Problems encountered in the use of CNC bending machines are often encountered but frequently overlooked, leading to more significant issues. Therefore, it is essential to pay close attention to these problems. Both in China's national standards and in the CE certification of electromechanical products in the European Union, it is considered a mandatory and crucial aspect. It explicitly states that "the above-mentioned basic elements should not be sacrificed to obtain high performance in terms of personnel and property safety, control response consistency, and maintenance convenience." However, in practical maintenance and modification work, the following phenomena still occur:
Non-uniform power supply:
Some manufacturers of CNC bending machines add additional mechanisms for the convenience of wiring or for the interaction with other equipment. The power supply for these additional mechanisms is not disconnected from the power switch of the equipment but is connected separately, which violates the standard requirement of "connecting mechanical and electrical equipment to a single power source." In case of an emergency, turning off the power switch of the equipment does not cut off all power sources, posing a potential hazard to personnel.
Improper connection of the neutral line to PE:
Within electrical equipment, the neutral line and the protective grounding circuit should not be connected. The PEN terminal should not be used for multiple purposes inside the electrical control cabinet. We have encountered more than one case of modified machine tools where, due to the addition of components requiring AC 220V power supply, one phase of the three-phase 380V power was directly connected to the PE terminal of the machine tool to create AC 220V. After adding three-phase electrical components, the neutral line connected to the PE terminal. Obviously, if there is a mechanical fault at any point, the equipment becomes an energized body with a 220VAC voltage to the ground, posing a direct electric shock hazard to personnel.
In general, according to national standards and regulations, CNC bending machine manufacturers should only use the neutral line with the user's consent. When using the neutral line, it should be clearly indicated in the technical documentation (installation and circuit diagrams), and a single insulated terminal marked with "N" should be provided for the neutral line. Therefore, when modifying machine tools, it is crucial not to improperly connect the neutral line to PE to ensure the safety of production and maintenance personnel.
Lack of protection for electrical equipment:
According to national standard GB/T 5226.1, electric motors with a continuous power of 0.5 kW and above should have thermal protection measures. The accumulation of heat in electric motors is a significant cause of motor burnout. In the process of modifying machine tools, using "time-current" protection devices that are incompatible with the electric motor often fails to provide effective protection when the time constants of the protection device and the winding of the protected device differ significantly. In our practical work, we have observed that maintenance and modified machine tools often have problems in this aspect.
Insufficient Protection Level of Electrical Cabinets
The ventilation of electrical cabinets in CNC bending machines and other machine tools has long been internationally recognized as "positive pressure." However, this approach has not been implemented in some machine tool modifications. We encountered a domestic machine tool that used negative pressure ventilation, where cutting fluid from the tool holder entered the electrical cabinet through a flexible hose. Moisture from cable trenches, dust generated during cutting, and other contaminants entered the cabinet through various channels and gaps. After approximately one year of use, the circuit boards and electrical components inside the electrical cabinet were covered in a layer of dirt, and there were noticeable traces of condensation. The frequent malfunctions of the machine tool disrupted normal production. GB5226.1-13.3 clearly states that "the enclosure of control devices should generally have a protection level of no less than IP54." It is evident that deviating from the standards comes at a cost.
Hidden Hazards in Control Circuits
It is well known that for signals involving personnel and equipment safety, such as emergency stop signals, overtravel signals, and alarm signals, they should be implemented as "stop by cutting off power" and "cutting off all power conductors to the controlled components" (GB/T5226.1-9.4.2.1). We encountered a privately-owned enterprise that designed a modified CNC bending machine, and the limit switches used normally open contacts, meaning that the circuit was "open" (low level) when the machine was operating within the normal range. When the moving parts exceeded the travel limit and pressed the limit switch, the circuit closed (high level). During the machine's debugging process, the maintenance personnel unplugged the connector to measure the circuit but failed to plug it back in promptly. As a result, the circuit remained in the open state during the trial run, and the controller confirmed that the machine was "normal," leading to damage to the screw support bearing seat. It is imaginable that if the motion chain circuit disconnects due to mechanical reasons during user operation, the resulting property damage or personal injury should be borne by whom?
Grounding one side of the relay coil has long been recognized as an effective safety measure to prevent accidental operation. However, in some modified machine tool equipment, it is common to have control contacts on both sides of the relay coil or one side connected to the power supply and the other side connected to the contact point. As the wiring runs up and down the machine tool, if insulation failure due to mechanical wear occurs at a certain point and causes a short circuit to ground, the relay may malfunction, with unimaginable consequences. This WeChat account on metal processing provides valuable content and is worth following. It is worth mentioning that some designers of electrical modifications often design the control output terminals as collector-open circuit form to directly drive the relay coil circuit for convenience. However, this also hides the potential for accidents.
Incomplete Warning Labels
Some modified machine tool equipment fails to provide the corresponding warning signs as required by GB5226.1-18.2. Often, these modifications are not adequately explained in the technical documentation and are only verbally warned during training for operators.
