Wire Electrical Discharge Machining (WEDM) is an important component of precision and specialty machining. When processing complex-shaped workpieces with high hardness and high melting point conductive materials, WEDM is often the best or only option. The development of wire-cutting technology cannot be separated from the synchronous development of electrode wire technology. The cutting efficiency and cutting quality of wire-cutting machines are closely related to the performance of the electrode wire, and breakthroughs in electrode wire technology often lead to innovation in wire-cutting machine design. Therefore, the selection of electrode wire and analysis of wire breakage faults are particularly important.
一、Selection of electrode wire in wire-cutting processing
Wire-cutting processing interacts in various aspects such as processing materials, cutting speed, profile accuracy, surface quality, and factory operation mode. Only by selecting a suitable electrode wire can the factory optimize the processing efficiency, processing cost, and processing quality as a whole.
1.1 Performance of electrode wire
High-performance electrode wire must be an organic combination of various useful characteristics. The electrode wire must be able to withstand peak cutting currents exceeding 700A or average currents exceeding 45A, and energy transmission must be very efficient. The electrode wire must have good tensile strength and elongation. High-efficiency and high-precision wire-cutting machines require electrode wires with minimal geometric errors. Additionally, electrode wires with low melting points and high vapor pressure can help blow away waste residue.
二、Types and applications of electrode wire
Brass is an alloy of copper and zinc, and the most common ratio is 65% copper and 35% zinc. Theoretically, the higher the proportion of zinc, the better. However, in the actual manufacturing process, when the proportion of zinc exceeds 40%, the single-phase crystalline structure of the electrode wire becomes a duplex structure of α and 3, making the material too brittle to be drawn into very thin wires. The tensile strength of ordinary brass wire is between 490 and 900/mm2. However, brass wire has the following drawbacks: processing speed cannot be increased, surface quality is not good, and processing accuracy is not high. Especially when processing thicker workpieces, due to poor flushing performance, significant straightness errors may occur.
Brass wire is mainly used in situations where the processing volume is low, the processing accuracy, especially the surface quality, is not high, or where small size, thin thickness, low workpiece hardness, or thickness not exceeding 80 to 100mm are required. At the same time, some new brass wires with superior cost performance have emerged on the market, such as ultra-clean brass wire, super-hard brass wire, and high-speed brass wire.
2、Coated electrode wire
By adding a layer of zinc to the outside of the brass wire, zinc-coated electrode wire is produced. Its main advantages are high cutting speed, not easily breaking, good surface quality of processed workpieces, no copper accumulation, and improvement of the alteration layer. The machining accuracy is improved, especially the shape error of the sharp corner and the straightness error of the thick workpiece, etc., are all improved compared to brass wire. The loss of components such as wire guides is reduced, and the wire guide is less prone to blockage, contaminating related components.
Core materials for coated electrode wire mainly include brass, bronze, and steel. Coating materials include zinc, bronze, copper-zinc alloy, and silver. Currently, the more mature coated electrode wires on the market include ordinary galvanized electrode wire, high-precision galvanized electrode wire for processing, high-speed coated electrode wire for processing, high-difficulty coated electrode wire for processing, and ultra-precision electrode wire for processing.
三、Analysis of wire breakage in wire cutting
Wire breakage is one of the common faults in wire cutting processing. It not only affects processing efficiency but also damages the integrity and smoothness of the processing surface, making unmanned wire cutting difficult. The reasons for wire breakage in wire cutting are manifold, as analyzed below.
The pulse power supply is one of the main equipment in wire cutting machine tools, and its electrical standard parameters directly affect the service life of the electrode wire. When the pulse interval decreases, the average current increases, and the cutting speed accelerates, but the pulse interval cannot be too small to avoid arcing and wire breakage. Peak current is one of the main factors determining the energy of a single pulse and also one of the main causes of wire breakage. When its value increases, the cutting speed increases, the roughness of the workpiece surface deteriorates, the electrode wire wear increases significantly, and even wire breakage can occur. Open-circuit voltage can cause changes in discharge peak current and electrical processing gap. When the open-circuit voltage increases, the processing gap increases, chip removal becomes easier, and the cutting speed and processing stability are improved, but it can cause electrode wire vibration, affecting processing accuracy and increasing the wear and tear of the electrode wire, making it thinner or causing wire breakage. Based on the above analysis, the pulse power supply should have the performance of low electrode wire wear. Studies have shown that reducing the rise rate of the discharge power supply can reduce the wear of the electrode wire, which is not only beneficial to improving processing accuracy but also can extend the service life of the electrode wire. This is particularly important for rapid wire feed wire cutting processing. In addition, by adding a certain value of inductance in series in the rectangular wave discharge power supply loop, a front and rear edge buffer waveform can be obtained, and the wear of the electrode wire can be reduced.
If the deviation value caused by vibration exceeds the elastic limit of the electrode wire, wire breakage will occur. In addition, a balanced and stable wire feeding mechanism is required. Therefore, improving the manufacturing quality of the entire wire feeding mechanism is beneficial to reducing electrode wire vibration and extending its service life. If the working fluid splashes at the inlet due to the movement of the electrode wire during processing, it cannot enter the kerf, and the electrode wire cannot be fully cooled, which can easily cause wire breakage due to electrode wire burning. Therefore, the cooling system of the machine tool must be improved, and cooling must be sufficient.
1、Wire feeding speed
The wire feeding speed directly affects the residence time and discharge frequency of the electrode wire in the machining zone, thereby affecting the wear of the electrode wire. Generally, high-speed wire feeding is advantageous for cooling and lubrication, while low-speed wire feeding is advantageous for reducing the impact of electrode wire wear on processing accuracy. Especially for processing thick workpieces, the wear of the electrode wire can cause taper on the processing surface. In general, the wire feeding speed can be determined according to the thickness of the workpiece and the cutting speed.
2、 Feed Speed
If the preset feed speed is set too fast, frequent short circuit phenomena will occur, and the cutting speed will decrease. The upper and lower surfaces of the workpiece will turn brown, and it may even cause wire breakage. Conversely, if the speed is too slow, the erosion rate of the workpiece will be greatly reduced, and there will be frequent open circuit and short circuit phenomena, which will easily cause wire breakage. The ideal state of wire EDM processing is for the feed speed to track the wire running speed. Overtracking and undertracking are the main factors that cause unstable processing and can easily cause wire breakage. Therefore, it is necessary to improve the self-control performance of the feed control system and manually adjust the feed rate appropriately.