Maintaining CNC equipment is a challenging task due to the wide variety of CNC systems, their diverse forms and structures, varied design methods, and varied fault phenomena. While mastering the mechanical structure and electrical control principles, it is essential to analyze rationally, apply knowledge flexibly, and summarize experiences effectively to achieve optimal results. Based on the principles, the approach should be to narrow down the fault range and eliminate it gradually, starting with the easier faults. To ensure the safe operation of machine tools, linear axes are typically equipped with two protective "lines of defense": soft limits (parameter setting limits) and hard limits (travel switch limits). Limit problems are one of the common faults in CNC machine tools, yet related information is scarce. The following analysis and explanation of the main causes of "limit alarms" will be provided. I. Open Circuit in Related Control Circuit or Damaged Limit Switch This cause has a relatively high incidence of "limit alarms." External components are greatly affected by the environment, such as mechanical collisions, dust accumulation, corrosion, and friction, which can easily lead to damage to the relevant limit switches and open circuits in the control circuit, resulting in "limit alarm" messages. There were also cases where the overtravel switch could not reset after being pressed. The handling of this type of fault is relatively straightforward; simply repair or replace the damaged switch or wire. When there is a broken circuit or poor contact in the wires, careful wiring and observation are required. For example, an XK755 CNC milling machine using a FANUC 0-M CNC system suddenly displayed "X+, X-, Y+, Y- hard limit" alarms during machining, even though the machine was actually within its normal machining range. Based on this phenomenon, poor wiring contact or a broken circuit was the most likely cause. The 24V voltage supplied to the limit circuit on the terminal block in the electrical cabinet was measured and found to be normal. Following the wiring path step by step, when a wiring connector on the right side of the machine body was manually rotated, the alarm on the screen momentarily disappeared, only to reappear when the rotation was released. Therefore, the connector was removed, and a careful inspection revealed that the two soldered wires inside had come loose. Rotating the connector by hand allowed the two ends of the broken wires to touch, causing the aforementioned phenomenon. After resoldering the connector, the machine returned to normal operation. II. Improper Operation, Malfunctions, or Machine Tool Malfunction The primary cause is hard limit alarms. Generally, recovery requires direct remedial measures. Utilizing the machine tool's own overtravel release function or short-circuiting is a common maintenance method. To gain valuable production time, we should focus on the individual characteristics of the equipment and system during troubleshooting, finding reliable shortcuts to solve problems flexibly and quickly. 1. Handling Based on Machine Tool Structure Most machine tools are equipped with an "overtravel release" contact. Once a "hard limit" alarm occurs, after confirming that the hard limit switch is engaged, close the contact and manually move the machine out of the limit position in the opposite direction to clear the alarm. A few machine tools may lack this button. In this case, an equivalent short-circuit measure should be taken at the corresponding point to force the condition to be met, and then the machine tool should be moved out of the limit position. For example, an imported HX-151 vertical five-axis machining center experienced an "X-axis hard limit" alarm. This machining center did not have an "overtravel release" button. Due to the machine tool's structure, the X+ direction limit switch is "hidden," requiring the pedal to be moved and the protective plate removed, which is time-consuming and laborious, delaying production. Therefore, a solution was found to short-circuit the corresponding terminal points of equal potential on the terminal block in the electrical cabinet, specifically points 3230 and 3232 on the machine tool's terminal block (or directly short-circuit between PLC input points A305.3 and A306.6). The machine tool was then moved back within its travel range, resolving the fault. 2. Understanding the Limitations and Characteristics of CNC Systems In routine maintenance, we also encounter unique situations due to limitations in the CNC system's design software. Handling such problems requires a comprehensive understanding of the individual characteristics and performance of the CNC system. While exploring and summarizing, it's essential to keep records and, if possible, receive necessary technical training. For example: The blade shot peening economical CNC machine tool designed by our factory's technicians controls four axes: X, Y, Z, and A (where A is the rotary axis). The CNC system is the Swai M2000 developed by the Southwest Automation Research Institute, which adopts an open-loop control mode. The following two representative fault phenomena occurred: (1) Due to improper operation, the lower left