The grinding principle and wear mechanism of grinding wheels on cutting tools

The grinding principle and wear mechanism of grinding wheels on cutting tools.

 

1. Grinding Process Principles

Grinding is also a type of cutting process, and the grinding wheel can be considered as a milling cutter with numerous tiny cutting edges when used for machining tools. During the grinding process, the tool surface is subjected to friction and scratching by the grinding wheel, and the protruding and relatively sharp abrasive grains on the wheel surface cut into the material, forming chips. The grinding process is essentially a combination of cutting, scratching, and sliding actions. The chips are small in size and vary in shape, including ribbon-like chips, segmented chips, and some melted and burned chip ashes, as well as metal dust.

 

Grinding is divided into three stages: the initial grinding stage, the stable stage, and the finishing stage. In the initial grinding stage, the actual grinding depth is smaller than the radial feed rate. This is due to the elastic deformation of the machine tool, workpiece, and fixture system during the initial grinding stage. When the system's elastic deformation reaches a certain level, it enters the stable stage. During continued feeding, the actual grinding depth is essentially equal to the radial feed rate. In the finishing stage, as the elastic deformation of the process system is gradually eliminated, the actual grinding depth becomes greater than zero.

Grinding with a grinding wheel has the following characteristics: high precision and low surface roughness. The grinding wheel has a self-sharpening function, allowing the abrasive grains to cut the workpiece with relatively sharp edges. The radial force component is large. Similar to turning, the cutting force during grinding can be decomposed into three mutually perpendicular components, but the radial force component is larger. The grinding temperature is high. Because grinding with a grinding wheel involves negative rake angle cutting and a very high cutting speed, the grinding temperature is high. The grinding wheel has a self-sharpening effect, allowing the abrasive grains to continuously cut the workpiece with relatively sharp edges. The grinding motion consists of the main motion, the radial feed speed, and the axial feed speed. The main motion is the rotational motion of the grinding wheel; the linear velocity of the outer circumference of the grinding wheel is the main motion speed; the radial feed rate refers to the distance the workpiece moves radially relative to the grinding wheel during each double (single) stroke of the worktable; and the axial feed rate refers to the distance the workpiece moves axially relative to the grinding wheel during each revolution or each stroke of the worktable.

 

2. Grinding Wheel Wear Patterns and Causes

During the grinding process of cutting tools, the grinding wheel will experience varying degrees of wear due to various factors, including physical, chemical, and mechanical effects, leading to a decrease in grinding ability and affecting the accuracy of the helical groove. If a severely worn grinding wheel is not replaced and continues to be used, it will cause vibrations, noise, and other phenomena. Extensive research on grinding wheel wear has shown that the main forms of wear are abrasive wear, fracture wear, and clogging/adhesion.

 

2.1 Abrasive Wear

During the grinding process, each abrasive grain experiences wear, exhibiting different degrees of wear facets, as shown in the C-C plane in the figure below. As the number of blunted abrasive grains increases, the grinding wheel exhibits blunting characteristics, such as a significant increase in grinding force, workpiece surface burning, and chatter during the machining process, leading to a serious decline in the processing quality of the machined parts.

2.2 Fracture and Wear

The fracture and wear of grinding wheels can be divided into two types: abrasive grain fracture and abrasive grain shedding. Abrasive grain fracture refers to the phenomenon where, when the stress acting on the abrasive grain exceeds its own strength, a part of the abrasive grain breaks off in the form of small fragments. Abrasive grain shedding refers to the fracture of the binder between the abrasive grains, causing the abrasive grains to detach from the grinding wheel. This creates voids where the detached grains were located. The detachment of fractured abrasive grains from the grinding wheel leads to tangential wear of the workpiece, making it impossible to guarantee the dimensional accuracy of the part. However, the formation of new cutting edges from blunted abrasive grains, influenced by abrasive grain fracture and shedding, can be defined as the "self-sharpening" effect of the grinding wheel.

 

2.3 Clogging and Adhesion

During the grinding process, due to increased temperature and pressure, the workpiece material that is removed adheres to the abrasive grains as they pass through the grinding zone. Whether or not the adhered material contacts the workpiece is a major cause of abrasive grain fracture and shedding. The grinding ability is also related to the adhered material. The adhered material can also clog the gaps between the abrasive grains. Severe clogging can also lead to abrasive grain fracture and even shedding, significantly reducing the grinding wheel's grinding ability.

 

To explore the nature of grinding wheel wear, numerous scholars have studied the causes of grinding wheel wear.

 

Currently, the causes of grinding wheel wear are classified into the following types:

• Abrasive Wear: Friction is generated by the relative motion between the abrasive grains and the workpiece, leading to mechanical wear of the abrasive grains. This wear gradually forms over time as grinding progresses. When grinding, if the workpiece structure is uneven and contains hard points with higher hardness, the relative sliding friction between the abrasive grains and the hard points will exacerbate the mechanical wear of the abrasive grains. Plastic Wear: When the grinding temperature reaches a certain level, the abrasive grains will deform due to plasticity. The thermal hardness of the workpiece material directly affects the plastic wear of the grinding wheel. As the abrasive grains pass through the grinding zone, their temperature rises. When it reaches the melting point of the workpiece material, if the thermal hardness on the shear plane is greater than the thermal hardness in the abrasive grain contact area, the abrasive grains will undergo corresponding plastic deformation in the contact area, leading to abrasive wear.
• Oxidative Wear: Certain gases in the air can stimulate grinding. When the grinding process is carried out in a vacuum, the grinding of low-carbon steel with an alumina grinding wheel is not as smooth as in air. Analysis shows that the rotation of the grinding wheel drives air flow, reducing the temperature in the grinding zone. At high temperatures, the workpiece and chips undergo oxidation, forming an oxide film on the surface, preventing adhesive wear on the workpiece surface.
• Chemical Wear: During grinding, the surface of the grinding wheel and the surface of the workpiece exhibit a complex spatial distribution. Increased grinding speed leads to higher grinding temperatures, causing chemical reactions between the abrasive material, the workpiece material, and the grinding fluid. The various chemical elements produced by these chemical reactions may undergo further multi-stage chemical reactions. The chemical reaction between the abrasive material and the workpiece material is an important factor in the chemical wear of the grinding wheel.
• Diffusion Wear: When the grinding wheel grinds the workpiece, the elements on the surface of the grinding wheel and the workpiece diffuse at high temperatures, weakening the surface layer of the abrasive grains and causing wear. Two closely contacting metal materials, under high temperature and pressure, will undergo diffusion in the contact area after a certain grinding time, leading to wear of the grinding wheel.
• Thermal Stress Fracture Wear: During the grinding of the workpiece, the abrasive grains instantly reach a high temperature and then cool rapidly under the action of the grinding fluid. Under repeated intermittent cooling and heating, the thermal stress in the abrasive grains increases, leading to cracking and fracturing on the surface of the abrasive grains. Thermal stress is mainly related to thermal conductivity, coefficient of thermal expansion, and grinding fluid. Thermal conductivity is inversely proportional to thermal stress, while the coefficient of thermal expansion is directly proportional to thermal stress. The better the performance of the grinding fluid, the lower the workpiece surface temperature and the greater the thermal stress.