Reasons and Prevention of Broken Wire in High-speed Wire EDM Wire Cutting

Abstract : In view of the common phenomenon of wire breakage in high-speed thread cutting machine, six factors that can be used to break the electrode wire are analyzed in detail: the factors of the electrode wire, the factors of the wire feeding mechanism, the factors of the workpiece, the factors of the electrical parameters, and the factors of the working fluid. The use of factors, such as the operation, put forward a corresponding method of prevention, to improve the surface quality and accuracy of the workpiece, improve the processing efficiency of a certain reference.
Keywords: High Speed Wire EDM; Electrode Wire; Reasons for Broken Wires; Prevention

EDM wire-cutting machine tools are usually divided into two categories according to the operating speed of the electrodes: one is a high-speed wire-cut WEDM machine tool. The electrode wire of this machine tool is used for high-speed reciprocating motion. The general wire speed is 6-11m/s. Another type is low-speed wire-wound wire EDM machines. The electrode wire of such machine tools is used for low-speed unidirectional motion, and the general wire speed is lower than 0.2m/s. At this stage, high-speed WEDM wire-cutting machine tools are still the main models produced and used in China.

Due to the high speed of wire cutting and the single wire bidirectional reciprocation, wire breakage is a common problem. This article makes a comprehensive analysis and research on the causes of high-speed wire breakage and proposes corresponding preventive and remedial measures.

1 Broken wire causes and prevention

1.1 Electrode wire factors

(1) The material of the electrode wire. In the high-speed wire EDM process, the electrode wire is usually made of molybdenum wire, and it requires good conductivity and toughness. Because of the good conductivity, the resistance per unit length is small, otherwise, the energy consumed in the filament resistance is much, the energy supplied by the processing power supply to the gap is small, the energy consumed in the electrode wire is much, the wire electrode is hot, and the wire is easily broken; Poorly, it is also easy to break the wire in frequent hot and cold changes. In addition, the quality of the electrode wire should be ensured, such as small surface roughness and high dimensional accuracy. Otherwise, the electrode wire may be easily broken during processing.

(2) The diameter of the electrode wire. The thickness of the electrode wire is an important factor affecting the broken wire. Usually, the thicker wire electrode is selected as far as possible under the condition of satisfying the processing requirements. If the diameter of the electrode wire is too small, the current is small and the incision is narrow, which is not conducive to chip removal and stable processing, and the possibility of wire breakage is high. In addition, the thicker electrode wire can increase the tension of the electrode wire, reduce the jitter of the electrode wire, and it is not easy to break the wire. Therefore, when the electrode wire is worn to a certain extent, it is necessary to replace the electrode wire in time to prevent the wire from being too thin and broken.

1.2 factors of wire feeding mechanism

The wire breakage of the electrode wire caused by the wire feeding mechanism is mainly caused by the improper installation or wear of the guide wheel, the conductive block, the block wire block, and the like in the wire feeding mechanism, and the looseness of the wire electrode.

(1) The guide wheel, conductive block, gear block and other components should be installed correctly, the clearance should be appropriate, the rotation should be flexible, the guide wheel must not allow large radial runout or axial turbulence, and the wire electrode must not be allowed to lap Or clamp wire, to reduce the electrode wire jitter, improve wire electrode movement accuracy, otherwise it will easily lead to broken wire electrode. In addition, the related parts and components must be cleaned frequently to eliminate the dirt. When the guide wheels and conductive parts wear to a certain degree, they must be replaced.

(2) The looseness of the electrode wire can easily cause broken wires. The wire on the electrode wire and the tight wire are an important part of the wire cutting, which directly affects the tension of the electrode wire, is too loose, the tension of the electrode wire is too small, the shake is too large, and the wire breaks easily, and the workpiece quality and the cutting speed are affected; Wire tension is too high, it is also easy to break the wire. Therefore, the tightness of the electrode wire should be appropriate. When the hole or end face is processed, it is easy to break the wire at the initial discharge position. At this time, the tension force may be slightly smaller. After working for a period of time, the wire electrode is loosened. Make adjustments and allow tension to stabilize.

1.3 Workpiece Factors

The high-speed wire-cut WEDM process has certain requirements on the workpiece material and dimensions.

(1) Materials with good forging performance, good hardenability, small heat treatment distortion, and good surface quality are suitable for wire cutting. With interlayers and clips, or workpieces with rough cut surfaces, due to the non-conductive oxidation residues, bumps and bumps can lead to broken wires, burnt wire, or deep marks on the surface of the workpiece. In severe cases, the workpiece is scrapped. Therefore, before the on-line cutting process, the residue should be removed, the surface of the workpiece should be leveled with whetstone, and the appropriate electrical parameters should be selected so as not to break the wire.

