First, the cause of abnormal precision machining failure

Causes of Abnormal Machining Abnormal Faults are concealed and difficult to diagnose. Five main reasons are summarized as follows: the feed unit of the machine tool is changed or changed; the zero offset of each axis of the machine tool is abnormal; the axial backlash is abnormal; and the motor Abnormal operating conditions, that is, electrical and control part of the exception; mechanical failure, such as screw, bearings, shaft couplings and other components. In addition the preparation of processing procedures, the choice of tools and human factors, may also lead to abnormal processing accuracy.

Second, CNC machine tool fault diagnosis principles

1. The first external external CNC machine tool is a mechanical, hydraulic, electrical machine tool, so the occurrence of the failure will be reflected by the three comprehensively. Maintenance personnel should first be carried out from the outside one by one investigation, try to avoid unpacking, disassembly, otherwise it will expand the fault, the machine loss of accuracy, reduce performance.

2. First machinery after the electrical In general, mechanical faults easier to detect, and CNC system fault diagnosis is more difficult. Before troubleshooting, first pay attention to rule out mechanical failure, often can achieve a multiplier effect.

3. The first static after the first move in the static state machine power, through understanding, observation, testing, analysis, confirmed as non-destructive failure, before the machine power; under operating conditions, the dynamic observation, Inspection and testing, find fault. For destructive faults, you must first remove the danger before the power.

4. First simple and complex When multiple faults appear intertwined with each other, sometimes can not start, should first solve the easy problems, then solve the more difficult problems. After the simple problem is often solved, the difficult problem may also become easy.

Third, CNC machine tool fault diagnosis method

1. Intuitive method: (Wang Wenqu Qie) Q - machine fault phenomena, processing conditions; see -CRT alarm information, alarm lights, capacitors and other components deformation smoke charred, protector tripping, etc .; listen - abnormal sound; smell - electrical components coke smelling and other smell; touch - fever, vibration, poor contact and so on.

2. Parameter check method: The parameters are usually stored in RAM. Sometimes the battery voltage is insufficient. If the system is not powered on for a long time or external interference will cause the parameters to be lost or confused, the relevant parameters should be checked and calibrated according to the fault characteristics.

3. Isolation method: some faults, it is difficult to distinguish between the numerical control part, or servo system or mechanical parts, often using the isolation method.

4. The same type of swap replaces a template that is suspected of being faulty with a spare board of the same function or exchanges templates or units of the same function.

5. Function program test method G, M, S, T, the function of all the instructions to write some small programs, run these programs in the diagnosis of failure, you can determine the lack of function.

Fourth, the abnormal precision machining fault diagnosis and processing examples

1. Mechanical failure lead to abnormal machining accuracy

Symptom: A SV-1000 vertical machining center, the use of Frank system. In the process of connecting rod mold, suddenly found that the Z-axis feed abnormalities, resulting in at least 1mm cutting error (Z-direction overcut).

Fault diagnosis: The investigation learned that the fault occurred suddenly. The machine is in jog mode. Under the mode of manual input data, each axis runs normally, and the reference point is normal. There is no alarm and the possibility of hard fault of the electric control part is eliminated. Should mainly check the following aspects one by one.

Check the machining block that is running when the machine accuracy is abnormal, especially the tool length compensation, proofreading and calculation of the machining coordinate system (G54-G59).

In the jogging mode, the Z-axis motion was repeated. After seeing, touching and listening, it was found that the Z-direction motion noise was abnormal, especially when the jog was fast and the noise was more obvious. Judging from this, there may be mechanical hazards.

Check machine Z axis accuracy. Move the Z-axis with a hand-pulse generator (set its magnification to a 1 × 100 gear, that is, change every step, the motor feeds 0.1mm) and observe the motion of the Z-axis with a dial indicator. After the unidirectional motion is normal, the motor moves forward as a starting point. Each time the pulsator changes, the actual distance of the Z axis of the machine tool d = d1 = d2 = d3 = ... = 0.1mm, which indicates that the motor runs well and the positioning accuracy is good . The return to the machine tool changes in the actual movement can be divided into four stages: (1) machine tool motion distance d1> d = 0.1mm (slope greater than 1); (2) showed d1 = 0.1mm> d2> d3 The slope is less than 1); (3) the machine mechanism does not actually move, showing the most standard backlash; (4) the machine movement distance equals to the fixed value of the pulsator (the slope is equal to 1) and returns to the normal movement of the machine tool. No matter how the backlash compensation, the characteristics of its performance is: In addition to (3) stage of compensation, the other sections of the changes still exist, in particular, (1) stage seriously affect the machining accuracy. Compensation found that the greater the gap compensation, (1) the greater the distance of phase movement.

