The causes of abnormal machining precision defects are very hidden and difficult to diagnose. Today I have summarized the 4 diagnostic principles and 5 diagnostic methods for you. Do you know them all?
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Causes of abnormal machining precision failures
Five main reasons: the power unit of the machine tool is changed or modified; the zero offset of each axis of the machine tool is abnormal; the axial play is abnormal; the working condition of the engine is abnormal, that is, the electrical and control parts; are abnormal; mechanical failure, such as screws, bearings, couplings and other components.
In addition, processing program preparation, tool selection, and human factors can also lead to abnormal processing accuracy.
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Principles of fault diagnosis of CNC machine tools
1/. Look at the outside first, then the inside. CNC machine tools are machine tools that integrate mechanics, hydraulics and electricity, so the occurrence of defects will also be comprehensively reflected by these three elements. Maintenance personnel should first carry out inspections one by one from outside to inside, and try to avoid random unpacking and disassembly, otherwise the fault will be amplified, the machine tool will lose precision and performance will be reduced.
2/. Mechanical first, then electrical. Generally speaking, mechanical faults are easier to detect, but diagnosing CNC system faults is more difficult. Before troubleshooting, be sure to eliminate mechanical faults first, which often achieves twice the result with half the effort.
3/. Static first, then dynamic. Firstly, in the static state of the machine tool off, through understanding, observation, testing and analysis, the machine tool can only be powered on after confirming that this is a non-destructive defect. ; under operating conditions, carry out dynamic observation, inspection and testing to detect defects. In the event of destructive faults, the danger must be eliminated before the device can be switched on again.
4/. Simple at first, then complex. When multiple defects are intertwined and hidden and you don’t know where to start, you need to solve the easy problems first, then the more difficult ones. Often, once simple problems are solved, difficult problems can become easier.
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CNC machine tool fault diagnosis method
1/. Intuitive method (look, hear, ask and guess) to ask – machine tool fault phenomena, processing conditions, etc. ; watch – CRT alarm information, alarm lights, deformation of capacitors and other components, smoke and burnt, protector tripping, etc. ; listen – anomaly Sound; odor – the smell of burnt electrical components and other odors touch – heat, vibration and poor contact, etc.
2/. Parameter Check Method Parameters are usually stored in RAM. Sometimes the battery voltage is insufficient, the system is not turned on for a long time, or external interference will cause settings to be lost or confused. calibrated according to the characteristics of the fault.
3/. Isolation method: For some faults, it is difficult to distinguish whether they are caused by the CNC part, the servo system or the mechanical part. The isolation method is often used.
4/. The same-type swap method replaces the suspected defective module with a replacement board with the same function, or swaps models or units with the same function.
5/. Functional program testing method: Write a few small programs for all instructions of G, M, S and T functions. Run these programs during fault diagnosis to determine the lack of functions.
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Examples of machining precision abnormal fault diagnosis and treatment
1/. Mechanical failure results in abnormal processing accuracy
Fault phenomenon: An SV-1000 vertical machining center uses the Frank system. During the processing process of the connecting rod mold, it was suddenly discovered that the Z axis feed was abnormal, causing a cutting error of at least 1mm (Z direction overcut).
Fault Diagnosis: During the investigation, it was learned that the fault occurred suddenly. The machine tool jogs and each axis operates normally in manual data entry mode and returns to the reference point normally. There is no alarm prompt, and the possibility of a serious fault in the electrical control part is eliminated. The following aspects should mainly be checked one by one.
Check the segment of the processing program running when the machine tool accuracy is abnormal, especially the tool length compensation, calibration and calculation of the processing coordinate system (G54- G59).
In jogging mode, the Z axis was moved repeatedly and the movement status was diagnosed by sight, touch and hearing. It was found that the noise in Z direction movement was abnormal, especially in fast jogging mode. more obvious. Judging from this, the machines may contain hidden dangers.
Check the accuracy of the Z axis of the machine tool. Move the Z axis with a manual pulse generator (set the magnification to 1 × 100, i.e. the motor advances 0.1 mm for each step) and observe the movement of the Z axis with a dial indicator. Once unidirectional movement remains normal, it serves as the starting point for forward movement. Each time the pulse changes by one step, the actual distance of the Z axis movement of the machine tool is d=d1=d2=d3=… …=0.1mm, which shows that the motor runs well and the positioning accuracy is also good.
