1. Grinding Principles
Grinding is essentially a form of cutting. During the grinding of cutting tools, the grinding wheel can be regarded as a milling cutter with an extremely large number of microscopic cutting edges. The surface of the tool is subjected to friction, scratching, and actual cutting by the sharp abrasive grains protruding from the grinding wheel surface, generating grinding chips. Thus, the grinding process is a combination of three actions: cutting, ploughing (or scratching), and rubbing.
The resulting grinding chips are very fine and vary in shape—ranging from continuous ribbon-like chips and segmented chips to molten residues and metallic dust.
Grinding typically progresses through three stages: initial grinding, steady-state grinding, and spark-out (finishing) grinding.
In the initial stage, the actual depth of cut is slightly less than the radial feed due to elastic deformation within the machine-tool-workpiece-fixture system.
Once this elastic deformation stabilizes, the process enters the steady-state stage, where the actual depth of cut closely matches the radial feed.
In the spark-out stage, as the elastic deformation gradually recovers, the actual depth of cut becomes greater than zero, ensuring final dimensional accuracy and surface finish.
Key Characteristics of Grinding with Abrasive Wheels:
High precision and low surface roughness: Abrasive wheels possess a self-sharpening ("self-dressing") capability, allowing fresh, sharp cutting edges to continuously engage the workpiece.
Large radial force component: Similar to turning, grinding forces can be resolved into three mutually perpendicular components, but the radial force is significantly larger.
High grinding temperatures: Due to the negative rake angle of abrasive grains and extremely high cutting speeds, substantial heat is generated.
Self-sharpening effect: Worn or dulled grains fracture or dislodge, exposing new sharp edges—this is known as the "self-truing" or "self-sharpening" property.
Grinding motion composition:
Primary motion: Rotation of the grinding wheel (peripheral speed).
Radial feed: Incremental movement of the workpiece toward the wheel per stroke (or double stroke).
Axial feed: Longitudinal movement of the workpiece relative to the wheel per revolution or per table stroke.
2. Forms and Causes of Grinding Wheel Wear
During tool grinding, grinding wheels experience wear due to mechanical, physical, and chemical interactions, which reduces their cutting efficiency and compromises machining accuracy (e.g., spiral flute geometry). Continued use of a severely worn wheel leads to vibration, noise, and poor surface quality.
Extensive research has identified three primary wear modes:
2.1 Attritious (Abrasive) Wear
Each abrasive grain develops a wear flat (e.g., the C–C plane shown in diagrams) during grinding. As more grains become blunt, the wheel exhibits signs of dulling—increased grinding forces, workpiece burn, chatter, and degraded surface quality.
2.2 Fracture Wear
This includes two subtypes:
Grain fracture: When stress on an abrasive grain exceeds its strength, fragments break off, creating a new cutting edge.
Grain pull-out: The bond between grains and the wheel matrix fails, causing entire grains to detach and leave voids.
Although grain loss may cause dimensional inaccuracies, the exposure of fresh, sharp edges through fracture or pull-out contributes to the wheel’s beneficial self-sharpening ("self-dressing") effect.
2.3 Loading and Adhesion
During grinding, high temperature and pressure cause workpiece material to adhere to abrasive grains. This adhered material:
Blocks inter-granular pores,
Reduces cutting efficiency,
May induce further grain fracture or pull-out when it contacts the workpiece.
Severe loading significantly diminishes the wheel’s grinding performance.
3. Fundamental Causes of Grinding Wheel Wear
Scholars have categorized the underlying mechanisms of wheel wear as follows:
(1) Abrasive (Mechanical) Wear
Caused by relative sliding between abrasive grains and the workpiece. Grooves on worn grain flats align with the cutting direction. Hard inclusions or non-uniform microstructures in the workpiece accelerate this wear.
(2) Plastic Wear
At elevated temperatures, abrasive grains may undergo plastic deformation. If the thermal hardness of the workpiece material exceeds that of the grain at the contact zone (especially near the workpiece melting point), the grain deforms plastically and wears rapidly.
(3) Oxidation Wear
Atmospheric oxygen promotes oxidation of both workpiece and swarf at high temperatures, forming oxide films that reduce adhesion and improve grinding smoothness. Experiments show grinding in vacuum (e.g., with alumina wheels on low-carbon steel) is less efficient than in air.
(4) Chemical Wear
High-speed grinding generates high temperatures that trigger chemical reactions among the abrasive, workpiece material, and coolant. These reactions (including multi-stage processes) degrade grain integrity.
(5) Diffusion Wear
At high temperatures and pressures, atoms from the workpiece and abrasive diffuse across the contact interface, weakening the grain surface and accelerating wear.
(6) Thermal Shock (Thermo-Mechanical) Fracture
Abrasive grains experience rapid heating during contact and sudden cooling by coolant. Repeated thermal cycling induces high thermal stresses, leading to micro-cracking and grain fracture. This effect intensifies with:
Lower thermal conductivity of the grain,
Higher coefficient of thermal expansion,
More effective (i.e., faster-cooling) grinding fluids.
来源 中研高科智能制造
