CNC machine tool is the key equipment of modern manufacturing industry, 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, each problem will have its root cause of the production, we always find their solution to the root causes.
Failure analysis method
First, the common faults and their classification
1. According to the breakdown of the site classification
⑴ host failure CNC machine tool host usually refers to the composition of CNC machine tools, lubrication, cooling, chip evacuation, hydraulic, pneumatic and protective parts. Host common faults are:
1) Due to mechanical parts installation, commissioning, improper use of the operation caused by mechanical transmission failure
2) due to rail, spindle and other moving parts of the interference, friction and other causes of failure
3) due to mechanical parts damage, poor connection and other causes of failure, and so on
Host failure is mainly manifested as transmission noise, poor machining accuracy, running resistance, mechanical parts do not move, mechanical parts damaged and so on. Poor lubrication, hydraulic and pneumatic system blockages and poor sealings are common causes of host failure. CNC machine tools on a regular basis to maintain and maintain the control and eradication of "three leaks" phenomenon is to reduce the host part of the important measures to failure.
⑵ electrical control system fault from the type of components used.According to common customs, electrical control system fault is usually divided into "weak" and "strong" fault two categories,
"Weak" part refers to the control system to electronic components, integrated circuit-based control section. The weak part of CNC machine tools, including CNC, PLC, MDI / CRT and servo drive unit, the output unit for the output.
There are hardware faults and software faults in the "weak power" fault Hardware faults refer to the faults of the integrated circuit chips, discrete electronic components, connectors and external connecting components of the above parts. Software failure refers to the normal operation of the hardware out of germanium, data loss and other failures are common. Processing error, changes or loss of system procedures and parameters, computer operations and so on.
"Power" refers to the control system in the main circuit or high voltage, high-power circuits in the relay, contactors, switches, fuses, power transformers, motors, electromagnets, travel switches and other electrical components and their components Control circuit. Although this part of the fault repair, diagnosis more convenient, but because of its high voltage, high current working condition, the probability of failure than the "weak" part must be paid enough attention to maintenance personnel.
2. According to the nature of the breakdown
⑴ Deterministic failure Deterministic failure refers to the control system host hardware damage or as long as certain conditions are met, CNC machine tools will inevitable failure. This type of failure is the most common in CNC machine tools, but because it has a certain regularity, it also brings convenience to the maintenance of certainty with a non-recoverable fault, if the fault occurs, if it is not repaired, the machine will not Automatically back to normal.But just find out the root cause of the failure, the machine can be restored to normal immediately after maintenance. Proper use and careful maintenance is an important measure to eliminate or prevent failure.
⑵ stochastic fault Random fault refers to the numerical control machine tool in the process of accidental failure of such failures occur more hidden reasons, it is difficult to find out the regularity, so often referred to as "soft fault", random causes of failure Analysis and fault diagnosis are more difficult. In general, failures often occur due to factors such as the installation quality of components, the setting of parameters, the quality of components, the imperfect software design and the influence of work environment.
Stochastic fault recoverability, after the failure, restart the machine and other measures, the machine can usually be restored to normal, but in the course of operation, may occur the same fault.
Strengthen the CNC system maintenance inspection, to ensure that the electrical box seal, reliable installation, connection, the correct grounding and shielding is to reduce, to avoid such failures an important measure.
3. According to the form of failure instructions
⑴ a tape with the fault display CNC machine fault display can be divided into indicator light display and display two cases:
1) Indicator display Alarm indicator display Alarm refers to the alarm displayed by the status indicator (usually composed of LED tube or small indicator light) on each unit of the control system.According to the numerical control system status indicator, even if the display is faulty , You can still be roughly analyzed to determine the location and nature of the fault occurred, therefore, in the maintenance and troubleshooting should carefully check the status of these status indicator.
