What is the difference between WEDM-LS, WEDM-MS and WEDM-HS?

Time:2019-05-13 08:50:09 / Popularity: / Source:

In metal processing and manufacturing industry, everyone should be familiar with WEDM-LS, WEDM-MS and WEDM-HS. They all refer to WEDM machine, which are widely used in mould manufacturing industry. So what is difference between these three processing technologies?
  • Advantages of WEDM-LS
Precision, complex, long-life stamping die manufacturing precision and surface quality requirements continue to improve, fast wire processing technology can not meet requirements of precision mould manufacturing, this situation has promoted rapid development of slow wire processing technology, its various aspects of process indicators has reached a fairly high level which is irreplaceable by other processing technologies.
1. Development status of WEDM-LS processing technology
(1) Improved processing accuracy
Multiple cutting technology is fundamental means to improve accuracy and surface quality of WEDM-LS processing. Generally, cutting is performed by one cutting, secondary cutting is used to improve precision, and third cutting is performed to improve surface quality.
Due to hysteresis of wire when cutting corner, corner collapses. In order to improve corner cutting accuracy, more dynamic corner processing strategies have been adopted. Such as: automatically change processing speed, automatically adjust water pressure, control processing energy.
High-precision finishing circuit adopted by advanced WEDM-LS machine is an effective technique to improve straightness of workpiece, which makes precision of thick parts processing significantly improved; for micro-finishing of small rounded corners, narrow slits, narrow grooves and fine parts, top WEDM-LS machine can be cut with 0.02 ~ 0.03 mm wire.
In order to ensure high-precision machining, machine's mechanical precision, pulse power supply accuracy, servo control accuracy (including control of mechanical motion, pulse parameters, wire routing system and working fluid system) have reached a very high level. Water temperature cooling device is used to make internal temperature of machine tool same as water temperature, which reduces thermal deformation of machine tool. Closed-loop digital AC (straight) flow servo control system ensures excellent dynamic performance and high positioning accuracy. Machining accuracy can be controlled in several micrometers. Precise positioning can achieve 0.1μm equivalent control. Immersion processing can be used to reduce thermal deformation of workpiece; motor servo and closed loop wire tension control; high-precision tool setting can be realized by voltage modulation tool supply, tool setting accuracy can reach 0.002 mm without damaging workpiece, regardless of dryness and humidity.
(2) Surface quality is improving day by day
Advanced WEDM-LS machine uses a non-electrolytic pulse power supply with an average voltage of zero, destruction of electrolysis has been minimized. In addition, due to improvement of pulse power supply, high peaks and narrow pulse widths (microseconds) are generally used. Most of materials are gas phase throwing, taking a lot of heat, surface temperature of workpiece is not going up, and phenomenon of cracking is greatly reduced; not only processing efficiencyis high, but also surface quality is greatly improved. Electroless spark cutting process is carried out by using an electroless power source, and surface metamorphic layer can be controlled to be less than 2 μm. Wear resistance and grinding of cut carbide die edge are no different, even better than mechanical grinding, more and more parts are processed by cutting.
(3) Improvement of processing efficiency
Due to development of nanosecond large peak current pulse power supply technology and detection, control and anti-interference technology, processing efficiency of WEDM-LS machine is also constantly improving. Current processing speed of advanced WEDM-LS machine tool can reach 500mm2/min. Machining efficiency of workpieces with larger thicknesses has practical technical improvements. For example, when cutting workpieces with a thickness of 300 mm, machining efficiency can reach 170 mm2/min. For processing of workpieces with varying thickness, machining parameters are automatically adjusted by automatically detecting thickness of workpiece to prevent yarn breakage and achieve the highest machining efficiency in this state.
In addition, advanced automatic wire-drawing technology introduced by advanced WEDM-LS machine has an automatic threading time of <15 s to improve efficiency of processing operation; automatic double-wire automatic exchange technology enables automatic double-wire switching processing using electrode wires of 0.20 to 0.02 mm. The first cut is made with thick wire, general wire diameter is 0.25 mm to improve processing efficiency, and core can be cut; then filament is used for dressing, generally 0.10 mm filament is used to cut small round corner, improve accuracy and save 30% to 50% of overall cutting time.
(4) Development of automation, intelligence and information
In process of processing, in order to reduce human intervention and ensure expected process indicators, automation, intelligence and informationization of WEDM-LS processing have achieved corresponding development.
