Maybe you are exposed to EDM every day in production, but do you really know EDM well?
First of all, do you know how EDM technology comes from? In 1943, when Lazarenko and his wife, academicians of the former Soviet Academy of Sciences, studied the phenomenon and causes of spark discharge corrosion damage to switch contacts, they found that the instantaneous high temperature of electric spark could melt and gasify local metals and be corroded, and initiated and invented EDM.
So, what is the principle of EDM? Let’s understand the principle and micro mechanism of EDM through a video
EDM is a machining method that melts and erodes materials through the high temperature (instantaneous temperature up to 10000 ° C) generated in the micro process of pulse discharge.
1、 Micro process of a pulse discharge
Electrical discharge machining (EDM) is a process of metal removal by continuous discharge. Although the time of a pulse discharge is very short, it is a comprehensive process of electromagnetism, thermodynamics and hydrodynamics. In summary, the process of one pulse discharge can be divided into the following stages:
1) Ionization and breakdown of interelectrode dielectric and formation of discharge channel
When a pulse voltage is applied between the tool electrode and the workpiece, an electric field is immediately formed between the two poles. The electric field intensity is directly proportional to the voltage and inversely proportional to the distance. With the increase of inter electrode voltage or the decrease of inter electrode distance, the inter electrode electric field intensity will also increase. Because the micro surface of tool electrode and workpiece is uneven and the distance between poles is very small, the electric field intensity between poles is very uneven, and the electric field intensity at the nearest protruding point or tip between poles is generally the largest. When the electric field strength increases to a certain amount, the dielectric is broken down, the discharge gap resistance decreases rapidly from the insulation state to a fraction of an ohm, and the gap current rises rapidly to the maximum. Because the channel diameter is very small, the current density in the channel is very high. The gap voltage drops rapidly from the breakdown voltage to the spark maintenance voltage (generally about 20 ~ 30V), and the current rises from 0 to a certain peak current.2) Medium thermal decomposition, electrode material melting, vaporization thermal expansion
Once the interelectrode medium is ionized and broken down to form a discharge channel, the pulse power supply makes the electrons between the channels rush to the positive electrode and the positive ions rush to the negative electrode at high speed. Electric energy becomes kinetic energy, which is converted into heat energy by collision. Therefore, in the channel, the positive and negative surfaces become instantaneous heat sources respectively, reaching a very high temperature. The channel vaporizes the working liquid medium at high temperature, and then thermal cracking decomposition vaporization. The vaporized working liquid and metal vapor suddenly increase in volume, become bubbles in the discharge gap, expand rapidly and have the characteristics of explosion. Observing the EDM process, we can see that bubbles appear between the discharge gaps, the working fluid gradually turns black, and a slight and crisp explosion sound can be heard. EDM mainly relies on thermal expansion and local micro explosion to throw out the melted and vaporized electrode materials for erosion.
3) Throwing out of electrode material
The instantaneous high temperature of the discharge point on the surface of the channel and the positive and negative electrodes makes the working fluid vaporize and the metal materials melt and vaporize, and the thermal expansion produces a high instantaneous pressure. The pressure in the center of the channel is the highest, which makes the vaporized gas expand outward continuously, and the molten metal liquid and steam at the high pressure are pushed out and thrown into the working liquid. Due to the action of surface tension and cohesion, the thrown material has the smallest surface area and condenses into fine spherical particles during condensation.
When the molten and vaporized metal is thrown away from the electrode surface, it splashes everywhere. In addition to throwing most of it into the working solution and shrinking into small particles, a small part of it splashes, coatings and adsorbs on the opposite electrode surface. This phenomenon of mutual splashing, plating and adsorption can be used to reduce or compensate the loss of tool electrode in the machining process under some conditions. In fact, the process of etching and throwing out metal materials is relatively complex. At present, people’s understanding of this complex mechanism is still deepening.
4) Deionization of interelectrode medium
With the end of the pulse voltage, the pulse current also drops to zero rapidly, but there should be an interval after that to deionize the gap medium, that is, the charged particles in the discharge channel are compounded into neutral particles, restore the insulation strength of the medium at the discharge channel, and reduce the electrode surface temperature, so as to avoid repeated discharge at the same place next time, resulting in arc discharge, So as to ensure that the next breakdown discharge channel is formed at the nearest place between the two poles or at the place with the minimum resistivity. If you want to learn UG programming and don’t want to engage in bottom work, you can add QQ group 565120797 to learn CNC processing programming technology.
Therefore, in order to ensure the normal process of EDM, there should be sufficient pulse interval between two pulse discharges. In addition, there should be room to disperse and transfer the breakdown and discharge points, otherwise it is easy to form an arc if it is discharged only near one point.
2.Five different discharge states
Discharge state refers to the basic state of each pulse discharge in the discharge gap in EDM. There are generally five discharge states, as shown in the figure below.
Pulse parameters and pulse voltage and current waveforms
1) Open circuit (no-load pulse)
There is no breakdown in the discharge gap. There is a voltage greater than 50V on the gap, but there is no current flowing in the gap. It is in no-load state.
2) Spark discharge (working pulse, or effective pulse)
The insulation performance in the gap is good, and the working liquid medium can effectively throw out and erode the metal after being broken down. Its waveform is characterized by small sawtooth with high-frequency oscillation on voltage and waveform.
3) Short circuit (short circuit pulse)
The discharge gap is directly short circuited, which is caused by excessive instantaneous feed of servo feed system or overlap of electric corrosion products in the discharge gap. When the gap is short circuited, the current is large, but the voltage at both ends of the gap is very small, so it has no effect of etching.
4) Arc discharge (stable arc discharge)
Due to poor chip removal, the discharge points are concentrated in a certain part without dispersion, local heat accumulation, temperature rise and vicious cycle. At this time, spark discharge becomes arc discharge. Because the discharge point is fixed at a certain point or part, it is called stable arc, which often burns the electrode surface area. The waveform of arc discharge is characterized by the disappearance of small sawtooth with high-frequency oscillation.
5) Transient arc discharge (unstable arc discharge, or unstable spark discharge)
Transition arc discharge is the transition state between normal spark discharge and stable arc discharge, and it is the precursor of stable arc discharge. The waveform feature is that the breakdown delay is very small or close to zero, only becomes a spike, and the high-frequency component on the voltmeter becomes low or sparse sawtooth. The above discharge states occur alternately and probabilistically in actual machining (related to machining standards, feed rate, oil flushing, pollution, etc.), and more than two discharge states may occur alternately even in a single pulse discharge process.