CNC machine tools are the key equipment for modern manufacturing, and to a large extent affect the development of equipment manufacturing industry. However, in the actual production operations, there will be a variety of common failures, and each issue will have its roots. We will always find the solution to it.
Failure analysis methods First, common faults and their classification 1. According to the location of the fault occurred (1) Host failure The main machine of a CNC machine usually refers to the components of a CNC machine, such as machinery, lubrication, cooling, chip removal, hydraulics, pneumatics, and protection. The main faults of the host are:
1) Mechanical transmission failure caused by improper installation, debugging, operation and use of mechanical components, etc. 2) Failure due to interference, excessive friction, etc. of moving parts such as guide rails and spindles 3) Damage to mechanical parts, poor connection, etc. Causes of failures, etc. The host failures are mainly manifested as transmission noise, poor processing accuracy, large operating resistance, mechanical component movements, and mechanical component damage. Poor lubrication, blockage of hydraulic and pneumatic lines, and poor sealing are common causes of host failures. Regular maintenance and maintenance of CNC machine tools. Controlling and eradicating the "three leakage" phenomenon is an important measure to reduce some of the host's failures.
(2) Failure of the electrical control system from the type of components used. According to the usual habits, faults in electrical control systems are generally divided into two categories: "weak current" faults and "strong current" faults.
The “weakening” part refers to the control part of the control system that is dominated by electronic components and integrated circuits. The weak parts of CNC machine tools include CNC, PLC, MDI/CRT, servo drive units, and output and output units.
"Weak power" fault is divided into hardware fault and software fault. Hardware failure refers to the failure of the integrated circuit chips, discrete electronic components, connectors, and external connection components of each of the above components. Software failure refers to the failure of the action, data loss, etc. Processing program error, system program and parameter change or loss, computer operation error, etc.
“Strong” refers to the main circuit in the control system or the electrical components such as relays, contactors, switches, fuses, power transformers, motors, electromagnets, and travel switches in the high-voltage, high-power loops and their components. Control circuit. Although this part of the fault repair, diagnosis more convenient, but because it is in a high voltage, high current working condition, the probability of failure is higher than the "weak current" part. Must pay enough attention to maintenance personnel.
2. According to the nature of the fault classification (1) deterministic fault deterministic fault refers to the hardware damage in the control system host or as long as it meets certain conditions, the inevitable failure of the NC machine tool. This kind of fault phenomenon is most common on CNC machine tools, but because it has certain rules, it also brings convenience to maintenance. Certain faults are irreparable. Once a fault occurs, if it is not repaired, the machine tool will not Automatically returned to normal. However, as long as the root cause of the failure is found out, the machine can return to normal immediately after the maintenance is completed. Proper use and careful maintenance are important measures to prevent or avoid failures.
(2) Random failure Random failure is the accidental failure of the index control machine tool during the work process. The causes of such failures are more subtle, it is difficult to find out the regularity, so it is often called "soft failure", the reason of random failure Analysis and fault diagnosis are more difficult. In general, the occurrence of faults is often related to many factors such as the installation quality of components, the setting of parameters, the quality of components, the imperfection of software design, and the influence of the work environment.
Random faults are recoverable. After a fault occurs, the machine can usually recover by restarting the machine. However, the same fault may occur during operation.
Strengthen the maintenance and inspection of the CNC system, ensure the sealing of the electrical box, and ensure reliable installation and connection. Correct grounding and shielding are important measures to reduce and avoid such failures.
3. According to the indication form of fault classification (1) The fault display with reported faults The display of faults of CNC machine tools can be divided into two cases: indicator display and display display:
1) Indicator lamp display Alarm indicator display Alarm refers to the alarm displayed by the status indicators (generally composed of LED lamps or small indicator lamps) on each unit of the control system. According to the status indicator of the numerical control system, even when the monitor is faulty, the location and nature of the fault can be roughly analyzed and judged. Please check the status of these status indicators carefully during maintenance and troubleshooting.
