TAGUTI molybdenum wire application – Discipline on electrical discharge wire cutting (2)


TAGUTI molybdenum wire application – Discipline on electrical discharge wire cutting (2)

>This time we continue talking about the last topic “Discipline on electrical discharge wire cutting”, we talked about working solution, but not the end. Let’s discuss injection modes and directions. They have big effect on machining precision. First, let’s look at the injection modes: immersion, injected and immersion-injected mode. In immersion method, poor fluidity, unstable machining, Non-uniform discharge clearance and it is difficult to obtain the ideal processing precision in the EDM process; Injected mode is a kind of mode widely used in Chinese high speed wire cut EDM machines. Due to the characteristic, the working fluid can flow fast in the gaps, and more stable machining. But when injected, some air also is injected. Gas medium discharge will happen now and then. Removal feature and liquid medium are also different. Consequently the steps affect the machining precision. Compare immersion with injected mode, we can find that the later has more advantages, so it is used widely in high speed wire cut EDM machines. Slow speed wire cut EDM use immersion-injected mode, because the process makes precision parts, so this mode can show injected advantages and at the same time avoid the air.

OK, let’s see the injected directions, it has one way and both ways. No style is good, in the process, as the kerfs is too narrow, the dielectric coefficient of medium liquid is nonuniform in the discharging area, so the discharging gaps are not balance. You can see the parts not smooth and bad precision. If use one way to inject the liquid, the solution around entrance end is clean, but it is dirty around the exit end, thus the part will have an inclination; if choose two-way to inject the operating fluid, both of the ends are clean, but it is dirty in the middle. Dielectric coefficient is low, and that drum cutting surface. Workpieces are thicker, and you can see the phenomenon obviously.

c. The material and thickness
(a). The material affect the tech index,

At other special conditions, the materials have different chemical and physical characters, so result to different effect. When slow speed wire moving, kerosene as medium, making workpiece of brass materials, the process is very stable and the machining speed is very fast. Machining high melting point, high hardness, high brittleness material like cemented carbide, not stable and not fast as the brass parts. When make the steel, especially stainless steel, magnetic steel, unhardened or the Quenching but hardness is low, unstable process, slower speed and surface roughness is poor. When hard alloy steel, machining stably, low speed but good surface roughness. Different materials, there are different machining efforts. This is because of different materials, pulse discharging energy has the different distribution, conduction and transition on the poles. From the view of thermal point, material of electric spark machining has the relation with its melting point, boiling point.

b. Thickness
The thickness has big effects on the work liquid into & out of the work area, ectric corrosion product excluding, channel deionization, etc. Meanwhile Electrical discharge channel pressure on wire electrode shaking inhibition is related to the thickness of the workpiece. Workpiece thickness has some necessary effects on EDM machining stability and machining speed. If the workpiece material is thin, work liquid can come into and go out of discharging gaps, is good for chip removal and deionization, stable machining. But if the workpiece is too thin, to fixed wire scaffolds, the distance between the ends and roller is big of the wire. Then it is shaking easily, and has bad effects on machining precision and surface roughness, low utilization rate of pulse and low cutting speed; If the workpiece material is too thick, it is hard for the work liquid to come into and go out of the discharging gaps, so it is not good for chip removal and deionization, bad machining stability.

Workpiece thickness also has big machining speed. Under some certain conditions, the speed changes along with the thickness changing. In a general way, has a corresponding maximum processing speed of workpiece thickness.

2. Feed speed
a. The feed speed has an effect on machining speed,

When wire cut EDM machining, the workpiece continuously corrodes removing, in other word, there is a corrosion removing speed; on the other hand, in order to normal spark discharge, wire electrode must feed forward, that is to say, there will be a feed speed. In the normal processing, corrosion removing speed roughly equal to the feed speed, thus makes the discharging gaps maintaining in a normal range, and makes the wire cut EDM machining going on.

Corrosion removing speed has the relation with machine performance, workpiece material, electrical parameters, non-electrical parameters, etc. But once in a workpiece processing, it can be regarded as a constant; in domestic wire walking machine, there are a lot of machine tool whose feed rate need manual adjustment, it is a variable adjustable parameter at any time.

