The wire breakage has always been a problem that exists in the process of high speed wire cutting machine. The EDM wire breakage makes all wire cut process pause and has to start from the scratch, wastes a lot of time, destroys the integrity of machining surface and increases the difficulity level of unmanned processing. This article is aimed at analysing the wire breakage situation such as molybdenum wire and edm brass wire and putting forward the corresponding measurements, especially in the complex and practical wire edm process.
1. The analysis and solutions for wire breakage
1.1 Relate to the electrode wire
(1)Electrode wire tension & wire feeding speed
To the high speed wire electrical discharge machining (WEDM), the widely used electrode wires are molybdenum wires from wire diameter 0.06 mm to 0.25 mm because the TAGUTI molybdenum wires have higher tensile strength, lower wastage, long service life and high curl rate. To increase the tension of molybdenum wire can reduce the influence of electrode wire vibration so as to improve the process precision and wire cutting speed. The fluctuation of electrode molybdenum wires has a great impact on the processing stability. The reason for this vibration is: the tension of molybdenum wire in positive movement is different from that in negative movement; the tension of the electrode wire is elongated after worling for a period of time, which results in the tension declining (It is generally belived that the tension in the range of 12 to 15 N is most suitable). The declining tension of molybdenum wire may cause the vibration intensified and wire more easy breaking.
With the increase of wire winding speed, the processing speed advances in a certain range. While the improved wire walking speed contributes to the electrode wire and molybdenum wire will transmit the working fluid to a larger thickness of workpiece, it is beneficial to eliminate the corrosion products and enhance the electrical discharge stability. However, haste makes waste. If the wire walking speed is too high, the molybdenum wire shakes violently, which not only reduces the processing speed, but also worsens the machining accuracy and surface finishness and easily leads to wire breakage. Similarly, the wire walking speed can not be too low. Otherwise it is easy to break due to the large loss during the wire cut edm process. The generally appropriate speed of molybdenum wire is less than 10 m/s.
(2)The choice of electrode wire
The choice of electrode wire is nothing more than the type and diameter of EDM wire. In general, the material of electrode wire in electrical discharge machining should have good electrical conductivity, small electronic spillover ppower, large tensile strength and good resistance to electric corrosion. The EDM electrode wire itself may not be winded or knotted. The materials are usully molybdenum wire, tungsten wire, tungsten-molybdenum wire, brass wire, copper-tungsten wire and so on. The most used materials are molybdenum wire and brass wire in wire cut edm process. Tungsten wire processing can access a higher cutting speed, but the edm wire becomes brittle and easy to break after discharge. So it is used less in the application just when it is slower wire walking speed and weak electricity standards. Compare with tungsten wire, molybdenum wire is featured with low melting point, low tensile strength but good resilience. Thus, in the frequent changes of rapid hot and cold temperature, the molybdenum wire is not easy to become brittle and break. Although some performance is not as good as tungsten wire, molybdenum wire is still the most widely used in the edm electrode wire. Tungsten-molybdenum wire (half tungsten and half molybdenum) performs better than single tungsten wire and single molybdenum wire.
So the service life and cutting speed is higher than the molybdenum wire. But the price is expensive. EDM brass wire shows high cutting speed and stable processing, but poorer tensile strength and large loss. It is used in low speed wire electrical discharge machining. In summary, the type of EDM electrode wire depends on the processing conditions. Or it will often cause wire breakage in wire cut machines.
For the high speed wire electrical discharge machining process, the general electrode wire diameter is between 0.06 mm and 0.25 mm and commonly used in the range of 0.12 mm to 0.18 mm. To obtain a fine shape and a very small radius of the fillet in the wire cut process, choose 0.04 mm electrode wire. Proper selection of edm electrode wire will greatly reduce the wire breakage.
(3)New molybdenum wire and self-broken molybdenum wire
There is a layer of black oxide in the surface of new molybdenum wire. During the edm wire process, the wire cuts fast and the workpiece surface shows thick black. At this time the power source is too large, it is easy to break molybdenum wire. Therefore, to the new molybdenum wire, the processing power current should turn to reduce appropriately and restore the normal electrical parameters until the electrode wire turns white.
When the wire cut machine is not used for a long time, it’s found molybdenum wire has been broken when it is used. This is the temperature difference that makes the thermal expansion and contraction of the molybdenum material and coupled with the tension of molybdenum wire. So if the machine is not used, it is better to roll the wire barrel to the end and loosen the molybdenum wire.
(4) Here we introduce a kind of molybdenum wire to all of you — TAGUTI molybdenum wire! It adopts the cold drawn technology to produce molybdenum wire and this kind of molybdenum wire is made by TAGUTI factory for 20 years history. It keeps a hot selling both in China and overseas. There are quite competitive advantages for TAGUTI molybdenum wire: strict material selection of pure molybdenum materials and excellent electrical erosion resistance, high tensile strength, low ductility, Correct wire diameter to ensure the processing precision, Low wastage and long service life, high curl rate, Vacuum package, more than one year storage life. The diameter of 0.18 mm molybdenum wire is the most widely used in high speed wire cut edm machines. The common specifications of molybdenum wires are from diameter 0.10 mm to 0.30 mm.
1.2 Relate to the workpiece
(1) The wire breakage in thin workpiece during wire cutting
Thin workpiece commonly refers to its thickness below 3 mm. The main reason for molybdenum wire breakage: the open distance between upper and lower wire guide wheels is fixed and generally around 70 mm. In the process of high speed wire cut edm, when cutting the thin edm parts workpiece, the molybdenum wire loses the damping effect of edm coolant resulting from thick workpieces processing. Spark discharge influences the vibration of the molybdenum wire.
The solution is to adjust the process voltage to about 50 V, adjust the wire cut current of 0.3 A, and adjust the pulse width less than 10 mm. For reducing the shaking of molybdenum wire, reduce the phase voltage, to commutate the transition time slightly longer, to achieve soft commutation in the high speed wire edm. Adopt the method to increase the thickness, increase damping effect between the upper and lower guide wheel. It is simple to prevent molybdenum wire shaking by increasing the thickness and this method does not need to adjust the processing electrical parameters.
To be continued……