2) The display shows the alarm The display shows that the alarm is an alarm that shows the alarm number and alarm information via the CNC display. As the CNC system generally has a strong self-diagnostic function, if the system diagnostic software and display circuit is working properly, once the system fails, you can display the alarm and alarm text on the form of fault information. CNC system can display the alarm as few as dozens, as many as thousands, it is an important diagnostic information. In the display alarm, but also can be divided into NC alarm and PLC reported two categories. The former for the NC manufacturer set the drop display. It can control the system "Maintenance Manual" to determine the possible causes of the fault. The latter is set by the CNC machine manufacturer PLC alarm message text, belonging to the side of the machine so drop display. It can be controlled by the machine manufacturer's "Machine Repair Manual" in the relevant content to determine the cause of the malfunction.
⑵ no fault alarm display This type of fault occurs when there is no alarm machine tool and system display, the analysis of the diagnosis is usually more difficult. Need careful and careful analysis and judgment to be confirmed. Especially for some early CNC systems, there is more faulty alarm with no alarm display due to the poor diagnostic function of the system itself or no PLC alarm message text.
For no alarm display failure, usually specific analysis of the specific circumstances, according to changes before and after the failure to analyze and determine the principle analysis method and PLC program analysis method is to solve the main method of alarm display failure.
4. According to the cause of the malfunction classification
⑴ CNC machine failure itself occurs such failure is caused due to the reasons of the CNC machine itself, with the external use of environmental conditions has nothing to do. Most of the CNC machine tool failures are such failures.
⑵ CNC machine external fault This failure is due to external causes. Supply voltage is too low, too high, excessive fluctuations: power phase sequence is incorrect or unbalanced three-phase input voltage; Ambient temperature is too high: harmful gases, moisture, dust into: external vibration and interference are all caused by failure the reason.
In addition, the human factor is also caused by CNC machine tool failure of one of the external causes, according to statistics, the first time the use of CNC machine tools or unskilled workers to operate CNC machine tools, the first year of use, improper operation caused by external failures accounted for More than one third of the total machine failure.
In addition to the above common fault classification method, there are many other different classification methods. Such as: according to whether the failure occurred without destructive can be divided into destructive fault and non-destructive fault. According to the fault occurred and the need to repair the specific functional parts. Can be divided into numerical control device failure, feed servo system failure, spindle Drive system failure, white moving tool system failure, etc., this classification method commonly used in the maintenance.
Second, the basic method of fault analysis
Fault analysis is the first step of CNC machine maintenance, fault analysis, on the one hand you can quickly identify the cause of the fault troubleshooting: but also can prevent the occurrence and expansion of the role. In general, the main method of failure analysis of CNC machine tools are the following,
⑴ routine analysis routine analysis method is on the CNC machine, electricity, liquid and other parts of the routine inspection, in order to determine the cause of a fault. Routine analysis on a CNC machine typically includes the following:
1) Check the power supply specifications (including voltage, frequency, phase sequence, capacity, etc.) is required
2) Check CNC servo drive, spindle drive, motor, input / output signal connection is correct and reliable
3) Check if the printed circuit board inside the CNC servo drive and other devices is firmly installed and if there is any loose connection
4) Check CNC servo drive, spindle drive and other parts of the set-end, potentiometer settings, the adjustment is correct
5) Check the hydraulic, pneumatic, lubricating parts of the hydraulic pressure and pressure meet the machine requirements
6) Check electrical components, mechanical components for obvious damage, and so on
⑵ action analysis method Act analysis is by observing and monitoring the actual machine action to determine the action of the bad parts and thus to trace the root cause of a failure.
In general, CNC machine tools using hydraulic, pneumatic control of parts such as: automatic tool change device, exchange table devices, fixtures and transmission devices, etc. can be diagnosed by action to determine the cause of the malfunction.
⑶ state analysis method of state analysis is by monitoring the working status of the implementation of components to determine the cause of a fault method, this method is the most widely used in the maintenance of CNC machine tools.
In modern CNC system, the main parameters of servo feed system, spindle drive system, power module and other components can be dynamically and statically tested. These parameters include input / output voltage, input / output current, reference / actual speed, position The actual load of the situation and so on. In addition, all input / output signals of the CNC include the status of internal relays, timers, etc. It can also be checked by the diagnostic parameters of the CNC. By means of the state analysis method, it can be quickly found without any instrument or equipment according to the internal state of the system The reason for the failure, the most widely used in the maintenance of CNC machine tools, maintenance personnel must be proficient.
