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Key words: diamond grinding wheel, electric spark trimming, pulse power supply
Abstract: On the basis of the analysis of the request to pulse generator of EDM dressing of metal-bonded fine grain diamond grinding wheel,this paper develops a new kind of pulse generator applied to EDM dres~sing.The design and work principles are introduced
Keywords: diamond grinding wheel EDM dressing pulse generator
Ultra-precision grinding of diamond micro-grinding wheel is one of the research hotspots in the field of ultra-precision machining technology. Ultra-precision grinding of hard and brittle materials such as engineering ceramics with diamond micro-grinding wheels eliminates the need for grinding and polishing processes, not only meets the requirements of machining accuracy and surface quality, but also greatly increases productivity and reduces processing costs. One of the key technologies for ultra-precision grinding of diamond micro-powder wheels is the diamond micro-powder wheel dressing technology. 3. The electric spark trimming method is a method to effectively trim the metal bond diamond micronized grinding wheel. The electric spark dressing experiment of diamond grinding wheel shows that the discharge parameter is one of the key factors affecting the dressing efficiency, dressing precision, sharpening effect and grinding wheel grinding performance. Therefore, the pulse power supply for electric spark trimming is very important.
1 Analysis of the requirements of pulsed power supply for metal bond diamond micronized grinding wheel
The dressing of the grinding wheel consists of two parts: shaping and sharpening. The shaping is to make the grinding wheel surface and its axis of rotation meet a certain geometric relationship. For the flat grinding wheel, the roundness and cylindricity of the grinding wheel are guaranteed. Sharpening is to make the abrasive grains on the surface of the grinding wheel protrude to form a sharp cutting edge and chip space. The principle of the metal bond diamond micro-powder wheel electric spark dressing method is to generate a pulse spark discharge between the tool electrode and the high-speed rotating diamond grinding wheel, and the metal bond of the grinding wheel is etched away by the electric erosion, thereby removing the eccentricity of the grinding wheel and making the diamond The abrasive grains expose the surface of the grinding wheel. The pulse power supply is an important part of the electric spark trimming equipment. Its function is to convert the power frequency alternating current into a unidirectional pulse current of a certain frequency to supply the energy required for the electrode discharge gap to remove the metal bond. Since the electric spark trimming is carried out on the grinding machine, the processing object is a high-speed rotating, large-volume diamond grinding wheel, so the requirements for the pulse power supply have its particularity.
Due to the large volume of the diamond grinding wheel, the total volume of the metal bonding agent that needs to be etched in the process of removing the eccentricity of the grinding wheel is also large, which requires large discharge standards in the early stage of shaping and quick removal of the grinding wheel with high efficiency. eccentric. However, the shaping accuracy of the grinding wheel is affected by the discharge standard. In order to obtain higher shaping accuracy, a small EDM standard must be selected. Therefore, the EDM process requires that the pulse power supply can provide both large discharge standards and small discharge standards.
3. The discharge pit, metal melt and metal spatter are distributed on the surface of the grinding wheel after the electric spark is sharpened. According to the scanning electron micrograph and grinding experiment, the flatter the surface of the diamond micronized grinding wheel after the electric spark is sharpened, the more the protruding abrasive grains, the better the grinding effect. The discharge process of low-energy single pulse is analyzed. At the beginning of discharge, the plasma radius in the discharge channel is small, the energy density of the plasma is high and the distribution is relatively uniform, and the erosion of the metal bond is mainly in the form of evaporation. Will make the surface of the sharpened wheel flat. As the discharge continues, the discharge channel gradually expands, the current density and the channel temperature decrease, the removal mode of the metal bond changes to melt and throw, and the molten metal bond re-distributes and condenses after discharge to form irregular discharge marks. The discharge state is not good for sharpening quality. Moreover, according to the theory of heat transfer, if the spark discharge time is short, the heat penetration depth will be small, and the temperature rise will be limited to the vicinity of the surface of the grinding wheel, and the formed discharge pit will be relatively shallow. It can be seen from the above theoretical analysis that the pulsed power supply of the diamond micronized grinding wheel requires a pulse width to provide a narrow pulse width and low energy discharge parameter.
In the process of precision shaping and sharpening, due to the small discharge parameters, the discharge gap is also small. Although the high-speed rotation of the grinding wheel is conducive to the stability of the discharge state, abnormal phenomena such as "burning arc" or "short circuit" may occur, and the burn grinding wheel Large pits are formed on the surface or on the surface of the grinding wheel, so that the pre-shaping work has been abandoned. On the electroforming machine, the stability of the discharge state is controlled by the automatic feed adjustment system. In order to avoid installation errors, the diamond micronized grinding wheel must be trimmed in place, ie the dressing must be carried out on the grinding machine. There is no automatic feed adjustment system on the grinding machine. Moreover, due to the eccentricity of the diamond grinding wheel that needs to be trimmed, the gap between the tool electrode and the grinding wheel changes momentarily during the rotation of the grinding wheel, and the automatic feed adjustment system is not applicable in the electric spark trimming.