corner of the machine tool panel showed "hard limit" in the Y direction. The +Y travel switch was pressed, and the hard limit red indicator light was on. In manual mode, it was impossible to move out of the limit position in the opposite direction. Handling method and cause: The usual method of moving out and shorting could not eliminate the fault. Because the alarm was not cleared, the Y-axis movement operation in manual or handwheel mode was ineffective. In the absence of other possible causes, the problem of the CNC system was suspected. However, at this time, the CNC system did not show any signs of crashing or disorder, and all other axes could move normally. It was decided to short-circuit the +Y travel limit switch, turn off the machine tool power and wait a moment, then restart the machine tool. The alarm message disappeared and the red indicator light went out. The machine tool was then moved out of the limit position. Finally, the short-circuit wire was removed and everything returned to normal. In fact, after handling the fault multiple times, we realized that the fault was due to the fact that the CNC system has the function of keeping the previous coordinate position in memory when powered on. (2) The alarm message in the lower left corner of the machine tool operation panel CRT is "hard limit". The hard limit red indicator light is not lit. The actual position of the machine tool is still far away from the hard limit switch. At the same time, the machine tool coordinate display value is close to the maximum value of 99999999. The axis cannot be moved. Handling method: In response to the above phenomenon, it is first determined that the coordinate value has overflowed and exceeded the limit value of the machine tool memory. As it accumulates and increases, the coordinate data must be cleared to zero. The steps for clearing the mechanical coordinates of this system are as follows: ① Enter the "Monitoring" menu on the main page; ② No content is displayed on the page, ignore it (it is hidden), and enter the second item "Slave Monitoring"; ③ Next, press the third item "F3". At this time, you can see that the coordinates of each axis of the machine tool are zero and the alarm has been cleared. Special note: The machine tool must return to the reference point to establish the machine tool coordinate system. This situation occurs due to the limitation of the CNC system function program. When dealing with this, you should combine the characteristics of point (1) above. III. Failure of returning to the reference point, causing limit switch High-end CNC systems can usually use convenient and flexible parameter correction functions to maintain the machine tool. If the actual position of the machine tool does not exceed the limit position and a limit switch alarm occurs, you should first carefully check whether the stroke parameters are lost or changed. Regarding parameters, the most typical example is that some machine tools are prone to soft limit switch alarms when returning to the reference point, while the actual position of the machine tool is a certain distance away from the reference point. At this point, assuming the machine tool's hard limit function is working properly, adjust the soft limit parameter value appropriately based on 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 canceled, depending on the situation). After the machine tool returns to the reference point normally, the soft limit setting should be restored. Additionally, after replacing some equipment involving travel (such as motors, shaft connections, lead screws, etc.), their clearance and displacement are prone to change, which may also lead to failure to return to the reference point and generate a "limit alarm". For example, a THM6350 horizontal machining center manufactured by Ningjiang Machine Tool Co., Ltd., with a FANUC 0i-MA CNC system, experienced an alarm message "507 OVER TRAVEL +X" on the Y-axis during the return to the reference point process. There was a deceleration process, and repeated operations failed to return to the reference point, with the same alarm message appearing. This machining center uses a stop block method for returning to the reference point. Analysis and handling: It can be seen that the root cause of this fault is not the hard limit itself. Then, is it possible that the pulse for returning to the reference point mark does not appear after deceleration? If so, there are two possibilities: First, the grating did not detect the reference point pulse signal during the reference point homing process, or the reference point marker failed, or the reference point pulse selected by the reference point marker was lost during transmission or processing, or the measurement system hardware malfunctioned and had no ability to identify or process the reference point pulse signal. Second, the deceleration switch and the reference point marker were misaligned, and the reference point marker did not appear after the deceleration switch was reset. Check the relevant parameters one by one to ensure there are no changes or losses. Press down each switch directly by hand; if the deceleration signal changes from "0" to "1" in the PMC address X1009.0, it