2) The display shows the alarm. The display alarm is the alarm that can display the alarm number and alarm information through the CNC display. Because the numerical control system generally has a strong self-diagnosis function, if the system's diagnostic software and the display circuit are working properly, once the system fails, the fault information can be displayed in the form of an alarm number and a text on the display. There are dozens of alarms that can be displayed on the CNC system, and as many as thousands, it is an important information for fault diagnosis. In the monitor display alarm, it can be divided into NC alarm and PLC report. The former is a drop-down display for a CNC manufacturer. It can refer to the system's "Maintenance Manual" to determine the cause of the failure. The latter is the PLC alarm message text set by the CNC machine tool manufacturer, which belongs to the down display of the machine tool. It can be used to compare the contents of the "Machine Maintenance Manual" provided by the machine tool manufacturer. Determine the cause of the failure.
(2) Faults without alarm display When this type of fault occurs. There is no alarm display in the machine tool and system, and its analysis and diagnosis are usually difficult. It needs to be confirmed through careful and careful analysis. Especially for some early CNC systems, since the system's own diagnostic function is not strong, or there is no PLC alarm information text, there will be more failures without the alarm display.
For no alarm display failure, it is usually necessary to analyze the specific circumstances, according to the changes before and after the failure occurred. Analysis and judgment, principle analysis and PLC program analysis are the main methods to solve the problem of no alarm display failure.
4. According to the cause of failure classification (1) CNC machine tool itself failure This type of failure is caused by the reasons of the NC machine tool itself, and has nothing to do with the external use of environmental conditions. Most failures of CNC machine tools are such failures.
(2) External faults of numerically-controlled machine tools These types of faults are caused by external causes. Supply voltage is too low, too high, excessive fluctuations: Incorrect phase sequence of the power supply or unbalanced three-phase input voltage; Excessive ambient temperature: Harmful gas, moisture, dust, etc.: Exogenous vibration and interference are all caused by the fault. the reason.
In addition, human factors are also one of the external causes of the failure of numerical control machine tools. According to statistics, the first time the use of CNC machine tools or unskilled workers to operate CNC machine tools, in the first year of use, improper operation caused by external faults to account for More than one-third of the total machine tool failures.
In addition to the above common fault classification methods, there are many other different classification methods. Such as: whether or not there is damage when the fault occurs. Can be divided into two kinds of destructive failure and non-destructive failure. According to the fault occurred and need to repair the specific functional parts. Can be divided into numerical control device failure, feed servo system failure, spindle drive system failure, white blade change system failure, etc., this classification method is commonly used in maintenance.
Second, the basic method of failure analysis Failure analysis is the first step in the maintenance of CNC machine tools, through the failure analysis, on the one hand can quickly identify the causes of failure troubleshooting: At the same time can also play a role in preventing the occurrence and expansion of failure. In general, the main methods for fault analysis of CNC machine tools are the following:
(1) Routine Analysis Routine analysis is a routine check of the machine, electrical, and hydraulic components of a CNC machine tool to determine the cause of the failure. Routine analysis methods on CNC machine tools usually include the following:
1) Check whether the power supply specifications (including voltage, frequency, phase sequence, and capacity, etc.) meet the requirements. 2) Check if the CNC servo drive, spindle drive, motor, and input/output signal connections are correct and reliable. 3) Check the CNC servo drive, etc. Whether the printed circuit board in the device is firmly installed, and if there is any looseness in the connecting part 4) Check whether the setting end of the CNC servo drive, spindle drive, etc., potentiometer is set or adjusted correctly. 5) Check the hydraulic, pneumatic, and lubrication components. Whether the hydraulic pressure, air pressure, etc. meet the requirements of the machine tool 6) Check whether the electrical components and mechanical components have obvious damage, etc. (2) Motion analysis method The motion analysis method is used to observe and monitor the actual movement of the machine tool to determine the faulty part and thus A way to trace the root cause of a fault.
In general, CNC machine tools use hydraulic, pneumatic control parts such as: automatic tool changer, exchange workbench device, fixtures and transmission devices, etc., can determine the cause of the fault through the action diagnosis.
(3) State analysis The state analysis method is a method of determining the cause of failure by monitoring the operating state of the actuator. This method is most widely used in the maintenance process of CNC machine tools.
In the modern numerical control system, the main parameters of the servo feed system, spindle drive system, power supply module and other components can be dynamically and statically detected. These parameters include: input/output voltage, input/output current, given/actual speed, position Actual load conditions, etc. In addition, all input/output signals of the CNC system include the status of internal relays, timers, etc., and can also be checked by the diagnostic parameters of the CNC system. The status analysis method can be quickly found in the absence of equipment and equipment according to the internal status of the system. The cause of the failure is the most widely used in the maintenance of CNC machine tools, and maintenance personnel must be skilled.
