What causes the abnormal machining accuracy of CNC machine tools?

In production, it often encounters faults in the machining accuracy of CNC machine tools. Such faults are highly concealed and difficult to diagnose. The main causes of such failures are the following:

1) The machine feed unit has been changed or changed

2) The zero offset (NULLOFFSET) of each axis of the machine tool is abnormal.

3) Axial backlash (BACKLASH) anomaly

4) The motor is running abnormally, that is, the electrical and control parts are faulty.

5) In addition, the preparation of the machining program, the selection of the tool and the human factors may also result in 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 feed units are metric and imperial. Selecting the quality machine tool accessories to look for Titanium machinery, some processing in the process of machine tool repair, often affect the zero offset and gap changes, the fault should be adjusted and modified in time; on the other hand, due to mechanical wear or serious Looseness may also cause changes in the measured values ​​of the parameters. It is necessary to modify the parameters accordingly to meet the machining accuracy requirements of the machine.

2, the machining accuracy caused by mechanical failure is abnormal

A THM6350 horizontal machining center uses the FANUC0i-MA CNC 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). In the investigation, the fault occurred suddenly. 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 tool accuracy is abnormal, especially the tool length compensation and the machining coordinate system (G54 to G59) for proofreading and calculation.

(2) In the jog mode, the Z-axis is repeatedly moved, and the motion state is diagnosed by visual, touch, 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 tool. 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 movement of the machine tool d=d1=d2=d3...=0.1mm, which means that the motor runs well and the positioning accuracy is good.

Returning to the change of the actual motion 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 d=0.1mm>; 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 equals 1), and it returns to the normal movement of the machine tool.

No matter how the backlash (parameter 1851) is compensated, the characteristic is that, besides the third stage can compensate, 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 inspection, CNC mechanic training believes that there are several possible reasons: First, the motor has an abnormality; Second, there is a mechanical failure; Third, there is a certain gap. Select the quality machine tool accessories to look for Titanium, in order to further diagnose the fault, completely disconnect the motor and the lead screw, and inspect the motor and mechanical parts 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 bearing should be able to feel the orderly and smooth movement. After dismantling, the bearing was found to be damaged and a ball fell off. The machine returns to normal after replacement.

3, the machine tool electrical parameters are not optimized motor operation is abnormal

A CNC vertical milling machine with FANUC0-MJ CNC system. During the machining process, the X-axis accuracy was found to be abnormal. The inspection found that there was a certain gap in the X-axis and there was instability when the motor was started. When the X-axis motor is touched by hand, the motor shake is more serious, and it is not obvious when starting and stopping. It is more obvious in JOG mode.

The analysis believes that there are two reasons for the failure, one is that the mechanical backlash is large; the other is that the X-axis motor is working abnormally. Use the parameter function of the FANUC system to debug the motor. Firstly, the existing gap is compensated; the servo gain parameter and the N-pulse suppression function parameter are adjusted, the jitter of the X-axis motor is eliminated, and the machining accuracy of the machine tool returns to normal.

4, machine tool position loop abnormality or control logic is not appropriate

A TH61140 boring and milling machine machining center, CNC system is FANUC18i, full closed loop control. During the machining process, the Y-axis accuracy of the machine tool was found to be abnormal, the precision error was about 0.006mm, and the maximum error was 1.400mm. During the inspection, the machine tool has set the G54 workpiece coordinate system as required. In the MDI mode, a program running “G90G54Y80F100; M30;” is run in the G54 coordinate system. The mechanical coordinate value displayed on the display after the standby bed is over is “-1046.605”, and the value is recorded. Then, in the manual mode, the Y axis of the machine tool is jogged to any other position, and the above statement is executed again in the MDI mode. After the standby bed is stopped, it is found that the machine coordinate number of the machine tool is "-1046.992", the same as the first The number of displayed values ​​after the second execution is 0.387 mm. In the same way, the Y-axis is jogged to a different position, and the statement is executed repeatedly, and the numerical display value is indefinite. Using the dial indicator to detect the Y-axis, it is found that the actual error of the mechanical position is basically the same as the error displayed by the digital display. Therefore, the cause of the fault is that the Y-axis repeat positioning error is too large. Careful inspection of the backlash and positioning accuracy of the Y-axis and re-compensation have no effect. Therefore, there is a problem with the grating scale and system parameters, but why is such a large error, but the corresponding alarm information does not appear? Further inspection revealed that the axis was a vertical axis. When the Y-axis was released, the headstock fell down, causing an out-of-tolerance.

The PLC logic control program of the machine tool has been modified, that is, when the Y-axis is released, the Y-axis is first enabled to be loaded, and then the Y-axis is released; and when clamping, the shaft is first clamped and then Y is clamped. The axis is enabled to be removed. The machine fault has been resolved after adjustment.