Abstract : This paper introduces the relevant knowledge about CNC machine return reference. It analyzes the causes of return reference failures and provides examples for troubleshooting. Keywords : CNC machine, Fault analysis, Troubleshooting Chinese Classification Number: TG659 Document Code: B [align=center]Fault analysis and processing of CNC machine return reference Shi Yongcheng[/align] Abstract This paper introduces the know-how about CNC machine return reference. It analyzes the fault definition of machine return reference and discusses methods for example in detail. Key words : CNC machine, Fault analysis, Processing 1 Overview CNC machine return reference can be divided into two types based on the detection element: absolute pulse encoder and incremental pulse encoder. In systems using an absolute pulse encoder as the feedback element, after machine installation and commissioning, as long as the backup battery of the absolute pulse encoder is effective during normal use, it is not necessary to perform a return reference operation on each subsequent startup. In systems using incremental pulse encoders, the machine tool must first perform a reference point homing operation after each power-on to determine the machine's coordinate origin. Finding the reference point mainly depends on the zero-point switch, encoder, or grating ruler's zero-point pulse, and generally involves two methods: 1) Rapid axial movement in the predetermined direction. After the stop presses down the zero-point switch, the machine decelerates and continues moving forward until the stop disengages from the zero-point switch. The CNC system then begins searching for the zero point, and upon receiving the first zero-point pulse, the reference point position is determined. Machine tools equipped with FANUC and Beijing KND systems generally use this homing method. 2) Rapid axis movement in the predetermined direction. After the stop presses down the zero-point switch, the axis decelerates in the opposite direction. When it disengages from the zero-point switch again, the axis changes direction and moves towards the reference point. When the stop presses down the zero-point switch again, the CNC system begins searching for the zero point, and upon receiving the first zero-point pulse, the reference point position is determined. Machine tools equipped with SIEMENS, AB, and Huazhong systems generally use this homing method. The method and movement of the system are determined by the PLC program and the machine tool parameter settings of the CNC system. The axis movement speed is also set in the machine tool parameters. The process of the CNC machine tool returning to the reference point is completed by the cooperation of the PLC system and the CNC system. The CNC system issues a zero-point command, and then the axis moves in the predetermined direction, pressing against the zero-point switch (or disengaging from the zero-point switch). After that, the PLC sends a deceleration signal to the CNC system, and the CNC system decelerates in the predetermined direction. The measurement system receives the zero-point pulse, and after receiving the first pulse, it calculates the coordinate value. The process of returning to the reference point ends after all axes have found their reference points. Common faults in which a CNC machine tool cannot return to the reference point generally include the following: 1. Problems with the zero-point switch; 2. Problems with the encoder; 3. Problems with the system measurement board; 4. The zero-point switch is too close to the hard (soft) limit position; 5. System parameters are lost, etc. The following are some examples of the repair process I have encountered in my work. 2. Maintenance Examples Example 1) When the XH714 machining center is started and returns to the reference point, the X-axis moves in the opposite direction to the reference. This machine is equipped with a SIEMENS 810D CNC system, which adopts a semi-closed-loop control method and uses an incremental pulse encoder as the detection feedback element. Analysis: The process of the X-axis returning to the reference point after machine tool startup is as follows: The axis first moves rapidly. When the zero-point switch is pressed down by the stop block, the PLC input signal I32.2 changes from 1 to 0. Upon receiving this change signal, the CNC outputs a deceleration command, causing the X-axis to brake and move in the opposite direction at low speed. When the stop block releases the zero-point switch, the I32.2 signal changes from 0 to 1. The X-axis brakes and changes direction, moving towards the reference point at the return-to-reference-point speed. When the zero-point switch is pressed down again by the stop block, the I32.2 signal changes from 1 to 0. At this point, when the CNC receives the zero-position marker pulse I0 from the incremental pulse