Several reasons for the easy accumulation of carbon on electrodes

1. Improper use of machining specifications, exceeding the current density tolerance of the discharge cross-section, and excessively small pulse intervals

       The application of discharge energy is not arbitrary. To process faster, the current is applied larger. In the discharge machining criteria, it generally refers to the peak current, which is the current flowing through the discharge channel when the pulse is on. The current acts for a very short time, depending on the pulse on-time, that is, the pulse width. The current value displayed on the ammeter or simulated ammeter on the processing equipment is the average value of the pulse current, because the pulse is intermittent discharge. Generally speaking, the peak current is determined by the number of power amplifier components input into the discharge circuit and the voltage applied to the circuit, and has nothing to do with the pulse width and pulse interval; while the processing current can be changed by adjusting the pulse width and pulse interval, in addition to being related to the peak current. As shown in the figure.

The correct approach is to accurately estimate the discharge area and select the machining parameters based on the discharge area.

For example, consider a square with a discharge area of 3×3 (mm). When selecting discharge parameters, first choose the model number condition of □3, and then select an appropriate starting machining condition based on the reduction amount.

It should be noted here that the discharge current should be estimated based on the discharge area. The normal current value for this area should be controlled at around 1/9 of the current density value, and can be appropriately increased to no more than 2.5 A. If it is too high, it is prone to produce carbon deposits. Therefore, there is no need to make the reduction too large. As in the above example, a reduction of 0.1 on one side is basically sufficient.

Imported discharge equipment has relatively comprehensive and scientific processing parameter settings, and generally, users can simply select according to their needs. However, for some Taiwanese or domestic low-to-medium-end spark discharge machines, their processing parameters are generally not as detailed as those of imported machines, and there may even be no expert database for automatic selection. Instead, operators have to select processing conditions based on their own experience. The principle for selecting processing conditions is the same. First, estimate the processing area and determine the discharge current. As mentioned above, since the discharge current is related to pulse width and pulse interval, it is also necessary to consider the combination of these two factors. It is particularly important to set a reasonable or sufficient pulse interval. An insufficient pulse interval can prevent the discharge channel from deionizing in time, causing the discharge process to remain in a conducting state and resulting in arc discharge (i.e., carbon buildup). In uncertain situations, it is better to set the pulse interval larger to improve processing stability.

2. Poor chip removal

A. Impact of machining depth

       The smaller the discharge cross-sectional area and the deeper the machining depth, the more difficult it is to remove chips. For machining this type of cavity, improving the quality of chip removal is crucial. When it comes to electric discharge chip removal, there are currently two conventional methods: one is the flushing method, and the other is the immersion method. Additionally, the immersion method is often assisted by the flushing method to enhance the fluidity of the liquid and improve the chip removal effect. In terms of machining accuracy and chip removal efficiency, the immersion method is much better than the pure flushing method (see figure). The chip removal mechanism of the immersion machining method is to utilize the high-speed motion of the spindle to create a high-pressure or vacuum state in the cavity, causing strong turbulence of the liquid to achieve the purpose of chip removal. Imported equipment, due to the high acceleration of the spindle operation, is very suitable for this chip removal method, and the effect is very good. For the flushing machining method, due to the uneven flow direction and pressure of the liquid, the chip removal may not be clean, and where the chip removal material accumulates, it is easy to form secondary discharges, affecting the accuracy of the cavity; secondly, it is also prone to carbon deposition. Therefore, if conditions permit, the immersion machining method should be adopted as much as possible.

B. Impact of processing location

       The location of electric spark discharge is related to whether carbon buildup is easily formed. It is not necessarily true that open-type discharge machining has higher stability than machining blind holes. Sometimes, machining only one surface can actually lead to poorer machining stability than machining all surrounding surfaces simultaneously. The reason is as follows: electric spark chip removal mainly relies on the disturbance of the machining fluid, either through flushing or extrusion. The stronger the fluid's fluidity, the cleaner the chip removal. However, when only one surface is machined, the up-and-down motion cannot cause strong disturbance to the machining fluid. Moreover, if the distance of the spindle's up-and-down movement is relatively small, the erosion products cannot be separated from the discharge gap, which can easily lead to poorer machining stability and even carbon buildup. As shown in the figure.

       In this case, if the machine tool has good performance and can achieve three-axis linkage machining, the lateral servo mode should be adopted to allow the tool to slightly retract from the machining surface during back-off, so that the electroerosion products can be smoothly washed away. If conditions do not permit, try to increase the amplitude of the spindle movement to expose the machining surface to the machining fluid as much as possible, in order to remove the electroerosion products.

C. Impact of liquid flow treatment

       In electrical discharge machining (EDM), fluid flow management is crucial. The direction of fluid flow should be along the direction in which the erosion products are generated. If, as in the example above, the fluid flow direction is not parallel to the discharge gap but perpendicular to it, the discharge state may deteriorate. When machining the aforementioned items, relying solely on immersion processing may not yield good results; it is advisable to supplement it with a flushing method.

D. Impact of material quality

       The quality of materials is, of course, a primary reason for processing anomalies. Generally speaking, the probability of quality issues occurring in graphite materials is higher than in copper materials. This is due to the complexity of graphite manufacturing processes, and also because graphite is, after all, a non-metallic material, with many characteristics that differ from metallic materials. Quality issues in graphite typically manifest as loose material, easy slagging, and uneven discharge roughness. However, all of these phenomena do not necessarily equate to graphite quality issues. Poor processing conditions or improper use of processing conditions can also cause these problems. Only after ruling out these possibilities should one consider whether the issue lies with the material itself. Toyo Tanso provides genuine Japanese-made graphite with stable product quality and guaranteed service. Customers can rest assured in using our various grades of graphite products.