Talking about wire walking and slow wire walking in WEDM

Thu Jun 09 09:05:12 CST 2022

Talking about wire walking and slow wire walking in WEDM

1.In recent years, with the rapid development of my country's economy and the continuous advancement of industrial technology, people must continue to develop and innovate CNC machining technology, and wire cutting technology is a key technology in the field of machine tool processing. The wire-cutting CNC machine tool is a special machine tool for processing precision parts and metal molds with complex shapes. It is suitable as the last process of mold processing. It can process metal materials with higher hardness and keep it in a good cutting state.

1. Wire cutting machine-middle wire


1.1 Overview of Middle Wire


Middle wire feeding refers to a compound wire cutting machine, not that the wire speed is between high and low speeds. It is based on the premise of repeated cutting to reduce the error caused by material deformation and molybdenum wire loss during the cutting operation. The standard of workpiece quality can be adjusted by the wire transport speed, which is between high-speed and slow-speed wire transport. Rough machining generally uses high-speed wire (8~13mm/s), the first cutting is high-speed and stable, high-current is used, and high-current cutting is performed with a larger pulse width. The trimming process adopts low-speed wire transport (1~3mm/s), the working state of the wire is stable, and the upper and lower eye masks can control the jitter of the wire. The cutting accuracy is high, ensuring the accuracy of the workpiece, size and surface roughness (Ra is 1.4~1.7μm). Repeated trimming, the main purpose is to polish and polish. In the processing process, cutting also needs to pay attention to the deformation of the workpiece and the uneven size of the knife may affect the quality and accuracy. According to different materials, the processing amount is reserved, so that the internal stress is distorted and deformed, and the finishing processing is performed in multiple trimmings, so that the size of the workpiece is guaranteed.


1.2 The processing conditions of the wire cutting


1.2.1 The relationship between spark gap and cutting speed.

In daily processing, the fast and slow cutting speed determines the size of the spark gap. The cutting speed cannot exceed the corrosion speed. If it exceeds, a short circuit will occur. During this period, a certain cutting current needs to be maintained to keep the gap size consistent. The cutting speed is uniform to ensure the required size of the workpiece.

1.2.2 The relationship between spark gap and coolant.

During the machining operation, a large amount of waste and micro-particles will continue to be produced, and the main function of the coolant is to remove chips and eliminate the difference of electrical ions, which determines the size of the spark gap, especially when machining high-precision workpieces. And the amount of backlash compensation is taken into account.

1.2.3 Workpiece material.

The material of the workpiece should be a material with a small deformation and a high yield. If there is heat treatment quenching and tempering, the correct process parameters should be selected, the cooling should be uniform, and the residual stress generated during the heat treatment should be eliminated as much as possible.


1.3 Application of medium wire processing


At present, the application range of the middle wire processing technology is wide, including: various punching dies; micro-shaped holes, narrow slits and complex-shaped workpieces; templates and forming tools; metallurgical molds, cavity molds, wire drawing dies and forming dies. It can process hard materials, cut thin slices and metal materials, and can also process internal and external gears. It is suitable for the processing of various parts and small batches.


2. Wire cutting machine-slow wire walking


2.1 Overview of walking wire


Slow wire is a kind of wire cutting, which can process all conductive materials (such as metal materials, cemented carbide, carbide, graphite, etc.). During the electrical discharge machining, the spark generator generates sparks, and thereafter, within a certain period of time, the sparks and the workpiece produce continuous pits during machining. The temperature generated by the spark is 8 000~12 000°C. The energy released by the spark generator determines the size of the pit. Slow wire is suitable for processing complex shapes and precision small workpieces.


In the slow wire machine tool processing, the wire electrode running mode is low-speed one-way movement. According to the different requirements of the workpiece, the wire electrode running speed also needs to be adjusted continuously. Generally speaking, the wire speed is 0.2mm/s, the surface accuracy can reach the micron level, and the quality effect is close to the grinding level. In the slow-moving wire machine tool, the electrode wire is for one-time use, and its purpose is to maintain a stable working state in real time, and to move the wire uniformly, to avoid jitter, and the quality of the processed workpiece is good. The slow-moving wire cutting machine adopts a continuous wire supply method. During the machining of the machine tool, the electrode wire is lost after discharge, which can be continuously supplemented.