encoder, the X-axis continues to run to the parameter-set distance and stops. The reference point is established, and the return-to-reference-point process ends. This method of returning to the reference point avoids the damage caused to the machining center by abnormal operations such as returning to the reference point while it is already at the reference point. When the machining center's X-axis is already at the reference point, and a return-to-reference-point operation is performed, the initial signal of I32.2 is zero. The CNC detects this state and issues a movement command in the opposite direction to the return-to-reference-point direction. After the zero-point switch is released (i.e., I32.2 becomes 1), the X-axis brakes and changes direction, moving towards the reference point at the return-to-reference-point speed, repeating the aforementioned return-to-reference-point process. Based on the fault symptoms, it was suspected that after the zero-point switch was pressed, although the X-axis had left the reference point, the switch could not reset. PLC diagnostics confirmed this. The operator stated that all axes were in the middle position when the machine was started, ruling out the possibility of the stop block continuously pressing the zero-point switch due to fatigue failure of the switch spring caused by deceleration at the reference point. This also indicates that the deceleration switch had already failed before shutdown. Careful observation of the machining process revealed that the machining center stopped at the reference point after each machining cycle. This significantly increased the likelihood of zero-point switch failure and the probability of malfunction. This is likely the true cause of the fault. Because the NC code generated by CAM software programming is mostly in G28 reference point return format before the program ends (M30), it is recommended that CNC programmers add G-code instructions to return to the midpoint of each axis before the program ends (M30) and remove the G28 instructions to reduce the occurrence of this type of fault. Example 2) An overtravel alarm occurred when the XH713/4 machining center returned to the reference point. This machining center is equipped with a FANUC-OMD control system, which adopts a semi-closed loop control mode and uses an incremental pulse encoder as the detection feedback element. The reference point return is achieved by pressing the zero-point switch with a stop block, decelerating forward, disengaging from the zero-point switch, and starting to search for the zero point. Due to the failure of the CNC backup battery, the parameters were lost. After reloading the backup parameters with the computer, a soft limit overtravel alarm occurred when returning to the reference point, with each axis at the middle position of the travel range. At this time, if each axis is moved manually, even if its mechanical position is in the middle of the travel range, the CRT will still display the soft limit overtravel alarm for each axis position coordinate. This is because after the backup battery fails, when the machine is powered on after reinstalling the battery, the CNC interprets the current mechanical position as the return to the reference point. The solution is to first set the positive soft limit values ​​of each axis to their maximum values, then perform a three-axis return to the reference point to establish the correct machine zero point, and finally change the three-axis soft limits back to their original values. The specific steps are as follows: 1) In the OFFSET menu, set PWE=1. 2) Set the CNC parameters NO. 700, 702, and 704 (X, Y, Z) to their maximum values ​​for each axis. 3) Manually move X, Y, and Z away from the machine origin by a certain distance. 4) In reference point zeroing mode, manually return each axis to the reference point. 5) Carefully observe whether each axis is in the return to the reference point position, especially the Z axis related to ATC. If the position is inaccurate, repeat steps 3 and 4 until accurate. 6) Change the parameters changed in step 2 back to their original values. 7) Reset PWE to zero. This resolves the overtravel alarm issue during reference point retracement. Example 3) A V560 machining center experiences a soft limit overtravel alarm during Z-axis reference point retracement. This machining center uses a FANUC-OIMA control system with a semi-closed-loop control method. Reference point retracement involves pressing a stop block against the zero-point switch, decelerating forward, disengaging from the zero-point switch, and then starting the search for zero. Observing the Z-axis display on the CRT shows 6.01, indicating a soft limit overtravel. Testing confirmed that the manual reference point retracement process was not yet complete when the alarm occurred. During manual reference point retracement, the PMC signal