China's unique high-speed wire-cut wire electric discharge machine (WEDM-HS), due to its simple structure, low cost, good process effect, and low consumption during use, has been rapidly developed since its successful development in the late 1960s. It has become an indispensable process equipment in the manufacturing industry. By the end of 2004, the national annual output had exceeded 30,000 units, accounting for about 70% of the world's total output of wire EDM. However, due to the problem of its processing quality has not been effectively solved, and with the continuous improvement of the level of low-speed wire-cut wire EDM (WEDM-LS) technology produced abroad, the Chinese-specific WEDM-HS, which has made the Chinese proud, Now it is in a difficult situation to develop.
Proposal of multiple cutting techniques
Wire Cut Electro Discharge Machining (WEDM) is based on the phenomenon of electrical corrosion during pulse discharge between the wire and the workpiece, that is, a large amount of heat is generated every pulse to instantaneously melt and gas the local metal near the discharge point. The molten and vaporized metal is removed to form a small pit on the surface of the workpiece. If the pulse discharge energy used is large, the processing speed is fast; but the discharge pit is also large, and the surface roughness of the processing is poor.
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| Figure 1: TP series high speed wire cutting machine |
After the successful development of WEDM processing technology in the late 1950s, there has been a certain period of time for the cutting of a precision gauge with a precise specification. There is a certain contradiction between the cutting speed and the surface quality of the machine. In a cutting process, it is very difficult to obtain a high cutting speed and to obtain a very good surface quality.
With the development of the mold industry, many users have proposed that the EDM wire cutting process not only has a high cutting speed, but also requires good processing quality, otherwise it is difficult to meet the needs of mold development. In order to meet the needs of users, foreign low-speed wire-cut EDM wire cutting machines have developed and applied multiple cutting techniques, that is, the first cutting uses a large electric gauge to carry out high-speed rough cutting, and then uses fine gauge and fine gauge. The second, third or even fourth and fifth cuts will be used to level the surface to obtain better surface quality and processing accuracy. The application results of low-speed wire-cut EDM wire cutting process using multiple cutting technology show that multiple cutting is an effective way to solve the contradiction between cutting speed and surface quality and improve the effect of wire cutting integrated process.
While developing and applying multiple cutting technology on foreign low-speed wire EDM wire-cutting machines, a large number of multiple cutting tests have been carried out on WEDM-HS in China, but they have not been applied in production practice. The main reasons are: In the case of high-speed movement of the wire, the operation of the wire system is unstable, and the spatial shape of the wire is abnormally changed, so that the spatial position of the secondary cutting is difficult to ensure uniformity, and the round-trip cutting stripe of the high-speed wire cutting is obvious. It is difficult to achieve multiple cuts on WEDM-HS.
The results of a large number of experimental studies conducted by the predecessors indicate that the use of multiple cutting processes in China's unique high-speed wire EDM wire cutter is not only necessary but also possible, but the following conditions must be created:
1 According to the relevant national technical standards, strictly control the manufacturing precision of the high-speed wire EDM machine and the stability of the wire handling system.
2 In-depth study of the spatial shape change of the electrode wire during processing, and put forward effective measures to stabilize the spatial shape of the electric wire.
3 Take effective measures to suppress the generation of round-trip cut stripes.
4 Develop a high-frequency pulse power supply and its tracking control system that can meet the needs of multiple cutting.
5 In-depth study of WEDM-HS multiple cutting process, reasonable setting of rough cutting, finishing and fine polishing pulse parameters, processing track compensation, wire moving mode and its speed, and the development of corresponding multiple cutting software .
WEDM-HS equipment capable of multiple cutting
In order to improve the processing quality of WEDM-HS, Shanghai Massive Electronic Equipment Co., Ltd. developed on the basis of DK77 series high-speed wire-cut EDM wire-cutting machine, using the advanced concept of international precision mold processing equipment and many patented technologies developed by itself. A TP series high-speed wire EDM wire cutting machine capable of multiple cutting (Fig. 1). The reason why the TP series high-speed wire-cutting machine can be cut as many times as the low-speed wire-cutting machine is because of the following fruitful reforms of the original high-speed wire-cutting machine.
â–² In-depth study of the spatial shape change of the electrode wire, found that the deflection of the electrode wire produced during electrical discharge machining is related to the distance of the guide wheel from the surface of the workpiece. The farther the fulcrum of the wire is from the surface of the workpiece, the greater the deflection; and the deflection of P in the workpiece is very small, generally only 0.002~0.005mm. The patented technology of the high wear-resisting guide developed by the company can effectively stabilize the spatial position of the electrode wire and create conditions for multiple cutting.
â–² The patented technology of ultra-short-cut reciprocating cutting developed by ourselves can realize stripe-free cutting.
