molybdenum wire application, discipline on electrical discharge wire cutting

About the main tech indexes:
1/ The wire cut speed,

The speed is guaranteed some roughness on the surface in the wire cut process, in the unit time, the total area that the middle wire in the wire cuts on the workpieces, the unit is mm2/min. The max wire speed is at the condition of disregarding the directions and roughness, it gets to the max wire cut speed. In general, the speed is between 40 and 80 mm2/min. It has some relationship with electric current.

2/ Machining precision/ working accuracy,

Machining precision means the together of the precision in size, shape and position. Machining precision is an aggregative indicator, it includes the precisions in the control of cutting track, Accuracy of mechanical transmission, Workpiece clamping positioning accuracy and Pulse power parameter fluctuations, the error in the diameter of the electrodes, loss & shaking, The change of working liquid smudgy degree, proficiency of the operator.

3/ Surface roughness,

In China and Europe, we show arithmetical mean deviation of the (roughness) profile “Ra(μm)”, but in Japan, they use “Rmax”.

4/ Wire electrode wastage,

To high speed EDM machines, the wire will be reduced after cutting the area of 10 000 mm2 , the reduction is not less than 0.01mm. But to slow speed EDM machines, due to the wire is one-off, so it makes no sense to discuss about the wastage.

About: The influence of electric parameters on the technical index

1/ the peak discharge current has an effect on technic index,

The peak discharge current increases, single pulse energy increases, the discharging trace in workpiece increases. So the cutting speed is improved, surface roughness number value increases, the wastage of wire is increased but the machining precision reduces. So the bigger peak discharging current for the first cutting and machining the thicker workpieces. The current has the limited range, when reaches a certain critical value, if continue adding the current, then the stability of machining variation becomes bad, the machining speed becomes slow, and even broken wire.

2/ pulse width ti
At the same condition, adding the pulse width ti, improving the speed of the wire cutting, the worse of the surface roughness. This is because when add the width, single pulse energy increases, the discharging trace in workpiece increases. The same time the wire wastes too much. When the width reaches a certain critical value, the speed reduces instead.

3/ pulse interval

At the same conditions, reduce the interval, pulse frequency improves, so the times of discharging adds, and average current increases, and improve the cutting speed.

4/ polarity
The pulse width is narrow, so in fact we use positive polarity to make, if not, the speed will be slower and the big wastage.

Non-electrical parameter

1/ The choice of electrodes,
At present the common electrode materials are molybdenum wire, tungsten wire, Molybdenum-tungsten alloy wire, brass wire, Copper tungsten filament and etc.

Adopt tungsten wire, the higher speed, but after discharging, the wire is brittle and broken wire easily. So it is used little. Melting point of moly wire is lower than tungsten wire, Low tensile strength, but good tenacity. In the process of frequent change snap hot and cold, the wire can’t be embrittlemented and be broken wire easily.
Tungsten-molybdenum wire (half-half) whose machining effect is better than Tungsten wire and moly wire, and at the same time the mixture wire has the both wire’s characters, the service time is longer and the machining speed is higher than moly wire. Copper-tungsten has a better machining effect, but the tensile strength is a little worse, with more expensive and rarely source, so it is seldom used in the industry. Brass wire whose machining speed is faster, and good machining stability, but bad tensile strength and great loss.

At present, moly wire is widely used in high speed wire cut EDM machining and brass wire whose diameter is at least 0.1mm is used in slow speed wire cut EDM machining.

You can choose the EDM wire based on the machining needs and tech condition. Around a range of machining, you can choose the bigger diameters’ EDM wire. Bigger diameter, more tensile strength, bear more current, Strong electric gauge is used, that can improve the output pulse energy and improve the machining speed finally. In the meanwhile, thick wire electrode, kerfs width, the excluded condition of discharge products is good and stable processing. So in further, that can improve pulse efficiency and machining speed. But you have to consider that if the wire is much too thick, it is very hard to make sharp corners in workpieces. What is the result? The machining precision is reduced and kerfs are too wide that make the materials removal rate becomes big. It will have an effect in the speed; it is not wise to choose a much too smaller diameter, lower tensile strength and broken easily, narrow kerfs, bad excluded condition and unstable process to make the slower speed. The advantage of thin EDM electrodes is that you can get a smaller diameter sharp corners, the machining precision is improved in some degree. The diameter of moly wire between 0.10mm to 0.25mm is commonly used in high speed wire cut EDM machining.

2/ Wire travelling speed
To high speed wire cut EDM machines, in some special range, along with the improvement of the velocity, it is good for the end of the pulse discharge channel deionization quickly. Meanwhile, high speed movement can bring work fluid into thicker workpieces’ discharging gaps. So it is make for chip removal and guaranteed the stable machining process. We can see that in some special process, along with the increasing of the wire speed, improving the machining speed. The same way for slow speed wire cut machines. This kind of moving type is very stable, shaking little, so it can make good surface roughness and machining precision; but if it is too slow, the discharging products can’t be removed in time. The result is short circuit and unstable discharging in the EDM process.
The optimal speed for slow speed wire cut EDM machines have some related with the workpieces, EDM wire and diameter, etc. Now you can see the recommended speed in operating manual.

3/ EDM wire moves back and forth
You will find Black and white stripes in high speed EDM cut processing. EDM wire moves back and forth. It also makes inclination. When the wire works up and down, entrance and exit have different size. The wide-mouthed end is the entrance and the narrow one is exit, thus on the same cut surface the height of the ends are different. This point has an effect on the precision and surface roughness.
But you won’t find the above phenomenon in slow speed wire cut EDM machining. Because the wire is one-off, don’t need to change the directions, and keep work fluid and removal products balance in the discharging gaps, so you can’t see Black and white stripes in the workpiece. Due to slow speed system, the slow speed and stable state. In this good process, it is not easy to make big Machine shake and that avoid the stripes.

4/ The tension of the wire electrodes
a. At the start step, the tension becomes more and the cut speed becomes faster, do you know the reason? In the process, the wire electrode becomes smaller amplitude, the kerfs width is narrow, so the speed is faster. If the tension is very weak, one side it will shake a lot and result to short circuit frequently, unstably and the machining accuracy is not high; on the other side, the wire is too loose to make the electrodes curve seriously in the process of machining under discharging pressure, the cutting track becomes backward and off tracks. And in the machining process, there will be errors in shape and size. But the tension is too strong, the cut speed won’t continue increasing, on the contrary, it is broken wire easily. The reason of broken wire is due to the control of wire’s tensile strength. So at practice processing, the start cut wire’ tension should be weak to make sure the wire not broken. When in precision machining, the tension should be strong to reduce the wire’s shake to improve the machining precision.

b. At the same working conditions, different working fluids make different machining speed, surface roughness. The speed of WEDM is on the basis of the dielectric coefficient, mobility, detergency and so on. The fluids include kerosene, deionized water, emulsified liquid, detergent liquid, alcoholic solution, etc. But at the condition of the work solution like kerosene and alcoholic solution, the slow wire cut speed, free-burning, so it is rarely used now. At present, emulsified liquid is widely used in high speed wire cut EDM machining procedure, the speed is fast in the process. Deionized water is used in slow speed wire cut EDM machining procedure.
We discuss about some elements make a difference in the machining technic index. In the last paragraph, we talk a little about the operating solution, but time is limited, we will continue discussing it in next month. OK?