1 Introduction 

According to the cutting speed, the wire electric discharge machine can be divided into two types: fast wire and slow wire. The surface roughness of the workpiece processed by the wire cutting machine is generally in the range of Ra=1.25~2.5μm, and the wire cutting machine is slow. The surface roughness of the machined workpiece can usually reach Ra=0.16μm, and the roundness error, linear error and dimensional error of the slow wire cutting machine are much better than those of the wire cutting machine. Therefore, when processing high-precision parts, the wire is slow. Cutting machines are widely used.

Since the wire-cutting machine adopts the method of continuously supplying the wire electrode, that is, the wire electrode is processed during the movement process, even if the wire electrode is worn out, it can be continuously replenished, so that the machining accuracy of the component can be improved. However, there are many factors affecting the surface quality of the machining work when the EDM machine is working. In particular, the slow wire cutting machine needs to make a reasonable selection of the relevant processing parameters to ensure the surface quality of the workpiece. In the CAD/CAM Center of Shantou University, I used the slow wire cutting machine made by Sodick Company of Japan to carry out the wire cutting process of many injection molds and dies, and made some explorations and researches on the factors affecting the surface quality of the workpieces. , accumulated a number of effective work experience, as described below.

2 Measures and methods to improve the surface quality of wire-cut workpieces

Before analyzing the relevant factors affecting the surface quality of the WEDM, it is necessary to understand the loose contact-type light-pressure discharge phenomenon between the wire electrode and the workpiece during the EDM machining. Through years of observation and research, it is found that when the flexible electrode wire and the workpiece are close to the commonly considered discharge gap (for example, 8 to 10 μm), no spark discharge occurs, and even when the wire has contacted the workpiece, no gap is seen from the microscope. At the same time, sparks are often not seen, and normal spark discharge occurs only when the workpiece bends the tip of the wire and is offset by a certain distance (a few microns to tens of microns). At this time, the wire electrode is fed by 1 μm, the discharge gap is not reduced by 1 μm, but the wire is increased by a little tension between the wires, and the work is increased by a little lateral pressure. Obviously, only a slight contact pressure is maintained between the wire and the workpiece. After the spark discharge can be formed. According to this, there is a certain electrochemically generated insulating film medium between the wire and the workpiece. When the wire is in contact with the workpiece, the moving film causes the insulating film medium to be thinned due to its continuous movement. The degree of wear will cause a spark discharge. Since the discharge force always pushes the wire toward the direction opposite to its advancement during processing, this force will be the main cause of the wire lag. In addition to the above factors, the factors directly affecting the quality of wire cutting processing mainly involve personnel, equipment and materials. In order to improve the surface quality of the machined workpiece, the control method and improvement method of the processing quality can be considered from the three aspects of the most influential human factors, machine tools and material factors.

2.1 Control and improvement of human factors affecting the surface quality of wire-cut workpieces

The control and improvement of human factors mainly includes the determination of the processing technology and the choice of processing methods, which can be achieved by the following points:

(1) Perform a small number of multiple processing. For this quantity, it is generally determined by the processing parameters of the machine tool. According to the experience of using the slow wire cutting machine manufactured by Sodick Company of Japan, in addition to the first processing, the processing amount is generally gradually reduced from several tens of micrometers to several micrometers, especially It is the last time that the number of processing times is large, and the processing amount should be small, that is, several micrometers, so that the more the 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 machined workpiece, a small amount and multiple cutting methods should be used. A certain amount of allowance can be left during roughing or semi-finishing to compensate for the deformation caused by the original stress balance state and the machining allowance required for the final finishing, so that when the final finishing is completed, Satisfactory processing results. Small, multiple cuts allow the machined workpiece to have a single cut. Unmatched surface quality is a simple and easy way to control and improve the surface quality of the machined part. Kunshan copper wire

(2) 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 of the cutting surface.

(3) Correct selection of cutting parameters. For different rough and fine machining, the wire speed, wire tension and jet pressure should be adjusted according to the parameter table. In order to ensure higher precision and surface quality of the machined workpiece, the wire cutter can be adjusted appropriately. Wire speed and wire tension, although the manufacturers of wire cutting machine tools provide relevant parameters to adapt to different cutting conditions, but due to the material of the workpiece, the required processing accuracy and other factors, people can not completely copy the cutting introduced in the book. Conditions, based on these conditions, should be adjusted according to actual needs. For example, if a workpiece with a thickness of 27 mm is to be processed, no equivalent situation can be found in the processing condition table. Under such conditions, the cutting condition and the corresponding compensation amount must be adjusted according to the thickness between 20 mm and 30 mm. The main method is : When the thickness of the machined workpiece is close to which standard thickness, it is selected as the machining thickness to be set, and the compensation amount is selected according to the difference between the actual thickness of the workpiece and the standard thickness in the table.

