2.1 Effect of short circuit
"No contact processing" is the difference between EDM and ordinary cutting, as shown in Figure 1. During normal EDM, there is a discharge gap between the workpiece and the tool electrode. The workpiece is continuously eroded at a rate of Vw. The gap S will gradually expand. The electrode tool must be compensated by the velocity Vd to maintain the required discharge gap. If the amount Vd is greater than the erosion speed Vw of the workpiece, the gap S will gradually become smaller and the electrocorrosion product will be difficult to remove in time, so that secondary discharge will occur, so that the energy consumption will be remelted in the electrocorrosion product and the processing speed will also be increased. Reduce the burned surface. When the gap value is equal to zero (S=0), a short circuit is formed. Although the short circuit itself produces neither material erosion nor damage to the electrode, it creates a hot spot at the short circuit, and after several short-circuits, the automatic adjustment system makes it easy for the tool electrode to “pull arc†when it is retracted to eliminate the short circuit. ". For this reason, in the EDM process, especially after the coarse-medium-precision conversion processing specification, to maintain a certain discharge gap, the spindle feed amount knob must be re-adjusted to achieve a stable feed processing state, even if Under the same circumstance, when the processing area or processing depth has been greatly changed, the feed knob should be re-adjusted to prevent unstable processing and short circuit.
Arcing often occurs during EDM. Once it occurs, it will directly affect the stability of the process, reducing its processing speed and processing quality. In actual processing, in order to ensure the quality of processing and prevent arcing, we often take the following measures: (1) When processing small-area workpieces, the residual heat after discharge is not propagated and diffused in time due to the high concentration of its local electrocorrosion products. Taking a smaller peak current, the current density can be maintained in the range of 3-5A/cm2. (2) When processing large area workpieces, especially when machining slender cavities and complex-shaped workpieces, arcing is often prevented or reduced by raising the knife movement or increasing the fluctuating pressure. (3) In order to ensure the machining process and machining accuracy of the workpiece, the ratio of the pulse width ti to the pulse interval t0 should be selected within the range of 10:1-2:1 during roughing; the pulse width ti and pulse interval during finishing The ratio of t0 should be between 1:1-1:10 so as to improve the chip removal capacity and prevent secondary discharge and arc drawing.
2.3 Improper selection of processing parameters
The factors affecting the surface quality of EDM machining besides arcing and short-circuiting, improper matching of processing parameters is also an important factor affecting the quality of the machined surface. Processing parameters are divided into "offline parameters" and "online parameters."
2.3.1 Influence of Offline Parameters
Off-line parameters include polarity, peak voltage, etc. These parameters are usually pre-arranged before machining and do not change during processing. However, under certain conditions, they also need to change during processing. When the processing area of ​​the workpiece increases from small to large, the cavity becomes shallower and darker, the interference in the discharge process also increases. If the pre-set processing current is still used, it will affect the quality of the machined surface. When the surface will be burned, the discharge current which is less than the predetermined gauge is generally used during the initial stage of processing to make the transition process more stable. After about 1 minute of stabilization, the discharge current is adjusted to the preset value. In order to ensure the normal conduct of electrical discharge machining.
2.3.2 Impact of Online Parameters
The on-line parameters include inter-pulse, feedrate, discharge gap, servo-lifting knife movement, and flushing flow, which are set according to the processing requirements of the workpiece and the processing accuracy. The following main online parameters are discussed below:
(1) Pulse interval t0
In actual processing, when the pulse interval t0 decreases, the workpiece removal speed Vw increases rapidly, and the relative loss ratio p of the electrode also decreases accordingly, but if the clearance value t0 is too small, it will cause arcing, and for this reason, the operator only has to In order to ensure stable electrical discharge machining without drawing arcs, in principle, try to select a smaller value of the inter-pulse value t0 to obtain higher productivity. However, it should be noted that in the initial stage, the selection between the pulses should be slightly Larger to facilitate chip evacuation to ensure the quality of the workpiece.
(2) Discharge gap
In the EDM process, there is a certain discharge gap S between the tool and the workpiece (also known as the end clearance SF). In the rough machining, the operator has sufficient practical experience to ensure the low loss and high productivity. The discharge gap will take a larger value, so that it is safer in processing and minimizes the occurrence of small gaps and large electrode losses, thereby reducing short circuit and arcing. However, the discharge gap cannot be increased too much. Otherwise, the liquid between the two electrodes cannot be broken down, the pulse utilization rate is low, the energy transferred from the discharge channel to the workpiece is significantly reduced, the discharge is insufficient, and the amount of metal etching is insufficient. Normal electrical discharge machining. For example, when machining a complex-shaped workpiece, if the gap is too large, the influence of uneven electric field at the corners will be more pronounced. In the finishing process, in order to improve the machining accuracy, small standard parameters should be used to reduce the discharge gap. In this way, not only the accuracy of profiling can be improved, but also the gap variation that may occur during machining of small gaps is small. Since the discharge gap changes instantaneously in actual processing, it changes with the complexity of the processing area and shape. For this reason, in order to ensure the processing quality of the molding surface, during the processing, the processing parameters should be appropriately adjusted according to the processing accuracy of the workpiece to obtain a stable processing gap.