(4) Operation, program analysis method operation, and program analysis method is a method of confirming the cause of a fault through some special operation or programming a special test program segment. Such as manual single-step execution of automatic tool change, automatic exchange of table movements, execution of single-function machining instructions, etc., to perform motion and function detection. Through this method, the cause and component of the fault can be specifically determined, and the correctness of the program preparation can be checked.
(5) Self-diagnosis of the system The self-diagnosis of the CNC system is to use the system's internal self-diagnosis program or dedicated diagnostic software to perform self-diagnosis and test diagnostic methods on the system's internal hardware and system control software. It mainly includes one aspect of boot self-diagnosis, online monitoring and offline testing (see details below)
Third, the CNC fault diagnosis 1. Power-on self-diagnosis The so-called power-on self-diagnosis is a diagnostic performed automatically by the system's internal diagnostic program when the exponential control system is powered on. It is similar to the computer's boot diagnostics.
Power-on self-diagnosis can automatically check the key hardware in the system, such as: CPU, memory, I/O unit, CRT/MDI unit, paper tape reader, floppy drive, etc.; determine the installation, connection status and performance of the specified device: Some systems can also diagnose certain important chips such as PAM, ROM, and dedicated LSIs. The self-diagnosis of the CNC system is performed at the time of starting, and the normal operation can be entered only after all the items are confirmed.
The time of diagnosis determines that the ten-digit control system generally takes only a few seconds, but some require a few minutes. Self-diagnosis for power-on self-diagnosis is generally performed according to the specified steps. In the FANUC II system of FANUC as an example, during the execution of the diagnostic program, the seven-segment display on the system board is displayed as 9→8→7→6→5→4→3→2→1. The order of changes, the corresponding inspection content is:
9-Reset the CPU and start executing diagnostic commands:
8 - Perform a ROM test, which means that when the ROM check error occurs, the display becomes b;
7 - Clearing the RAM, the system clears the contents of the RAM to prepare for normal operation;
6 A pair of BAC (bus random control) chips are initialized. At this time, if the display changes to A, there is an error between the motherboard and the CRT; if it changes to C, it indicates a connection error: if it changes to F, it means that the I/O board or the connecting cable is defective: it becomes H, indicating that it is used. The connection unit identification number is incorrect. The lowercase letter c is used to indicate that the optical cable is transmitting. The display J indicates that the PLC or interface conversion circuit is defective.
5 - Check the MDI unit 4 - Initialize the CRT unit 3 - Display the initial screen of the CRT, eg software version number, serial number, etc. If the display changes to L at this time, it indicates that there is a problem with the PLC control software. If it changes to 0, it indicates that the system failed to initialize and there is a problem with the control software:
2 - indicates that the initialization of the system has been completed;
1 - Indicates that the system can operate normally. If the display changes to E, it indicates that the system's main board or ROM board, or the CNC control software is faulty.
After the initialization of the CRT is completed under normal conditions, if there is a fault in other parts, the CRT can display the alarm information.
2. On-line monitoring On-line monitoring can be divided into CNC internal program monitoring and external device monitoring. Two forms of CNC internal program monitoring is a method that automatically diagnoses, checks, and monitors the status of each part through the internal program of the system. The on-line monitoring range includes the CNC itself and servo units, servo motors, spindle servo units, spindle motors, and external devices connected to the CNC. Online monitoring is always in effect during system operation.
The internal program monitoring of CNC system includes interface signal display, internal status display and fault display.
(1) Interface signal display It can display the current status of all interface signals between CNC and PLC, CNC, and machine tool.
Refers to the disconnection and clearing of digital input/output signals to help analyze faults. The maintenance must understand the significance of each signal between CNC and PLC, CNC and machine tool, as well as the various conditions that should be possessed for the signal generation to be checked. The "function specification", "connection specification" provided by the manufacturer of the numerical control system and the "machine electrical schematic diagram" provided by the machine tool manufacturer are the technical guides for performing the above status checks.
