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Experience in self-diagnosis of failure of CNC machining center

With the development of computer technology, modern CNC offline diagnosis software is gradually being integrated with CNC control software, and some systems have introduced “expert systems” into fault diagnosis. This undoubtedly brings convenience to the maintenance and diagnosis of CNC machining centers. Through such software, the operator can diagnose CNC system or machine tool faults by simply doing some simple conversation operations on the MDI/CRT. Next is a set of self-diagnosis methods. The years of work experience of CNC masters are very valuable and worth collecting.

(1) Power-on self-diagnosis
The so-called power-on self-diagnosis is the diagnosis automatically executed by the system’s internal diagnostic program when the index control system is powered on. It is similar to the computer’s power-on diagnosis. Power-on self-diagnosis can automatically check the key hardware in the system, such as CPU, memory, I/O unit, MDI/CRT unit, paper tape reader, floppy drive and other devices; determine the installation, connection status and performance of the specified equipment; partly The system can also diagnose some important chips, such as RAM, ROM, and dedicated LSI. The self-diagnosis of the CNC system is carried out when the machine is turned on. Only when all the items are confirmed to be correct, can it enter the normal operation state. The diagnosis time is determined by the CNC system. Generally, it only takes a few seconds, but some take several minutes.

(2) Online monitoring
Online monitoring can be divided into two forms: CNC internal program monitoring and monitoring through external equipment. CNC internal program monitoring is a method of automatically diagnosing, checking and monitoring the status of each part through the internal program of the system. The scope of online monitoring includes the CNC itself and the servo unit, servo motor, spindle servo unit, spindle motor, external equipment, etc. connected to the CNC. Online monitoring is always effective during the working process of the system. The internal program monitoring of the numerical control system includes three aspects: interface signal display, internal status display and fault display.

(3) Interface signal display
It can display the current status of all interface signals between CNC and PLC, CNC and machine tool. Indicate the on-off status of digital input/output signals to help analyze faults. During maintenance, you must understand the meaning of each signal between CNC and PLC, CNC and machine tool, as well as the various conditions for signal generation and cancellation, so that you can perform corresponding inspections. The “function manual” and “connection manual” provided by the CNC system manufacturer and the “machine tool electrical schematic diagram” provided by the machining center manufacturer are the technical guidelines for the above state inspection.

(4) Internal status display
Generally speaking, using the internal status display function, you can display the following aspects: the external reasons that cause the cycle command (processing program) to not execute, such as: whether the CNC system is in the “in-position check”; whether it is in the “machine lock” state ;Whether the “waiting for speed arrival” signal is turned on; is it waiting for the measurement signal of the “position encoder” during the feed programming of each spindle revolution; whether it is waiting for the “spindle 1 revolution signal” during thread cutting; the feed rate Whether the magnification is set to 0, etc.

(5) Fault information display
In the numerical control system, the fault information is generally displayed on the CRT in the form of “alarm display”. The content of the alarm display differs depending on the CNC system. Most of this information appears in the form of “alarm number” plus text. The specific content and troubleshooting methods can be consulted on the “maintenance manual” provided by the CNC system manufacturer. Monitoring through external equipment refers to a method of using computers, PLC programmers and other equipment to automatically diagnose, check and monitor the status of each part of a CNC machine tool. For example, the PLC program is dynamically detected in the form of ladder diagram and function diagram through the computer and PLC programmer. It can read and check the PLC program when the machine tool manufacturer does not provide the PLC program, thereby speeding up the maintenance progress of the CNC machine tool .

(6) Offline test
Offline test is also called “offline diagnosis”, it is the test and inspection of the CNC system itself after the CNC system is separated from the machine tool. Through offline testing, the fault of the system can be further located, and the scope of the fault can be minimized. The offline test of the CNC system requires special diagnostic software or a special test device. Therefore, it can only be performed by the manufacturer of the CNC system or a special maintenance department.

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