Machining center machining accuracy abnormality troubleshooting - Database & Sql Blog Articles

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In production, it is often encountered that the machining accuracy of CNC machine tools is abnormal. Such faults are highly concealed and difficult to diagnose.
There are five main reasons for this type of failure:
(1) The machine feed unit has been changed or changed.
(2) The zero offset (NULL OFFSET) of each axis of the machine tool is abnormal.
(3) The axial backlash (BACKLASH) is abnormal.
(4) The motor is in an abnormal state, that is, the electrical and control parts are faulty.
(5) Mechanical failure, such as screw, bearing, shaft coupling and other components. In addition, the programming of the machining program, the selection of the tool and the human factors may also lead to abnormal machining accuracy.
1. System parameters change or change System parameters mainly include machine feed unit, zero offset, backlash and so on. For example, SIEMENS, FANUC numerical control system, its feeding units are metric and imperial. Some processes in the repair process of the machine tool often affect the change of the zero offset and the gap. The fault should be adjusted and modified in time according to the fault; on the other hand, the mechanical measured value may be changed due to serious mechanical wear or loose connection. It is necessary to make corresponding modifications to the parameters to meet the requirements of machine tool machining accuracy.
2. Abnormal machining accuracy caused by mechanical failure A THM6350 horizontal machining center adopts FANUC 0i-MA numerical control system. During the milling of the turbine blades, the Z-axis feed anomaly was suddenly found, resulting in a cutting error of at least 1 mm (Z-direction overcut). The investigation found that the fault was sudden. The machine runs normally in the jog and MDI modes, and the reference point is normal; there is no alarm prompt, and the possibility of hard fault in the electrical control part is excluded. The analysis believes that the following aspects should be checked one by one.
1) Check the machining program segment that is running when the machine accuracy is abnormal, especially the tool length compensation and the machining coordinate system (G54~G59) for proofreading and calculation.
2) In the jog mode, the Z-axis is repeatedly moved, and the motion state is diagnosed by visual, tactile, and listening. It is found that the Z-direction motion sound is abnormal, especially the fast jog, and the noise is more obvious. Judging from this, there may be hidden dangers in the mechanical aspect.
3) Check the Z-axis accuracy of the machine. The Z-axis is moved by the hand pulse generator. (The hand pulse magnification is set to 1×100, that is, the motor feeds 0.1mm for each step change), and the movement of the Z-axis is observed with the dial indicator. After the one-way motion accuracy remains normal, the forward motion as the starting point, the actual distance of the Z-axis motion of the machine tool is d=d1=d2=d3...=0.1mm, which means that the motor runs well and the positioning accuracy is good. Returning to the actual movement displacement of the machine tool can be divided into four stages:
1 machine tool movement distance d1>d=0.1mm (slope is greater than 1);
2 shows as d = 0.1 mm > d2 > d3 (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 hand pulse set value (slope is equal to 1), and the normal motion of the machine tool is restored.
Regardless of how the backlash (parameter 1851) is compensated, the characteristic is that, besides the third stage can compensate, the other sections still exist, especially the first stage seriously affects the machining accuracy of the machine tool. In the compensation, it is found that the larger the gap compensation, the larger the moving distance of the first segment.
Analysis of the above inspections suggests that there are several possible reasons: First, the motor has an abnormality; second, there is a mechanical failure; third, there is a certain gap. In order to further diagnose the fault, the motor and the lead screw are completely disengaged, and the motor and the mechanical part are inspected separately. The motor is running normally; in the diagnosis of the mechanical part, it is found that when the screw is moved by hand, the return movement initially has a very obvious feeling of vacancy. Under normal circumstances, the bearings should be able to feel the orderly and smooth movement. After dismantling, it was found that the bearing was damaged and there was a ball falling off. The machine returns to normal after replacement.