(2) The workpiece is too thin or the thin plate superimposed workpiece, easy to break the wire. The thin workpiece, the working fluid is easy to enter and fill the discharge gap. It is beneficial to chip removal and stable processing, but the electrode wire is prone to jitter, or due to misaligned deformation of the workpiece, resulting in wire clipping, resulting in broken wire. Therefore, if you want to cut the thin plate workpiece, you must use a thicker electrode wire or choose a reasonable electrical parameters, such as increasing the power pulse amplitude, increase the pulse width and other methods, so that the spark gap increases, thus preventing breakage. In addition, if the workpiece is too thick and the chip removal conditions are poor, it may easily lead to wire breakage.

1.4 Electrical Parameters

In the high-speed wire EDM process, the reasonable choice of electrical parameters is very important.

(1) When selecting, it is necessary to balance the processing speed, surface roughness and stability. In general, the parameters are strong and the processing speed is fast, but the surface roughness is large and it is also easy to break the filament. Therefore, in the selection of electrical parameters, must take into account the processing requirements of the workpiece, processing accuracy, roughness size and workpiece thickness and other factors to obtain satisfactory processing results.

(2) The cutting preset feed speed adjustment has a great influence on the cutting speed, machining accuracy and surface quality. The preset feed speed is adjusted too quickly, exceeding the possible removal rate of the workpiece. Frequent short-circuits may occur. The cutting speed is slow and the surface roughness is also poor. The upper and lower cross-section cuts are yellow and may even break; otherwise The feed rate adjustment is too slow, greatly lagging behind the removal rate of the workpiece, the poles will be biased to open, sometimes open and sometimes short circuit, the surface is also coke light brown, the upper and lower ends of the workpiece are also burned. Therefore, the preset feed speed should be appropriate, that is, the workpiece removal speed is equal to the feed speed. In this way, the machining surface is fine and bright, the wire pattern is uniform, the surface roughness is good, and the precision is high.

(3) Improper frequency conversion tracking adjustment is also an important cause of broken wire, because the tracking is too fast, it is easy to form a short circuit and the tension of the electrode wire is increased to cause broken wires. When the actual adjustment can be judged by the stability of the ammeter pointer, the more stable the pointer is, the better. If the pointer is not stable, try to make the pointer fluctuate in the 0-bit direction.

1.5 working fluid factors

In high-speed wire EDM machining, the working fluid is a pulsed discharge medium and has a great influence on processing. The working fluid generally requires a certain degree of insulation performance, better washing performance, better cooling performance and no pollution to the environment, and no harm to the human body. The configuration of the working fluid is also very important. When it is too thick and too light, it will cause broken wires, and the water-liquid concentration ratio will be in the range of 1:10-1:20. When the cutting speed is required to be high or when the workpiece is cut with a large thickness, the concentration is appropriately lower, so that the processing is relatively stable and it is not easy to break the filament. However, the concentration of the working fluid should not be too low. Otherwise, the insulating performance of the working fluid will be reduced, the resistivity will be reduced, and the cooling capacity will be enhanced. This will reduce the lubricating effect on the workpiece, and it is not conducive to chip evacuation and easy wire breakage. In order to ensure that the working fluid channel is non-blocking, the size of the working fluid flow should also be properly controlled. Generally, when the cutting is completed, the workpiece will be sagged due to the hydraulic force of the working fluid in the cutting section, the cutting part will fall off, and the electrode wire will be easily pinched off. At this time, the working fluid should be kept at a certain pressure and flow rate, and the corrosives should be discharged in time to prevent the occurrence of broken filaments. When cutting aluminum, copper and other soft materials, especially in the case of relatively thick cutting materials, in order to improve the processing efficiency and surface finish, reduce the phenomenon of broken wire, you can use some special coolant, but also conducive to environmental protection.

1.6 Operating Factors

In the high-speed wire-cut WEDM process, there are certain requirements for the operator in order to efficiently process the workpiece that meets the requirements. The operator must correctly understand the technical requirements of the drawings. The working fluid should be replaced in time, maintain a certain degree of cleanliness, and ensure that the upper and lower nozzles are not blocked and the flow rate is appropriate. The electrode wire is calibrated vertically and the workpiece is clamped correctly. Reasonably select the pulse power supply parameters, when the processing is unstable, adjust the frequency feedrate and so on. When switching function keys, do not perform during processing, so as to avoid short circuit caused by poor contact of the switch, or to cause wire burns, as well as broken wires. When the cutting is completed, the cut-off part will fall under the action of gravity, and the electrode wire will be easily pinched off. It can be fixed in time by means of magnet adsorption, clamp plate clamping and the like to avoid broken wires.

2 Conclusion

There are many factors that affect the wire breakage in high-speed wire-cut WEDM. The influence of various factors is often interdependent and mutually restrictive. The above analysis and research on some phenomena of high-speed wire-cut WEDM interrupted wire, and put forward corresponding preventive measures, this article is based on long-term work experience and access to a large amount of information summed up, I believe to improve the workpiece surface Quality and precision, improve processing efficiency have a certain reference.