indicates that the function is intact. Based on the fault symptoms, the overtravel signal is also intact. The focus should be on checking the reference point signal to rule out the possibility of signal loss or component damage. The distances of the deceleration switch and reference point switch are set by the manufacturer's standard, and the reference counter capacity is consistent with the standard, and generally, no changes or modifications are made during maintenance. Do not rush to use the tracing method to determine the first possible cause analyzed above; first, consider the problem from easy to difficult. See if it is due to a decrease or loss of the reference point marker's recognition ability. The decision was made to change the X value of parameter 1425 (FL speed after hitting the deceleration block) from 200 to 100. To ensure the balance of motion of each axis, the FL speed of other axes was simultaneously set to 100. After testing the return to the reference point, the machine tool returned to normal, validating this hypothesis. Therefore, the cause of the fault was a decrease in the ability to recognize the reference point marker, leading to the machine tool's failure to return to the reference point until the hard limit was engaged. IV. Changes or Loss of Machine Tool Parameters Due to External Interference This aspect mainly involves soft limit parameters. Poor workshop power quality, harsh processing environment, lightning, inadequate shielding measures, and other external factors can easily cause changes or loss of various parameters of CNC machine tools. While restoring the parameters, it is essential to identify the direct cause of the fault and take remedial measures. A horizontal machining center using a FANUC 0i-MA CNC system encountered a "501 OVER TRAVEL –X" error during machining, indicating negative overtravel. The machine's digital coordinate display values ​​far exceeded the set range of -99999999 to +99999999 (unit: μm), while the actual machine travel was within the travel range. Troubleshooting: The above phenomenon indicated that the machine's digital display data had changed due to interference and exceeded the soft limit setting range. Parameters 1320 and 1321 (coordinate values ​​of the negative boundary of the Y-axis stored travel detection) were modified in the parameter screen. Next, parameter 1320 was set to be less than parameter 1321, assuming the travel was infinite and disabling stored travel detection 1. The machine was then shut down and restarted, returning to the reference point. Parameters 1320 and 1321 were then restored to their original coordinate values. Furthermore, it was crucial to identify and immediately eliminate the direct cause of the data change to prevent recurrence and more serious consequences. The fault was ultimately confirmed to be caused by lightning interference. V. Impact of Coordinate System and CNC Program The machining program must strictly consider the machine tool's machining range. During machining, if the tool enters a prohibited area, a travel (soft and hard travel) limit alarm will occur. One possibility is that the program coordinate values ​​have been improperly increased due to operation (this is not detected by rigorous software simulation and filtering checks). Another possibility is that the machine tool's machining coordinate system (G54~G59) parameters are improperly set, causing the travel range to be exceeded when moving relative coordinates. For example, on a VMC1000C vertical machining center, after setting the machining coordinate system and compensation parameters, an "OVER TRAVEL –Y" alarm appears as soon as the program runs, indicating a negative hard limit on the Y-axis. Simultaneously, the interpolation statement is executed directly without executing the tool change statement (M06), and the tool path is incorrect. Handling process: Obviously, this hard limit alarm is only a warning. After confirming that there are no abnormalities in the system parameters and machining program, it is decided to further check whether the position loop is intact. Running another CNC program using G54 as the machining coordinate system without load, the machine tool worked normally, ruling out the possibility of a position loop fault. The fault was narrowed down to the machining coordinate system. The coordinate values ​​set in G58 were transferred to G54, and the original program's G54 was changed back to G58. A test run of the modified program with the modified machining coordinate system showed no problems. At this point, the problem was essentially determined to be with G58. Faults in the coordinate system establishment function of G54-G59 are relatively rare. Following the principle of starting with the easiest problem, we initially suspected that the coordinate system set in G58 was not being accepted by the system, but rather remembered as different data, as indicated by the incorrect path. Therefore, we cleared the data and re-entered it, and the machine tool returned to normal after a test run, proving the initial assessment correct. This fault was caused by non-standard input data, which affected the machine tool's coordinate system data, leading to an overtravel alarm.