DGNX9.3 input to the deceleration switch changed normally, indicating the deceleration switch was functioning correctly. Setting the CNC parameter NO. 704 (Z-axis soft limit) to the maximum value of 99999999 resolved the manual reference point retracement issue. Resetting NO. 704 to 6000 resolved the overtravel alarm during reference point retracement. Analysis: Since the deceleration switch was not faulty, but the return-to-reference-point process was incomplete, and a soft limit overtravel occurred, it indicated that the stop block was not loose, and the deceleration switch position was likely loose. Inspection revealed that the deceleration switch position was indeed loose. After readjusting the deceleration switch position and tightening the fixing screws, the problem was solved. However, it should be noted that once the deceleration switch position becomes loose, the screw pitch compensation parameters originally set at the machine tool's factory will be inaccurate. The machine tool's screw pitch compensation parameters need to be re-measured using a laser measuring instrument before being set again. Example 4) A CNC milling machine equipped with a Beijing KND-100M motor exhibited normal XZ axis operation during power-on return-to-reference-point operation, but alarm 222 "Y-axis servo not ready" occurred during Y-axis return-to-reference-point operation. Analysis: Based on the fault symptoms, a targeted inspection was conducted. Upon checking the servo drive module, alarm number 23 was found. The fault manual at this point provided the following explanations: 1) Excessive resistance in the ball screw movement or a problem with the ball screw itself. However, manual movement revealed no issues. 2) Damaged servo motor. Measurement of its windings also revealed no problems. 3) Insufficient load capacity or damage to the servo drive module, causing a malfunction in the control panel and generating an error alarm. The fault was resolved after replacing the X and Y axis servo drive modules with the same model. This indicates that the fault was due to unstable performance or poor contact of the Y axis servo drive module. However, the fault recurred a few days later, with alarm 212 (Servo Ready Not Ready) appearing again when the X axis returned to the reference point. Based on previous experience, checking the servo drive module revealed alarm 23 (Servo Ready Not Ready). It seemed easy to conclude that the original Y axis (now replaced with the X axis) servo drive module was completely damaged. However, to further confirm, replacing the X and Y axis servo drive modules with the same model again did not resolve the issue, indicating that this fault was unrelated to the servo drive module. Upon inspection, it was discovered that the stop block of the X-axis positive limit switch had moved towards the stop block of the deceleration switch. This caused the X-axis to hit the hard limit switch before the return-to-reference-point action was completed, triggering the CNC alarm. After readjusting the position of the hard limit switch and tightening the fixing screws, the machine tool returned to normal reference-point return. 3. Summary The inability to return to the reference point is a relatively common fault in CNC machine tools. This fault is generally caused by factors such as loose stops, malfunctioning deceleration switches, lost parameters, and inaccurate soft limit settings. Of course, damage to the encoder or grating ruler, as well as problems with the zero-point pulse of the encoder or grating ruler, can also cause this fault, although encoders and grating rulers are relatively reliable and have a lower probability of failure. As long as we master the relevant working principle of CNC machine tool return to reference point and the mechanical structure of the equipment, understand its operation method and action sequence, and conduct a thorough investigation and analysis of the fault phenomenon, we will definitely find the cause of the fault, check and repair it, eliminate the fault, and finally restore the machine tool to normal. References : 1. Wu Kuanming. CAN bus principle and application system design [M]. Beijing University of Aeronautics and Astronautics Press, 1996 2. Zeng Hao, Yuan Chuming et al. Research on DNC system for CIM environment [J]. Journal of Huazhong University of Science and Technology, 1996, (4) Author's profile : Shi Yongcheng (born in 1965), male, from Nantong, Jiangsu Province, Department of Mechatronics, Nantong Textile Vocational and Technical College, senior engineer, mainly engaged in teaching CNC technology, CAD/CAM application and CNC machine tool training. Contact Address: Department of Mechanical and Electrical Engineering, Nantong Textile Vocational and Technical College, No. 105 Qingnian East Road, Nantong City, 226000, China. Telephone: 0513-83525321, 13338069732. Email: [email protected]