In production, we often encounter troubles with abnormal machining accuracy of Heavy duty cnc milling machine. Such faults are highly concealed and difficult to diagnose. The main reasons for this type of failure are as follows:
1) The feed unit of the machine tool is changed or changed
2) The zero offset (NULLOFFSET) of each axis of the machine tool is abnormal
3) Abnormal axial backlash (BACKLASH)
4) The motor is running abnormally, that is, the electrical and control parts are faulty
5) In addition, the preparation of processing programs, the selection of tools and human factors may also cause abnormal processing accuracy.
1. Change or modification of system parameters
The system parameters mainly include machine tool feed unit, zero offset, backlash and so on. For example, SIEMENS and FANUC CNC systems have two feed units: metric and imperial systems. Choose quality machine tool accessories and look for Taihao Machinery. Some treatments during machine tool repair often affect the changes of zero offset and clearance. After troubleshooting, you should make timely adjustments and modifications; on the other hand, due to severe mechanical wear or connection Looseness may also cause changes in the measured values of the parameters, and the parameters need to be modified accordingly to meet the requirements of the machining accuracy of the machine tool.
2. Abnormal machining accuracy caused by mechanical failure
A 0 horizontal machining center adopts FANUC0i-MA CNC system. Once in the process of milling a steam turbine blade, it was suddenly found that the Z-axis feed was abnormal, causing a cutting error of at least 1 mm (Z-direction overcut). During the investigation: the failure occurred suddenly. In the jog and MDI operation modes, each axis of the machine tool runs normally, and the reference point return is normal; there is no alarm prompt, and the possibility of a hard fault in the electrical control part is eliminated. The analysis believes that the following aspects should be inspected one by one.
(1) Check the machining program segment that is 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 was moved repeatedly, and the motion state was diagnosed by sight, touch, and hearing. It was found that the Z-direction motion sound was abnormal, especially the rapid jog, and the noise was more obvious. Judging from this, there may be hidden dangers in machinery.
(3) Check the accuracy of the Z axis of the machine tool. Move the Z-axis with the MPG generator (set the MPG ratio to 1×100 gear, that is, the motor feeds 0.1mm for each step), and observe the movement of the Z-axis with the dial indicator. After the one-way movement accuracy remains normal, it is the positive movement as the starting point. Every time the MPG changes one step, the actual distance of the machine Z axis movement d=d1=d2=d3...=0.1mm, indicating that the motor is running well and the positioning accuracy is good.
The return to the actual movement displacement of the machine tool can be divided into four stages:
①The machine tool movement distance d1>d=0.1mm (the slope is greater than 1);
② Showed as d=0.1mm>; d2>d3 (the slope is less than 1);
③The machine tool mechanism does not actually move, showing the most standard backlash;
④The machine tool movement distance is equal to the MPG given value (the slope is equal to 1), and the machine returns to the normal movement.
No matter how the backlash (parameter 1851) is compensated, the characteristic is: in addition to the compensation in the third stage, other changes still exist, especially in the first stage, which seriously affects the machining accuracy of the machine tool. It is found in the compensation that the greater the gap compensation, the greater the movement distance of paragraph ①.
Analyzing the above inspections, CNC technician training believes that there are several possible reasons: one is that the motor is abnormal; the other is mechanical failure; and the third is that there is a certain gap. Choose quality machine tool accessories to look for Taihao. In order to further diagnose the fault, the motor and the lead screw are completely disconnected, and the motor and mechanical parts are checked separately. The motor is operating normally; in the diagnosis of the mechanical part, it is found that when the screw is turned by hand, there is a very obvious sense of vacancy at the beginning of the return movement. Under normal circumstances, you should be able to feel the orderly and smooth movement of the bearing. After disassembly and inspection, it was found that the bearing was indeed damaged, and one ball fell off. After the replacement, the machine tool returns to normal.
3. The electrical parameters of the machine tool are not optimized. The motor runs abnormally.
CNC car
A CNC vertical milling machine, equipped with FANUC0-TF CNC system. During the machining process, it was found that the X-axis accuracy was abnormal. The inspection found that there is a certain gap in the X axis, and the motor is unstable when it starts. When I touch the X-axis motor by hand, I feel that the motor shakes more seriously, and it is not obvious when starting and stopping, and it is more obvious in JOG mode.
Analysis believes that there are two reasons for the failure, one is the large mechanical backlash; the other is the abnormal operation of the X-axis motor. Use the parameter function of FANUC system to debug the motor. First, the existing gap was compensated; the servo gain parameters and the N pulse suppression function parameters were adjusted, the jitter of the X-axis motor was eliminated, and the machining accuracy of the machine tool returned to normal.
4. The position loop of the machine tool is abnormal or the control logic is incorrect
A boring and milling machine machining center, the CNC system is FANUC18i, fully closed loop control mode. During the machining process, it was found that the accuracy of the Y-axis of the machine tool was abnormal. The minimum accuracy error was about 0.006mm, and the large error could reach 1.400mm. During the inspection, the machine tool has set up the G54 workpiece coordinate system as required. In MDI mode, run a program in the G54 coordinate system, namely "G90G54Y80F100; M30;", the machine coordinate value displayed on the display after the standby bed runs is "-1046.605", record this value. Then in manual mode, jog the Y axis of the machine tool to any other position, execute the above statement again in MDI mode, after the standby bed stops, it is found that the digital display value of the machine coordinate is "-1046.992" at this time, and execute it at the same time The latter number shows a difference of 0.387mm compared to the value. In the same way, jog the Y-axis to a different position, execute the sentence repeatedly, and the value of the digital display is uncertain. The Y-axis was inspected with a dial indicator, and it was found that the actual error of the mechanical position was basically the same as the error displayed by the digital display, so it was considered that the cause of the failure was that the Y-axis repeated positioning error was too large. Carefully check the backlash and positioning accuracy of the Y-axis, and recompensate, but it will have no effect. Therefore, it is suspected that there is a problem with the grating ruler and system parameters, but why such a large error occurs, but the corresponding alarm message does not appear? Further inspection found that this axis is a vertical axis. When the Y axis is loosened, the headstock will drop downwards, causing an out-of-tolerance.
