Reason for the Sudden Decrease in CNC Machine Accuracy?

Reasons for Abnormal Processing Accuracy:

The machine tool feed unit is modified or changed;

The zero offset of each axis of the machine tool is abnormal;

Abnormal axial backlash;

The motor operating status is abnormal, that is, the electrical and control parts are abnormal;

Mechanical failure, such as screws, bearings, couplings and other components.

In addition, the preparation of processing programs, tool selection and human factors may also lead to abnormal processing accuracy.

CNC Machine fault diagnosis principles:

1. First look at the outside and then the inside. CNC machine are machine tools that integrate mechanics, hydraulics, and electricity, so the occurrence of faults will also be comprehensively reflected by these three. Maintenance personnel should first conduct inspections one by one from the outside to the inside, and try to avoid random unpacking and disassembly. Otherwise, the fault will be magnified, the machine tool will lose accuracy and performance will be reduced.

2. Mechanical first, then electrical. Generally speaking, mechanical faults are easier to detect, while the diagnosis of CNC system faults is more difficult. Before troubleshooting, first pay attention to eliminating mechanical faults, which can often achieve twice the result with half the effort.

3. Static first and then dynamic. First, in the static state of the machine tool with power off, through understanding, observation, testing and analysis, the machine tool can only be powered on after confirming that it is a non-destructive fault; under operating conditions, conduct dynamic observation, Inspect and test to find faults. For destructive faults, the danger must be eliminated before power can be turned on.

4. Simple first, then complex. When multiple faults are intertwined and covered up, and you have no idea where to start, you should solve the easy problems first and then the more difficult ones. Often after simple problems are solved, difficult problems may become easier.

CNC Machine Fault Diagnosis Methods:

1. Intuitive method (look, hear, ask and guess) to ask - fault phenomena of machine tools, processing conditions, etc.; look - CRT alarm information, alarm lights, capacitors and other compon

ents deformation, smoke and burnt, protector tripping, etc.; listen - abnormal sounds ; Smell - the smell of burnt electrical components and other odors; touch - heat, vibration and poor contact, etc.

2. Parameter checking method Parameters are usually stored in RAM. Sometimes the battery voltage is insufficient, 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: 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. Similar replacement method uses a spare board with the same function to replace the suspected faulty module, or exchanges modules or units with the same function.

5. Functional program testing method: Write some small programs for all the instructions of G, M, S, and T functions. Run these programs when diagnosing faults to determine the lack of functions.

Abnormal Machining Accuracy Fault Diagnosis and Examples

1. Mechanical Failure Causes Abnormal Processing Accuracy

Fault phenomenon: A vertical machining center using Frank system. During the process of processing the connecting rod mold, it was suddenly discovered that the Z-axis feed was abnormal, causing a cutting error of at least 1mm (overcutting in the Z direction).

Fault diagnosis: During the investigation, it was learned that the fault occurred suddenly. The machine tool is jogging, and each axis operates normally under manual data input mode, and returns to the reference point normally. There is no alarm prompt, and the possibility of a hard fault in the electrical control part is eliminated. The following aspects should be mainly checked one by one.

Check the processing program segment that is running when the machine tool accuracy is abnormal, especially the tool length compensation, calibration and calculation of the processing coordinate system (G54-G59).

In the jog mode, the Z-axis is repeatedly moved, and the movement status is diagnosed through sight, touch, and listening. It is found that the noise in the Z-direction movement is abnormal, especially in rapid jog, the noise is more obvious. Judging from this, there may be hidden dangers in machinery.

Check the Z-axis accuracy of the machine tool. Move the Z-axis with a hand-operated pulse generator (set the magnification to 1×100, that is, the motor feeds 0.1mm for each step), and observe the movement of the Z-axis with a dial indicator. After the one-way motion remains normal, it serves as the starting point of forward motion. Every time the pulser changes 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 running well and the positioning accuracy is also good. good.

Returning to the changes in the actual movement displacement of the machine tool, it can be divided into four stages: (1) The movement distance of the machine tool d1>d=0.1mm (slope is greater than 1); (2) The expression is d1=0.1mm>d2>d3 ( The slope is less than 1); (3) The machine tool mechanism does not actually move, showing the most standard backlash; (4) The machine tool movement distance is equal to the predetermined value of the pulser (slope is equal to 1), and the machine tool returns to normal motion.

No matter how the backlash is compensated, its characteristic is that except for the compensation in stage (3), changes in other stages still exist, especially stage (1), which seriously affects the machining accuracy of the machine tool. During the compensation, it was found that the greater the gap compensation, the greater the distance moved in stage (1). ​

After analyzing the above inspection, it is believed that there are several possible reasons: first, there is an abnormality in the motor, 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 motor is running normally; during the diagnosis of the mechanical part, it was found that when the screw is turned by hand, there is a large sense of vacancy at the beginning 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 off. The machine returned to normal after replacement.

2. CNC Machine Position Problems Lead to Abnormal Processing Accuracy

Fault phenomenon: A vertical CNC milling machine equipped with Beijing KND-10M system. During jogging or machining, Z-axis abnormality was found. ​

Fault diagnosis: The 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 when the Z-axis moves upward in the jogging mode, and the motor feels shaking. When moving downward, the shaking is not so obvious; there is no shaking when stopping. It is more obvious during the processing.

There are three reasons for the failure: first, the screw backlash is very large; second, the Z-axis motor is working abnormally; third, the pulley is damaged to the point of uneven stress.

But one thing to note is that there is no jitter when stopping and the up and down motion is uneven, so the problem of abnormal motor operation can be eliminated. Therefore, the mechanical part is diagnosed first, and no abnormalities are found during the diagnostic test, which is within the tolerance. Using the rule of elimination, the only remaining problem was the belt. When inspecting 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 on the inner side of the belt. It was obviously caused by uneven force. , what is the cause? During the diagnosis, it was found that there was a problem with the placement of the motor, that is, the angular position of the clamping was asymmetrical, causing uneven stress. ​

Troubleshooting: Just reinstall the motor, align the angle, measure the distance (motor and Z-axis bearing), and make sure the belt (length) is even on both sides. In this way, the uneven movement of the Z-axis up and down along with the noise and jitter are eliminated, and the Z-axis machining returns to normal.

3. The system parameters are not optimized and the motor operates abnormally.

The system parameters that lead to abnormal machining accuracy mainly include machine tool feed unit, zero offset, backlash, etc. For example, the Frank CNC system has two feed units: metric and imperial. During the machine tool repair process, local processing often affects changes in zero offset and clearance. Timely adjustments and modifications should be made after the fault is resolved. On the other hand, serious mechanical wear or loose connections may also cause the measured parameter values to change. Changes in parameters require corresponding modifications to meet the requirements of machine tool processing accuracy.

Fault phenomenon: A vertical CNC milling machine equipped with KND-10M system. During the machining process, it was found that the X-axis accuracy was abnormal.

Fault diagnosis: The inspection found that there is a certain gap 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 the inching mode. Analysis believes that there are two reasons for the failure: first, the screw backlash is very large; second, the X-axis motor is working abnormally.

Troubleshooting: Use the parameter function of the KND-10M system to debug the motor. First, the existing gap 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 machine tool processing accuracy returns to normal.