This paper analyzes various related factors that affect the surface quality of workpieces in the process of EDM,and proposes measures and methods to improve the accuracy and surface quality of workpieces processed by wire cutting.
1. Introduction
According to the cutting speed, wire EDM can be divided into two types: HS-WEDM and LS-WEDM . The surface roughness of the workpiece processed by HS-WEDM is generally in the range of Ra=1.0 ~ 2.5μm, while the surface roughness of the workpiece processed by LS-WEDM can usually reach Ra=0.16μm, and the roundness error, linear error and dimensional error of LS-WEDM are much better than those of HS-WEDM, so when processing high-precision parts, the Lower-speed wire cutting machines are widely used.
Since LS-WEDM adopts the method of continuous supply of brass wire, during the processing, even if the brass wire is worn out, it can be continuously supplemented, so the machining accuracy of the parts can be improved. However, there are many factors that affect the surface quality of the WEDM when it is working. In particular, the LS-WEDM needs to reasonably select the relevant processing parameters to ensure the surface quality of the workpiece. TAGUTI used the ALN400 lower-speed wire cutting machine manufactured by Japan Sodick to conduct many wire cutting experiments on injection molds and stamping dies, and did some exploration and research in analyzing the relevant factors affecting the surface quality of the wire cutting workpiece. Some effective work experiences are presented below.
2. Measures and methods for improving the surface quality of workpieces processed by wire cutting
Before analyzing the relevant factors affecting the working surface quality of WEDM, it is necessary to learn the loose contact light-voltage discharge phenomenon existing between the brass wire and the workpiece during WEDM. Through years of observation and research, it has been found that when the flexible brass wire and the workpiece are close to the generally considered discharge gap (for example, 8-10 μm), no spark discharge occurs, and even when the brass wire contacts the workpiece (gap can not be seen from the microscope), the spark is often not seen. The normal spark discharge occurs only when the workpiece bends the brass wire and offsets it by a certain distance (several microns to tens of microns).
Every time the brass wire is fed by 1 μm, the discharge gap does not decrease by 1 μm, however, the brass wire increases a little tension between the wires, the workpiece increases a little lateral pressure. Obviously, spark discharges can only be formed after a certain slight contact pressure is maintained between the brass wire and the workpiece. Based on this, this paper believes that there is a certain electrochemically generated insulating film medium between the brass wire and the workpiece. When the brass wire and the workpiece are in contact with the workpiece, it is in constant motion, and the moving friction reduces the insulating film medium to an acceptable level. Spark discharge occurs only to the extent of breakdown.
Since the discharge force always pushes the brass wire in the opposite direction to the direction it is advancing, this force will be the main factor causing the brass wire to lag. In addition to the above factors, the factors that have a direct impact on the quality of wire cutting mainly involve personnel, equipment, materials and so on. In order to improve the surface quality of the processed workpiece, the control methods and improvement methods for the processing quality can be considered from the three most influential aspects which are human factors, machine tool factors and material factors.
2.1 Control and improvement of human factors affecting surface quality of wire-cut workpieces
The control and improvement of human factors mainly include the determination of processing technology and the selection of processing methods, which can be achieved through the following points:
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Carry out a small amount of multiple processing
For this amount, it is generally determined by the processing parameters of the machine tool. According to the experience of using the ALN400 manufactured by Japan Sodick, except for the first processing, the processing amount is generally gradually reduced from tens of microns to several microns. Especially the last time with more processing times, the processing amount should be small, that is, several microns. The more processing times, the better the surface quality of the workpiece.
Since reducing the deformation of the material during wire cutting can effectively improve the surface quality of the processed workpiece, a small amount and multiple cutting methods should be used. A certain allowance can be left during roughing or semi-finishing to compensate for the deformation of the material due to the destruction of the original stress balance state and the machining allowance required for the last finishing, so that a more satisfactory processing effect can be obtained when the final finishing is carried out. A small amount and multiple cuts can make the processed workpiece have an incomparable surface quality in a single cut, and it is a simple and easy method and measure to control and improve the surface quality of the processed workpiece.
(2)Reasonable arrangement of cutting routes.
Try 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 arrangement of the cutting route under the action of the fixture during the cutting process, resulting in a decrease in the quality of the cutting surface.
(3)Correct selection of cutting parameters
For different rough and finish machining, the wire speed, wire tension and jet pressure should be properly adjusted based on the parameter table. In order to ensure that the workpiece has higher precision and surface quality, the wire speed and wire tension can be appropriately adjusted. Although the manufacturer of the wire cutting machine provides relevant parameters to adapt to different cutting conditions, due to the influence of the workpiece material, the required machining accuracy and other factors, people cannot completely copy the cutting conditions introduced in the manual. On the basis of these conditions, adjustments should be made according to actual needs.
For example, if you want to process a workpiece with a thickness of 27mm, you cannot find a corresponding situation in the processing condition table. Under this condition, you must make adjustments according to the cutting conditions and the corresponding compensation amount with a thickness between 20mm and 30mm. The main method is : Whichever standard thickness the workpiece thickness is close to is selected as the processing thickness that should be set, and the compensation amount is selected proportionally according to the difference between the actual thickness of the workpiece and the standard thickness in the table.
(4)Adopting veneer processing
In order to achieve high precision and high surface quality of the workpiece, veneer processing can be used to make the distance between the upper nozzle and the workpiece as close as possible (about 0.05 ~ 0.10mm), In this way, it can avoid that the upper nozzle is far away from the workpiece and the amplitude of the brass wire is too large to affect the surface quality of the workpiece.
