Cause Analysis And Solution Of Position Limit Alarm Of CNC Machine Tools

Due to the wide variety of CNC machining centers, different equipment forms and structures, various design methods and different fault phenomena, it is very difficult to maintain CNC equipment. While mastering the mechanical structure and electrical control principle, we must analyze reasonably, flexibly apply and be good at summarizing, so as to achieve twice the result with half the effort. Based on the principle, reduce the fault scope and eliminate it from easy to difficult. In order to ensure the safe operation of the machine tool, the linear axis of the machine tool is usually set with two protective "lines" of soft limit (parameter setting limit) and hard limit (travel switch limit). Position limit problem is one of the common faults of CNC machine tools, and relevant data are rarely mentioned. The main causes of position limit alarm are analyzed and explained below.

1. Open circuit of relevant control circuit or damage of position limit switch

   The occurrence rate of position limit alarm caused by this reason is relatively high. Because external components are greatly affected by the environment, such as mechanical collision, dust accumulation, corrosion, friction and other factors, it is easy to cause damage to relevant position limit switches and open circuit of control circuit, and generate position limit alarm information at the same time. It also happens that the overtravel switch cannot be reset after being pressed. This kind of fault can be handled directly. The damaged switches and wires can be repaired or replaced. In case of open circuit or poor contact of the conductor, it is necessary to carefully calibrate and observe. For example, a CNC milling machine adopts FANUC 0-m CNC system. In the process of machining, the alarm of "x +, X -, y +, y-hard limit" suddenly appears, but in fact, the machine tool is within the normal machining range. According to the above phenomena, it is estimated that the line has the greatest possibility of poor contact or open circuit. Measure the 24V voltage supplied to the position limit circuit on the wiring board in the electrical cabinet, and the voltage value is normal. Search one by one according to the line direction. When turning a line connector on the right side of the base by hand, it is found that the alarm on the screen disappears instantly and reappears in the release room. Therefore, the connector should be removed and carefully inspected. It was found that the two wires welded inside had fallen off. During the process of rotating inward by hand, the two ends of the wire circuit breaker could touch, so there were the above changes. After re welding the joint, the machine tool returns to normal.

2. Nonstandard operation, misoperation or machine tool out of control

   Among them, it is mainly caused by hard position limit alarm. Generally speaking, it can be recovered only through direct remedial measures. Using the over range release function or short circuit method of the machine tool itself is the usual method of daily maintenance. In order to gain valuable production time, we should firmly grasp the individual characteristics of equipment and system in the processing process, find a reliable shortcut and solve problems flexibly and quickly.

3. The process of returning to the reference point failed, causing the position limit.

   The relatively high-end CNC system can usually use the convenient and flexible parameter correction function to maintain the machine tool. If the actual position of the machine tool does not exceed the limit position and there is a position limit alarm, first carefully check whether the parameters of the stroke are lost or changed. For parameters, the most typical example is that some machine tools are prone to soft position limit alarm when returning to the reference point, and the actual position of the machine tool is a certain distance from the reference point. At this time, when the hard position limit function of the machine tool is in good condition, the soft position limit parameter value shall be appropriately modified to a large value according to the displacement of the stop point from the reference point mark when the machine tool alarms (sometimes it needs to be set to the maximum value or cancelled, depending on the situation). After the standby machine returns to the reference point again, the soft position limit setting shall be restored. In addition, after replacing some equipment involving travel (such as motor, spindle connection, lead screw, etc.), its clearance and displacement are easy to change, and there may also be failure to return to the reference point, resulting in position limit alarm.

4. Machine tool parameters are changed or lost due to external interference

   In this regard, soft position limit parameters are the common casue. External factors such as poor power quality in the workshop, bad processing environment, lightning and inadequate shielding measures are very easy to lead to changes or losses of various parameters of CNC machine tools. While restoring the parameters, the direct cause of the fault must be found out and remedial measures must be taken.

5. Influence of coordinate system and CNC program

   The processing range of the machine tool must be strictly considered in the preparation of the processing program. In the processing process, once the tool enters the prohibited area, the travel (soft travel and hard travel) position limit alarm will appear. One is that the program coordinate value is increased due to improper operation (the program filtering inspection does not exist through strict simulation by software); the other is that the parameters of the machining coordinate system (g54 ~ G59) of the machine tool are improperly set, which exceeds the travel range when running the relative coordinates.