⑷ operation, programming analysis method of operation, programming analysis method is through some special operation or preparation of a special test program segment, a method of identifying the cause of the malfunction. Such as manual single-step implementation of automatic tool change, automatic exchange of table movements, the implementation of single-function processing instructions and other methods of action and function testing. Through this method, you can determine the specific causes and components of the fault occurred, check the correctness of the programming.
⑸ system self-diagnosis CNC system self-diagnosis is the use of internal self-diagnostic program or a dedicated diagnostic software, the system's internal hardware and system control software for self-diagnosis, test diagnostic methods. It includes boot self-diagnosis, online monitoring and offline testing of this aspect (see below)
Third, CNC fault self-diagnosis
1. Power on self-diagnosis
The so-called power-on self-diagnosis refers to the numerical control system is powered on, diagnosed by the system's internal diagnostic procedures automatically, it is similar to the computer's boot diagnosis.
Power on self-diagnosis can check the key hardware in the system, such as CPU, memory, I / O unit, CRT / MDI unit, paper tape reader and floppy disk drive; Check the installation, connection status and performance of the specified equipment: Some systems can also diagnose some important chips, such as PAM, ROM, dedicated LSI, etc. CNC system self-diagnosis at boot time, only when all items are confirmed to be correct before they can enter the normal operation.
The timing of the diagnosis ten CNC system usually only a few seconds, but some take a few minutes. Power on self-diagnosis generally carried out according to the provisions of the steps to FANUC FANUCII company as an example of diagnostic procedures in the implementation process, the system on the seven-segment display by 9 → 8 → 7 → 6 → 5 → 4 → 3 → 2 → 1 The order of change, the corresponding check is:
9 - CPU reset, start executing diagnostic instructions:
8- ROM test, said ROM check error, the display becomes b;
7 - RAM cleared, the system cleared the contents of RAM, to prepare for normal operation;
6 A pair of BAC (bus random control) chip initialization. At this point, if the display becomes A, that motherboard and CRT
Between the transmission error; become C, said the connection error: F, said I / O board or connection cable is bad: to H, said the connection unit identification number is wrong; lowercase letters c said the cable transmission error ; Display J, said poor PLC or interface conversion circuit and so on.
5- Check the MDI unit
4 - CRT unit initialization
3- display CRT initial screen, such as: software version number, serial number and so on. At this point if the display becomes L, that PLC
Control software problems: into O, the system failed to initialize, the control software problems:
2 - said the system has completed the initialization work;
1- indicates that the system is ready for operation. If the display changes to E, it indicates the system's main board or ROM board, or the CNC control software is faulty.
Under normal circumstances CRT initialization is complete, if other parts of the fault, CRT can display the alarm message.
2. Online monitoring
Online monitoring can be divided into CNC internal program monitoring and monitoring by external devices in two forms Internal CNC program monitoring is through the system internal procedures for the status of various parts of the automatic diagnosis, inspection and monitoring methods. The scope of on-line monitoring includes the CNC itself and the servo units, servomotors, spindle servo units, spindle motors, external devices, etc. connected to the CNC. Online monitoring always takes effect during system work.
Internal control of CNC system includes interface signal display, internal status display and fault display.
⑴ interface signal shows that it can display the current state of all interface signals between CNC and PLC, CNC and machine tools.
Refers to the digital input / output signal on-off celebration, to help analyze the fault. When repairing, it is necessary to understand the meaning of each signal between CNC and PLC, CNC and machine tool, as well as the various conditions that should be possessed by signal canceling to check correspondingly. The "Function Manual", "Connection Manual" provided by the CNC system manufacturer and the "Electrical Schematic of Machine Tool" provided by the machine manufacturer are the technical guides for the above status check.