Complete process expert system of WEDM-LS machine tool gives a set of parameters according to processing requirements. It not only includes common electrode wire numbers and corresponding workpiece materials, but also provides processing parameters for special materials such as PCD, PCBN, etc., depending on whether upper and lower nozzles are in contact with workpiece, how large distance is, whether it is refined in wind cut, or open surface finishing, accuracy, surface roughness and efficiency which are priority processing strategies to generate their respective standards; discharge expert system copes with random factors in cutting, and uses adaptive control strategies and automatic control functions in process of cutting, section change, center cutting, near edge cutting, large section high-speed cutting, etc, to obtain stable and efficient processing.
Automatic non-porous detection function is also very practical. When pre-hole is forgotten to be played or hole is offset, machine will automatically move to next pre-hole, which will prevent machine from being stopped during unmanned operation. When a short circuit occurs after wearing wire, position of eliminating short circuit can be automatically searched, reliability of continuous unmanned operation is improved.
CNC system of machine tool is equipped with a standardized mechanical interface to form an intelligent manufacturing system. Robot of 3R or EROWA system robot can easily realize automatic exchange of workpieces (tray), expert system and automatic wire exchange technology of electrode wire can automatically complete entire machining process.
2. Technical level of each grade WEDM-LS machine
WEDM-LS machine can be divided into four grades: top grade, high grade, mid-range and entry. Typical manufacturers of WEDM-LS machines include Swiss GF processing program (formerly AgieCharmilles), Japanese Makino, Japan's Mitsubishi, Japan's Sodick, Western Japan, and Japan's Frank.
(1) Top-level WEDM-LS machine
This WEDM-LS machine represents the highest level currently, mainly manufactured in Switzerland and Japan. Machining accuracy of this type of machine can be guaranteed within ±0.002mm, maximum processing efficiency can reach 400-500mm2/min, surface roughness can reach Ra0.05μm, surface quality is perfect, and surface has almost no metamorphic layer. Micro-finishing is performed using a wire of Φ0.02 mm. Most of main engines have a heat balance system, and some machines are cut in oil. This type of machine tool has complete functions and high degree of automation. It can directly complete precision mould manufacturing, and processed mold life has reached level of mechanical grinding.
(2) High-grade WEDM-LS machine
This type of machine is basically produced by Swiss and Japanese companies. It has automatic threading function, non-resistive anti-electrolysis power supply, and overall thermal constant system. It can be cut with Φ0.07 mm electrode wire with an accuracy of ±0.003 mm. Efficiency can reach more than 300mm2/min, surface roughness can reach Ra<0.2μm, and it can detect cross-section change of workpiece in time and optimize discharge power in real time. These machines are also widely used in precision stamping.
(3) Mid-range WEDM-LS machine
This machine is generally produced by Swiss and Japanese companies in China. Technical level of some Taiwanese machines can also reach this level. Its configuration and performance meet requirements of most domestic precision wire cutting processing. Generally, it adopts non-resistive anti-electrolysis power supply with water-immersed processing and taper cutting function. Practical maximum processing efficiency is 150-200 mm2/min, the best surface roughness is Ra<0.4μm, cutting precision can reach ±0.005mm, and electrode wire of Φ0.1mm and above is generally used for cutting. Collision protection system avoids damage caused by programming errors or mishandling, and is equipped with an optional automatic threading mechanism.
(4) Entry-level WEDM-LS machine
This machine tool is generally a Taiwan machine tool or a domestically developed machine tool. Its configuration and performance meet processing requirements of common domestic mould manufacturing and parts. Generally, process of cutting and repairing, or cutting and repairing two. Can be used to stably achieve a surface finish of about 0.8 μm in Ra, and a processing accuracy of ±0.008 mm. Most of them can only be cut with a wire of 0.15 mm or more. In processing of surface microstructure and corners, there is a certain gap with advanced machine tools.