Returning to the changes in the actual movement of the machine tool, it can be divided into four stages: (1) The movement distance of the machine tool d1>d=0.1mm (the slope is greater than 1); the expression is d1=0.1 mm>d2>d3 (the slope is less than 1); (3) The machine tool mechanism does not actually move, showing the most standard clearance (4) The movement distance of the machine tool is equal to the predetermined distance; value of the pulser (the slope is equal to 1), and the machine tool returns to normal movement.
Regardless of how the clearance is compensated, its characteristic is that except for compensation in step (3), changes always exist in other steps, especially in step (1), which seriously affects the machining precision of the machine tool. During compensation, it was found that the greater the gap compensation, the greater the distance traveled in step (1).
After analyzing the above inspection, we believe that there are several possible reasons: first, there is an abnormality in the engine, second, there is a mechanical failure, and third, there is a gap in the screw. In order to further diagnose the fault, completely disengage the motor and screw, and inspect the motor and mechanical parts respectively. The inspection result is that the engine is operating normally; When diagnosing the mechanical part, it was found that when the screw is turned by hand, there is a great feeling of emptiness at the start of the return movement. Under normal circumstances, you should be able to feel the bearing moving in an orderly and smooth manner.
Troubleshooting: After disassembly and inspection, it was found that the bearing was indeed damaged and the balls fell out. The machine returned to normal after replacement.
2/. Machine tool position problems lead to abnormal machining accuracy.
Fault phenomenon: a vertical CNC milling machine produced in Hangzhou, equipped with the Beijing KND-10M system. While jogging or machining, a Z axis abnormality was detected.
Fault Diagnosis: Inspection found that the Z axis moved up and down unevenly and made noise, and there was a certain gap. When the motor starts, there is an unstable noise and uneven force in the upward movement of the Z axis in jogging mode, and the motor shakes when going down, the shaking is not so obvious that there has no jerks when stopping. This is more obvious during processing.
There are three reasons for the failure: first, the screw clearance is very large; second, the Z axis motor works abnormally. Third, the pulley is damaged to the point of being under uneven stress.
But one thing to note is that there is no jitter when stopping, and the up and down movement is uneven, so the problem of abnormal motor operation can be eliminated. Therefore, the mechanical part is diagnosed first, and no abnormality is found during the diagnostic test, which is within the tolerance. Using the rule of elimination, the only problem remaining was the belt. Upon inspection of the belt, I found that this belt had just been replaced. However, when I carefully inspected the belt, I found that there were varying degrees of damage inside. side of the belt. This was obviously due to uneven force, what is the cause? During diagnosis, it was found that there was a problem with the placement of the motor, i.e. the angular position of the clamp was asymmetrical, causing uneven stress.
Troubleshooting: Simply reinstall the motor, align the angle, measure the distance (motor and Z-axis bearing) and make sure the belt (length) is equal on both sides. In this way, the irregular movement of the Z axis up and down as well as noise and jitter are eliminated and the machining on the Z axis returns to normal.
3/. System parameters are not optimized and the engine operates abnormally.
The system parameters that lead to abnormal machining accuracy mainly include the machine tool power unit, work offset, backlash, etc. For example, the Frank CNC system has two power units: metric and imperial. During the repair process of the machine tool, local processing often affects the zero offset and backlash changes. Timely adjustments and modifications should be made after the fault is resolved. On the other hand, serious mechanical wear or loose connections can also cause measured parameter values. For a change. Changes in parameters require corresponding modifications to meet the processing precision requirements of the machine tool.
Fault phenomenon: a vertical CNC milling machine produced in Hangzhou, equipped with the Beijing KND-10M system. During the machining process, it was found that the accuracy of the X axis was abnormal.
Fault Diagnosis: Inspection found that there is a certain deviation in the X axis and the motor is unstable when starting. When you touch the X-axis motor with your hand, you feel that the motor pulls strongly, but the pull is not obvious when it stops, especially in inching mode. The analysis believes that there are two reasons for the failure: first, the screw clearance is very large; second, the X axis motor works abnormally.
Troubleshooting: Use the KND-10M system parameter function to debug the motor. First, the existing deviation is compensated, and then the servo system parameters and pulse suppression function parameters are adjusted. The jitter of the X-axis motor is eliminated, and the processing precision of the machine tool returns to normal.
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