(5)Pay attention to the fixation of the workpiece
When the workpiece is about to be cut, its connection strength with the parent material is bound to decrease. At this time, it is necessary to prevent the workpiece from being deflected due to the impact of the machining fluid, because once the deflection occurs, the cutting gap will be changed, which will affect the workpiece. If the surface quality is heavy, the workpiece will be cut off and scrapped, so we must find a way to fix the workpiece to be processed.
2.2 Control and improvement of machine tool factors that affect the surface quality of workpieces processed by wire cutting.
LS-WEDM is a high-tech and high-precision machine tool. The maintenance of machine tools is very important, because the high precision and high quality of processed workpieces are directly established on the basis of the high precision of the machine tool, the working state of the machine tool must be checked before each machining in order to provide conditions for obtaining high-quality machining workpieces. The points to be noted and the measures to be taken are as follows:
(1)Brass wire exposed to the air for a long time cannot be used for machining high-precision parts.
Because if the surface of the brass wire has been oxidized, it will affect the surface quality of the workpiece, so care should be taken not to damage the brass wire when storing the brass wire. In order to avoid oxidation of brass wire in contact with air, the quality of brass wire must be checked before processing. In addition, the tension of the brass wire also has a great influence on the surface quality of the workpiece. For workpieces with high surface quality requirements, the tension of the brass wire should be increased as much as possible under the premise of continuous wire.
(2) The LS-WEDM generally uses deionized water as the working fluid.
Spark discharge must be carried out in a liquid medium with certain insulating properties. The insulating properties of the working fluid can compress the discharge channel after breakdown, so that the spark discharge is confined to a small channel radius, forming instantaneous and local high temperature to melt and gas. After the discharge is completed, the discharge gap is quickly restored to an insulating state. If the insulation performance is too low, electrolysis will be generated and the breakdown spark discharge will not be formed; if the insulation performance is too high, the discharge gap will be small, the chip removal will be difficult, and the cutting speed will be reduced. Generally, the resistivity should be 5×104～10x104cm. The display of the resistivity meter must be observed before processing. Especially when the machine tool is just started, it is often found that the resistivity is not within this range. At this time, do not rush to process, let the machine tool run for a period of time to reach the required resistivity before starting the formal processing.
In order to ensure the machining accuracy, it is necessary to increase the resistivity of the working fluid. When it is found that the resistivity of the water is no longer increased, the ion exchange resin should be replaced. In addition, it is necessary to check the conditions related to the coolant, check the liquid volume of the processing fluid, and also check the filter pressure gauge, and its pressure value should be above 2.0×13-3 Pa. When the working fluid flows in the reverse direction from the dirty lateral cleaning tank, the filter needs to be replaced to ensure that the insulating performance, washing performance and cooling performance of the machining fluid meet the requirements.
(3)Power feed contacts must be checked for wear.
Generally, after 50-100 hours of processing, thelower-speed wire cutting machine must consider changing the cutting position of the conductive block or replace the conductive block. When it is dirty, it needs to be cleaned 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 workpiece.
(4)Check the rotation of the rollers.
If the roller does not rotate well, it should be replaced. It is also necessary to carefully check the damage and degree of dirt on the upper and lower wire guide. If there is dirty, use cleaning fluid to remove the dirt. If there is damage, it should be replaced in time. In addition, the bad condition of the power feed contact, roller and EDM nozzle will also cause the vibration of the brass wire. At this time, even if the machined surface can perform a good discharge, the machined surface is prone to crests or streaks due to the vibration of the brass wire , which eventually causes the surface roughness of the workpiece to deteriorate
(5)Maintain a stable supply voltage.
Unstable power supply voltage will cause instability at both ends of the electrode and the workpiece, resulting in unstable breakdown discharge process and affecting the surface quality of the workpiece
2.3Control and improvement of material factors affecting surface quality of wire-cut workpieces
In order to process WEDM products with high dimensional accuracy and good surface quality, careful consideration must be given to the workpiece materials used, which should mainly be started from the following aspects:
(1)Due to the different workpiece materials, the melting point, vaporization point, and thermal conductivity, even if the same processing method is used, the obtained workpiece surface quality is not the same, Therefore, the workpiece material must be selected according to the actual surface quality required. For example, in order to achieve high precision, cemented carbide materials must be selected, and stainless steel or unquenched high carbon steel should not be selected, otherwise it will be difficult to achieve the required requirements
(2)Since the internal residual stress of the workpiece material has a great influence on the processing, when the material after heat treatment is processed, the relative balance of the residual stress inside the material will be destroyed due to the removal of metal in a large area and the cutting process, which may affect the machining accuracy and surface quality of the part. In order to avoid these situations, materials with good forgeability, good hardenability and small thermal deformation should be selected.
(3)In the process of cutting, the parameters should be adjusted to the best fit state to reduce the phenomenon of wire breakage. Because there will be two discharges where the broken wire occurs, the surface quality of the workpiece will be reduced. In addition, in the process of cutting, it should also pay attention to the sound of the machine tool, the sound of normal processing should be like “chi-chi”, it is very smooth . Note that the spark during normal machining should be blue, not red. Furthermore, during normal processing, the pointer of the ammeter and voltmeter of the machine tool should be stable or have a small amplitude. At this time, the feed speed is uniform and stable, which is the guarantee of high precision and high quality of the wire cutting workpiece.
3. Conclusion
There are many factors that affect the surface quality of workpieces processed by WEDM, but as long as they are systematically and scientifically analyzed, such complex and messy factors can be controlled and deployed, thereby improving and improving the accuracy and surface quality of the workpiece.
