CNC Knowledge: Why does the precision of machine tools suddenly decrease?

1. Causes of abnormal processing accuracy failures

The causes of abnormal machining precision defects are very hidden and difficult to diagnose. Five main reasons are summarized: the feed unit of the machine tool is modified or changed; the zero offset of each axis of the machine tool is abnormal; abnormal; abnormal operating condition of the engine, i.e. abnormality of electrical and control parts, 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.

2. Principles of fault diagnosis of CNC machine tools

First external then internal CNC machine tool is a machine tool that integrates mechanical, hydraulic and electrical components, so its failure will also be reflected by these three components. 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.

First mechanical, then electrical. Generally speaking, mechanical faults are easier to detect, while 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.

Static first, then dynamic. Firstly, in the static state of the machine tool when the power is turned off, through understanding, observation, testing and analysis, the machine tool can only be turned on after having confirmed that it is not a machine tool. destructive defect; Under operating conditions, carry out dynamic observation, inspection and testing to find faults. In the event of destructive faults, the danger must be eliminated before switching on the voltage.

First simple, 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.

3. Methods for fault diagnosis of CNC machine tools

Intuitive method: (Look, hear and ask) Learn about the machine tool fault phenomenon, processing conditions, etc. ; watch CRT alarm information, alarm lights, capacitors and other components for deformation, smoke and burns, protector tripping, etc. ; listen for abnormal sounds; odor: burning smell and other odors from electrical components; heat to the touch, vibration, poor contact, etc.

 Parameter verification 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. Relevant parameters must be checked and calibrated. depending on the characteristics of the defect.

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, so the isolation method is often used.

The same-swap method uses a spare board with the same function to replace the suspected defective module, or swaps modules or units with the same function.

Functional program testing method: write a few small programs with all the instructions of G, M, S, T functions, run these programs when fault diagnosis to determine the lack of functions.

4. Examples of fault diagnosis and treatment of abnormal machining precision

A. Mechanical failure results in abnormal processing accuracy

Fault phenomenon: An SV-1000 vertical machining center uses the Frank system. During the processing 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 is moved repeatedly. After visual, tactile and auditory diagnosis of its movement state, it is found that the movement noise in the Z direction is abnormal, especially in fast jogging mode, the noise is 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, indicating that the motor is working well and that 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 machine tool movement distance d1>d=0.1 mm (the slope is greater than 1).

(2) It is represented by d1=0.1mm>d2>d3 (slope less than 1).

(3) The machine tool mechanism does not actually move and has the most standard clearance.

(4) The moving distance of the machine tool is equal to the predetermined value of the kicker (the slope is equal to 1), and the machine tool returns to its normal movement.

Regardless of how the clearance is compensated, its characteristic is that except for compensation in step (3), modifications 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.

B. Improper control logic results in abnormal processing accuracy

Fault phenomenon: a machining center produced by a Shanghai machine tool manufacturer, the system is Frank. During the machining process, it was found that the precision of the X axis of the machine tool was abnormal, with a minimum precision error of 0.008mm and one. maximum of 1.2 mm. Fault Diagnosis: During inspection, the machine tool was It is necessary to adjust the coordinate system of the G54 workpiece. In manual data entry mode, run a program in the G54 coordinate system, namely “GOOG90G54X60.OY70.OF150;M30;”. After the operation of the machine tool is completed, the mechanical coordinate value displayed on the screen is (X axis) “-1025.243”, saving Reduce this value. Then, in manual mode, move the machine tool to any other position and run the program segment again in manual data entry mode. After stopping the machine tool, it is found that the coordinate value of the machine tool is displayed as “-1024.891”. , identical to the previous execution. The difference between the final values ​​is 0.352 mm. Follow the same method, move the X axis jog to different positions and run this program segment repeatedly, but the values ​​displayed on the screen. are different (unstable).

Carefully check the X axis with a dial indicator and find that the actual error of the mechanical position is basically consistent with the error displayed by the numbers. Therefore, the cause of the defect is believed to be excessive repeated positioning error of the X-axis. Check the clearance and positioning accuracy of the X axis and compensate the error value again, but it has no effect. Therefore, it is suspected that there is a problem with the network rule and system settings. But why such a large error occurred, but no corresponding alarm message appeared. Further inspection revealed that this axis is a vertical axis and when the X axis is released, the spindle box falls downward causing the error.

Troubleshooting: The PLC logic control program of the machine tool has been changed, that is, when the X axis is loosened, the X axis is activated and loaded first, and then the X axis is loose when the X axis; is tightened, the X axis is tightened first. After that, remove the validation. After adjustment, the problem of the machine tool was solved.

C. 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, an abnormality in the Z axis 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 unstable noise and uneven force in the upward movement of the Z axis in jogging mode, and the motor shakes when it goes down, the shaking is not so obvious when it stops ; shake. This is most evident during the treatment process. The analysis believes that 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 subjected to 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 uneven movement of the Z axis up and down as well as noise and jitter are eliminated, and the Z axis processing returns to normal.

D. 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, then the servo system parameters and pulse suppression function parameters are adjusted. The jitter of the X-axis motor is eliminated, and the machining precision of the machine tool returns to normal.