The malfunction of the machining precision of CNC machine tools is often encountered in the production. This kind of fault has strong concealment and difficulty in diagnosis. There are five main reasons for this kind of fault.
(1) the machine tool feed unit has been changed or changed.
(2) the zero point bias (NULLOFFSET) anomaly of each axis of the machine tool.
(3) abnormal reverse clearance (BACKLASH) in the axial direction.
(4) the motor operating state is abnormal, that is, electrical and control parts of the fault.
(5) mechanical faults, such as wire rod, bearing, shaft coupling and other components. In addition, the preparation of the processing program, the choice of the tool and the human factors may also lead to the abnormal machining precision.
1. abnormal machining precision caused by mechanical failure
A THM6350 horizontal processing center, using the FANUC0i-MA CNC system. At one time, during the process of milling turbine blades, the abnormal feed of Z axis was suddenly found, causing at least 1mm cutting error (Z over cutting). It was found in the survey that the failure occurred suddenly. The machine is running normally under the way of point operation and MDI operation, and the reference points are normal. Without any alarm, the possibility of hard failure of the electrical control part is excluded. It is believed that the following aspects should be examined one by one.
(1) check the process section of the normal operation when the precision of the machine tool is abnormal, especially the proofreading and calculation of the tool length compensation and the machining coordinate system (G54 to G59).
(2) in the way of point movement, the Z axis of repeated movement is diagnosed by movement, state of sight, touch and listen. It is found that Z is abnormal to motion sound, especially fast moving, and the noise is more obvious. It can be judged that there may be a hidden danger in the mechanical aspects.
(3) check the accuracy of the Z axis of the machine tool. Hand pulse generator shaft (mobile Z, hand pulse rate set at 1 x 100 gear, which changes every step motor, 0.1mm), with the motion of the Z axis observation indicator. The actual distance of the Z axis motion of the machine tool is d=d1=d2=d3 when the one-way motion accuracy is kept normal as the forward motion of the starting point. =0.1mm, indicating that the motor is running well and the positioning accuracy is good. Change and return machine actual displacement, can be divided into four stages: the machine movement distance of D1 and d=0.1mm (the slope is greater than 1); the show was d=0.1mm > D2 > D3 (less than 1); the actual machine does not move, showing the reverse gap the standard; the machine tool movement distance with the hand vein (equal to the given value is equal to the slope of 1), restored to the normal movement of the machine.
No matter how to compensate for the backlash (parameter 1851), it shows the following characteristics: in addition to stage 3, it can compensate for all the other changes, especially in the first stage, which seriously affects the machining accuracy of machine tools. It is found that the greater the gap compensation is, the greater the movement distance of the first section is.
Analysis of the above inspections, numerical control technician training that there are several possible reasons: first, the motor is abnormal; the two is mechanical failure; three, there is a certain gap. In order to further diagnose the fault, the motor and the screw are completely removed, and the motor and the mechanical part are checked respectively.
The motor runs normally; in the diagnosis of the mechanical part, there is a very obvious vacancy in the movement of the motor when the hand disc is moved. In normal circumstances, it should be able to feel the orderly and smooth movement of the bearing. It was found that the bearings had been damaged and a ball fell off. After the replacement, the machine is restored to normal.
2. change or change of system parameters
The system parameters mainly include machine tool feed unit, zero point bias, reverse gap and so on. For example, SIEMENS, FANUC CNC system, its feed unit has two kinds of public system and British system. Some of the processing machine repair process, often affect the change of zero bias and fault clearance, should be dealt with timely adjustment and modification; on the other hand, due to mechanical wear or links may also be caused by loose change parameters of the actual value, need to be revised accordingly to meet the requirements of precision machining parameters.
3. electrical parameters of machine tool do not optimize motor operation abnormality
A numerical control vertical milling machine, equipped with FANUC0-MJ numerical control system. In the process of processing, it is found that the accuracy of the X axis is abnormal. It is found that there is a certain gap in the X axis, and there is an unstable phenomenon when the motor is started. When touching the X axis motor with hand, the jitter of the motor is serious, it is not obvious when starting and stopping, and it is more obvious in the way of JOG.
After guiding system parameter changes or changes, the electrical parameters of mechanical failure, the machine without abnormal operation, optimization of motor position loop machine control logic is nothing wrong or unusual, is a common cause of the machining precision of NC machine tools in the production of abnormal fault, find fault and processing, the machine can be returned to normal.
The analysis shows that there are two reasons for the fault, one is the large mechanical reverse gap, and two is the abnormal working of the X axis motor. Using the parameter function of the FANUC system, the motor is debugged. First, the gap is compensated, and the servo gain parameters and N pulse suppression function parameters are adjusted. The X axis motor's jitter is eliminated, and the machining accuracy of machine tool returns to normal.