In EDM process, due to EDM parameter selection of inappropriate, or processing chip discharge is not so good reasons to produce concentrated discharge arcing, such as electric sparks are scarce and red sparks, the electric discharge sound has become irregular, then produce the so-called unstable discharge. Processing is not stable, the processing current is not in accordance with the set of the current work, the working current is small, and in the process of processing will appear a variety of processing defects, resulting in waste. Often the following abnormal discharge phenomenon.
(1) the discharge is not stable at the start of the process
At the beginning of the processing of graphite electrode, due to the workpiece being chip cutting, burr, resulting in concentrated discharge; and due to the discharge energy (high peak current, pulse width), and pulse interval is too narrow, jet pressure is too large, resulting in at the beginning of processing discharge instability. Therefore, the machining process of the first complete removal of the workpiece on the workpiece, burr, and the removal of heat treatment due to the oxide film, paint, rust, etc.. At the start, the current is set at a small value, and then gradually increased to the peak current, and the jet pressure is set to a small number of.
(2) a granular projection
In the process of electric spark machining of graphite electrode, the machining depth is too deep to produce electric arc, and the granular material is formed on the workpiece, which causes the workpiece to be scrapped (as shown in Figure 1).
Pulse width is set too large, the angle of the edge of the electrode to generate granular protuberance, thus causing short circuit, causing arcing; electric corrosion product processing chip too much, will not be drained; processing liquid nozzle angle setting is wrong, the machining liquid can not be fully spray into the gap, electric corrosion product processing chip can not be fully discharged; processing depth, processing chips can not be fully discharged, stranded at the bottom.
Shorten the pulse width ton, longer pulse interval toff, inhibit the production of granular protrusion on the generation and electric corrosion product processing chip, and as far as possible to the side of the nozzle is arranged on the electrode; if the processing depth and improve the number of vibrations of the electrode and to accelerate the speed of vibration.
(3) machining of the ground
In EDM process and pulse interval is too small, the electrode vibration speed slow, jet pressure is weak, resulting in electric corrosion product processing chips can not be fully discharged and many electric corrosion product bonded on the electrode surface, forming carbon block, in the electrode motion of easy detachment, in ground processing produce depression (Figure 2).
Extend the pulse interval, improve the electrode vibration velocity, jet pressure increases, also can in the process to suspend processing, with a brush cleaning electrode surface and processing the bottom surface of the processing chip.
(4) the entrance space is large
Because of the graphite electrode lifting speed is too slow, at the lower end of the processing time is too long, the absolute loss, and at the entrance to the electrode of the discharge gap exist electric corrosion product, resulting in "secondary discharge" (machined surface due to the involvement of electrical corrosion products such as again for unnecessary discharge), the discharge gap to expand, in the depth direction of processing produce processing angle, entrance space (as shown in Figure 3). Therefore, in the process of processing, the graphite electrode can not be too slow.
(5) the process of machining the bottom surface or the bottom surface roughness is not uniform
Due to the pulse interval is too small, especially jet pressure uneven, inter electrode gap is too small, electrical corrosion products can not fully discharged, and at the bottom of the machining surface is not uniformly distributed, so along with the processing continuously, on the bottom surface bending (as shown in Figure 4 (a)) or cause at the end of processing rough surface is not uniform (as shown in Figure 4 (b) below).
Increasing the pulse interval, setting the constant jet pressure, increasing the gap between the two electrodes, often check the chip removal.