Analysis Of The Causes Of Wire Breakage in Wire Cutting

Wire cutting is a unique EDM wire cutting processing mode invented in my country and is widely used. The advantages of EDM wire cutting are that it can process parts that have been heat-treated such as quenching, special-shaped parts, and remove less waste. The tool electrode is usually a molybdenum wire with a diameter of 0.10~0.18mm. The wire is very easy to break during the processing, which not only delays production time and increases production costs, but also reduces the processing quality of parts. The reasons are analyzed as follows:
1. Wire breakage related to the workpiece
(1) After the workpiece is heat-treated, there is internal stress inside the workpiece, which causes the internal stress to be released during the cutting process, clamping the key wire and causing wire breakage. If the wire threading hole is processed before the workpiece is heat-treated, cutting from the inside of the workpiece can avoid wire breakage caused by internal stress.
(2) After cutting the workpiece, due to the large weight of the waste, the key wire is clamped at the moment of falling, causing wire breakage. When the cutting is almost completed, the waste and the workpiece can be sucked by a magnet at the same time, or clamped with a clamp (such as a pressure plate), and the waste can be removed after the processing is completed.
(3) Casting parts may have sand holes and pores during the casting process, and non-conductive impurities inside the workpiece, which may cause wire breakage during the cutting process. For such parts, if conditions permit, a detection tool can be used to detect whether the material inside the part is uniform. If conditions are not met, the machine tool instruments should be monitored at any time during the cutting process, and voltage or current mutations should be handled in a timely manner.
(4) The entry point of the workpiece or the threading hole may contain non-conductive oxides and other impurities after heat treatment, which may make it impossible to cut and cause wire breakage. In this case, you can use a file or a grinding wheel to grind the entry point of the workpiece, remove the non-conductive material, and then expose the conductive part before cutting.
(5) The workpiece surface coating (such as plastic film, paint, etc.) is not conductive and causes wire breakage. The workpiece is connected to the positive pole of the pulse power supply, and the key wire is connected to the negative pole of the pulse power supply. If the workpiece has poor contact with the positive pole of the pulse power supply due to the coating layer, the discharge machining cannot be performed and the molybdenum wire may be broken. Therefore, it is necessary to ensure that the workpiece and the positive pole of the pulse power supply are reliably connected. If necessary, the surface coating of the workpiece should be removed first.
2. Wire breakage related to working fluid
(1) Wire breakage caused by unreasonable concentration of working fluid. The working fluid concentration should be reasonable. First, we should choose a good quality working fluid with good water quality. Then, we should prepare the working fluid according to the different processing technology index requirements of the parts. The ratio is generally 5%~20%. Usually, the EDM wire cutting machine works for 8 hours a day. After 8~10 days of continuous use, it is necessary to replace the new working fluid, otherwise the wire will break easily. For workpieces with large thickness or high cutting speed, the working fluid concentration can be reduced by about 5%~8%, so that the processing is stable; for workpieces with high processing quality requirements, the working fluid ratio can be increased to 10%~20%.
(2) Wire breakage caused by insufficient flushing of the working fluid. One of the functions of the working fluid is to flush the cutting seam, cool the key wire and workpiece, and remove the etched material. If the impact force of the working fluid is too large when it is sprayed, it may cause the key wire to deviate and the discharge to be uneven; if the impact force is too small, the working fluid will not be sprayed enough, and it will not be able to flush into the cutting seam, and it will not be able to discharge, resulting in poor discharge conditions and the inability to remove the etched material, causing wire breakage. Therefore, it is necessary to regularly check whether the nozzle and the return channel are blocked, and the working fluid spraying speed should be reasonable. For thick parts, the working fluid spraying speed can be increased so that the working fluid can fully enter the cutting seam for cooling and chip removal.
(3) Insufficient working fluid or blockage will cause machining without cutting fluid, and the key wire will burn out quickly. Therefore, during the operation of the machine tool, it is necessary to check whether the working fluid is sufficient and whether the circulation channel is unobstructed.
