Abnormal Wire Cutting Processing And Analysis
Slow wire cutting has an indispensable position in the manufacture of precision molds, especially in the manufacture of stamping dies. The punching die, the punch fixing plate, the die and the unloading plate and the like all require a slow wire cutting process. Processing quality and efficiency directly affect the assembly performance, product quality, mold life, manufacturing cycle and cost of mold parts, so it is of great significance to prevent the occurrence of abnormal processing.
1. Low processing efficiency
The slow-cut wire cutting process parts use multiple cutting processes. When the processing efficiency is low, it is necessary to distinguish whether it occurs in the main cutting process or the trimming process.
(1) Main cutting efficiency is low
In actual production, whether or not effective high-pressure flushing is the main reason for the efficiency of the main cutting process of slow wire cutting. The wire drawing speed of the wire cutting process is slow, and the processing requires high-pressure flushing to remove chips. If the upper and lower nozzles are not attached to the surface of the workpiece during processing, or if cutting occurs along the edge, the high-pressure water cannot be effectively punched into the slit, resulting in a significant reduction in processing efficiency.
For special machining (such as special materials, complex shaped step workpieces), frequent broken wires have to be modified to reduce the discharge energy to avoid broken wires. At this time, the pulse gap should be preferentially increased to reduce the frequency of the pulse discharge, which can usually solve the problem effectively. Avoiding wire breakage by reducing the machining current is also an effective method, but this method will reduce the processing efficiency more. Therefore, it is only considered to reduce the machining current only if the pulse gap is still broken.
Regular maintenance of the slow-moving wire-cutting machine is the basis for high-efficiency machining. The electrode wires obtain discharge energy through the conductive blocks, and it is necessary to ensure that they are in good contact. When the depth of the wear groove of the conductive block exceeds the radius of the wire, the position of the conductive block must be replaced. The surface of the conductive block needs to be sanded to remove the oxide layer to increase its conductivity.
If you do not clean the lower guide wire core for a long time, it will block (using the brass wire will drop the copper powder). The cooling water of the conductive block or even the flushing of the lower nozzle will seriously affect the processing efficiency. The maintenance of the lower guide wire core seat is required. When the reading of the water pressure gauge in the process differs greatly from the process setting value, the nozzle should be inspected for damage. It will affect the actual pressure of the high pressure flushing, which will greatly reduce the processing efficiency and replace it if necessary.
(2) Low processing efficiency
If the workpiece is deformed during the cutting process, the efficiency of the finishing process will be significantly affected. Lightly, the processing efficiency is reduced, and even when the deformation is large, there is even a situation in which the repair is not performed. In order to prevent deformation of the slow-cut wire cutting mold, certain measures must be taken. Firstly, the material with small deformation should be selected, and the correct heat treatment process should be adopted. Secondly, the processing technology should be arranged reasonably. The large cavity or the narrow and complicated cavity can pre-process the release stress of the cutting part, and the large cavity retains multiple sections. For the retention amount, the punched parts should be machined through the threaded holes for closed contour processing. Finally, the cutting path is optimized, including the position of the clamping, the retention amount, and the introduction path.
In order to obtain reliable processing quality, the machining parameters generated by the machine tool expert system are generally conservative, and the amount of material reserved between the two cuttings will be relatively large. In actual production, by modifying the offset and reducing the material allowance, the efficiency of the trimming process can be greatly improved. However, it should be noted that the material allowance cannot be reduced too much, otherwise it will lead to poor handling of the machined surface.
The more the number of trimming processes, the better the surface roughness, and the more refined it will double the processing time. Therefore, the number of cuts should be determined according to the processing requirements of the mold parts, so as to be sufficient, the number of repairs should be reduced as much as possible to meet the processing requirements, so as to ensure high-efficiency mold production. For more than four times of multiple cutting, because the final finishing energy is very small, its discharge state is affected by various factors, and it is easy to have extremely low processing efficiency. This situation can be used for the last refinement. The constant speed cutting is set to ensure the processing speed and the surface roughness that meets the requirements.
2. Surface quality problems
The surface quality problems of processing mainly include surface stripe, roughness is not required, and surface metamorphic layer is too thick.
(1) Surface stripes
According to the process specified by the slow wire cutting machine, if there is streaking on the surface, first of all, it depends on whether the quality of the wire is defective. It is easy to use the inferior electrode wire which is cheap on the market, and the problem of processing stripe is easy to use. Try to use genuine consumables. Similarly, the use of low-quality workpiece materials containing impurities can also result in dense streaks on the machined surface.
