The Analysis and Measurement for Wire Breakage in High Speed Wire Cut EDM Machine Part TWO
(2)The wire breakage of thick workpiece during wire cutting.
By thick workpiece, we generally mean the thickness of workpiece is more than 100mm. The wire breakage of thick workpiece wire cutting may occur when the wire cut machine generates sparks or during the wire cutting process or when the edm parts are finished. The main reasons of wire breakage are as follows.
① Wire breakage happens at the very beginning of wire cut edm process.
The molybdenum wire feeds from the outside of the workpiece and the molybdenum wire breakage takes place once a spark is produced. This is because molybdenum wire is in the outside of workpiece at the beginning of wire cut. The upper and lower wire guide wheels have large distance. There is no damping of molybdenum wire itself so the molybdenum wire keeps jitter. The gap between molybdenum wire and workpiece is in a poor state. The excessive edm emulsion results in lower insulation resistance and poor arc performance so that the discharge gap also contains the arc discharge and then the brass wire burns out. In the electrical discharge machining process, arc discharge is the principal factor of corrosion damage to the negative cathode. Coupled with the poor gap clearance, it is easy to form arc discharge. As long as the arc is concentrated in a certain section, it will cause wire breakage. Moreover, the larger the short circuit current is, the more severe the burn of the arc to the molybdenum wire is and the greater possibility the wire breakage happens.
② Wire breakage happens during the wire cutting process.
When the molybdenum wire cuts into the workpiece, the edm emulsion is difficult to penetrate in to the workpiece dut to the narrow slit. The electrical erosion contents in the slit can’t be removed so that the process conditions of wire cut edm deteriorate. The electric erosion debris happen discharge in the narrow slit which causes the widening kerf. Compared to the thin workpiece of wire cutting, now it is in the same situation the gap is in the poor condition. The formation of pulse discharges arc. When the arc discharge focused on a certain period, the molybdenum wire will be blown soon.
③ Wire breakage happens in the near end of wire cutting process.
The molybdenum wire breakage happens when the wire cut part is almost finished but still a few millimeters and even tens of microns. In addition to the above reasons that we have mentioned, there are also workpiece weight, workpiece material caused by internal stress deformation, resulting in folder wire pull off. So the solution is to invent self-made simple fixture and have heat treatment of the materials before wire cut process.
(3) The wire breakage caused by workpiece containing non-conductive material.
The materials with normal appearance happen a sudden short circuit phenomenon in the normal cutting. In any canse it can not be ruled out. This situation is mostly took place when the materials in the forging or smelting contain impurities. These impurities do not have good electrical conductivity, resulting in the short circuit in the wire edm process and ultimate molybdenum wire breakage. The solution is to program for cutting each 0.05 ~ 0.1mm and reverting back 0.5 ~ 1mm in the wire cut process. Use repeatedly in wire edm and increase the cooling fluid flow. In general, this can help wash away the impurities and restore the normal wire cutting.
(4) The workpiece for wire cutting edm is almostly in flat polish. According to the normal process, it should be demagnetized after grinding. If the wire cut part is not demagnetized, the electrical corrosion particles can be easily absorbed in the slit in the wire cut edm process, particularly when the workpiece is thick. Non-demagnetization can lead to uneven cutting and feeding, increase surface roughness and eventually cause the short citcuit and molybdenum wire breakage.
(5) The wire breakage happens when the workpiece automatically moves to the center hole in the wire cut process.
This is because the hole wall in the edm machine retains oil, burr or some non-conductive materials. When the molybdenum wire moves to the hole wall without spark discharge, this will result that the machine can not automatically change the direction of the workpiece and the workpiece bends molybdenum wire with the top, and finally break off the molybdenum wire. So please be sure to clean the process hole before wire cut process.
1.3 Related to the pulsed power supply
(1) Molybdenum wire breakage is caused by the large machining current and spark discharge anomalies. Most of the molybdenum wire failure is because the pulse power output has become a result of DC output. From the pulse power output stage to the multivibrator step by step check the waveform, replace the damaged components, so that the output pulse waveform to meet the requirements can be put into use.
(2) the output current exceeds the limit broken wire. In the process of spark discharge suddenly become blue arc discharge, current exceeds the limit, the molybdenum wire blown. With the oscilloscope measured input and output part of the oscillation are no waveform. So determine the fault in the oscillation part. Check the transistor found the internal power of the power is restored, the internal pole of the pole is opened, the inner pole of the pole is broken down and the tube is replaced. Another situation of the sudden molybdenum wire breakage in the wire cutting process is that the current is above the limit. Check the power tube, which measured a tube of the internal breakdown of the pole.
(3) Molybdenum wire breakage happens because of burnt point in molybdenum wire. Once the molybdenum wire has the burn point like “knots”, it is prone to break the molybdenum wire. It is considered that this is the machining chip (anode material) adhered to the electrode wire, which acts to concentrate the discharge on the electrode wire. In this case, if the cooling and heat dissipation conditions are poor, So that the continuous discharge in the processing of chips can continue to have other adhesion in the vicinity of the point, thus creating a vicious cycle, resulting in the occurrence of the molybdenum wire phenomenon of burns.
As to problems why the chip will be attached to the molybdenum wire electrode, the main reason is related to the pulse parameters and the cooling conditions of the discharge gap during the wire erosion. The solution is to increase the pulse load power supply voltage amplitude, or adopt the use of double pulse method that is similar to the so-called group pulse, which can reduce the possibility of edm machining chip adhesion to the electrode molybdenum wire, increase the cooling fluid flow and improve the cooling conditions.
Thus for the high speed wire cutting, the key issue is to solve the aspects of high efficiency cutting, wire wear solution, guide wire and location. It is also found that the material removal rate increased significantly at a higher molybdenum wire winding speed, with the straightness got worse at the meanwhile both in edm emulsion dielectric and atmosphere.
To be continued……