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Several Reasons Why Graphite Electrodes Are Prone to Carbon Deposition

Ⅰ. Improper use of graphite electrode processing standards, exceeding the current density capacity of the discharge cross-section


The application of discharge energy is not arbitrary. If you want to process faster, you need to apply a larger current. The discharge machining standard generally refers to the peak current, that is, the current flowing through the discharge channel when the pulse is on, and the time for the current to act. Very short, depending on the pulse on time, i.e. the pulse width.


On the processing equipment, the current value displayed by the ammeter or the analog ammeter is the average value of the pulse current of the graphite electrode, because the pulse is intermittently discharged.


Generally speaking, the peak current is determined by the number of power amplifier components in the input and discharge circuit and the voltage applied by the circuit, and has nothing to do with the pulse width and pulse interval; while the processing current is not only related to the peak current, but also can be adjusted by adjusting the pulse width and pulse interval. 


The discharge current density of the graphite electrode is about 6-8(A)/cm2. If the value exceeds this value, the probability of abnormal discharge will increase, and the probability of carbon deposition will also increase. The correct treatment method is to correctly estimate the discharge area and select the processing parameters according to the discharge area.


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


It must be noted here that the discharge current of the graphite electrode should be estimated according to the discharge area. The normal value of the current of this area should be controlled at about 1/9 of the current density value, and it can be appropriately increased, preferably not more than 2.5A, if it is too large, it is easy to generate carbon deposits. Therefore, for some small graphite electrodes, it is not necessary to reduce the amount of graphite electrodes too much.


Ⅱ. Poor chip removal of graphite electrodes


1. The influence of processing depth


The smaller the discharge cross-sectional area and the deeper the machining depth, the more difficult chip evacuation is. For the machining of this type of cavity, improving the chip evacuation quality is the key.


2. The influence of processing parts


The spark discharge site is related to whether it is easy to form carbon deposits, not necessarily. Open discharge machining of graphite electrodes has higher discharge stability than machining blind holes. Sometimes machining only one surface is more likely to cause poor machining stability than machining all peripheral surfaces at the same time.


The reasons are as follows. EDM chip removal mainly depends on the disturbance of the machining fluid, or scouring or extrusion. The stronger the fluidity of the liquid, the more thorough the chip removal.


In the case of machining only one surface, the up and down movement of the graphite electrode cannot cause strong disturbance of the machining fluid. Moreover, if the distance between the up and down movement of the main shaft is relatively small, the electro-corrosion products cannot escape from the discharge gap, which is likely to cause poor processing stability and even carbon deposition.


In this case, if the machine tool has good performance and can realize three-axis simultaneous machining, it is better to use the lateral servo method to make the graphite electrode slightly away from the machining surface when retreating, so that the electrical corrosion products can be washed away smoothly If there is no condition, try to increase the range of motion of the spindle, so that the processing surface is exposed to the processing fluid as much as possible, so as to eliminate the electrical corrosion.


3. The influence of liquid flow treatment


In electric discharge machining, the flow treatment is very important. The direction of liquid flow should follow the direction of the graphite electrode galvanic corrosion products. If the flow direction is not parallel to the discharge gap but perpendicular to the discharge gap, then the discharge state may become worse.


4. The influence of graphite electrode material quality


Of course, the quality of graphite 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 manufacturing process of graphite is complicated, and after all, it is a non-metallic material. different from metal materials.


The quality problems of graphite are generally manifested in loose material, easy slag drop, uneven discharge roughness, etc., but all these phenomena do not necessarily mean that the quality of graphite is a problem. Poor processing conditions and improper processing conditions can also cause these problems. Problem, only after these possibilities are ruled out, consider whether the graphite electrode material is a problem.


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