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Analysis of the reasons for reducing the machining precision of CNC machine tools

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Analysis of the reasons for reducing the machining precision of CNC machine tools

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There are five main reasons: the machine tool feed unit is changed or changed, the machine tool feed unit is changed or changed, the zero point bias of the machine tool axis, the axial reverse gap anomaly, the abnormal motor running state, the abnormal electrical and control part, the mechanical fault, such as silk. Bars, bearings, shaft couplet and other components. In addition, the processing program, tool selection and human factors may lead to abnormal machining accuracy.

1. Causes of abnormal machining accuracy
   
There are five main reasons: the machine tool feed unit is changed or changed, the machine tool feed unit is changed or changed, the zero point bias of the machine tool axis, the axial reverse gap anomaly, the abnormal motor running state, the abnormal electrical and control part, the mechanical fault, such as silk. Bars, bearings, shaft couplet and other components. In addition, the processing program, tool selection and human factors may lead to abnormal machining accuracy.
   
2. Principle of fault diagnosis for CNC machine tools
   
1. first, the external and internal CNC machine tools are machine tools, hydraulic, electrical as a whole, so the occurrence of their faults will be reflected by the three. The maintenance personnel should be checked out one by one before the outside, as far as possible to avoid the random opening and disassembly, otherwise the failure will be expanded to reduce the accuracy and reduce the performance of the machine.
   

2. in the first place, the mechanical fault is easy to detect, but the fault diagnosis of CNC system is more difficult. Before troubleshooting, first of all, we should pay attention to eliminating mechanical faults, which often achieve twice the result with half the effort.
 
3. first static and post motion in the static state of the machine cutting off, through understanding, observation, testing, analysis, confirmed as a non destructive fault, the machine can be electrified, under the operating conditions, dynamic observation, test and test, find fault. For destructive failure, danger must be removed before power is switched on.
 
4. first, simple and complex, when there are a variety of faults, intertwined and covered up, when it is impossible to start, we should first solve the easy problems and then solve the difficult problems. Often difficult problems can be easily solved after simple problems are solved.

   
3. Fault diagnosis method of NC machine tool
   
  1. intuitionistic method: (look and smell) ask - the failure phenomenon of machine tool, processing condition, and so on; look at -CRT alarm information, alarm indicator lamp, capacitor and other components deformed smoke and burn, protective device release and so on; listen to abnormal sound; smell electric components coke paste taste and other odor; touch - fever, vibration, contact malpractice, etc.
   
  2. parameter check method: the parameters are usually stored in RAM, sometimes the battery voltage is not enough, the system will lose or confuse the parameters for a long time, and the parameters should be checked and proofread according to the characteristics of the fault.

3. isolation method: some faults are difficult to distinguish between the numerical control part and the servo system or mechanical part, and the isolation method is often adopted.
 
4. similar matching methods replace the template with suspected failure with the same function spare board, or exchange the template or unit with the same function.
The 5. functional program tests the G, M, S, T, and all the instructions of the function to write some small programs. In the diagnosis of failure, the program can be run to judge the lack of function.

   
 4. Fault diagnosis and processing example of machining accuracy.
   

1. mechanical faults lead to abnormal machining accuracy
 
Fault phenomenon: a SV-1000 vertical machining center adopts Frank system. During the processing of connecting rod die, Z axis feeding was found to be abnormal, resulting in at least 1mm cutting error (Z direction cutting).
 
Fault diagnosis: in the investigation, we know that the fault occurred suddenly. The machine tool is moving at the point, operating under the manual input data mode operation, and returning to the reference point normal, without any alarm prompt, the possibility of the electrical control part of the hard fault is excluded. The following aspects should be inspected one by one.
 
Check the machining process segments that are running in the abnormal accuracy of machine tools, especially tool length compensation, machining coordinate system (G54-G59) proofreading and calculation.
 
In the point moving way, the Z axis is repeatedly moved. After seeing, touching, listening, and diagnosing the motion state of the Z, it is found that the noise of the motion is abnormal, especially the fast motion, and the noise is more obvious. It can be judged that there may be a hidden danger in the mechanical aspects.
 
Check the accuracy of the Z axis of the machine tool. A hand pulse generator is used to move the Z axis. (the ratio is fixed to a 1 x 100 position, that is, every step of the change, the motor feed is 0.1mm), and the motion of the Z axis is observed with the dial indicator. When the unidirectional motion is kept normal, as the starting point of the forward movement, the actual distance of the Z axis movement of the machine tool is d=d1=d2=d3= per step. =0.1mm shows that the motor works well and the positioning accuracy is good. The actual movement of the machine can be divided into four stages: (1) the motion distance of the machine tool d1>d=0.1mm (the slope is more than 1); (2) showing d1=0.1mm>d2>d3 (less than 1 slope); (3) the machine tool mechanism is actually not moving, showing the most standard reverse gap; (4) machine tool moving distance and pulser The fixed value is equal (slope equal to 1), and the normal motion of machine tool is resumed. No matter how to compensate for the reverse gap, it shows that the changes of the other segments still exist except (3) phase compensation, especially in the (1) stage, which seriously affects the machining accuracy of the machine tools. In compensation, it is found that the larger the gap compensation, the greater the distance between (1) stages.
Analysis of the above inspection found that there are several possible reasons: first, the motor is abnormal, the two is mechanical failure, and three is the gap between the lead screw. In order to further diagnose the fault, the motor and the lead screw are completely disconnected, and the motor and mechanical parts are checked separately. The result of the test is that the motor is running normally. In the diagnosis of the mechanical part, it is found that when using the hand plate to move the lead screw, there is a great sense of vacancy in the beginning of returning to motion. Under normal circumstances, bearings should be able to sense the orderly and smooth movement of bearings.
 
