Analysis Of Wire Breakage Causes in Medium-speed Wire Cutting Machines

1. Tightness of molybdenum wire
If the molybdenum wire is installed too loosely, the molybdenum wire will shake violently, which will not only cause wire breakage, but also directly affect the surface roughness of the workpiece due to the shaking of the molybdenum wire. However, the molybdenum wire cannot be installed too tightly. If it is too tight, the internal stress will increase, which will also cause wire breakage. Therefore, the tightness of the molybdenum wire should be appropriate during the cutting process. The newly installed molybdenum wire should be tightened before processing, and the force should not be too large when tightening the wire. After a period of processing, the molybdenum wire becomes loose due to its own stretching. When the elongation is large, the vibration of the molybdenum wire will be aggravated or the molybdenum wire will overlap on the wire storage barrel. The wire will be unstable and cause wire breakage. The tightness of the molybdenum wire should be checked frequently. If there is any looseness, it should be tightened in time.
Molybdenum wire installation. The molybdenum wire should be wound around the wire storage barrel according to the specified direction, and both ends should be fixed at the same time. When winding the wire, generally 10mm is left at each end of the wire storage drum, and the middle is fully wound without overlapping. The width is not less than half of the length of the wire storage drum, so as to avoid frequent motor reversing and accelerated damage to the machine parts, and also to prevent the molybdenum wire from frequently participating in cutting and breaking the wire.
There is a wire stop rod at the molybdenum wire lead-out point on the machine tool. The wire stop rod is a guide column made of two rubies. The wire stop rod does not roll like the guide wheel. They are in direct contact with the molybdenum wire and make sliding friction. Therefore, it wears very quickly. After a short time of use, deep grooves will form where the column contacts the molybdenum wire. It must be checked, turned over and replaced in time, otherwise there will be overlapping wires and broken wires.
2. Wire transport mechanism
The wire transport mechanism of the wire cutting machine is mainly composed of a wire storage drum, a wire rack and a guide wheel. When the accuracy of the wire transport mechanism decreases (mainly the transmission bearing), it will cause radial runout and axial movement of the wire storage drum. The radial runout of the wire storage drum will reduce the tension of the electrode wire, causing the wire to loosen. In severe cases, the molybdenum wire will be pulled out of the guide wheel groove and broken. The axial movement of the wire storage drum will cause uneven wire arrangement and wire overlap. The shaft and bearings of the wire storage drum often produce gaps due to wear, which can easily cause wire shaking and wire breakage. Therefore, the worn shaft and bearings must be replaced in time. When the wire storage drum is reversing, if the high-frequency power supply is not cut off, the molybdenum wire will be overheated in a short time and burn out the molybdenum wire. Therefore, it is necessary to check whether the travel switch at the rear end of the wire storage drum is malfunctioning. The wire storage drum and guide wheel must be kept flexible, otherwise the wire transportation system will vibrate and break during the reciprocating movement. After winding the wire, check whether the molybdenum wire shakes during the no-load wire running. If shaking occurs, analyze the cause. The limit stopper at the rear end of the wire storage drum must be adjusted to prevent the wire storage drum from rushing out of the limit stroke and breaking the wire. The stopper in the wire blocking device contacts and rubs with the fast-moving molybdenum wire, which is easy to produce grooves and cause the clamped wire to break, so it also needs to be replaced in time. The wear of the guide wheel bearing will directly affect the wire guide accuracy. In addition, when the V-groove, gem limit block, and conductive block of the guide wheel are worn, the grooves will also cause excessive friction of the electrode wire, which is easy to break the molybdenum wire. This phenomenon generally occurs when the machine tool has been used for a long time, the workpiece is thick, and the wire feeding mechanism is not easy to clean. Therefore, the accuracy of the wire feeding mechanism should be checked regularly during the use of the machine tool, and the easily worn parts should be replaced in time.
3. Workpiece
Workpiece material: For unforged and unquenched materials, it is best to use low-temperature tempering to eliminate internal stress before online cutting. Because if the internal stress of the workpiece is not eliminated, some workpieces will crack during cutting, breaking the molybdenum wire; some will cause the gap to deform, and the molybdenum wire will be clamped or broken. For example, quenched T8 steel is used as little as possible in online cutting and is prone to wire breakage. When cutting thick aluminum materials, due to the difficulty of chip removal, the conductive block is worn out, so pay attention to timely replacement.
Workpiece clamping: Although the workpiece is subjected to very little force during wire cutting, it is still necessary to firmly clamp the workpiece to prevent wire breakage due to changes in the workpiece position during processing. At the same time, wire breakage caused by the weight of the workpiece and the elastic deformation of the workpiece material should be avoided. When processing heavy workpieces, you can use a magnet to absorb the workpiece that is about to fall when the processing is about to end, or manually protect the falling workpiece so that it falls slowly and parallelly to prevent wire breakage.
4. Electrical parameters
Improper selection of electrical parameters is also an important cause of wire breakage. Therefore, it is necessary to select reasonable electrical parameters according to the thickness of the workpiece, and open up the pulse interval to facilitate the discharge of molten metal particles. At the same time, the peak current and no-load voltage should not be too high, otherwise the energy of a single pulse will increase, the cutting speed will increase, and concentrated discharge and arcing will easily occur, causing wire breakage. The general no-load voltage is about 100V. In EDM, arc discharge is the main factor causing negative electrode corrosion damage. In addition, if the gap is not appropriate, it is easy for a pulse to form arc discharge. As long as the arc discharge is concentrated in a certain section, it will cause wire breakage.
Choose the appropriate discharge gap according to the thickness of the workpiece: the discharge gap cannot be too small, otherwise it is easy to cause a short circuit, which is not conducive to cooling and the discharge of electrolytic corrosion products; if the discharge gap is too large, it will affect the surface roughness and processing speed. When cutting thicker workpieces, large pulse width current should be used as much as possible, and the discharge gap should also be larger and longer to enhance the chip removal effect and improve the cutting stability.