construction is quite dry. Some construction workers have a weak sense of standardization and are very dry during construction, causing equipment accidents. There is a piece of equipment, the electrician wiring is convenient for the figure, the power supply line is connected to the lower port of the fuse, and the load end is connected to the upper port of the fuse. When other personnel repaired, after unplugging the fuse, it was mistaken that the lower end of the fuse had been powered off, and a false alarm was made during the measurement, and the electric shock was almost hit. As for some modified CNC machine tools, it is not uncommon for technical reform workers and construction personnel to write or attach the device identification directly to the device. After some repair parts are replaced, the mark will disappear. It is extremely difficult for the next maintenance work, and it takes a lot of trouble.
3.8 Some CNC machine tools have been modified, especially some minor changes, but they are not revised or explained in the relevant technical documents, causing the documents to be inconsistent with the actual situation. Some equipment has been modified several times to become a "three no" product with wireless number, no device identification, no circuit or mechanical pattern. The equipment management department of a general enterprise often manages the technical documents provided by the manufacturer more strictly, but does not pay attention to the records after the maintenance changes. The longer this situation lasts, the more serious the problem. The dependence of CNC equipment on “specific people” or specific maintenance units is more obvious. Thereby reducing the market competitiveness of enterprises and restricting the development of enterprises. Therefore, for enterprises with CNC equipment, in the process of equipment transformation, we must pay special attention to the information provided by the transformation party, whether it meets the requirements of GB. This has not been confirmed, and it turns out to be a big gap in enterprise equipment management.
For CNC system manufacturers, when designing CNC software, large variables must be set aside for PLC and machine parameters to be developed for open CNC systems. For the NC reformers, they are required to have a deep understanding of the working principle of the CNC, the machine electrical and machine tool structure and machining program preparation, a good grasp of PLC programming, and familiarity with the meaning and choice of each parameter. Only in this way can the CNC, servo system, machine tool electrical system and mechanical parts be integrated into one, and finally handed over to the user a large-scale, special-purpose CNC machine tool with individual characteristics that has been modified or remanufactured. Greater economic benefits enable the assets to gain effective value.
In short, the transformation of CNC machine tools has a good development prospect, but it puts higher requirements on CNC system manufacturers and CNC machine tool reformers. China's machine tool transformation industry has also entered the new industry dominated by numerical control technology from the old industry. As a new economic growth industry, machine tool transformation is an "eternal" subject, and it will be full of vitality.