(2) The internal status display generally uses the internal status display function, which can display the following aspects:
1) An external reason that causes a loop instruction (machining program) not to be executed. Such as: CNC system is in "in place check": whether in the "machine lock" state: whether in the "wait for speed to reach" signal is turned on: whether to wait for the "position encoder" measurement signal when the spindle per revolution feed programming ; During thread cutting, is it waiting for the 'spindle I turn signal' feedrate override is set to 0%, and so on.
2) The reset status display indicates whether the system is in the "emergency stop" state or the "external reset" signal is on.
3) TH alarm status display. It can show the position of the tape error hole in the alarm.
4) The memory contents and the abnormal state of the bubble memory are displayed.
5) Display of position following error.
6) Control information display of the servo slamming part 7) Input pulse display of position measuring elements such as encoder and grating, etc. (3) Fault information is displayed in the CNC system, and fault information is generally displayed in the CRT in the form of “alarm display”. The contents of the alarm display differ according to the numerical control system. Most of this information appears in the form of "alarm number" plus text, and the specific contents and exclusion methods can be found on the "maintenance manual" provided by the manufacturer of the CNC system.
Monitoring through external equipment refers to a method of automatically diagnosing, checking, and monitoring the status of each part of a CNC machine tool using a computer, a PLC programmer, and other equipment. Such as: through the computer, PLC programmer to PLC program in the form of ladder diagrams, function diagrams for dynamic detection, it can not provide PLC program in the machine tool manufacturer, the PLC program dynamic waveform display and other content, usually also need help Necessary online monitoring equipment.
With the development of computer network technology, as a kind of external equipment online monitoring, remote diagnosis technology through network connection is further popularization and improvement. Through the network, the CNC system manufacturer can directly inspect and monitor the working conditions of the products it produces at the site, solve the problems in the system in time, and provide guidance and assistance for on-site maintenance personnel.
3. Off-line testing Off-line testing, also known as “off-line diagnosis”, is the testing and inspection of the CNC system itself after the NC system is detached from the machine tool. The off-line test can be used to further locate the fault of the system and strive to minimize the fault scope. For example, through offline testing of printed circuit boards, the fault range can be located on a certain part of the printed circuit board or even a certain chip or device. This is necessary to repair the printed circuit board. The off-line test of the CNC system requires special diagnostic software or special test equipment. Therefore, it can only be performed at the manufacturer of the CNC system or a dedicated maintenance department.
With the development of computer technology, modern CNC offline diagnostic software is gradually integrating with CNC control software. Some systems have introduced "expert system" into fault diagnosis. Through such software, the operator can diagnose the fault of the CNC system or the machine tool by simply performing some simple conversation operations on the CRT/MDI.
CNC machine tools common mechanical failures and preventive measures A. Spindle component failure Due to the use of speed motor, CNC machine tool spindle box structure is relatively simple, prone to failure parts of the spindle is the automatic tool clamping mechanism, automatic speed control device. In order to ensure that the tool holder does not loosen during operation or during power outage, the automatic tool clamping mechanism adopts spring clamping and sends a clamping or relaxing signal with the limit switch. If the tool cannot be released after clamping, consider adjusting the pressure of the loose cylinder and the stroke switch or adjusting the nut on the disc spring to reduce the amount of spring pressure. In addition, spindle heating and spindle box noise problems can not be ignored, this time mainly consider cleaning the spindle box, adjust the amount of lubricant to ensure that the spindle box cleanliness and replace the spindle bearings, repair or replacement spindle gear.
Second, feed the transmission chain failure In the numerical control machine feed transmission system, commonly used ball screw, static pressure screw nut pair, rolling guide, static pressure rail and plastic rail. Therefore, the failure of the feed transmission chain mainly reflects the decline in the quality of movement. Such as: the mechanical parts did not move to the specified position, the operation is interrupted, the positioning accuracy is reduced, the backlash increases, crawling, bearing noise becomes louder (after a crash) and so on.
For such failures can be prevented by the following measures:
(1) Improving the transmission precision Adjusting the pre-tightening force of each pair of movements, adjusting the loosening link, eliminating the transmission gap, shortening the transmission chain, and setting the reduction gear in the transmission chain can also improve the transmission accuracy.
