Grinding processing is a widely used metal cutting method, mainly for processing hard materials that are difficult to cut with traditional cutting tools, as well as materials with high surface quality and dimensional accuracy requirements. With the emergence and application of a large number of new materials demanding precision and quality of parts, the growth rate of grinding processing applications exceeds that of other traditional processing methods. In grinding processing, the size, shape, and distribution of abrasive particles play an important role in the machining process. However, when processing ductile metals, there may be rapid blockage and passivation of the grinding wheel, resulting in a shortened lifespan of the grinding wheel. To avoid the adverse effects of grinding wheel blockage and passivation, it is necessary to analyze the mechanism and causes of grinding wheel blockage.
1. Formation of debris
The grinding process is achieved by cutting a certain amount of workpiece material to obtain higher surface quality and accuracy, and the commonly used tool is a grinding wheel. A grinding wheel is a loose body formed by pressing, drying, and sintering abrasives and binders. Its individual abrasive grain is a tiny cutting edge with a large negative rake angle and blunt edge radius. The high-speed moving abrasive particles enter the workpiece after sliding and plowing, and the cutting layer material has obvious sliding along the shear plane, forming short and thin chips. These chips are heated to a high temperature in the grinding zone, then oxidized and melted, and solidified into particle spheres. There are also some branches on the sphere surface, which is the main form of chips. When grinding acid resistant stainless steel Cr20Ni24Si4Ti, a large amount of spherical grinding debris was found, accompanied by strip-shaped, section shaped grinding debris and ash. Many of these grinding debris will fill the air holes of the grinding wheel and adhere to the periphery of the abrasive, causing blockage of the grinding wheel, reducing grinding accuracy, burning the workpiece, and shortening the life of the grinding wheel.
2. Analysis of Types and Mechanisms of Grinding Wheel Blockage
2.1 Types of blockage in grinding wheels
The types of wheel blockage include embedded type, adhesive type, and hybrid type. Embedded blockage is a blockage state in which abrasive chips are embedded in the gaps of the working surface of the grinding wheel; Adhesive blockage is a blockage state in which debris melts and accumulates on abrasive particles and binders; Mixed blockage refers to both embedded and adhesive blockages.
2.2 Mechanism analysis of embedded blockage
External factors: Grinding processing has an important characteristic, generally with Fy/Fz greater than 2-10. The harder the workpiece material, the smaller the plasticity, and the higher the ratio. Under strong positive pressure, the grinding debris in the grinding area is mechanically squeezed into the gaps on the surface of the grinding wheel. Grinding debris slides out along the front of the abrasive grain, and several layers of debris accumulate in the local area in front of the abrasive grain. Under the high-speed rotation of the grinding wheel, a vortex area of airflow is formed behind the abrasive grain, and the air pressure in the vortex area is significantly reduced. Under the negative pressure, some of the debris adheres to the back of the abrasive grain, forming an adhesive blockage on the cutting surface behind the abrasive grain. Most of the attachments are ash and particles.
The role of electrostatic field: In certain small areas of the grinding zone, a small electric field composed of the grinding wheel and the workpiece is formed. Under the action of the electric field, some of the grinding debris will exhibit polarity. According to the principle of opposite polarity attraction, the grinding debris with the opposite polarity to the grinding wheel will be adsorbed on the working surface of the grinding wheel. By utilizing the significant mechanical pressure between the grinding wheel and the workpiece, the abrasive particles adsorbed on the surface of the grinding wheel can be stably embedded in the gaps between the grinding wheel surfaces.
2.3 Mechanism analysis of adhesive blockage
Melting adhesion: During the grinding process, the vast majority of input power is converted into grinding heat, causing the grinding point temperature to reach over 1200k. The grinding debris quickly oxidizes when exposed to air, forming low melting point metal oxides. When heated at high temperatures in the grinding area, they become melted or slightly melted, covering the surface of the grinding wheel. When this part of the surface of the grinding wheel participates in grinding again, it is squeezed or strengthened under the action of grinding force, increasing the affinity and adhesion with the grinding wheel. Some are squeezed and adhered to the raised groove surface of the workpiece surface. Through multiple random grinding processes, a large amount of debris adheres around the abrasive grains, increasing the grinding force and temperature. This leads to a vicious cycle, exacerbating blockages until the abrasive grains break or detach.
Chemical bonding: The chemical affinity between different elements is another important reason for adhesive blockage. The abrasive particles and the ground material come into contact at high temperatures, and the temperature factor enhances their activity and affinity. When certain conditions are met, it leads to a chemical reaction, causing the abrasive particles and debris to form a crystal on the surface of the grinding wheel that loses cutting ability.
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