2.2 Cutting accuracy


2.2.1 Multiple cutting technology.


Trimming technology is a fundamental means to improve machining accuracy and surface quality. It is a comprehensive application technology involving manufacturing technology, numerical control technology, intelligent technology, pulse power technology, precision transmission and control technology. The main purpose of the first cutting is to shape the part, the main purpose of the second cutting is to improve the accuracy of the part, and the main purpose of the third (or more than three times) cutting is to improve the surface roughness of the workpiece. According to the required precision and surface roughness of the workpiece, the number of cutting knives should be selected reasonably. The number of cutting knives is relatively high, usually 7-9 knives. Implement a small number of multiple cuts, the first is rough cut, the discharge parameters are large, and the main purpose is to cut the part. Most of the metal materials are removed, and the second time is trimming. The discharge parameters are reduced, and the trajectory is reduced in stages. The more processing times, the better the surface quality of the workpiece. During cutting, the surface profile change curve of the part is shown in Figure 2, where the roughness Ra is the arithmetic mean deviation of the profile, and the roughness Rz is the maximum height of the profile.
2.2.2 Corner strategy.


In the process of some workpieces, when cutting corners, the electrode wire will stop and cause the corner to collapse. In order to improve cutting accuracy and avoid such problems, people need to adopt more corner strategies, such as changing the silk path, groove clearing, internal round clearing, and V-shaped clearing to avoid corner collapse caused by the stop of the wire.

2.2.3 Horizontal and vertical state.


The workpiece and the electrode wire are in a horizontal and vertical state, and the upper and lower water nozzles are in veneer processing with the workpiece, and the gap is 0.1mm. The upper and lower water flushing liquid can play a good role in carbon emission and cooling, thereby improving the processing efficiency .


2.3 The main part of the machine tool


The processing equipment of slow-moving wire is mainly composed of the body, worktable, wire-moving mechanism, lead screw, working fluid circulation refrigeration system, servo motor and ion sensor.


2.3.1 Body.


The fuselage is the basic body of the machine tool. It is the internal placement of other components and the electrical appliances inside the machine tool. It is an appearance box structure, including the fuselage guide rails.


2.3.2 Workbench.


The workbench is used to clamp workpieces and fixtures. It is composed of a guide rail, a lead screw, and a motor transmission mechanism, and the lead screw is displaced in a spiral manner.


2.3.3 Wire walking mechanism.


The wire-moving mechanism is mainly composed of a wire-moving wheel and a wire-moving motor, including a unidirectional wire-moving formed by a ceramic wheel, a tension wheel and other mechanisms.


2.3.4 Working fluid circulation system.


The system is used to filter water quality, and the deionized water must meet the requirements of working fluid circulation processing. The water tank is the storage place of the working fluid. There are dirty water tank and clean water tank; the ion sensor is used for resistivity measurement to measure the ion value flowing into the resin tank. The resin bucket is a container for storing resin, which is an extract from petroleum, used for deionization and better filtration of water quality.


2.4 The difference and advantages and disadvantages of medium and slow wire


The middle wire feed belongs to the category of reciprocating high-speed wire EDM wire cutting machine tools. The molybdenum wire (called electrode wire) that runs continuously is used as the electrode to perform pulse spark discharge to remove metal from the workpiece. The error is reduced through multiple cuttings until the cut is formed. . The quality of the workpiece is between the high-speed wire and the slow wire, so it is called the middle wire. The middle wire is an upgraded product of the fast wire, so it can also be called the fast wire that can be cut multiple times. Its processing speed is close to fast-moving wire, and its processing quality tends to be slow-moving wire. The wire speed is 1~12m/s, which can be adjusted as required.
Sodick AQ400Ls slow walking wire


The slow-moving wire uses a continuously running copper wire (called electrode wire) as the electrode, and generally performs unidirectional movement at a running speed of less than 0.2m/s, so that the electrode and the workpiece are in a deionized water environment for pulse spark discharge. The high temperature generated during processing will gradually consume deionized water. Slow wire is used to process various complicated and precise small workpieces. Generally, the wire speed is lower than 0.2m/s and the accuracy is 0.001mm. After one trimming, the roughness Ra can reach 0.8μm, and after trimming four, the roughness Ra can reach 0.33μm, and the surface roughness quality is close to the grinding level. As a one-time use, the wire electrode has a stable working condition, uniform wire walking, small electrode wire jitter, and good processing quality.


The slow-moving wire adopts advanced power supply technology to realize high-speed processing, and the maximum processing speed can reach 350mm/min.


 3. Conclusion


Wire-cutting machine tools have high processing precision and strong functions, but the processing cost is high. If you want to give full play to the role of the machine tool and create good economic benefits, it is necessary to conduct a reasonable processing technology analysis and technical performance analysis of the workpiece, fully understand the structure and performance of the machine tool, master the operation skills of the machine tool, and maximize the potential of the machine tool ,Increase productivity.