(2) Improve the transmission stiffness. Adjusting the pre-tightening force of the screw nut pair and the supporting part and choosing the reasonable size of the screw itself is an effective measure to improve the stiffness of the transmission. Insufficient stiffness can also cause crawling and vibration on the table or pallets and reverse dead zones, affecting transmission accuracy.
(3) Improve the accuracy of movement. In the premise of meeting the strength and rigidity of components, the quality of moving parts should be reduced as much as possible, and the diameter and mass of rotating parts should be reduced to reduce the inertia of moving parts and improve the movement accuracy.
(4) Guideway Roller guides are sensitive to stolen goods and must have good protection devices. Moreover, the preload of rolling guides should be properly selected. Traction should be significantly increased as a result of excessive assembly. Hydrostatic guide rails shall have a set of oil supply system with good filtering effect.
Third, the automatic tool changer failure Automatic tool changer failure is mainly manifested in: magazine movement failure, positioning error is too large, the robot handle holder handle instability, robot movement error and so on. If the failure is serious, the tool change will be stuck and the machine will be forced to stop working.
1. Tool magazine movement failure If mechanical reasons such as the loose coupling of the motor shaft and the worm shaft or the mechanical coupling is too tight, the tool magazine can not be rotated. At this time, the screws on the coupling must be tightened. If the tool magazine is not rotated, it belongs to the motor rotation fault or transmission error. If the knife sleeve does not clamp the tool, adjust the adjustment screw on the knife sleeve to compress the spring. When the knife sleeve is not in place, check the position of the fork or the installation of the limit switch. And adjust the situation.
2. Tool changer failure If the tool clamp is not tight and the tool is missing, adjust the clamping jaw spring to increase the pressure, or replace the robot clamping pin. If the tool is clamped and then loose, adjust the loose lock spring. Nut, so that the maximum load does not exceed the rated value. If the tool changes during the tool change, the spindle box does not return to the tool change point or the tool change point drifts during tool change. The spindle box should be operated again to bring it back to the tool change position and reset the tool change point.
Fourth, the movement position of each axis of the trip switch press failure In the numerical control machine tool, in order to ensure the reliability of automated work, the use of a large number of movement detection position switch machine tools. After a long-term operation, the movement characteristics of the moving parts change, and the reliability of the stroke switch pressing device and the quality characteristics of the stroke switch itself have a great influence on the performance of the entire machine. Generally, timely inspection and replacement of the travel switch can eliminate the influence of such bad switch on the machine tool.
5. Miscellaneous auxiliary device failure 1. Hydraulic pump The hydraulic pump should be equipped with a variable pump to reduce the filter installed in the heating oil tank of the hydraulic system. It should be regularly cleaned with gasoline or ultrasonic vibration. Common failures are primarily wear, cracks, and mechanical damage to the pump body. At this time, parts must be overhauled or replaced.
2. Air pressure system In the air pressure system used for tool or workpiece clamping, safety protection door switch and spindle taper hole blowing, the water separation filter should be regularly drained and periodically cleaned to ensure the sensitivity of moving parts in pneumatic components. Faults such as failure of the valve core, air leakage, damage to the pneumatic components, and malfunctioning of the valve are all caused by poor lubrication, so the oil mist filter should be cleaned regularly. In addition, the sealing of the pneumatic system should also be checked frequently.
3. Lubrication system includes lubrication of machine tool guides, transmission gears, ball screws, spindle boxes, etc. The filter in the lubrication pump needs to be cleaned and replaced regularly, and it should be replaced once a year.
4. Cooling system It cools and punches the tool and workpiece. Coolant nozzles should be cleaned regularly.
5. Chip removal device Chip removal device is an accessory with independent functions, mainly to ensure automatic cutting plus smooth and reduce the heat generated by CNC machine tools. Therefore, the chip evacuation device should be capable of automatic chip evacuation in time, and its installation position should generally be as close as possible to the cutting area of the tool.
VI. Conclusion CNC machine tools are computerized, mechanical and electrical integration of automated processing equipment, the use of CNC machine tools is a technical application project. Correct prevention and effective maintenance are the basic guarantee for improving the efficiency of CNC machine tools. Although there are few opportunities for common mechanical failures, it should not be overlooked that the causes of failures should be comprehensively analyzed and judged, and the downtime should be shortened as much as possible so as to facilitate the efficient performance of CNC machine tools.