⑵ internal status display In general, the use of internal status is not functional, you can display the following aspects:
1) An external cause that caused the loop instruction (machining program) not to be executed. Such as: whether the CNC system is in "check in place": whether it is in the state of "machine locked": whether it is in "waiting for speed reached" signal: whether to wait for measuring signal of "position encoder" Is it waiting for the feedrate override of the spindle I signal to be set to 0% during thread cutting, and so on?
2) Reset status display, indicating whether the system is in "emergency stop" status or "external reset" signal is on.
3) TH alarm status display. It shows the position of the wrong hole in the tape when it is alarmed.
4) Display of memory contents and abnormal state of bubble memory.
5) Position follow error display.
6) servo control part of the Luo information display
7) Encoder, grating and other position measuring device input pulse display and so on
⑶ fault information displayed in the numerical control system, fault information is generally "alarm display" in the form of CRT display. The contents of the alarm display according to the different numerical control system. Most of these messages are "alarm number", plus text in the form of specific content and troubleshooting methods in CNC system manufacturers to provide "maintenance manual" can be accessed.
Through the external device monitoring refers to the use of computers, PLC programmer and other equipment, CNC machine parts of the state of automatic diagnosis, inspection and monitoring of a method. Such as: through the computer, PLC programmer to PLC program in the form of ladder diagram, function chart for dynamic detection, it can be in the machine manufacturer did not provide the PLC program, the PLC program read dynamic waveform display and so on, usually also need help The necessary online monitoring equipment.
With the development of computer network technology, as a kind of on-line monitoring of external devices, the remote diagnosis technology by network connection is further popularized and perfected. Through the network, CNC system manufacturers can directly detect the production of its products at the scene to monitor the situation, the timely resolution of problems in the system for on-site maintenance staff to provide guidance and assistance.
Offline test
Offline testing, also known as "off-line diagnosis", it is the CNC system and machine separation, the CNC system itself for testing and inspection. By off-line test system fault can be further located, and strive to minimize the scope of the fault. Such as: Offline testing of printed circuit boards, you can locate the fault range to a certain part of the printed circuit board or even a chip or device, which is printed circuit board repair is necessary. Off-line testing of a CNC system requires dedicated diagnostic software or special test equipment so it can only be performed by a manufacturer of the control system or by a dedicated service department.
With the development of computer technology, modern CNC off-line diagnosis software is gradually integrated with CNC control software and some systems have introduced "expert system" into fault diagnosis. With this software, the operator can diagnose a CNC system or a machine tool malfunction simply by making some simple session operations on the CRT / MDI.
Common Mechanical Faults of CNC Machine Tools and Precautionary Measures
First, spindle parts failure
As a result of the use of speed motor, CNC machine tool spindle box structure is relatively simple, prone to failure of the parts inside the spindle automatic tool clamping mechanism, automatic speed control devices. In order to ensure that the tool holder will not loose itself during work or power failure, the tool automatic clamping mechanism adopts the spring clamping and matches the travel switch to issue the clamping or relaxation signal. If the tool can not be released after clamping, then consider adjusting the pressure and stroke cylinder knife switch device or adjust the disc spring on the nut, reducing the amount of spring pressure. In addition, the spindle heating and headstock noise problems, can not be ignored, this time the main consideration to clean the spindle box, adjust the amount of lubricant to ensure the headstock cleanliness and replacement of spindle bearings, repair or replacement of the spindle box gear.
Second, feed drive chain failure
In CNC machine tool feed transmission system, commonly used ball screw, hydrostatic screw nut, rolling guide, hydrostatic guide and plastic guide. Therefore, there is a fault in the feed chain, the main reflection of the decline in the quality of exercise. Such as: mechanical parts are not moved to the specified location, operation interruption, positioning accuracy decreases, the reverse gap increases, crawling, bearing noise becomes larger (after a collision) and so on.
For such failures can be prevented by the following measures:
(1) improve the transmission accuracy Adjust the preload of the movement, adjust loose links, eliminating the transmission gap, shorten the transmission chain and set the reduction gear in the transmission chain, but also improve transmission accuracy.
