After cutting some time, the dark spot appear on every part of wire. The dark spot usually is several to decade millimeters long, the space between dark spots usually is several to decades centimeters long. The dark spot is burnt and carbonized by continuous arc discharge.

1 How to cause “Wire Dark Spot”

Because the ionization can’t remove effectively that causes continuous arc discharge, large numbers of carbon are separated out by arc resistance heat, the wire is carbonized too. The basic conditions causing Wire Dark Spot are including the work piece much thick (long discharge gap), low dielectric coefficient of the fluid (low insulating recovery ability) and DC magnitude (over 10mA). The impurity going into discharge gap is the inducement of this phenomenon. Wire Dark Spot causes the result like drawing arc burnt under the discharge process. If the offset between work piece and electrode occurs, it leads to drawing arc burnt each other. It has to be cleaned by the operator.

2 Development of “Wire Dark Spot”

In case the spot that induces drawing arc burnt in the gap still is existing, repeating arc discharge occurs between this spot and contact point of wire easily, thus the second dark spot occurs too. At same time, a series of charred spot stay in the gap of work piece, fine carbon grains spray into the fluid and enter into the gap again to lead to cross infection. At the moment, changing wire or the fluid or work piece does not work. After period of time, the induced element causing cross infection is absent, in the same condition can go on cutting the same material.

3 Presentation and Observation of “Wire Dark Spot”

The ammeter of pulse source swings as wide range because of arc discharge, short & open circuit and carbon grains. The spark discharge alternately appears with red light, yellow light and white light. The primary dark spot became thicker because of burnt and carbonization, after going through the gap and being ablated several times, it became thinner again, meantime became crispy. Because Wire Dark Spots regularly range on the wire cylinder, people try to find out the regularity. But it is not conformity with the perimeter of wire cylinder, the perimeter of guider wheel and the distance between the conducting block and some part. If there is some regularity; it’s the distance between the burnt start-pinot and the work piece exit.

4 Analysis and Solution of “Wire Dark Spot”

Once the phenomenon of Wire Dark Spot occurs, should start from the three genetic elements to resolve it. Firstly, confirm the quality of pulse generator, as long as there is no DC magnitude to prevent from arc extinguishing, usually not lead to the breakage wire of “Wire Dark Spot”. Secondly, pay attention to the effective composition of water, dirt and coolant not including a certain quantity of salt and alkali which affects dielectric insulation. Thirdly, pay attention to the material, for the thick work piece, arc burnt easily occurs and not easy to be removed, especially the oxidized material, forging material with interlayer and the quenching material without forging. Once “Wire Dark Spot” happens, if only keep any one of material, wire and water, it’s possible for “Wire Dark Spot” to happen again. The solution is: thoroughly change the wire and the fluid, clean well the machine; use large pulse space & pulse width, low current and high voltage to start. After the process is stable, gradually increase the current until 2.5A.