What Is Electrolytic Grinding (ECM)?
Electrolytic Grinding (ECM)
Basic Principle of Electrolytic Grinding
Electrolytic grinding is a grinding process that combines the electrochemical anodic dissolution of metal with mechanical grinding. The schematic diagram of the electrolytic grinding principle is shown in Figure 6-26.
During grinding, the workpiece is connected to the positive pole of the DC power supply, and the electrolytic grinding wheel (also known as the conductive grinding wheel) is connected to the negative pole of the DC power supply. The abrasive grains protruding from the electrolytic grinding wheel maintain a certain electrolytic gap between the two poles, and a specific amount of electrolyte is injected into this gap. When the DC power is connected, the metal surface of the workpiece (anode) undergoes electrochemical dissolution, and the metal atoms on the surface lose electrons and become ions, dissolving into the electrolyte. At the same time, the oxygen in the electrolyte combines with the metal ions to form an extremely thin oxide film on the workpiece surface. This oxide film has a relatively high resistance, which slows down the anodic dissolution process. At this point, the high-speed rotating grinding wheel continuously scrapes away this oxide film, and it is carried away by the electrolyte. As a result of the combined and alternating action of anodic dissolution and mechanical grinding, the workpiece surface is continuously etched away, forming a smooth surface and achieving the required dimensional accuracy.
In the process of electrolytic grinding, the metal is mainly corroded by the electrochemical anodic dissolution action, and the electrolytic grinding wheel only serves to grind away the anodic passivation film of the electrolytic product and level the surface.
Characteristics of Electrolytic Grinding
① Wide processing range, high production efficiency Since electrolytic grinding is mainly an electrolytic action, any metal material with high hardness and high toughness can be processed by simply selecting a suitable electrolyte. For example, when grinding cemented carbide, the processing efficiency of electrolytic grinding is 3 to 5 times higher compared to grinding with ordinary diamond grinding wheels.
② High machining accuracy, good surface quality Because the function of the grinding wheel is to scrape off the oxide film, rather than grinding the metal, the grinding force and grinding heat are very small, and phenomena such as grinding burrs, cracks, burns, and so on will not occur. The general surface roughness can be less than 0.16 um, and the machining accuracy is similar to mechanical grinding.
③ Small grinding wheel wear For example, when grinding cemented carbide, the wear amount of a silicon carbide grinding wheel in ordinary mechanical grinding is about 400% ~ 600% of the weight of the cemented carbide removed; with electrolytic grinding, the wear amount of the grinding wheel is only 50% ~ 100% of the cemented carbide removed. Small grinding wheel wear helps to improve machining accuracy.
Although the electrolytes used for electrolytic grinding are weakly corrosive passivating electrolytes (such as NaNO3, NaNO2 etc.), the machine tool and fixtures still need to take anti-salt measures, and the cutting edge of molds (blanking dies) should not be ground extremely sharp when processed by electrolytic grinding.
Application of Electrolytic Grinding in Mold Processing
① Processing of difficult-to-machine materials Electrolytic grinding is independent of the hardness of the workpiece, so it is remarkably effective when used for processing difficult-to-machine materials with high hardness. For example, processing the flat surface of cemented carbide molds with a vertical electrolytic surface grinding machine not only yields high production efficiency but also good processing quality.
② Reduce processing steps and ensure grinding quality In the past, when manufacturing various segmented molds (pieced-together molds), it was necessary to rough process according to the shape of the segments, followed by flat grinding and form finish grinding after heat treatment, resulting in many steps. Moreover, in order to prevent thermal deformation, a relatively large finish grinding allowance was often left. When grinding molds, a large number of diamond grinding wheels were consumed, which increased mold costs. When electrolytic grinding is adopted, cemented carbide segments can be used directly without rough processing, thereby reducing processing steps, and grinding heat, cracks, burns, and deformation will not occur during grinding, which can well guarantee the grinding quality.
③ Improve processing efficiency Mechanical form grinding generally uses a single-piece grinding wheel (or a formed grinding wheel) for plunge grinding on surface grinders, form grinders, or optical profile grinders. Using an ordinary grinding wheel often requires frequent dressing due to the large wear of the grinding wheel, and the time taken for grinding wheel dressing and forming takes up a lot of time, thus extending the working hours. If electrolytic grinding is used, the wear of the grinding wheel is small, and the dressing time of the grinding wheel is short, which helps to improve processing efficiency.
