Pulse power, wire feeding device, and working fluid are all possible causes of molybdenum wire breakage, as detailed in the following analysis.

Relating to the pulse power

The machining current is very large, and the spark discharge is abnormal, resulting in molybdenum wire breakage. This kind of failure is mostly caused by the output of the pulse power supply has become DC output. Check the waveform step by step from the output stage of the pulse power supply to the multi-vibrator, replace the damaged components, and put it into use only when the output is the pulse waveform that meets the requirements.

The output current exceeds the limit and the molybdenum wire is broken. During the high speed wire EDM machining process, the spark discharge suddenly changed to a blue arc discharge, and the current exceeded the limit and the molybdenum wire was burnt. There is no waveform output when measuring the input terminal and the oscillation part with an oscilloscope. It can be judged that the fault lies in the oscillation part. Check that there is an audion. Internal open circuit between the poles of the meritorious service, internal breakdown between the poles, replace this tube, the high-frequency power supply returns to normal.

Another situation is that the molybdenum wire is suddenly broken during processing and the current is above the limit. Using an oscilloscope to measure the output end of the high-frequency power supply. Its waveform amplitude is reduced, and there is a negative wave, and the pulse width meets the requirements, and the frequency, pulse width and amplitude of the measured push-level waveform meet the requirements. Judge the fault in the power amplifier part. Inspect the power tube, and measured the internal breakdown between the ce of one of the tubes, so that the final current was directly applied between the steel wire and the workpiece, causing the molybdenum wire to be burned by the arc. Replace the tube and return to normal.

The molybdenum wire has burnt points and wire breaks. Once a pimple-like burn spot appears on the molybdenum wire, it is very easy to break the wire. It is generally believed that this is caused by the machining debris (anode material) adhering to the electrode wire. The adhering matter acts to concentrate the discharge on the electrode wire. At this time, if the cooling and heat dissipation conditions are poor, it is likely to cause the temperature at the place rise, so that other machining chips may continue to adhere to the vicinity of the point during the continuous discharge, which causes a vicious circle and finally causes burns at the place.

As for the problem why machining chips will adhere to the electrode molybdenum wire, the main reason is related to the pulse parameters and the cooling condition of the discharge gap. The solution is to increase the amplitude of the no-load voltage of the pulse power supply, or use a double pulse method similar to the usual grouped pulse, which can reduce the possibility of processing chips sticking to the electrode wire and increase the coolant flow, improve cooling conditions.

There is an ablation point on the molybdenum wire and the molybdenum wire is broken. In the forehead of the steel wire, there is an ablation point every other segment (about 10mm). Slightly like a mildew spot, serious ablation spots of molybdenum wire can be clearly seen. This is caused by the arc drawn between the electrode wire and the workpiece. For some reason, the workpiece is drawn at point A and point B on the molybdenum wire. The electrode wire is moving. The arc is drawn longer and longer, and point A and the closest point B’ begin to arc again, and the cycle repeats, that is, regularly spaced pits are formed, which greatly reduces the strength of the electrode wire. The main reason for this phenomenon is the unbalanced output of the final stage of the feed system. This phenomenon can be eliminated by adjusting the feed system.

Relating to the wire feeding device and working fluid

The root cause of molybdenum wire breakage related to the wire walking device is the poor accuracy of the device, especially the wear of the different wheels, which will increase the jitter of the molybdenum wire, destroy the normal gap of spark discharge, and easily cause large current concentrated discharge. Thereby it increases the chance of wire breakage. The accuracy of the guide wheel mechanism can be checked from 3 aspects.

The V-shaped groove of the guide wheel becomes wider. This will cause the electrode wire to move back and forth in the Y axis direction, which is manifested in the phenomenon of non-feeding or skipping when the wire storage drum is reversed.

The bottom diameter of the V-shaped groove of the guide wheel is not round. This is caused by the damage of the bearing supporting the guide wheel, the molybdenum wire did not enter the V-shaped groove of the guide wheel during processing, or the guide wheel was jammed by dirt, and the molybdenum wire was pulled out of the deep groove. When the wire storage drum was shaken by hand You will find that the electrode wire is shifted back and forth in the X axis direction.

Poor contact between the conductive shaft and the conductive wheel causes the molybdenum wire breakage. During processing, it was found that the ammeter pointer oscillated to the left and right, and the feed speed was uneven. Sometimes the ammeter pointer returned to zero and the console was fed very quickly. Because there was no discharge, the molybdenum wire was finally broken. At this time, replace the new conductive wheel and conductive shaft.

For workpieces that require high cutting speed or large thickness, the ratio of the working fluid can be appropriately lighter at a concentration of about 5% to 8%, so that the processing is more stable and the molybdenum wire is not easily broken.
The working fluid is dirty, and the overall performance deteriorates after a long time is an important cause of broken wires. In practice, you can measure whether the working fluid is getting worse: when the processing current is about 2A, the cutting speed is about 40mm2/min, and it works for 8 hours a day. The effect is best after two days of use. If you continue to use it for 8-10 days, it will break easily. , It must be replaced with a new working fluid.

Some studies believe that the working fluid configured with high-purity water works stably and there is less molybdenum wire breakage during processing. The reason is that in the process of ion exchange purification, the high-purity water removes some ions harmful to electrolytic corrosion processing, such as calcium ions, magnesium ions, etc., so that in the processing process, although the electrolytic corrosion products are interposed, the working fluid Various ions are mixed in, but the processing is stable due to the removal of harmful ions.