(3) Effective control of flushing flow
Oil flushing and pumping are the main means to effectively eliminate the electric corrosion between poles in EDM. Generally, when the slender hole is processed, blind oil holes are often used for oil circulation. The effect is better than pumping oil. It is mainly used to clean the working fluid. Forced impulse discharge gap, so that the working fluid with the erosion products from the electrode side clearance removed, but in the actual processing, flush flow pressure is not static, it should be based on the workpiece geometry and processing depth and processing area and the Make adjustments accordingly. We know that in the actual machining of electric sparks, especially in the finishing process, an important reason for the electrode loss is to increase with the increase of the flow rate (pressure) of the flushing liquid. When the deep blind hole is machined or the machining area increases, it increases. The oil flushing pressure, although it can increase the gap discharging force, the erosive force of the electrode surface is also increased. Instead, the galvanic corrosion product is not easy to anti-stick to the electrode surface, which further increases the degree of electrode wear and discharge at the same time. During processing, the concentration of free carbon also decreases as the flow rate of the rush oil increases, which affects the formation of the black film and the uneven flow field. In addition, the partial thickness of the black film at the working site of the electrode is greatly affected by the local erosion and anti-sticking of the electrode. The processing accuracy. Therefore, when processing a deep-type slender blind hole, the size of the oil pressure should be determined by the processing accuracy and the specific processing conditions, and the flushing pressure cannot be increased blindly. Therefore, as long as the processing can be carried out stably and the necessary evacuation is ensured, the processing personnel should adjust the rushing pressure at any time according to their actual processing conditions. For example, when processing the through hole, when the electrode penetrates the workpiece quickly, the oil pressure should be reduced. However, when high-frequency finishing, because of the small gap, chip removal difficulties, but at this time should increase the oil pressure.
(4) Reasonably adjust servo movement
In EDM, the tool electrode is automatically lifted periodically, depending on the size of the workpiece area and the machining accuracy, or by changing the periodic blade lift ratio to increase the chip removal force and increase the pulse pause time. The average current during processing limits the concentration of the electrolytic corrosion product to a large enough extent to achieve sufficient conditions for chip removal. Of course, we know that in EDM machining, lifting a knife means a loss of time. Therefore, only when the normal flushing is not enough, it is easy to consider raising the knife when arcing occurs, and it is necessary to minimize the electrode elevation and The processing time ratio, in the processing according to the requirements of the processing technology, adjusts the knife-lifting ratio in accordance with the specific conditions to ensure the quality of the processing, thereby increasing the productivity.
2.4 Effect of working fluid quality
In electrical discharge machining, the working fluid should be kept in circulation, reducing the accumulation of impurities (metal corrosion products, bubbles, carbon particles, etc.) in the liquid. If the impurities in the liquid increase, the chips are unfavorable, causing burns, short circuits, and insufficient deionization. The formation of secondary discharge, thereby affecting the quality of the surface of the processing.
2.5 Improper processing safety measures
In EDM, the working fluid level must be higher than the workpiece and the electrode at a certain distance, generally about 30 ~ 100mm, because EDM directly uses electric energy, and the exposed parts of the tool electrode and other high voltage of 100V ~ 300V, plus The kerosene discharges for a long time, kerosene will decompose hydrogen, acetylene, ethylene, methane, and a small amount of carbon monoxide and oil mist smoke under the transient local high temperature, and the fire easily burns in the event of open flame. Therefore, in actual processing, the following phenomena must not be allowed:
(1) The machining liquid level is lower than the workpiece (2) The spark caused by the collision of the electrode and the nozzle (3) The spark discharge is accidentally dropped when the connection between the electrode and the spindle is not firm (4) A part of the electrode and the fixture are produced during machining. Unexpected discharge. (5) The accidental cracking of the insulation case caused a spark discharge between the wire and the workpiece holder.
In addition, because the high-frequency pulse power source emits high-frequency electromagnetic waves of a certain intensity to the surroundings during operation, when the human body is too close or exposed to radiation for a long time, it will affect the health of the body; while the working fluid kerosene will evaporate at room temperature and evaporate. The kerosene vapor contains alkanes, aromatic hydrocarbons, cyclic hydrocarbons, and a small amount of organic components. These are toxic gases. Inhaling human body for a long period of time will affect people's health. For this reason, operators must maintain indoor air flow. At the same time, the operator must work on an insulating plate with a withstand voltage of 20 kV or more. During the processing, the electrode must not be touched. The machine tool must be kept clean to prevent moisture, maintain the insulation strength of the machine tool, keep the oil path open, and ensure stable operation. EDM processing to achieve satisfactory processing results.
The following are the relevant experimental data in EDM, which shows the influence of disturbance factors in the EDM process on the quality and process effect of the molding surface:
Experimental conditions:
Experimental equipment: E46PM EDM machine 1;
Experimental electrode: copper electrode, ψ5mm;
Experimental model: A3 steel plate, thickness 10mm, quenched;
Processing method: short pulse positive electrode finishing;
Pulse power supply: Unidirectional pulse power supply;
Working fluid: EDM machine-specific working kerosene;
Processing parts: hole diameter of the forming workpiece ψ = 5mm hole depth H = 1.5mm;
From Table 1 to Table 3 above, we can see that there are many factors that affect the quality of the forming surface in EDM, and they are mutually restrictive. These experimental data show that in the actual processing of EDM, whether or not the processing parameters are properly selected will directly affect the process indicators and discharge stability of the workpiece.
3 Conclusion
The understanding and research of various interference factors in EDM are important guarantees for achieving safe and stable EDM and improving the surface quality and processing accuracy of the parts. Therefore, engineers and technicians engaged in electric discharge machining must cause heights. The importance attached.