Tap water has the advantages of good fluidity, non-flammability, and fast cooling. However, when tap water is used as the working fluid directly, the electrical resistivity of the ions in the water is low, about 5 kΩ·cm, which not only affects the discharge gap deionization, prolongs the time for recovery of the insulation, but also produces electrolysis. Therefore, the working fluid of the slow wire EDM wire cutting process generally uses deionized water. The general resistivity should be 10 ~ 100kΩ · cm, the specific value depends on the workpiece material, thickness and processing accuracy. When machining steel with brass wire, the resistivity of the working fluid should be low, which can increase the cutting speed, but vice versa when machining cemented carbide.
Before the processing, you must observe the display of the resistivity meter. Especially when the machine is just starting, it is often found that the resistivity is not within this range. At this time, do not rush to process, and let the machine start working for a period of time to reach the desired resistivity. In order to ensure the processing accuracy, it is necessary to increase the resistivity of the working fluid. When it is found that the resistivity of water is no longer increased, the ion exchange resin should be replaced.
In addition, the conditions related to the coolant must be checked. In the slow wire EDM wire cutting process, the working fluid sent to the processing area is usually casted, or the workpiece can be fully immersed. Therefore, check the fluid volume of the machining fluid and filter the pressure gauge. When the machining fluid flows backward from the dirty tank to the cleaning tank, the filter needs to be replaced to ensure that the insulation performance, washing performance and cooling performance of the working fluid meet the requirements.
When special finishing is performed with a slow wire EDM machine, kerosene with higher insulation properties can also be used as the working fluid.
(3) Selection and correction of electrode wire The wire electrode for slow wire EDM wire cutting is mostly made of copper wire, brass wire, brass plus aluminum, brass plus zinc, brass galvanized and so on. For precision wire EDM, the tension of the wire should be increased as much as possible under the premise of continuous wire. Molybdenum wire or tungsten wire can also be used.
At present, the wire diameter specifications of the domestic electrode wire are 0.10 mm, 0.15 mm, 0.20 mm, 0.25 mm, 0.30 mm, 0.33 mm, 0.35 mm, etc., and the wire diameter error is generally within 2 gm of the soil. The electrode wire produced abroad has a wire diameter of at least 0.03mm or even 0.01~0.003mm, which is used for precision micro-EDM cutting of clear angle and narrow slit. The surface of the electrode wire exposed to the air for a long time is in contact with the air and is easily oxidized, and cannot be used for processing high-precision parts. Therefore, care should be taken not to damage the packaging film of the wire when storing the wire. The quality of the wire must be checked before processing. In one of the following cases, the verticality correction of the wire must be re-executed: the wire cutting machine generally must consider replacing the guide wheel or its bearing after processing for 50-100 hours; changing the cutting position of the conductive block or replacing the conductive block; When it is dirty, it needs to be washed with washing liquid. It must be noted that when changing the position of the conductive block or replacing the conductive block, the verticality of the wire electrode must be re-corrected to ensure the accuracy and surface quality of the machined workpiece.
(4) Machining of the threading hole In the actual production and processing, in order to prevent the residual stress deformation inside the workpiece blank and the thermal stress deformation caused by the discharge, whether it is processing the concave mold type closed workpiece or the convex mold type workpiece, it should first The threaded hole of a certain diameter is processed at a suitable position for closed cutting to avoid open cutting. If the workpiece has been rough cut on the fast wire EDM machine and then further processed on the slow wire EDM machine, the wire hole is not broken.
(5) Fixing and aligning the workpiece Prepare the workpiece processed by the slow wire EDM machine. In the previous process, an accurate datum should be machined to clamp on the slow wire EDM machine. Just looking for it. The machine tool accessories should be fully utilized to clamp the workpiece. For the clamping of some structurally complex and easily deformable workpieces, special fixtures can be designed and manufactured if necessary.
After the workpiece is mounted on the machine tool, you can use the machine's contact sensing, automatic alignment of the center and other functions or use the dial gauge to find the exact position of the workpiece, in order to set the origin of the coordinate system, determine the starting point of programming. When looking for the right time, you should pay attention to the operation several times, and strive to position accurately and minimize the error.
When the workpiece line is cut, its connection strength with the parent material is bound to decrease. At this time, it is necessary to fix the workpiece to prevent the workpiece from being deflected due to the impact of the working fluid, thereby changing the cutting gap, which affects the surface quality of the workpiece. The workpiece is scrapped.
2. Perform a small number of multiple cuts
A small amount and multiple cutting methods refer to the same surface being cut twice or more times by the same diameter electrode wire, and the finishing allowance is reserved before the first cutting process, and then the remaining finishing allowance is left. , using the finishing conditions, using the same trajectory program to reduce the offset amount in stages, and then cutting. Generally, it can be divided into 1~5 times of cutting. Except for the first processing, the processing amount is generally gradually reduced from several tens of micrometers to several micrometers, especially the last time that the processing times are more, the processing amount should be smaller, that is, several Micron. Small, multiple cuts allow the workpiece to have an unmatched surface quality with a single cut, and the more the number of passes, the better the surface quality of the workpiece. The specific values ​​are generally determined by the machine's processing parameters.
The use of a small number of multiple cutting methods can reduce the deformation of the workpiece material during wire cutting, effectively improve the machining accuracy of the workpiece and improve the surface quality, and leave a certain margin during roughing or semi-finishing to compensate the stress balance of the material. The deformation caused by the destruction and the machining allowance required for the final finishing, and finally finishing, can obtain a satisfactory processing effect. It is a simple and easy way to control and improve the quality of the machined surface, but with reduced productivity.
3. Arrange the cutting route reasonably
The guiding ideology of this measure is to avoid destroying the original internal stress balance of the workpiece material, and prevent the workpiece material from being significantly deformed due to the unreasonable cutting route arrangement during the cutting process, resulting in a decrease in the quality and precision of the cutting surface. . In general, a reasonable cutting route should arrange the cutting section separating the workpiece from the clamping part at the end of the total cutting program, and set the pause point to the position near the holding end of the blank.
4. Correct selection of cutting parameters
Wire parameters, water parameters and electrical parameters should be properly controlled and adjusted during slow wire EDM wire cutting.
When the wire tension is large, the amplitude of the vibration is reduced, the discharge efficiency is relatively increased, and the cutting speed can be increased. The high wire speed can reduce the chance of wire breakage and short circuit, and increase the cutting speed, but too high will increase the vibration of the wire and affect the cutting speed. In order to ensure higher machining accuracy and surface quality of the workpiece, the wire speed and wire tension parameters provided by the machine tool builder can be appropriately adjusted.
Increasing the pressure and flow rate of the working fluid, it is easy to discharge the etching material, and the cutting speed can be increased, but if it is too high, the electrode wire will vibrate, affecting the cutting speed, and the laminar flow can be maintained.
Short pulse width, high peak current, and positive polarity processing are widely used in roughing. Extremely short pulse widths (hundred nanoseconds) and single pulse energy (several microjoules) are used for finishing to significantly improve the quality of the machined surface.
In addition, a stable supply voltage should be maintained. Unstable power supply voltage can cause instability between the electrodes and the workpiece, which can lead to instability of the breakdown discharge process and affect the processing quality of the workpiece.
5. Control the distance between the upper guide and the workpiece
In the slow wire EDM wire cutting process, the distance can be closely processed, that is, the distance between the upper guide and the workpiece is as close as possible (about 0.05 to 0.10 mm), so as to avoid the influence of the electrode wire amplitude being too large due to the long distance. Processing quality.
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