For the processing of tapered parts, the surface is prone to streaking, and a softer brass wire with better flexibility can be used to obtain a significant improvement. When machining a die, it often happens that a dent appears at the infeed, which can be improved by arcing in and out during programming.
Processing should ensure that the machine tool is running smoothly, the wire tension is stable, and the wire speed and tension are calibrated if necessary. The wear of the conductive block should be rotated or replaced in time to clean the upper and lower guide wire components, including the guide wire guide, the guide wire core seat, the lower guide wheel, and the guide wheel bearing.
(2) Surface roughness does not meet the requirements
For abnormal conditions where the surface is not completely polished and the surface roughness does not meet the requirements, it should be checked whether the offset of the program is reasonable. It is possible to appropriately increase the amount of material between the two knives to ensure that the last trimming can completely repair the surface of the previous processing.
A slight deformation of the cutting workpiece will also result in a surface roughness that is not satisfactory, and the surface after the trimming is partially uneven. At this time, it is necessary to control the occurrence of machining deformation. For large, thick-walled, complex-shaped, uneven-thickness mold parts, it is advisable to increase the number of cuts to reduce and remove surface defects.
(3) The surface metamorphic layer is too thick
The surface of the slow-cut wire-cutting mold part produces a metamorphic layer of a certain thickness. The metamorphic layer causes the hardness of the mold to decrease, which is prone to early wear and affect the service life of the mold.
The processing uses pure water as the working fluid, and the conduction of ions in the water causes destructive electrolysis. Therefore, it is necessary to add an ion exchange resin to the water to control the conductivity of the water. The higher the conductivity of the deionized water, the thicker the metamorphic layer of the machined surface. Therefore, the wire cutting process should control the conductivity of the deionized water within a certain range. The finishing should ensure that the conductivity of the deionized water is less than 10 μS. When the allowable range is exceeded, the ion exchange resin should be replaced in time. When processing cemented carbide and PCD tool parts and having strict requirements on the thickness of the surface metamorphic layer, an oily medium with high insulation performance can be used as the working fluid. The oily medium has high insulation performance, and it is difficult to break through the discharge under the same voltage condition, without electrolytic corrosion, and can obtain a superior surface quality than the processing with deionized water, and the processed surface has almost no metamorphic layer.
The anti-electrolytic processing technology of the slow-cut wire cutting machine tool can effectively reduce the damage of electrolysis during processing to a minimum. The processing principle is to apply a reverse polarity voltage during the processing time (pulse gap) where no discharge occurs, and the alternating positive and negative pulse voltages make the average voltage of the gap zero, preventing the occurrence of such harmful phenomena of electrolysis.
The electrode wire is made of galvanized wire, which is not easy to drop copper powder in the processing, the surface of the processing does not accumulate copper, the surface deterioration layer can be improved, and the surface hardness of the workpiece is higher.
3. Machining accuracy problem
Slow wire cutting can handle high-precision mold parts. In actual production, due to improper operation or unreasonable process, the processing accuracy may cause problems. Generally, the following situations may occur.
(1) Large shape error
In general, machining deformation is the main reason for the large shape error, and can be improved from the processing technology by referring to the above related introduction.
The pressure of the spark channel in the process produces a large back thrust to the wire, which causes a slight bending of the wire, and the hysteresis of the wire is likely to cause the corner to collapse when the corner is cut. In order to improve the corner cutting accuracy, the corner control function of the machine tool can be used to improve the corner accuracy problem. When machining to the corner position, the machine automatically reduces the machining energy, reduces the flushing pressure, reduces the servo advance, and uses a delay to control the corner accuracy. The corner strategy is divided into an inner corner and an outer corner, and their protection strengths can be set separately. The corner strategy should be applied flexibly according to the corner accuracy requirements.
For microfinishing of small rounded corners, narrow slits, and fine parts, filaments can be cut to obtain a smaller inner corner radius. The corners processed using galvanized wire have high precision, especially the shape error of the sharp corners is significantly improved compared to the brass wire.
If the tension of the wire is too small, the rushing pressure is too high, and the wire is shaken, the processing is unstable, resulting in an increase in geometric error, and reasonable control is required to reduce the shape error.