Reasons for carbon deposition on electrode

2020-12-17254

        1. Improper use of processing standards, beyond the discharge cross-section current density capacity, pulse to pulse is too small

        The application of discharge energy is not random. If you want to process faster, the current should be applied a little higher. In the EDM standards, it generally refers to the peak current, that is, the current flowing through the discharge channel when the pulse is on. The current action time is very short, which depends on the pulse on-off time, that is, pulse width. On the processing equipment, the current value displayed by ammeter or analog ammeter is the average value of pulse current, because the pulse is intermittent discharge. Generally speaking, the peak current is determined by the number of power amplifier components in the discharge circuit and the voltage applied by the circuit, and has nothing to do with the pulse width and pulse to pulse; besides the peak current, the processing current can be changed by adjusting the pulse width and pulse to pulse. As shown in the figure.

        The discharge current density of graphite electrode is about 6 ~ 8 (a) / cm2. If the discharge current density exceeds this value, the probability of abnormal discharge increases and the probability of carbon deposition increases.

        The correct processing method is to estimate the discharge area correctly and select the processing parameters according to the discharge area.

        For example, there is a square electrode with a discharge area of 3 × 3 (mm). When selecting the discharge parameters, first select the model number condition of ﹣ 3, and then select a suitable initial processing condition according to the reduction of the electrode.

        It should be noted here that the discharge current should be estimated according to the discharge area. The normal current value of this area should be controlled at about 1 / 9 of the current density value, which can be appropriately increased to no more than 2.5A. If it is too large, it is easy to produce carbon deposition. Therefore, for some small electrodes, it is not necessary to reduce the electrode too much. For example, the reduction of 0.1 electrode on one side is enough.

        The processing parameters of imported discharge equipment are relatively complete and scientific. Generally, it can be selected according to the needs. However, some Taiwan machines or domestic low and medium gear spark discharge machines have less detailed processing parameters than imported machines. There is even no expert database for automatic selection, so the processing conditions can only be selected according to the experience of operators. The principle of selecting the processing conditions is the same. First, estimate the processing area and determine the discharge current. As mentioned above, the discharge current is related to the pulse width and pulse to pulse ratio, so it is necessary to consider the matching relationship between the two. Special attention should be paid to setting a reasonable or sufficient pulse to pulse time. If there is not enough pulse time, the discharge channel will not be able to eliminate ionization, and the discharge process is always in the conduction state Arc discharge (i.e. carbon deposition) is formed. In the case of uncertainty, it is better to set the pulse gap larger to improve the machining stability.

        2. Poor chip removal

        A. Influence of machining depth

        The smaller the discharge cross-section area is, the deeper the machining depth is, the more difficult chip removal is. It is crucial to improve the chip removal quality for processing this type of cavity. When it comes to EDM chip removal, there are two conventional methods at present, one is flushing method, the other is immersion method, and the other is using flushing method to improve the fluidity of liquid to enhance the chip removal effect. In terms of machining accuracy and chip removal effect, the immersion method is much better than the simple flushing method. (see Fig.) the chip removal mechanism of the immersion processing method is to use the high-speed movement of the spindle to cause the cavity to be in or high-pressure or vacuum state, causing strong disturbance of the liquid to achieve the purpose of chip removal. The imported equipment, because of the high acceleration of the spindle, is very suitable for this method of chip removal, and the effect is very good. For the flushing type processing method, due to the uneven flow direction and pressure of the liquid flow, the chip removal may be not clean. The place where the debris is gathered is easy to form two discharges, which affects the accuracy of the mold cavity; secondly, it is easy to cause carbon deposition, so the immersion processing method should be used as far as possible when the conditions permit.

        B. Influence of processing position

        It is not necessarily that the stability of open EDM is higher than that of blind hole machining. Machining only one surface sometimes leads to worse machining stability than machining all peripheral surfaces at the same time. The main reasons for the cleaning or discharge of chips are as follows. In the case of machining only one surface, the up and down movement of the electrode can not cause strong disturbance of the processing fluid. Moreover, if the distance between the up and down runout of the spindle is relatively small, the corrosion products can not leave the discharge gap, which is easy to cause poor machining stability and even carbon deposition. As shown in the figure.

        In this case, if the performance of the machine tool is good and the three-axis linkage machining can be realized, the lateral servo processing is adopted to make the electrode slightly away from the processing surface when it is retreated, so that the corrosion products can be washed away smoothly. If there is no condition, increase the motion range of the spindle as much as possible, so that the machining surface is exposed to the processing fluid as much as possible, so as to eliminate the electric erosion.

        C. Effect of liquid flow treatment

        Liquid flow treatment is very important in EDM. The direction of liquid flow should be along the direction of erosion products. If the direction of liquid flow is not parallel to the discharge gap but perpendicular to the discharge gap in the above example, the discharge state may become worse. When processing the above-mentioned articles, however, the effect may not be very good by immersion processing, supplemented by flushing method.

        D. Influence of electrode material quality

        Of course, the quality of electrode materials is also the main reason for abnormal processing. Generally speaking, the probability of quality problems of graphite materials is higher than that of copper materials. The reason is that the graphite manufacturing process is complex