1 Overview
Since WEDM (Wire-EDM) has the advantages of being free from the influence of the hardness of the processed material, no processing stress, high processing precision, and the ability to process complex-shaped workpieces and can perform unmanned machining, wire cutting It is considered to be a major current metal processing method.
In the wire cutting process, the electrode wire associated with the power source cuts the workpiece by the energy generated by the discharge striking the surface of the workpiece. The workpiece must be a conductor. In order to prevent the occurrence of a short circuit during processing, an insulating working fluid must flow between the electrode wire and the workpiece, so that proper conditions can be generated to generate the discharge. Usually, the insulating liquid uses deionized water. It has the advantages of low cost, low fire, and easy production safety.
At the same pulse energy, discharge cutting in water can increase the processing productivity. To reduce the wire loss and improve the surface quality of the workpiece being machined, but when the pulse power applied to the discharge gap on the no-load pulse voltage contains a direct current component, the water will be electro-discharge wire cutting will produce electrolytic corrosion state, For example, water electrolysis and anode metal atom dissolution are observed on the electrode wire and the workpiece surface. This leads to the loss of control of the electric erosion process on the surface of the workpiece to be machined, which impairs the surface accuracy, increases the rust or oxidation of the workpiece surface, and weakens the mechanical strength of the workpiece. In addition, hydrogen and oxygen produced by water electrolysis constitute an "explosive" gas mixture, which will explode during spark discharge, causing defects on the surface of the machined workpiece, damaging the electrode wire, moving the fixed electrode, and reducing the machining accuracy.
2 Analysis of Electrolytic Corrosion Discharge State in WEDM Wire EDM Process
In the on-line cutting process, the electrolytic rust discharge state is very common. This irritating discharge state is mainly related to the current flowing through the working fluid, as shown in FIGS. 1 and 2 . In Fig. 1 the electrode wire is connected to the negative pole of the pulsed power supply, and the anode of the workpiece electrode is connected to the positive pole of the pulsed power supply. Fig. 2(a) and Fig. 2(b) show the mechanism waveform of the discharge state during the wire cutting process. The graph shows the voltage V applied to the positive and negative electrodes as a function of time and the discharge current I flowing through the machining gap. The pulse power source emits a pulse voltage during each cycle. During the preparation discharge, the voltage V reaches the voltage peak value Va, and the voltage at both ends of the discharge instantaneous gap drops to the discharge gap voltage Ve. With the end of discharge, the voltage across the gap drops to zero volts. At the same time, the current flowing between the electrodes during discharge is Ie, and the gap current after the discharge is Ir.
Fig.1 Electrolytic corrosion discharge state
Figure 2 gap voltage and current waveform during wire cutting
First, the dissolution of anode metal atoms during processing is the main cause of electrolytic corrosion. Since water is immersed between the electrodes and the conductivity of the water is very low, it is typically several mega Siemens per centimeter to several tens of megabits Siemens per centimeter, so that the gap voltage forms a "leakage current" Ia therebetween. Under the action of leakage current I, the metal atoms dissolved on the surface of the workpiece are ionized to release metal cations and free electrons, as shown in the following equation:
Second, water electrolysis can also accelerate the rate of electrolytic corrosion. Although water is a weak electrolyte, it can decompose positive ion H+ and negative ion OH-, as shown in the following equation:
During the processing, under the action of the electric field, the positive ion H+ and the negative ion OH- move to the cathode and the anode, respectively, and near the surface of the wire electrode, the hydrogen ion H+ releases electrons after the electrons are taken out, thereby increasing the risk of explosion caused by the discharge. In addition, the discharge gap during processing is very small, and the metal cation Mn+ dissociated by the anode has a large chance of reacting with the hydroxide ion to form M(OH)n precipitate deposited on the surface of the workpiece. In this way, the mechanical strength and surface accuracy of the workpiece will be affected.
Since the processing time is usually a few hours or days, and the workpiece is often steel, the anode metal dissolved by the electrolytic corrosion discharge, the anode metal hydroxide deposition, and the hydrogen generated by the process are sparked and exploded. The explosion phenomenon will become Can not be tolerated. Because it not only destroys the accuracy of the metal surface, changes the geometry of the metal surface, weakens the mechanical strength of the workpiece, and at the same time, it also destroys all parts of the machine tool that are in contact with water during the machining process and that can conduct electricity, such as worktables and fixtures. And conductive blocks and so on. After finishing the processing, it is impossible to finish the finishing of the metal workpiece that has been eroded by electroplating. It is impossible to achieve the desired effect. In addition, the machine parts must be carefully measured and additionally corrected. Therefore, since the introduction of EDM, manufacturers and users have been trying hard to find a good way to control the electrolytic discharge state of rust.