(2) to improve the transmission stiffness. Adjusting the screw nut, the preload of the supporting components and the proper selection of the size of the screw itself are effective measures to improve the rigidity of the transmission. Insufficient stiffness can also cause creeping and vibration in the workbench or pallet as well as reverse dead zones, affecting drive accuracy.
(3) improve the movement accuracy. Under the premise of meeting the strength and rigidity of components, the quality of moving parts is reduced as much as possible, the diameter and mass of rotating parts are reduced, the inertia of moving parts is reduced, and the precision of motion is improved.
(4) rail rolling rail is more sensitive to the stolen goods, there must be good guards, and the preload of the rolling guide to choose properly, over the General Assembly to significantly increase the traction. Static pressure guide rail should have a good filtering effect of the oil supply system.
Third, automatic tool change device failure
Automatic tool changer failure mainly in: knife movement fault, positioning error is too large, manipulator holding the handle instability, such as larger robot movement error. Serious problems will result in tool change stuck, the machine was forced to stop working.
1. Tool movement error
If the coupling between the motor shaft and the worm shaft is loose or the mechanical coupling is too tight, the tool magazine will not rotate. In this case, the screws on the coupling must be tightened. If the rotation of the tool magazine is not in place, it will be caused by motor rotation failure or transmission error. If there is a knife sleeve can not clamp the tool, you need to adjust the adjustment screw on the knife sleeve, the compression spring, the top tight clamping pin When the sleeve is not up / down, you should check the fork position or limit switch installation And adjust the situation.
2. Tool change robot fault
If the tool holder is not tight, the knife, then adjust the clamping jaw spring, so that the pressure increases, or replace the mechanical clamping chuck If the tool clamping loose, should adjust the loose lock spring nut, the maximum load is not Over rating. If the knife swap knife, it belongs to the tool changer headstock did not return to the tool change point or tool change caused by drift, should re-operate the spindle box, make it back to the tool change position, reset the tool change point.
Fourth, the position of each axis movement switch pressure joint failure
In CNC machine tools, in order to ensure the reliability of automation, using a large number of detection of the position of the travel switch machine. After long-term operation, the movement characteristics of the moving parts change, the reliability of the travel switch pressing device and the quality characteristics of the travel switch itself have a great impact on the whole machine performance. Generally check and replace the trip switch in time to eliminate the impact of such a switch on the machine.
Fifth, ancillary equipment failure
Hydraulic system
Variable displacement pumps should be used on the hydraulic pump to reduce the heat generated in the hydraulic system. Filters installed in the tank should be periodically cleaned with gasoline or ultrasonic vibration. Common failures are mainly pump body wear, cracks and mechanical damage at this time must generally overhaul or replacement parts.
2 pressure system
For the tool or workpiece clamping, safety door switch and the spindle cone blowing blowing pressure system, the water separator should be regularly drain, regular cleaning, to ensure that the pneumatic components of the sensitivity of moving parts. Valve heart failure, air leakage, pneumatic components and malfunction failure caused by poor lubrication, so the oil mist cleaner should be cleaned regularly. In addition, the tightness of the pneumatic system should also be checked regularly.
3. Lubrication system
Including the machine tool guide, transmission gear, ball screw, spindle box lubrication. Lubricating pump filters need regular cleaning, replacement, the general should be replaced once a year.
4. Cooling system
It acts as a tool for cooling and punching the workpiece. Coolant nozzles should be cleaned regularly.
Chip removal device
Chip removal device is an independent function of the annex, the main guarantee for automatic cutting plus smooth and reduce the CNC machine tool fever. Therefore, chip evacuation device should be able to timely chip evacuation, the installation location should generally be as close as possible to the cutting area of the tool.
VI Conclusion
CNC machine tool is the use of computer control, mechanical and electrical integration of automated processing equipment, the use of CNC machine tools is a technology application project. Correct prevention and effective maintenance is to improve the efficiency of CNC machine tools to ensure the basic. For common mechanical faults, although there is not much chance of occurrence, we should not neglect that we should analyze and judge the root cause of faults comprehensively, shorten the downtime as much as possible to facilitate the efficient performance of CNC machine tools.
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