Numerical control machine tools adopt digital control systems, which can achieve multi axis linkage, machining three-dimensional shapes, and producing complex geometric parts, thus gaining popularity among people. In recent years, with the widespread application of CNC machine tools, people have gained a considerable understanding of CNC technology and can troubleshoot some common faults, thereby improving the utilization rate of machine tools. However, they still find some uncommon faults quite tricky. Below are several examples that the author has encountered in the maintenance of CNC machine tools in recent years, for your reference.

As we all know, rotary encoders or grating rulers are generally used as position feedback components on CNC machine tools. After each start-up of the machine tool, a reference point must be found to determine the coordinate point of the machine tool, which is commonly referred to as "returning to zero". After a malfunction occurs in the rotary encoder, it is generally not possible to perform the "return to zero" operation, and an alarm will be triggered due to the inability to find the correct reference point. Here are several special faults encountered.

Fault phenomenon - Taiwan made CNC lathe, using FANUC-0 system, generates 400 # alarm (servo alarm) when the tool contacts the workpiece during processing. Diagnosis and exclusion check: The machining program is correct, the mechanical transmission parts of each axis are unobstructed, and the movement is flexible. The diagnostic parameters showed X-axis overload, so all parts of the motor were checked, but they were normal. The power supply voltage and brake coil voltage were also normal. All parts of the cables and connectors are also normal. Replacing the servo unit, axis card, and power supply unit still cannot eliminate the fault. Later, we contacted the manufacturer to replace the encoder inside the electric motor and resolved the issue.

Fault phenomenon: One machine adopts the Siemens SINUMERIKSYSTEM 840C turning unit. After starting up, the X-axis cannot return to the reference point. The X-axis can slow down but does not stop during the "return to zero" process, with each action stroke not exceeding 40mm until the hard limit is pressed. The panel coordinate value suddenly changes, and the displayed value is very large. At the same time, the "X AXIS SW LIMITSWATCH MINUS" alarm is displayed.

Diagnosis and troubleshooting check: The parameter settings inside the machine tool are correct, and no faults were found in external devices such as cable connections. The machine tool can operate manually and display coordinate values. The machine tool can locate, indicating that the grating ruler should not be damaged. After checking, it was found that the grating ruler is a German "HEIDENHAIN" product. After investigation, it was found that HEIDENHAIN grating ruler uses a different zeroing method compared to other companies' products. In order to avoid searching for reference points on a large scale, reference marks were encoded by distance. A groove was added next to the grating engraving line, which can be used to find the reference position through two adjacent reference marks. That is, the "zero point" can be found within any 40mm (or 80mm, depending on the grating ruler model). After removing the cover of the machine tool, it was found that due to prolonged use, oil mist entered the grating ruler, the zero point mark was obscured, and there was no zero point pulse output, causing the machine tool to be unable to find the zero point. Because the device is maintenance free, we contacted the manufacturer for replacement and the fault was resolved.

Fault phenomenon: One machine tool cannot return to the correct reference point.

Diagnosis and exclusion: This machine tool uses the FANUC-0M system, and there is no deceleration collision block on the machine tool, only a hard limit collision stop device. Everyone is familiar with the working principle of the machine tool's "return to zero". Generally, it moves in the axial setting direction. When the zero switch is pressed, it decelerates. After disengaging from the zero switch, the CNC system sets the received zero pulse as the machine tool reference point (there are roughly three specific ways to return to zero). After contacting the manufacturer, the fault was resolved in the following manner. After turning on the machine, manually move the shaft to the hard limit position, eliminate the "limit alarm", and then shake the shaft to 5mm away from the limit switch. Change parameters 20 and 21 before turning off the machine. After turning on the machine again, the fault is eliminated.

For machine tool failures caused by sudden power outages, interference, or misoperation, we do not need to perform tedious operations in sequence. Sometimes, as long as we master the basic rules, we can easily solve them.

The tool changing mechanism of a general CNC machine tool consists of four parts: the disc is pushed out, the disc rotates, the disc is pushed in, and the disc is clamped. When the tool changing mechanism malfunctions or the tool cannot be positioned and clamped, it can be restored manually by following the above steps, but it is quite troublesome. In fact, sometimes we can discover its patterns as long as we observe carefully.

Fault phenomenon: The four bladed disc rotates into place but is not clamped properly

Diagnosis and exclusion: According to the electrical schematic of the machine tool, search for the corresponding solenoid valve wiring. The machine tool I/O display indicates that the cutterhead of the machine tool is in place but not clamped. Open the electrical cabinet and find the solenoid valve wiring pushed in by the cutterhead. From the relay, it can be seen that it is currently in an unpowered state. Find a temporary wire to quickly power up the solenoid valve and solve the problem.

So when dealing with malfunctions in CNC machine tools, we need to consider both their universality and specificity. After a malfunction occurs, both parties need to consider it thoroughly in order to accurately and quickly solve the problem.