3 Method of controlling electrolysis rust discharge state by slow-cut wire cutting EDM
Based on the predecessors' experience, a power supply device for controlling the rust discharge state of the electrolytic wire by slow-cut wire cutting was developed, as shown in FIG. 3 . It is suitable for different gauge machining from roughing to finishing. It is based on the traditional WEDM power supply device to add a group to control the electrolysis corrosion discharge status system. As a typical anode protection device, this system can control a series of unfavorable phenomena such as electrolysis of electrolysis and anode metal atomization due to electrolysis corrosion discharge in a conventional wire-cut power supply device. The workpiece in the on-line cutting process circuit is used as an anode and at the same time in the electrolytic corrosion control circuit. The workpiece is again used as a cathode. The electrolytic corrosion control circuit has at least one or more electrolytic control electrodes, the number of which is determined by the complexity of the workpiece. In Fig. 3, two electrolytic control electrodes are used, which are immersed in water or continuously sprayed with water, while the control voltage is applied between them and the workpiece. During operation, currents Ia and Ie are generated in the electrolytic control loop. These two currents are used to cancel the electrolytic corrosion discharge generated by the process discharge circuit leakage current Ia and the discharge current Ie. As a result, the dissolution of the anode metal and the electrolysis of the working fluid due to electrolytic corrosion in the conventional processing method are eliminated.
Figure 3 Control electrolysis rust discharge state control power supply diagram
Slow Walking Wire Cutting Control Electrolytic Corrosion Discharge State Power MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) are used as switching elements in power systems. Equivalent DC power supplies E and E (100V to 150V) apply operating voltage directly to the electrode wire, workpiece and AE (power electrolytic corrosion) through the power MOSFETs M1, M2, M3, M4 and the current limiting resistors R1, R2, R3, R4. Control) between the electrodes. The controller Con1 controls M1, M2; the controller Con2 controls M3, M4, and their output pulse signals mutually differ in phase by 180°. In order to adapt to different processing materials and machining processes, the controller Con1 and the controller Con2 adjust the pulse width and pulse signal between the wire electrode and the workpiece and between the workpiece and the AE electrode, respectively. The detection circuit K1 and the detection circuit K2 detect the voltage signal between the AE electrode and the workpiece and the workpiece and the electrode wire, ie, the processing state signal and the control electrolytic discharge state signal, respectively, and these two signals are pre-specified in the controller Con1 and the controller Con2. After the signal is calculated, M1, M2, M3, and M4 are controlled to prevent the over-current from damaging the power MOSFET during the process. The current sensor CK is used to detect the current signal during the machining process, thereby ensuring the stability of the machining process. In this circuit, the resistances of R1 and R2 are 3 to 4 times that of R3 and R4. In this way, the absolute values ​​of the currents Ia and Ie do not exceed the leakage current Ia and the discharge current Ie. Otherwise, the metal cation dissolved from the AE electrode will deposit on the surface of the workpiece, which not only fails to control the electrolytic corrosion but also affects the surface roughness and surface accuracy of the workpiece.
Slow walking wire cutting control electrolytic rust discharge state power supply performance indicators are: pulse width adjustment range from 1 ~ 100us, pulse adjustment range from 10 ~ 1000us, minimum duty cycle is 0.1%; discharge peak current is 50A, discharge peak voltage Adjustable from 100 to 150V. In order to limit the actual metal corrosion, metals with high corrosion resistance are used as AE electrode materials, such as: stainless steel, titanium, graphite, and substances with anti-corrosion coating such as platinum.
4 Conclusion
This article discusses the basic principle of electrolysis rust discharge state in the process of slow-cut wire cutting, and develops a power supply device that controls the rust discharge state of electrolysis to control the process of electrolysis rust discharge. This power supply structure is simple. The high performance-to-price ratio has no effect on the processing speed. It has the characteristic of bi-directional current flow in the workpiece. This is achieved by adding another auxiliary system to the conventional processing power supply. During the working process, the control of the electrolytic current is smaller than the normal processing current, so that a good processing effect can be obtained. The voltage waveform during its operation is shown in Figure 4. The upper part is the gap voltage waveform of WEDM normal discharge, the pulse width is 8us, and the interval between pulses is 15us; the lower part is the voltage waveform for controlling the electrolytic rust discharge state, the pulse width is 4us, and the interval between the pulses is 20us; it can be seen that the control electrolysis The rust circuit flows through a very small control current because it can be seen from its voltage waveform that there is a voltage drop of about 20V. This control of the electrolysis rust discharge state power supply device better completes the control of electrolytic corrosion. The surface accuracy and mechanical strength of the workpiece are much improved compared to conventional machining methods. In short, as a new manufacturing method, the power supply device that controls the electrolytic rust discharge state will play an important role in high-precision wire cutting processing, and its application will be more extensive.
Figure 4 discharge gap voltage during normal processing and control electrolytic corrosion voltage waveform
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