Some Japanese scholars study EDM trimming techniques on CNC grinding machines or grinding centers. Based on the detected gap voltage values, they feed the grinder at 100%, 20%, and 0% of the set feed rate to achieve controlled discharge. The purpose of the state. At present, in China, the factory is still widely used in ordinary precision grinding machines (feeding by manual operation), CNC grinding machines and grinding machining centers are still very rare. It is especially difficult to achieve discharge state control on a common grinding machine. This requires some technical processing on the pulse power supply to ensure the spark trimming effect.
2 Design idea of ​​pulse power supply for electric spark trimming
According to the special requirements of pulsed power supply for metal bond grinding powder grinding wheel, a combined pulse power supply consisting of VMOS tube pulse power supply and RC pulse power supply is designed. In order to eliminate the abnormal discharge during the trimming process, the cut-off function is designed in the power supply.
VMOS tube pulse power supply uses VMOS FET as a switching element to obtain one-way pulse. It has high pulse frequency, easy adjustment of pulse parameters and good pulse waveform, which can meet the requirements of pulse power supply in the electric spark roughing stage. . The RC pulse power supply has a simple structure, reliable operation, low cost, and can obtain a narrow pulse width and a small single pulse energy at a low power, and can meet the requirements of the electric spark precision shaping and sharpening stage with relatively simple equipment.
In the stage of precision shaping and sharpening, there are sometimes abnormal phenomena such as "burning arc" or "short circuit". In order to ensure the quality of the trimming, the cutting function is designed in the pulse power supply for electric spark trimming. When an abnormal phenomenon occurs in the discharge gap and is detected by the detection circuit, the control circuit turns off the VMOS tube, cuts off the processing pulse and turns on the indicator light, thus eliminating abnormal discharge and fully eliminating the gap and notifying the operator. When the VMOS tube is turned off for a predetermined time, the circuit is turned back on and the trimming continues. If abnormal discharge occurs several times in a short time, the operator can know the situation through the indicator light, then the wheel can be lifted to remove the obstacle.
3 Design and characteristics of pulse power supply for electric spark trimming
Figure 1 is a block schematic diagram of the pulse power supply for electric spark trimming designed in this paper. The main vibration level is 555 oscillator circuit, which emits a series of rectangular pulse signals with adjustable period and duty cycle. After amplification by the amplification stage, the VMOS FET is controlled to be turned on and off. When the initial grinding of the diamond grinding wheel is carried out, the discharge standard used is large, the gap between the diamond grinding wheel and the tool electrode is also large, and abnormal discharge is not easy to occur, and even if abnormal discharge occurs, the hazard is not large. At this time, the switches k1 and k3 are turned off, and k2 and a are turned on. At this time, the circuit is actually a VMOS tube pulse power supply. When performing spark precision shaping and sharpening, the switches k1 and k3 are closed, k2 and b are connected, and k4 selects the appropriate capacitance and resistance. When the VMOS tube is turned on, the power supply is like a RC type tension pulse power supply, and the small capacity capacitor C outputs a series of pulses with a narrow pulse width for electric spark trimming. When the VMOS tube is turned off, the capacitor stops charging and discharging, and the discharge gap is sufficient. Elimination of ionization. Figure 2 shows the voltage waveform during precision trimming and sharpening. Ua is the output voltage waveform of the VMOS tube, and Ub is the waveform between the diamond grinding wheel and the tool electrode. When an abnormal discharge occurs, the VMOS tube is turned off, and the voltage between the output of the VMOS tube and the grinding wheel and the tool electrode is zero. 
This pulse power supply for electric spark trimming has the following characteristics:
1 The combined pulse power supply consisting of VMOS tube pulse power supply and RC type relaxation pulse power supply meets the high requirements of pulsed power supply for diamond micro-powder wheel EDM trimming with simple equipment.
2 In the case of precision dressing and sharpening, if abnormal discharge occurs, the power supply cuts off the pulse and automatically turns on after a delay, which solves the problem that the ordinary precision grinding machine cannot control the discharge state in time.
4 Conclusion
The diamond wheel EDM dressing method can be applied to any grinding wheel with conductive material as the bonding agent. The machining force is small, and it has the superiority of other dressing methods in the fast and high precision dressing of small diameter and very thin grinding wheel and forming grinding wheel. Many scholars are exploring the mechanism of EDM trimming and the development of trimming technology. Practice has proved that the pulse power supply for EDM trimming designed in this paper combines the advantages of VMOS tube pulse power supply and RC relaxation pulse power supply, and satisfies the metal bond diamond micro-powder wheel EDM on a common grinding machine with simple equipment. Special requirements for finishing.
Abstract: According to the requirements of pulsed power supply for metal bond grinding powder grinding wheel, a new type of pulse power supply for electric spark trimming was developed. The design, working principle and characteristics of the power supply were introduced.