Normal WEDM processing will ensure the roughly equal feed speed and removal to make feed steady. If the feed speed is too fast (over tracking), that is to say, the feed speed is much faster than removal speed, the discharging gaps will be smaller and smaller, so the result is short circuit. When short circuit happens, he wire can produce a short circuit and fast back immediately. When back to a certain distance, wire electrode feed forward at a speed greater than removal speed, then in the circles of short circuit, the fallback. Such frequent short circuit phenomenon gives rise to two results: on one hand, destabilize the processing, on the other hand caused by broken wires; But if the feed speed is too slow (owe tracking), namely the electrode wire feeding speed obviously falls behind the removal in speed, the distance between the wire electrode and workpiece is more and more big, the open circuit. A temporary halt appears in the process of corrosion removal, the whole processing speed will be naturally greatly reduced. This shows in wire-cutting processing adjusting the feed speed does not have the ability to improve the processing speed by itself, but it can guarantee the stability of the processing.

b. Feed speed has effects on workpiece surface quality,

Improper feeding speed regulation, not only can cause frequent short circuit, open circuit, but also affect machining unstably and broken wire; if the feed speed is too slow (owe tracking), namely the electrode wire feeding speed obviously falls behind the removal in speed, the distance between the wire electrode and workpiece is more and more big, the open circuit. This shows in wire-cutting processing adjusting the feed speed does not have the ability to improve the processing speed by itself, but it can guarantee the stability of the processing. The surface roughness of the workpiece, resulting in instability stripe, or appear on the surface of the ablation phenomenon. There are some conditions to discuss:
a. Feed sped is too high. When the workpiece removal in linear velocity is lower than the feed speed, frequent short circuit, caused by processing is not stable, the average processing speed is reduced, machining surface of coke, brownish, upper and lower end face of the workpiece were burnt.
b. The feed speed is too slow. Now the workpiece corrosion removing speed is greater than feed speed, it also often appears open circuit phenomenon, resulting in processing cannot be continued, the processing surface is coked, hazel, upper and lower end faces of the workpiece has burning phenomenon.
c. Feed speed is a little lower. Removal speed is a little more than feed speed, Machining surface coarser, white, two ends are black and white stripes.
d. Feed speed is appropriate. This removal in the linear velocity and feed speed match, machining surface of thin and bright, wire and line is balance. Therefore, in this case, it can get good surface roughness, high precision machining effect.

3. The spark channel pressure effects on technical index,
In liquid medium pulse discharge, the discharge pressure is with the nature of the explosion, a strong impact on the liquid, gas, removals near to the discharge place, make it to round jet, at the same time accompanied by light and sound effects. The pressure of the spark channel on the wire electrode, result in big backward and bending of wire electrode.

4. Choose the WEDM machining tech reasonably,
a. Catch the key point of the questions,

We can see that there are many elements in the wire cut EDM machining. And there are some relations between the elements. You have to catch the key point in the fact machining.
b. Try your best to minimize the number of broken wires,
In the processing, broken wire is a very common question, the result is also very serious. On one side, broken wire can have a very bad effect on machining speed, especially the broken wire during the middle of the process in high speed EDM machines. On the other side, it also has a very bad effect on the surface roughness of the workpiece. The questions of broken wire are not taught by books or somebody says, just based on your experience in practice. It can be said that in the wire-cutting processing, it is an important symbol in dealing with broken wires in the operating practice.

OK, this topic is over, it takes our two months to discuss the topic. We will change to a new topic next month. I am Amy from SUZHOU TAGUTI EDM Co., Ltd. We can supply EDM filters, Moly wire and other parts in EDM industry. If any need, visit TAGUTI website: www.filter-supplier.com.

This is December of the year of 2015, the end the year and a new year is coming. Hope you finish your 2015 dreams and make a suitable plan for the year of 2016. And all your dreams come true. OK, it is time to say goodbye to all of you. Hope you enjoy the end of year. Let’s see next year again. See you in 2016.

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