  • Advantages of WEDM-MS technology
Medium-speed Wire Cut Electrical Discharge Machining (MS-WEDM) belongs to category of reciprocating high-speed wire EDM wire cutting machine. It is a multi-cutting function on high-speed reciprocating wire EDM machine.It is commonly known as “medium wire cutting”. So-called middle wire processing does not mean that wire speed is between high speed and low speed, but a composite wire cutting machine. That is, wire routing principle is to use high-speed (8-12m/s) wire for roughing and low-speed (1-3m/s) wire for finishing, so work is relatively stable, jitter is small, and error caused by material deformation and molybdenum wire loss is reduced by multiple cutting, so that processing quality is relatively improved, and processing quality can be between high speed wire machine and low speed wire machine. Therefore, user said that "WEDM-MS" is actually a reciprocating wire EDM machine that draws on processing technology of some low-speed wire-cutting machines and realizes stripless cutting and multiple cutting.
In practice of WEDM-MS, it is concluded that the first cutting task in multiple cutting is mainly high-speed stable cutting, high peak current can be selected, and current pulse cutting can be performed with a longer pulse width to obtain a higher cutting speed. Task of second cutting is to refine and ensure dimensional accuracy of machining. Medium gauge can be selected so that roughness Ra after second cutting is between 1.4 and 1.7 μm. In order to achieve purpose of finishing, low-speed wire-traveling method is usually adopted, wire-traveling speed is 1~3m/s, tracking feed rate is limited to a certain range to eliminate round-trip cutting stripe and obtain required processing dimensional accuracy . Third, fourth or more cutting task (currently middle wire control software can achieve up to seven cuts) is polishing which can be polished with a minimum pulse width (currently the smallest can be divided to 1μs). Peak current varies with quality of machined surface. In fact, finishing process is an electric spark grinding process with a small amount of machining and does not change size of workpiece.
Wire-traveling method uses a low-speed wire-feeding speed limit feed like second cutting. In process of machining, cutting process needs to pay attention to deformation process. Because workpiece is cut in line, it will produce non-directionality due to influence of original internal stress and machining thermal stress generated by spark discharge. Irregular deformation makes thickness of cutting behind knife uneven, which affects processing quality and processing accuracy. Therefore, different machining allowances should be reserved according to different materials, so that workpiece can fully release internal stress and complete torsional deformation, and there can be sufficient margin for finishing cutting in multiple subsequent cuttings, so that final size of the workpiece can be guaranteed.
  • Advantages of WEDM-HS processing technology
As name implies, fast wire processing is named because wire is used for high-speed wire running during cutting process. Fast wire processing machine tool is China's original electric processing machine tool. After decades of continuous improvement and development, it has become an important processing method in mould manufacturing industry. It can meet requirements of medium, low-grade mould manufacturing and other complex parts manufacturing, and has a considerable weight in middle and low-end market.
Biggest advantage of WEDM-HS machine is its good performance and price ratio. Because of its practicability and economy, it has a large market in China. Traditional fast wire processing adopts a one-time cutting process, processing efficiency is about 40 mm 2 /min, surface roughness is about 3.0 μm, surface has obvious lines, and processing precision is about 0.02 mm.
In order to improve processing quality, manufacturer of WEDM-HS machine tool has improved relevant parts of machine tool by referring to multiple cutting process, and so-called "WEDM-MS machine" has appeared. A large number of process technologies such as numerical control precision, pulse power supply, wire routing system and process database of machine tools have been greatly improved. Machining accuracy and surface quality have been improved.
At present, technical index of WEDM-HS machine tool: efficiency of one cutting is 180mm2/min, surface roughness after multiple cutting is Ra≤1.0μm, average efficiency at this time is 50mm2/min, surface gloss is processed and there is no obvious cutting streak, processing accuracy 0.01mm, electrode wire loss ≤0. 01 mm /200,000 mm2. These performance indicators seem to have been relatively good, but it should be noted that obtaining these indicators requires ensuring good links. Once there are discrepancies, such as concentration of working fluid and loss of wire, stability of processing quality will be affected.
  • Summary
Due to strong demand for diversification of products in society, proportion of multi-variety, small- and medium-sized batch production has increased significantly. It is difficult to adapt to high-efficiency, high-quality and diversified processing requirements using traditional ordinary processing equipment. But WEDM-MS has characteristics of simple structure, low cost and low consumption, so it also has room for survival. Compared with WEDM-HS machine, WEDM-LS machine has obvious advantages in terms of processing efficiency, precision and surface quality. Although price of WEDM-LS machine is relatively expensive, with rapid development of mould manufacturing industry, it will be more applied to various processing fields.

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