The increasing use of numerical control (NC) devices has made it essential to ensure their efficient operation. When a failure occurs, the goal is to restore the device to normal function as quickly as possible. To achieve this, maintenance personnel must possess a high level of expertise, including in-depth knowledge of electromechanical integration, computer principles, NC technology, PLC systems, automatic control, and drive mechanisms, along with basic machining skills and simple NC programming. Additionally, they should have a good command of English to understand technical documentation.
Maintaining up-to-date data is also crucial—this includes machine diagrams, electrical schematics, system manuals, PLC ladder diagrams, and parameter backups. Having spare parts on hand can significantly speed up repairs. Experience and familiarity with common maintenance techniques are equally important. Over the years, I have gained considerable experience in NC equipment maintenance and developed a set of effective troubleshooting methods, which I will now share for reference.
**Identifying the Fault Phenomenon**
When an NC machine fails, the first step is to understand what exactly went wrong. This involves talking to the operator to gather details about when the fault occurred, under what conditions, and what led to the problem. Observing the machine during the failure can help clarify the sequence of events, making it easier to diagnose and resolve the issue.
For example, during an automatic grinding process on a CNC cylindrical grinder using the Bryant TEACHABLE III system, the dresser of the grinding wheel broke off. By removing the wheel and running the machine, we observed that the dressing process was normal until the dresser reached the upper limit switch, causing a sudden stop. Further investigation revealed a feedback issue with the E-axis encoder. After testing and replacing components, the machine returned to normal operation.
Modern NC systems often have built-in self-diagnostic capabilities. They can detect faults and generate alarms, such as power supply issues or low hydraulic pressure. These alarms provide valuable clues for quick resolution.
For instance, a CNC channel grinder with the Siemens 810 system displayed an alarm “BATTERY ALARM POWER SUPPLY†upon startup. Replacing the battery resolved the issue. Another machine with the Siemens 3 system showed no display after power-up, but by checking the CPU board’s LED status, we identified a low battery voltage and replaced it accordingly.
Some alarms directly point to the cause, while others only reflect the symptoms. In such cases, careful analysis and inspection are necessary. For example, a FANUC 0TC lathe displayed alarm 2043: “HYD. PRESSURE DOWN.†Checking the hydraulic system confirmed low pressure, and adjusting it restored normal operation.
**Using the CNC Status Display Function**
Many NC systems offer PLC status monitoring features, allowing technicians to check input/output signals, timers, and counters. This helps identify problems based on the machine’s working principle and electrical schematic.
In one case, a FANUC 0TC lathe triggered an X-axis overrange alarm, but the axis wasn’t actually out of range. Using the PMC status display, we found that the XC limit switch was incorrectly registering as active. Replacing the switch fixed the issue.
Another example involved a MITSUBISHI MELDAS L3 lathe where the turret wouldn’t rotate. Monitoring the PLC output showed that the relay controlling the hydraulic cylinder was not activating. Replacing the relay restored proper function.
**Using the PLC Ladder Diagram**
PLC ladder diagrams are critical for diagnosing many NC failures. By analyzing these diagrams, technicians can trace logic errors and identify faulty components.
A Siemens 810 grinding machine failed to return to the reference point without displaying any error. Checking the PLC diagram revealed that a signal from a proximity switch was not closing properly. After repairing the mechanical linkage, the machine resumed normal operation.
In another case, a Siemens 3TT milling machine experienced spindle speed issues during an automatic cycle. Monitoring the ladder diagram showed a brief loss of signal from a hydraulic pressure switch. Adjusting the hydraulic system resolved the problem.
**Using the Exchange Method to Locate Faults**
For complex failures, swapping suspected components with known good ones can quickly isolate the issue. This method is especially useful when dealing with control boards or sensors.
For example, a Bryant TEACHABLE III internal grinder displayed an E-axis following error. Replacing the position control board resolved the issue, but further checks revealed a faulty encoder coupling, which was then replaced.
Following general troubleshooting principles—starting with external components, checking mechanical and electrical systems, and prioritizing simple solutions—can greatly improve efficiency. Combining these methods ensures faster and more accurate diagnoses.
In summary, mastering the working principles of both the NC system and the machine tool is key to effective troubleshooting. With observation, analysis, and systematic testing, even complex failures can be resolved efficiently.
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