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.


(2) The processing size is unqualified

After cutting, the actual measured size exceeds the tolerance range of the required size of the pattern, and the size is too large or too small. In this case, it is generally possible to precisely control the machining size by correcting the program offset. For example, when cutting a punch, the program offset can be reduced when the size is too large. It should be noted that this adjustment is based on the inherent accuracy and stability of the machine itself, and the machine is in good maintenance. At the same time, the influence of environmental factors on precision machining is also very large. The indoor temperature should reach 21±2°C, the humidity should be 40-80%, and there should be no direct sunlight or airflow.

(3) Taper phenomenon

The wire cutting speed of the slow wire cutting process is slow, and the wire is lost in the main cutting process. Although the electrode wire for the wire cutting process is used once, the wire loss is inevitably present. Therefore, the slitting feature is wide and narrow, which is the main reason for the slight taper phenomenon in the processing of the mold parts. When the punch is cut, the upper end of the part is small, the lower end is large, and a certain taper is exhibited, and the dimensional error is about 6 μm.

Obviously, this problem can be solved by appropriately increasing the wire speed of the machine tool. However, this method will increase the processing cost. In actual production, the taper compensation function can be used in programming, that is, a small taper is added to the straight body part to correct the difference in accuracy. In addition, it is possible to appropriately increase the tension of the wire, and at the same time, it is necessary to confirm that the wire is subjected to accurate vertical alignment, and the upper and lower nozzles are intact without damage, and the low-pressure flushing flow during the trimming is correctly adjusted.

(4) concave or convex situation

The upper and lower dimensions of the workpiece are inconsistent. In actual production, this is called a concave heart. The main reason for this is that the servo feed state of the wire and the workpiece is not in a good state during processing. Normal processing should ensure that the servo feed speed of the wire is approximately equal to the erosion speed, and the feed is even and stable. If the servo feed rate is too high (close to short circuit), that is, the servo feed speed of the wire is obviously higher than the erosion speed. When the punch is cut, the bulge will be bad; the servo feed rate will be too slow. In the case of a concave core, the concave mold is reversed. According to this law, the servo feed speed of the machine tool can be adjusted to effectively solve the problem of dimensional accuracy of concave or convex in the mold part.

(5) Large position accuracy error

The accuracy of the cavity position on the stencil is highly dependent on the positioning before machining. In order to achieve high-precision positioning accuracy, the workpiece must have a precise, unambiguous datum. The wire must be subjected to automatic vertical alignment. Do not use the spark correction method because it is difficult to obtain a high precision of verticality, which will affect the positioning accuracy.

Using the positioning method in four sides can achieve higher positioning accuracy, and the sensing errors of the two reference sides can cancel each other out. Positioning and finding the edge should be done several times to check and confirm the positioning accuracy. If there is a large positional accuracy error in the skip processing, check if the machining is deformed. For multi-cavity machining, when programming the process, you can rough-process all the holes first, fully release the stress of the material, and then perform the trimming process uniformly, which can achieve higher precision jump accuracy.

Do not use incremental programming when programming, so as to avoid large differences after error stacking. The shaft of the machine tool should be maintained on time, filled with lubricating oil, and the mechanical precision can be detected and corrected in time to make the machine tool in good precision.

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