1.General situation of (PCBN) grinding of polycrystalline cubic boron nitride
In recent years, the use of polycrystalline cubic boron nitride (PCBN) makes it possible for precision hard cutting of workpieces with hardness above 58HRC, achieving the surface quality of turning instead of grinding. Compared with high-speed steel and cemented carbide materials, PCBN cutting tools can greatly improve workpiece surface quality and machining speed, and improve machining efficiency and tool life. At present, the main means of machining the cutting edge of PCBN cutting tools at home and abroad are mechanical grinding, electric spark discharge grinding and so on.
Mechanical grinding, that is, grinding the machined surface of the PCBN cutting tool with a high-speed rotating diamond wheel. Mechanical grinding has the advantages of simple process and low surface roughness of cutting tool edge. However, due to the difficulty of grinding wheel shaping, serious grinding wheel loss and high cost, it is difficult to realize the machining of cutting tools with sharp and thin cutting edge or complex cutting edge, and the scope of application is limited. EDM is a self-excited discharge machining process based on the principle of pulse discharge and corrosion removing. The physical process of discharge and corrosion removing is a comprehensive process of electromagnetism, thermodynamics and hydrodynamics, and the mechanism of EDM is a kind of electrophysical process. However, EDM belongs to non-contact machining and has no mechanical cutting force, so it does not need to consider its mechanical characteristics in making tool electrodes. It has the advantages of simple tool electrode forming and low relative loss, and can be effectively applied to the machining of PCBN cutting tools with sharp and thin edge and complex cutting edge. In this paper, according to the characteristics of superhard cutting tool products, combined with the principle of EDM grinding, the machining efficiency and quality of EDM grinding are described in details, such as electrode rotation cutting speed, peak current and pulse width.
2.Influencing factors of EDM grinding
2.1Cutting speed of electrode rotation
When the electrode is not rotated, the material removal of the workpiece is the lowest and the surface roughness of the workpiece is the largest, and the machining speed and quality are obviously improved after the electrode rotation, because the electrode rotation improves the discharge machining conditions and accelerates the etching speed of the material. With the increase of the rotation speed of the electrode, the removal amount of the workpiece material also increases, but the increase slows down gradually, and the loss of the electrode also increases, this is because the wear of the electrode mainly occurs in the initial stage of the discharge machining, with the increase of the electrode speed, the discharge point shifts rapidly, and the discharge frequency increases obviously, which leads to the increase of the electrode loss; with the increase of the electrode speed, the surface roughness of the workpiece decreases at first, and then increases. In terms of machining quality and efficiency, the influence of electrode cutting speed is less than that of peak current and pulse width.
With the increase of the peak current Ip, the removal amount of the material increases, the electrode loss decreases, and the surface roughness of the workpiece material increases. However, the changing trends of the three are different: when the peak current is less than 5A, the material removal and surface roughness of the workpiece increase slowly, while the electrode loss decreases sharply; when the peak current is greater than 10A, the material removal and surface roughness of the workpiece increase rapidly, and the decreasing trend of electrode loss slows down. At the same time, when the peak current is less than 8A, the PCBN grain size has little effect on the machining efficiency and quality, but when the peak current is greater than 10A, the PCBN grain size has a certain effect on the machining efficiency and quality of the workpiece.
With the increase of pulse width, the removal amount of PCBN material increases rapidly. The reason may be that the longer the pulse width and discharge time, the greater the energy generated between the electrode and the PCBN semifinished product, thus accelerating the removal of PCBN materials. At the same time, the electrode loss decreases rapidly with the increase of pulse width. The electrode loss mainly occurs at the front end of the pulse. When the pulse interval constant increases, the larger the pulse width is, the less the pulse number is, and the smaller the electrode loss is. With the increase of pulse width, the surface roughness of PCBN semifinished product and the thickness of metamorphic layer increase.
The grain size of workpiece PCBN has a certain influence on the efficiency and quality of EDM grinding, but the effect is not as significant as that of EDM process parameters. Considering the joint influence of cutting efficiency, surface quality and electrode wire loss, WEDM of PCBN compact should choose reasonable pulse width and use forward wire transport high pressure to supply liquid under the condition of high peak current and small pulse cutting.
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