Troubleshooting: after the disassembly inspection, it is found that the bearing is indeed damaged and the ball is falling off. After the replacement, the machine is restored to normal.

   
2. improper control logic leads to abnormal machining accuracy
   

Failure phenomenon: a processing center produced by Shanghai machine tool manufacturers, in the process of Frank. processing, it is found that the X axis precision of the machine tool is abnormal, the error of precision is 0.008mm, and the maximum is 1.2mm. fault diagnosis. In the inspection, the machine tool has set the G54 workpiece coordinate system according to the requirement. Under the manual input data operation, the G54 coordinate system is used to run a section of "GOOG90G54X60.OY70.OF150; M30;" the mechanical coordinate value displayed on the display bed after the end of the standby machine is (X axis) "-1025.243", and the value is recorded. Then, in the manual way, the machine tool point is moved to any other position, and the program section is run under the manual input data mode. After the standby bed is stopped, it is found that the numerical value of the machine tool coordinates is displayed as "-1024.891", and the numerical comparison after the previous execution is different 0.352mm. according to the same method, the X axis is used. The dots move to different locations and execute the program repeatedly, while the values displayed on the display are different (unstable). A dial meter is used to examine the X axis carefully. It is found that the actual error of the mechanical position is basically the same as that of the digital display. Therefore, the reason for the fault is that the error of the repeated positioning of the X axis is too large. The reverse clearance and positioning accuracy of the X shaft are checked, and the error value is re compensated. The result is not effective. So it is doubted that grating ruler and system parameters are in doubt. But why do we have such a large error, but there is no corresponding alarm information further inspection found that this axis is a vertical axis, when the X axis is loosened, the spindle box falls down, resulting in error.
 
Troubleshooting: to modify the PLC logic control program of the machine tool, that is, when the X axis is loosened, the X axis can be loaded first and then the X axis is loosened; and when the X axis is clamped, the X axis is clamped first, then the energy can be removed. The fault of the machine tool was solved after the adjustment.
3. machine position problem leads to abnormal machining precision
 
Fault phenomenon: a vertical CNC milling machine produced in Hangzhou is equipped with Beijing KND-10M system. In the process of point movement or processing, the Z axis is found to be abnormal.
 
Fault diagnosis: inspection found that the Z axis moves up and down unevenly and has noise, and there is a certain gap. When the motor is started, the motion of the Z axis is unstable and the force is uneven, and the jitter of the sensory motor is more severe. While the downward movement, it does not shake so clearly; when the stop is stopped, the motion is not jitter, and it is more obvious in the process of processing. The analysis shows that there are three reasons for the failure: one is the large backlash of the lead screw; the other is the abnormal operation of the Z axis motor; the three is that the pulley is damaged to the uneven force. But one thing to note is that it does not vibrate when it stops, and the movement is uneven. Therefore, the problem of abnormal motor operation can be ruled out. Therefore, the first part of the mechanical part of the diagnosis, in the course of diagnostic testing found no abnormalities, within the tolerance. Using the rule of exclusion, the rest of the belt is only the problem of belt. When detecting the belt, it is found that the belt has just changed soon. But when examining the belt carefully, it is found that the inner side of the belt is damaged in varying degrees, and it is obviously unequal to the force. Asymmetry of angle position causes unequal stress.
Troubleshooting: as long as the motor is reloaded and aligned, the distance (motor and Z shaft bearings) is measured, and the two sides of the belt are evenly distributed. In this way, the Z axis moves up and down unevenly, and the noise and jitter are eliminated. Z axis processing returns to normal.
 
4. the system parameters are not optimized and the motor operation is abnormal
 
The system parameters leading to abnormal machining accuracy mainly include machine feed unit, zero offset, reverse gap, etc. For example, the Frank CNC system has two kinds of feed units: metric system and British system. In the process of machine tool repair, local treatment often affects the change of zero point bias and gap. After the fault treatment, it should be adjusted and modified at the right time. On the other hand, because of the serious mechanical wear or the loosening of the connection position, the parameters of the parameters can be changed, and the corresponding modification of the parameters can be made to meet the machine tool. The requirement of machining precision.
 
Fault phenomenon: a vertical CNC milling machine produced in Hangzhou is equipped with Beijing KND-10M system. In the process of processing, it is found that the accuracy of the X axis is abnormal.
 
Fault diagnosis: inspection found that there was a certain gap in the X axis, and there was an unstable phenomenon when the motor started. When the X axis motor is touched by hand, the motor is pulled more strongly, and the pulling is not obvious when stopping, especially in the point of moving mode. The analysis shows that there are two reasons for the malfunction: the first is the large backlash of the screw rod; the two is the abnormal operation of the X axis motor.

Fault handling: using the parameter function of KND-10M system to debug the motor. First, the existing gaps are compensated, and the parameters of the servo system and the pulse suppression function are adjusted. The jitter of the X axis motor is eliminated and the machining precision of the machine is restored to normal.