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Which causes the abnormal malfunction of machining precision of NC machine tool


 Source: International Metalworking Network


In the production often will encounter the numerical control machine tool processing precision fault. This kind of fault has strong concealment and difficulty in diagnosis. The main causes of such failures are:

1)machine feed unit changed or changed
2)Abnormal zero offset (NULLOFFSET) of each axis of the machine tool
3)Abnormal axial backlash (BACKLASH)
4) Abnormal operating state of motor, i.e. electrical and control part failure
5)In addition, machining programming, tool selection and human factors may also lead to abnormal machining accuracy.

1.Changes or changes in system parameters

System parameters mainly include machine tool feed unit, zero offset, backlash and so on. For example, SIEMENS, FANUC numerical control system, its feed unit has two kinds of metric system and imperial system. Select high-quality machine tool accessories to identify Tihao Machinery. Some treatments during machine tool repair often affect the zero offset and gap changes. After the fault is processed, it should be adjusted and modified in a timely manner; on the other hand, due to severe mechanical wear or connection Loosening may also cause changes in the actual measured values of the parameters, and the parameters need to be modified accordingly to meet the requirements of machine tool machining accuracy.


2.Abnormal machining accuracy caused by mechanical failure

One THM6350 horizontal machining center with FANUC0i-MA CNC system. Once in the process of milling the turbine blades, it is suddenly found that the Z shaft feed anomaly causes at least 1 mm of cutting error (Z overcutting). Investigation: the fault happened suddenly. Each axis of the machine tool is running normally under the mode of point movement and MDI operation, and the return reference point is normal; without any alarm warning, the possibility of hard fault in the electrical control part is eliminated. According to the analysis, the following aspects should be examined one by one.

(1)Check the machining program segments that are running when the machine tool accuracy is abnormal, especially the tool length compensation, the proofreading and calculation of the machining coordinate system (G54~G59).

(2)In the jog mode, the Z axis is repeatedly moved, and the movement state is diagnosed through sight, touch, and hearing, and the sound of the Z-direction movement is found to be abnormal, especially when the jog is fast, the noise is more obvious. Judging from this, there may be hidden dangers in mechanical aspects.

(3)Check the accuracy Z the machine tool shaft. move the Z axis with the hand pulse generator (set the hand pulse rate as a retaining position of 1×100, i.e., the motor feed 0.1 mm per step change), and observe the movement of the Z axis with the percentile. the forward motion as the starting point after the one-way motion accuracy remains normal. for each step of the hand pulse change, the actual distance of the machine tool Z axis motion d=d1=d2=d3…=0.1 mm, indicating that the motor is running well and the positioning accuracy is good.

① The change of the actual moving displacement of the machine tool can be divided into four stages:

②The moving distance d1>d=0.1 mm( slope greater than 1);

③showing d=0.1 mmd>; d2d>d3( slope less than 1);

④ The machine tool mechanism has not actually moved, showing the most standard reverse clearance;

The moving distance of the machine tool is equal to the given value of the hand pulse (the slope is equal to 1), and the normal movement of the machine tool is restored.

Analyzing the above inspection, NC mechanic training thinks that there are several possible reasons: first, the motor has abnormal; second, there is a mechanical fault; third, there is a certain gap. Select the quality machine tool accessories to identify Tihao, in order to further diagnose the fault, the motor and the lead screw are completely removed, and the motor and mechanical parts are checked respectively. The motor is running normally; in the diagnosis of the mechanical part, it is found that when the lead screw is moved by hand disc, the initial return motion has a very obvious vacancy feeling. And normally, the bearing should be able to feel orderly and smooth movement. It was found that the bearing had been damaged and one ball had fallen off. the machine tool returned to normal after replacement.


3.Electrical parameters of machine tools are not optimized.

A CNC vertical milling machine, equipped with FANUC0-MJ CNC system. during machining, abnormal accuracy of X axis was found. Check found that there is a certain gap in the X shaft, and there is instability when the motor starts. When touching the X shaft motor by hand, it feels that the motor jitter is more serious, the start and stop is not obvious, and the JOG mode is more obvious.

According to the analysis, there are two reasons for the failure, one is that the mechanical reverse clearance is large, the other is that the X shaft motor is abnormal. By using the parameter function of the FANUC system, the motor is debugged. The existing gap is compensated first, and the servo gain parameters and N pulse suppression function parameters are adjusted to X the jitter of the shaft motor is eliminated, and the machining accuracy of the machine tool is restored to normal.


4.machine tool position ring abnormal or control logic improper

One TH61140 boring and milling machine machining center, CNC system for FANUC18i, full closed loop control mode. The machining process found that the machine tool Y shaft accuracy abnormal, the minimum accuracy error is about 0.006 mm, the maximum error can reach 1.400 mm. in the inspection, the machine tool has set the G54 workpiece coordinate system according to the requirements. Under the MDI mode, run a program in the G54 coordinate system that is “G90G54Y80F100;M30;”, the mechanical coordinate value displayed on the display after the machine tool is running is “-1046.605”, record the value. Then hand in hand Move the machine tool Y axis point to any other position, and execute the above statement again in the MDI mode. After the machine tool stops, it is found that the mechanical coordinate digital display value of the machine tool is “-1046.992”, which is 0.387 mm. different from the number display value after the first execution. According to the same method, move the Y axis points to different positions, execute the statement repeatedly, the indication value of digital display is uncertain. By measuring the Y axis with a percentile, it is found that the actual error of the mechanical position is basically consistent with the error displayed by the digital display, so it is considered that the fault is caused by the Y axis repeated positioning error is too large. Reverse clearance and positioning accuracy of Y axis Careful examination and compensation are ineffective. Therefore, it is suspected that there are problems such as grating ruler and system parameters, but why is there such a large error, but there is no corresponding alarm information? Further inspection found that the shaft is the shaft in the vertical direction, and when the Y shaft is loosened, the spindle box falls down, resulting in an excess difference.

The PLC logic control program of the machine tool has been modified, that is, when the Y shaft is loosened, the Y shaft can be loaded first, then the Y shaft is loosened; while when clamping, the shaft is clamped first, then the Y shaft can be removed. the machine tool fault can be solved after adjustmen