3. Broken wire related to the wire feeding mechanism
(1) Broken wire related to the conductive block. The conductive block usually presses or lifts the key wire a little. Since the molybdenum wire contacts the conductive block for a long time during operation, the conductive block will have grooves. If the grooves are too large, the aluminum wire will be clamped off. Therefore, the conductive block should be rotated a certain angle regularly, or the conductive block should be replaced directly.
(2) Broken wire related to the guide wheel. The molybdenum wire is guided by the guide wheel, so the accuracy of the guide wheel affects the operation of the molybdenum wire. The bearing supporting the guide wheel affects the axial and radial runout of the guide wheel, which in turn affects the stability of the key wire during EDM. Therefore, the machine tool maintenance instructions should be strictly followed to regularly inject grease or replace the bearings, or even directly replace the guide wheel assembly.
(3) Wire breakage caused by the tensioning mechanism. If the weight is too heavy during wire tensioning, wire breakage may occur during the wire tensioning process, or if the molybdenum wire exceeds the limit of elastic deformation, the aluminum wire will soon break due to frequent reversing, frequent discharge and cooling during operation. Therefore, a reasonable number of weights should be selected for tensioning when tensioning the wire.
(4) Wire breakage caused by the wire storage barrel. The radial runout of the wire storage barrel will cause a sudden change in tension during the molybdenum wire cutting process, which will break the molybdenum wire. The axial runout will also cause overlapping wires, which is more likely to cause wire breakage. Therefore, the accuracy of the wire storage barrel should be regularly tested and adjusted.
(5) Wire breakage caused by unreasonable winding of the molybdenum wire on the wire storage barrel. A molybdenum wire with a width of 5 to 10 mm should be reserved at both ends of the molybdenum wire storage barrel, otherwise the molybdenum wire will be easily broken due to uneven tension during reversing. If the key wire is overlapped on the wire storage barrel, it will also cause wire breakage. Therefore, the molybdenum wire should be arranged reasonably on the wire storage barrel when tensioning.
(6) Wire breakage caused by failure of the reversing mechanism of the motor running the wire storage barrel. The direction of the motor of the wire storage drum is adjusted by manually adjusting the pressure plate to adjust the axial stroke of the wire storage drum. After the switch pressure plate presses the stroke switch, the motor should be reversed. If the switch pressure plate is not fixed or the stroke switch is not pressed, or the stroke switch fails, the wire storage drum will overtravel and break the molybdenum wire. Therefore, the stroke switch and the switch pressure plate should be ensured to work reliably before the machine tool is operated.
(7) The molybdenum wire is not placed in the groove of the guide wheel, causing the wire to break. When the molybdenum wire is installed, if the molybdenum wire is not placed on the normal wire path, such as outside the guide wheel groove, the wire will break when the machine is turned on, and the consequences are very serious. Therefore, after the molybdenum wire is installed, be sure to check the wire path to see if the molybdenum wire is on the normal wire path.
(8) The molybdenum wire is broken due to thermal expansion and contraction. After the workpiece is processed, if the molybdenum wire stops at the middle section of the wire storage drum, if the molybdenum wire is too tight, it may break after cooling. Therefore, the molybdenum wire should stop at one end of the wire storage drum, and the wire head end should be loosened if the part is not processed.
4. Wire breakage related to programming:
(1) Wire breakage caused by unreasonable programming path for workpiece processing. A wire path that is easy to cause deformation of the workpiece during cutting is selected. When the workpiece is deformed, the molybdenum wire is clamped and broken, and the dimensional accuracy of the cut punch is low. The wire path that keeps the deformation of the workpiece as small as possible during the entire processing process should be selected, and the dimensional accuracy of the cut punch is high.
(2) Wire breakage caused by secondary cutting. If the wire breaks during cutting, the machine tool will have a retraction function. After reloading the new molybdenum wire, cutting will start from the beginning along the original cutting path. However, due to the gap after the first cutting, the discharge will be uneven in the second cutting, and the aluminum wire loss will be more serious. Once a thick part was cut, the wire broke seven times in a row in one night. Each time, the wire broke again before cutting to the first wire breaking point. After careful investigation of the cause, it was found that the wire breaking point was burned out. By changing the cutting path, the molybdenum wire was reversed and the part was processed smoothly without wire breakage.