Analysis of the above examination that there are several possible reasons: First, the motor is abnormal, the second is a mechanical failure, the third is the existence of lead screw clearance. In order to further diagnose the fault, the motor and the screw completely disconnected, respectively, the motor and mechanical parts were inspected. The test results are normal motor operation; found in the mechanical part of the diagnosis, hand-driven screw, the return of the initial movement has a great sense of vacancies. Under normal circumstances, should be able to feel the orderly and smooth movement of the bearings.

Troubleshooting: After demolition inspection found that the bearing is indeed damaged, and the ball off. Machine replacement returned to normal.

2. Control logic is not properly lead to abnormal machining accuracy

Symptom: A Shanghai machine tool manufacturer's machining center, the system is Frank. Machining process and found that the machine X axis accuracy anomalies, the minimum accuracy of the error is 0.008mm, the maximum is 1.2mm. Fault diagnosis: check, the machine has been in accordance with the It is required to set G54 workpiece coordinate system. In the manual input data mode operation, run a program in the G54 coordinate system as "GOOG90G54X60.OY70.OF150; M30;", the machine coordinate value displayed on the display after the running of the machine tool is (X axis) "-1025.243" Under this value. Then in the manual mode, the machine will be jogged to any other position, again in the manual input data mode operation just block, until the machine stopped, then found the machine coordinate value is displayed as "-1024.891", the same as the last execution After the comparison of the value of a difference of 0.352mm. In the same way, the X-axis jog will be moved to a different location, repeated execution of the program segment, and the display values ​​?? are different (unstable). Using a dial indicator on the X-axis to conduct a careful examination and found that the mechanical position of the actual error with the digital display of the error is basically the same, so that the cause of the error is the X axis repeat positioning error is too large. The X-axis backlash and positioning accuracy of the inspection, re-compensate for the error value, the results have no effect. So I suspect grating ruler and system parameters and other problems. But why have such a big error, but did not appear the corresponding alarm information Further examination found that this axis is the vertical axis, when the X-axis down the headstock down, resulting in error.

Troubleshooting: Machine PLC logic control program has been modified, that is, in the X-axis release, the first X-axis enable loading, and then release the X-axis; and X-axis clamping, the first X-axis clamp Tight, then enable to get rid of. After the adjustment machine fault can be solved.

3. Machine location problems lead to abnormal processing accuracy

Symptoms: a Hangzhou-made vertical CNC milling machine, equipped with Beijing KND-10M system. Z-axis abnormalities were found during jogging or machining.

Troubleshooting: Inspection found that the Z axis moves unevenly and noisy, and there is a certain gap. When the motor is started, there is unstable noise and uneven force in the Z-axis upward movement in the jog mode, and the motor jitter is more severe. When the downward movement is performed, no jittering is so obvious. When the jitter is not stopped, In the processing of the performance is more obvious. Analysis, the fault has three reasons: First, a large screw reverse gap; Second, Z axis motor abnormalities; Third, the pulley is damaged to uneven force. However, there is a problem to note is that when the stop does not shake, up and down movement is not uniform, so the motor can not work abnormal problems. Therefore, the first mechanical part of the diagnosis, in the diagnostic test did not find abnormalities within the tolerance. The use of exclusion rules, the only remaining belt problem, when the detection of the belt, found that the belt just for a short while, but when careful examination of the belt, the belt was found to varying degrees of damage inside, it is clear that the uneven force , What causes it? In the diagnosis found that the motor placed in question, that is, the angle of the clamping position caused by uneven force.

Troubleshooting: As long as the motor reassembly, alignment angle, measuring the distance (motor and Z axis bearings), both sides of the belt (length) should be uniform. In this way, the Z axis moves up and down unevenly and noise and jitter phenomenon is eliminated, Z axis machining back to normal.

4. The system parameters are not optimized, the motor is running abnormally

Leading to abnormal machining accuracy System parameters include machine feed unit, zero offset, backlash and so on. For example, Frank CNC system, the feed unit metric and imperial two. In the machine tool repair process for local processing, often affect the zero offset and gap changes, troubleshooting should be made after the timely adjustment and modification; the other hand, due to serious mechanical wear or loose connection may also result in measured parameters Changes in the need to make the appropriate changes to the parameters to meet the machining accuracy requirements.

Symptoms: a Hangzhou-made vertical CNC milling machine, equipped with Beijing KND-10M system. In the process of processing, we found abnormal X-axis accuracy.

Troubleshooting: Check the X-axis there is a certain gap, and there is instability when the motor starts. When you touch the X-axis motor by hand, the motor will feel more powerful and will not pull obviously when it stops. Especially when the jog mode is used, it is obvious. Analysis, the fault there are two reasons: First, a large anti-clearance screw; second is the X-axis motor abnormalities.

Troubleshooting: The use of KND-10M system parameters of the function, the motor debugging. First, the existence of the gap to compensate, and then adjust the servo system parameters and pulse suppression function parameters, X-axis motor jitter elimination, machine tool processing accuracy back to normal.

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