There are many kinds of wire breaks in wire cutting machine tools. Some experts once thought that the process of wire breaks began with the instability of the machining process. The unstable processing causes the discharge to be concentrated at one point, and the concentrated discharge causes the discharge to be concentrated in time, which increases the energy of the input gap and concentrates at one point, causing local high temperature and causing the electrode wire to be burnt out. This is the whole process of wire breaking. Then, according to the unstable signal of the processing process, taking effective measures to make it enter a stable state as soon as possible is the way to fundamentally overcome the problem of wire breakage.
pulse power supply
pulse power supply is an important part of wire cutting machine tool and a key device affecting wire cutting processing In high-speed wire cutting, the electrode wire is used repeatedly. If it is lost, it will directly affect the machining accuracy, and the probability of wire breakage will increase when the loss is large. Therefore, the wire cutting pulse power supply should have the performance of low loss of electrode wire.
coolant and cooling system
when the cooling system on the wire cutting machine tool is imperfect or the coolant is not selected well, the electrode wire cannot be fully cooled, which is easy to cause the electrode wire to be burnt out At present, water-based coolant has gradually replaced the traditional oil-based cooling oil in the market. It not only meets the requirements of international environmental protection, but also can fully cool the workpiece and electrode, effectively reducing the probability of wire breakage caused by this reason.
stable wire feeding mechanism and tightness of molybdenum wire
the vibration of the electrode wire in WEDM is like a spindle, with large amplitude in the middle and small at both ends. If the difference caused by the vibration exceeds the elastic limit of the electrode wire, the wire will be broken. Similarly, if the molybdenum wire is installed too loosely, the molybdenum wire jitters severely, which will not only cause wire breakage, but also directly affect the surface roughness of the workpiece. However, the molybdenum wire shall not 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 shall be appropriate during the cutting process. However, due to the manual adjustment, it is difficult to master the degree, which depends entirely on the experience of monthly operators. In addition, the loss of molybdenum wire in the long-time processing process will also lead to the relaxation of molybdenum wire. To effectively solve this problem, it is necessary to have a molybdenum wire constant tension mechanism. At present, most of them are mechanical and one-way wire tightening devices, which can not really solve this problem. Only the two-way closed-loop constant tension dynamic servo control system can be used.
wire conveying mechanism
the wire transport mechanism of wire cutting machine is mainly composed of wire storage cylinder, wire rack and guide wheel. When the accuracy of the wire conveying mechanism decreases (mainly the drive bearing), it will cause the radial runout and axial movement of the wire storage cylinder. The radial runout of the wire storage cylinder will reduce the tension of the electrode wire, resulting in loose wire. In serious cases, the molybdenum wire will be pulled out of the guide wheel groove. The axial movement of the wire storage cylinder will lead to uneven wire arrangement and wire stacking. The shaft, bearing and other parts of the wire storage cylinder often produce clearance due to wear, which is also easy to cause wire shaking and wire breakage. Therefore, the worn shaft, bearing and other parts must be replaced in time. If the high-frequency power supply is not cut off during the reversing of the wire storage cylinder, the molybdenum wire will be burnt out due to the high temperature of the molybdenum wire in a short time. Therefore, it is necessary to check whether the travel switch at the rear end of the wire storage cylinder is out of order. Keep the wire storage cylinder and guide wheel rotating flexibly, otherwise the wire will be broken due to the vibration of the wire conveying system during the round-trip movement. After wire winding, no-load wire walking shall be conducted to check whether the molybdenum wire jitters. If jitters occur, the cause shall be analyzed. The limit stop at the rear end of the wire storage cylinder must be adjusted to avoid wire breakage due to the wire storage cylinder rushing out of the limit stroke. The stop block in the wire stopper device contacts and rubs with the fast-moving molybdenum wire, which is easy to produce grooves and cause breakage of the wires, so it also needs to be replaced in time. The wear of the guide wheel bearing will directly affect the accuracy of the guide wire. In addition, the grooves generated after the wear of the guide wheel's V-groove, gem limit block and conductive block will also cause excessive friction of the electrode wire, which is easy to pull off the molybdenum wire. This phenomenon usually occurs when the machine tool is used for a long time, the workpiece is thick, and the wire conveying mechanism is not easy to clean. Therefore, the accuracy of the wire conveying mechanism should be regularly checked during the use of the machine tool, and the easily worn parts should be replaced in time.
workpiece material
for materials that are not forged or quenched, low temperature tempering should be used to eliminate the internal stress before online cutting, because if the internal stress of the workpiece is not eliminated, some workpieces will crack and break the molybdenum wire during cutting; Some will deform the gap and clip or snap the molybdenum wire. For example, the quenched T8 steel shall be used as little as possible in the on-line cutting process and wire breakage. When cutting thick aluminum materials, due to the difficulty of chip removal and large wear of the conductive block, pay attention to timely replace the workpiece clamping: Although the workpiece bears very little force during the wire cutting process, it is still necessary to firmly clamp the workpiece to prevent wire breakage due to the change of workpiece position during the processing process. At the same time, it is necessary to avoid wire breakage due to the dead weight of the workpiece and the elastic deformation of the workpiece material. When processing heavy workpieces, you can use magnets to absorb the falling workpieces at the end of processing, or manually protect the falling workpieces to make them fall slowly in parallel to prevent wire breakage.
electrical parameters
improper selection of electrical parameters is also an important reason for wire breakage. Therefore, reasonable electrical parameters should be selected according to the thickness of the workpiece, and the pulse interval should be widened 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 be increased, the cutting speed will be accelerated, and it is easy to produce concentrated discharge and arcing, resulting in wire breakage. Generally, the no-load voltage is about 100V.
In EDM, arc discharge is the main factor causing corrosion damage to the negative electrode. In addition, the improper gap is easy to form an arc discharge in a certain pulse. As long as the arc discharge is concentrated in a certain section, it will cause wire breakage. Select the appropriate discharge gap according to the thickness of the workpiece: the discharge gap should not be too small, otherwise it is easy to produce short circuit, which is not conducive to cooling and discharge of electric corrosion; Excessive discharge gap will affect surface roughness and machining speed.
When cutting workpieces with large thickness, large pulse width current shall be selected as much as possible, and the discharge gap shall be larger to enhance the chip removal effect and improve the cutting stability. However, the selection of electrical parameters requires good processing experience, which is a difficult problem for novices. This requires the WEDM system itself to have its own expert database. At present, the bmxp intelligent WEDM programming and control system can well solve this problem.