(4) The use of distance processing. In order to achieve high precision and high surface quality of the workpiece, a close-fitting process can be used, that is, the distance between the upper nozzle and the workpiece should be as close as possible (about 0.05 to 0.10 mm), so that the wire electrode can be avoided because the upper nozzle is far from the workpiece. Excessive amplitude affects the surface quality of the machined workpiece.

(5) Pay attention to the fixing of the machined workpiece. When the workpiece is cut, the connection strength with the parent material is bound to decrease. At this time, the workpiece is prevented from being skewed due to the impact of the machining fluid, because once the deflection occurs, the cutting gap is changed, and the workpiece is affected by the light. The surface quality, the heavy ones make the workpiece cut and scrapped, so we must find a way to fix the workpiece.

 2.2 Control and improvement of machine tool factors affecting the surface quality of wire-cutting workpieces. The wire-cut wire EDM machine belongs to high-tech and high-precision machine tools. The maintenance of machine tools is very important because the high precision and high quality of the machined workpieces are direct. Based on the high precision of the machine tool, it is necessary to check the working state of the machine before each machining to provide conditions for obtaining high quality workpieces. The links to be noted and the measures to be taken are as follows

(1) Electrode wire exposed to air for a long time cannot be used to process high-precision parts. If the surface of the electrode wire has been oxidized, it will affect the surface quality of the workpiece. Therefore, care should be taken not to damage the wire when storing the wire. The film is packaged so that the wire is not oxidized by contact with air. The quality of the wire must be checked before processing. In addition, the tension of the wire has a great influence on the surface quality of the machined workpiece. The workpiece with high surface quality requirements should be as high as possible under the premise of the wire.

(2) The slow wire cutting machine generally uses deionized water as the working fluid. The spark discharge must be carried out in a liquid medium with a certain insulating property. The insulating property of the working fluid can compress the discharge channel after the breakdown, thereby limiting the spark discharge in 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 become an insulated state. The insulation performance is too low, and electrolysis will not occur to form a breakdown spark discharge; if the insulation performance is too high, the discharge gap is small, the chip discharge is difficult, and the cutting speed is lowered. Generally, the resistivity should be 5×104～10×104Ω·cm. Before the processing, the display of the resistivity meter must be observed. Especially when the machine is just starting, it is often found that the resistivity is not within this range. Formal processing begins when it is first run for a certain period of time to achieve 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, it is necessary to check the conditions related to the coolant, check the liquid volume of the working fluid, and check the filter pressure gauge. The pressure value should be 2.0×103Pa or more. When the machining fluid flows backward from the turbid lateral cleaning tank, the filter needs to be replaced to ensure that the insulation performance, washing performance and cooling performance of the processing fluid meet the requirements.

(3) The wear of the conductive block must be checked. The slow wire cutting machine generally has to consider changing the cutting position of the conductive block or replacing the conductive block after processing for 50 to 100 hours, and cleaning with a washing liquid when there is dirt. It must be noted that when changing the position of the conductive block or replacing the guide block, the verticality of the wire electrode must be re-corrected to ensure the accuracy and surface quality of the machined workpiece.

(4) Check the rotation of the pulley. If the rotation is not good, it should be replaced. The damage of the upper and lower nozzles and the degree of dirt must be carefully checked. The cleaning liquid should be used to remove the dirt. If there is damage, it should be replaced in time. Also check the condition of the wire inside the wire cartridge. If it is too full, it will affect the smooth running of the wire and the processing accuracy will be affected. In addition, the poor condition of the conductive block, the pulley and the upper and lower nozzles may cause the vibration of the wire electrode. Even if the machined surface can perform a good discharge, the surface of the wire is easily generated with peaks or stripes due to the vibration of the wire electrode. Causes the surface roughness of the workpiece to deteriorate.

(5) Maintain a stable power supply voltage. Unstable power supply voltage will cause instability between the electrodes and the workpiece, which will cause the breakdown discharge process to be unstable and affect the surface quality of the workpiece.