Analysis of Seven Unsafe Phenomena in the Design, Installation, Maintenance and Reformation of CNC Machine Tools

Analysis of Seven Unsafe Phenomena in the Design, Installation, Maintenance and Reformation of CNC Machine Tools

This question is very important. Whether in the relevant national standards of our country, or in the CE certification of electromechanical products of the European Community, it is an important content of mandatory implementation. For the “safety of personnel and property, the consistency of control response and the convenience of maintenance”, it is clear that “it is not advisable to sacrifice the above basic elements to obtain high performance.” (IEC204.1, GB5226.1) But in actual maintenance and reconstruction work, The following phenomena still occur:
Phenomenon 1: Randomly connect the neutral line to the PE.
Take my country’s national standard GB/T5226.1 as an example, in which paragraph 5.1 clearly stipulates, “In electrical equipment, the neutral line and the protective grounding circuit should not be connected. Also, the PEN dual-use terminal should not be used inside the mechanical electrical cabinet. “We have encountered more than one case of modified machine tools. Due to the addition of components, AC220V power supply is required. Therefore, one phase of the three-phase 380V and the machine PE grounding terminal were connected to form AC220V. After adding three-phase electrical components, The lead out of the neutral wire is connected to the PE terminal. Obviously, if somewhere is disconnected due to a mechanical failure, the equipment will become a live body with a voltage of 220VAC to the ground. The equipment at this time can cause direct electric shock injury to personnel.
In general, CNC bending machine machine tool manufacturers stipulate in accordance with national standards that “the center line can only be used with the user’s consent. When using the center line, it should be clearly stated in the mechanical technical file (installation diagram and circuit diagram), and the center line should be marked There are single-use insulated terminals with N.” Therefore, when the machine tool is rebuilt, it must not be trouble-free to connect the neutral line to the PE to ensure the safety of production and maintenance personnel.
Phenomenon 2: Not a single power supply
Some CNC bending machine equipment adds some mechanisms. Because of the convenience of wiring or the action associated with other equipment, the electricity of the new mechanism is not drawn from the power cut-off switch of the equipment, but the other leads are violated. “Connect mechanical and electrical equipment to a single power source” standard (GB/T5226.1─5.1). In an emergency, turning off the power switch on the device cannot cut off all the power to the device, which is a potential hazard to personnel.
Phenomenon 3: Insufficient protection level of electrical cabinet
We have been commissioned by someone to carry out the acceptance inspection after the modification of a certain equipment. The electrical cabinet of the equipment is more than one person high (about 1800mm), and the single door is about 800mm wide, and there is no door switch. Anyone can open the door of the electric cabinet without using any tools and enter the inside of the electric cabinet. The protection grade of the components in the electric cabinet is IP1.1, R12. Obviously, the electrical cabinet structure of this equipment violates the following regulations in GB/T5226.1─6.2.1: “The protective shell (ie, door, cover, cover, etc.) is allowed to be opened only under one of the following conditions: a ) A key or tool must be used to open the protective shell by a skilled technician or trainee. It may not be suitable to open electrical equipment during operation.
b) Before opening the protective shell, cut off its internal live parts; this technical requirement can be realized by the interlocking mechanism of the door and the cut-off switch (such as the power cut-off switch), so that the door can only be opened after the cut-off switch is turned on, and only after the door is closed. Turn on the switch. c) Only when the protection level of all live parts directly contacting electricity is at least IP2X or IPXXB (see IEC529), it is allowed to open the case without keys or tools and without cutting off the live parts. “Therefore, the protection level of the electrical cabinet of this equipment is far from reaching “Only skilled technicians can be allowed to enter and comply with special technical requirements (IEC364─4─41 IEC─4─47 IEC439─1). “The hidden safety hazards buried by this can be imagined.
The ventilation of the electrical cabinets of CNC bending machines, machine tools and other equipment has long been recognized internationally as a “positive pressure” method, but it has not been implemented in some machine tool transformations. We have encountered a domestic machine tool that uses negative pressure ventilation, and the cutting fluid on the tool holder enters the electric cabinet along the snakeskin hose. Moisture in the cable trench, cutting dust, etc. enter the electrical cabinet along the channels and gaps. After the machine was used for about a year, a layer of dirt and obvious traces of water vapor (condensation) appeared on the circuit board and various electrical parts in the electrical cabinet. The machine has frequent failures, affecting the normal production. In GB5226.1─13.3, it is clearly stipulated that “the casing of the control device should generally have a protection level of no less than IP54 (see IEC529).” It can be seen that if you do not act according to the standard, you must pay a price.
Phenomenon 4: Under-protection of electrical equipment
In the national standard GB/T5226.1, there must be thermal protection measures for motors of 0.5kw and above that work continuously. The failure of the motor’s heat accumulation is an important reason for the motor to burn out. In the process of machine tool equipment transformation, the “time-current” protection device used in the process that does not match the motor, that is, when the time constants of the protection device and the protected device’s windings are greatly different from each other, they often fail to provide effective protection. We have seen in actual work that maintenance and transformation of machine tools often have problems in this regard.
Phenomenon 5: hidden dangers of buried control lines
As we all know, for such signals related to the safety of personnel and equipment, such as emergency stop signals, overtravel signals, alarm signals, etc., they should be “stop by power off” and “cut off all energized wires of controlled devices” (GB/ T5226.1─9.4.2.1) We once encountered a CNC bending machine machine designed and modified by a private enterprise. The limit travel switch uses a normally open contact, which means that the circuit is “disconnected” when the machine does not overtravel normally. (Low level), when the moving part over-travel presses the travel switch, the circuit is closed (high level). During the debugging of the machine tool, the maintenance personnel measure the circuit and failed to insert the plug in time after unplugging the connected plug. As a result, the line was always in the normally open state during the test run, and the controller confirmed that the machine tool was “normal”, which caused the lead screw support bearing seat to be damaged. It is conceivable that if the movement towline is disconnected due to mechanical reasons during the user’s use, who should be responsible for the property loss or personal injury caused?
The “grounding of one side of the relay coil” in the control circuit has long been regarded as an effective safety measure to avoid malfunctions. However, in some modified machine tools, it is common to have control contacts on both sides of the relay coil, or one end is connected to a power source, and the other end is a contact point. As the circuit is put on and off on the machine tool equipment, if a place is short-circuited to ground due to mechanical wear and insulation failure, the relay will malfunction and the consequences will be disastrous. It is worth mentioning that, for the convenience of diagrams, some electrical renovation designers often design the control output terminal as an open-collector form to directly drive the relay line diagram, but it also lays down the hidden danger of accidents.
Phenomenon 6: Incomplete safety operation protection function
When there is no protective door interlocking function for the equipment with the risk of splashing, there is no two-hand control mode that can only operate when it is necessary to confirm that the operator’s hands have left the dangerous action area. When the lack of phase causes the electrical appliances to burn out, the three-phase linkage circuit breaker is not installed and the three-way insurance is used respectively, which directly constitutes a hazard to personnel and property.
Phenomenon 7: Incomplete warning signs
Some modified machine tools failed to give corresponding warning signs in accordance with the requirements of GB5226.1-18.2. Often, the revised technical documents are not well explained, and only verbally warns the operators during the training.