â–² Some new structures have been adopted to improve the manufacturing precision of the WEDM machine. For the TP series wire cutting machine that has passed the inspection, the actual machine tool accuracy parameters are higher than the national standard GB7926-2005.
▲ The developed high-frequency pulse power supply can meet the needs of multiple cutting: (1) greatly increase the pulse current amplitude; (2) minimize the minimum duration of pulse discharge (minimum pulse width is less than 1μs); (3) can effectively control the pulse current Rate of increase.
â–² Reformed the tracking control method: (1) The first high-current rapid cutting adopts over-tracking control to avoid the occurrence of broken wire phenomenon; (2) The second-time and third-time polishing cutting adopts the control method of speed-limiting feeding. To improve the quality of the cut surface.
â–² Based on the process test, multiple cutting software was developed to facilitate user operation.
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| Figure 2: Multiple cut parts |
Multiple cutting process parameter settings
â—† The first cutting task is high speed and stable cutting
(1) Pulse parameters: High peak current is selected, and the current pulse cutting is performed for a longer pulse width to obtain a higher cutting speed.
(2) The compensation amount of the center wire of the wire is small:
f = 1/2φd +δ+ △ + S where f is the compensation amount (mm); δ is the discharge gap (mm) at the first cutting; φd is the wire diameter (mm); △ is reserved for the first Machining allowance for secondary cutting (mm); S is the finishing allowance (mm).
In the case of high peak current coarse gauge quasi-cutting, the unilateral discharge gap is about 0.02 mm; the finishing allowance is very small, generally only 0.003 mm; and the machining allowance â–³ depends on the surface roughness after the first cutting and Machine tool accuracy, in the range of 0.03 ~ 0.04mm. In this way, the compensation amount of the first cutting should be between 0.05 and 0.06 mm. If the selection is large, the speed of the second cutting will be affected. If it is small, it is difficult to eliminate the trace of the first cutting.
(3) Wire-cutting method: the whole process is used for round-trip wire, and the wire-cutting speed is 11m/s.
â—† The task of the second cutting is finishing, ensuring the dimensional accuracy of the machining
(1) Pulse parameters: The medium standard is selected so that the roughness Ra after the second cutting is between 1.4 and 1.7 μm.
(2) Compensation amount f: Since the second cutting is finishing, the discharge gap is small, δ is less than 0.01 mm, and the processing quality required for the third cutting is very small, only a few micrometers, and the two add up to 0.01mm. Therefore, the compensation amount f of the second cutting can be about 1/2d + 0.01mm.
(3) Wire-cutting method: In order to achieve the purpose of finishing, the ultra-short-travel wire-traveling method is usually adopted, and the tracking feed speed is limited to a certain range to eliminate the round-trip cutting stripe and obtain the required processing dimensional accuracy.
â—† The task of the third cutting is polishing
(1) Pulse parameters: The light is trimmed with a minimum pulse width, and the peak current varies with the quality of the machined surface.
(2) Compensation amount f: Theoretically, the radius of the wire is added with a discharge gap of 0.003 mm. In fact, the finishing process is an electric spark grinding, and the machining amount is small, and the size of the workpiece is not changed. Therefore, the ideal effect can be obtained by simply using the radius of the electrode as a compensation amount.
(3) Wire-traveling method: It can be used for the second-time cutting with ultra-short stroke round-trip speed limit feed.
Multiple cutting process effects
The TP series high-speed wire EDM wire cutting machine developed and produced by Shanghai Mass Electronic Equipment Co., Ltd. can be cut many times. At present, it has sold more than 600 sets on the market. Users generally report that the wire cutting machine has better processing quality. Wenzhou also refers to this wire cutting machine as a "medium speed wire machine". Said "medium speed wire", not the wire moving speed between "high speed" and "low speed", but the wire cutting processing quality is higher than the "high speed wire machine", and approaching the low speed wire machine. After three cuttings of the TP series wire cutting machine, the optimum surface roughness Ra≤1μm, the dimensional difference △≤0.006mm; far higher than the regulation of Ra≤2.51μm and △≤0.015mm specified in GB7926-2005.
The parts shown in Fig. 2 were processed by a TP25 high-speed wire cutting machine. The electric grade wire was φ0.18 mm molybdenum wire, and the workpiece was 40 mm thick quenched Cr12.
in conclusion
The long-standing processing quality problems of China's unique high-speed wire EDM wire cutting machine can be solved by multiple cutting processes. The high-speed wire-cut EDM wire cutting machine capable of achieving multiple cuttings can greatly improve the quality of wire cutting processing while retaining the characteristics of simple structure, low cost, good process effect and low consumption. However, not all high-speed wire cutters can perform multiple cuts, but the necessary reforms should be made like the TP series wire cutters to create the necessary conditions for multiple cuts.
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