Manual reference point operation is the premise of establishing the machine coordinate system. The first action of most CNC machine tools after starting up is usually manual operation back to the reference point. If the reference point fails, the program processing will not be possible. The inaccurate position of the reference point will affect the machining accuracy and even a crash. It is necessary to analyze and troubleshoot the problem of returning to the reference point.

1. The main reference point of failure type <br> <br> a CNC machine operator training center has 20 CNC machine tools, and are purchased in batches, many types of systems, training students in the process of operation, has The reference point fault has appeared many times, and it has been eliminated in time through analysis and research.

According to work experience, there are five types of referencing faults on CNC machines with different models and different systems: over-travel and alarm; no reference point, reference point indicator is off; reference point position Unstable; return to the reference point full pitch offset; alarm when returning to the reference point, and there is alarm information.

2. Refer to the reference point troubleshooting method

There are two cases for the occurrence of overtravel and alarm. One case is caused by the position of the coordinate axis being too small from the reference point before the reference point is returned. After the over-travel protection is released, the coordinate axis is moved to the stroke range, and the reference point operation is returned again to eliminate the fault; the other is the fault caused by the looseness or position change of the “reference point deceleration” block, and the adjustment is fixed again. The stop can eliminate the fault.

When the reference point is undershoot, the reference point indicator does not light. Through the diagnostic function of the machine tool, check the corresponding “Position Following Error” (DGN800～802) under the diagnosis page. If the tracking error exceeds the allowable range of positioning accuracy, you can adjust the “offset” potentiometer of the servo drive to make “ After the value of the position following error (DGN800802) is close to "0", the fault can be eliminated.

For faults where the position of the reference point is unstable, first check the position and contact of the stop, then check the pulse encoder "zero pulse" for any problems, and finally check the gap between the motor and the lead screw. If the unstable displacement error is small, it is generally the last case, and the gap should be eliminated to eliminate the loose connection.

Reference point full pitch offset. When machining a workpiece, a stable displacement error always occurs, and it is exactly equal to the pitch of the lead screw. Then, the problem is at the reference point. The reference point is usually set on the first "zero pulse" after the "reference point deceleration" stop is released. If the encoder is released, the encoder will happen to be near the “zero pulse” and the full pitch offset will occur. The position of the retarding stop needs to be re-adjusted. It is best to have a half-pitch between the deceleration stop release position and the encoder "zero pulse" position.

For the alarm when returning to the reference point, and there is a fault with the alarm information. For the alarm information, check the machine manual and handle it accordingly. The information prompt may belong to: the encoder "zero pulse" is bad and the system scale is bad, the shielded line is bad, and the system parameter setting is wrong. For hardware failure, it needs to be repaired or replaced; for parameter setting error, it needs to be reset according to the backup parameters.

3. Conclusion

When the reference point fault occurs during the operation of the CNC machine tool, it mainly occurs in the above aspects. According to the principle of the reference point return, it can be stepped out in sequence to find the key point and apply the correct method to eliminate the fault.

For more articles, please refer to "Metal Processing" Cold Processing 2008 No. 1

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