Precision Grinding Processes and Wheel Wear Mechanisms

Grinding Process and Wheel Fundamentals

Grinding is a material removal process in which a rotating wheel with abrasive particles removes material from the workpiece surface. The key elements involved include:

• Grinding wheel
• Workpiece
• Relative motion between the tool and workpiece
• Coolant to reduce heat

The grinding wheel rotates at high speed; abrasive particles present on it act as tiny cutting tools, each removing a small amount of material from the workpiece.

Common Types of Grinding Operations

The primary types of grinding include surface grinding, cylindrical grinding, centerless grinding, and internal grinding.

A grinding wheel is a multipoint cutting tool used in the grinding operation to remove material from the workpiece. It consists of abrasive grains held together with a bonding material, where each abrasive grain acts as a cutting edge that removes material from the workpiece.

Understanding Grinding Wheel Wear

Grinding wheel wear is a critical factor that directly impacts part quality, process efficiency, and safety:

• Dimensional Accuracy & Surface Quality: As wear occurs, the effective diameter reduces and cutting geometry changes, causing inconsistent results.
• Process Stability: Wheel wear affects grinding forces and vibration; uneven or excessive wear causes chatter or waviness on the workpiece and loss of geometric tolerances.
• Dressing & Conditioning Cycle: Wear determines when to dress the wheel. Dressing too early wastes material; dressing too late causes poor quality or wheel loading.
• Thermal Effects: A worn wheel is less efficient, converting more energy into heat rather than removing material, risking metallurgical damage to the workpiece.
• Safety: Uneven wear causes wheel imbalance or structural weakness, posing a serious risk of wheel fracture.

Three Mechanisms of Grinding Wheel Wear

1. Grain Fracture: Occurs when a portion of the grain breaks off, but the rest of the grain remains bonded to the wheel. The edges of the fractured area become the new cutting edge. The tendency of a grain to fracture is called friability—high friability means grains fracture more readily due to cutting force on the grain.
2. Attritious Wear: Involves gradual dulling of the abrasive grain's cutting edge due to repeated rubbing contact with the workpiece. It occurs through abrasion, adhesion, diffusion, and chemical reaction at the grain-workpiece interface, forming a small flat region called a wear flat on the grain tip. As the wear flat grows, the grain loses its cutting ability and begins to rub or plow instead of cut, leading to increased grinding forces, heat generation, and surface damage.
3. Bond Fracture: Occurs when the bond material holding the abrasive grains breaks down, causing entire grains to be pulled out before they are fully worn. This is actually desirable to a controlled extent, as it exposes new sharp grains, giving the wheel a self-sharpening characteristic. Excessive bond fracture leads to rapid wheel wear, reduced wheel life, and poor economics. It depends on bond type, grain size, and grinding condition.

Analysis of Wear Regions

The relationship between the volume of wheel wear and the volume of material removed is typically divided into three regions:

• Region 1 (Initial): Grains are initially sharp; wear is accelerated due to grain fracture, which corresponds to the break-in period.
• Region 2 (Steady State): The wear rate is constant, giving a linear relationship between wheel wear and material removed. This is characterized by attritious wear with some grain and bond fracture.
• Region 3 (Final): Grains become dull; plowing and rubbing increase relative to cutting. Grinding efficiency decreases, and wheel wear volume increases faster compared to material removed.

Friability and Abrasive Applications

Define the friability of a grinding wheel, explain the factors affecting friability, and identify the applications of conventional and super abrasives.

Friability is the property of an abrasive grain to break into small sharp fragments during grinding. As the grain breaks, new sharp cutting edges are formed, which keeps the grinding wheel sharp. This is known as self-sharpening.

Factors Influencing Grain Friability

• Crystal Structure: Crystal structure decides how easily the grain breaks.
• Grain Size: Coarse grains break more easily than fine grains.
• Grinding Temperature: High temperature creates thermal stress and increases fracture.
• Grinding Force: Higher grinding force causes grains to fracture faster.
• Bond Strength: A weak bond releases grains easily, while a strong bond holds them longer.

Applications of Conventional and Super Abrasives

Conventional Abrasives:

• Brown Aluminum Oxide: Grinding of mild steel and forgings.
• White Aluminum Oxide: Grinding hardened steel and High-Speed Steel (HSS) tools.
• Silicon Carbide: Grinding cast iron, glass, and ceramics.

Super Abrasives:

• Diamond: Grinding carbides, ceramics, and glass.
• Cubic Boron Nitride (CBN): Grinding hardened steel and superalloys.

Precision Finishing: Honing and Lapping

The Honing Process

Honing is a precision finishing process used to improve surface finish, dimensional accuracy, and roundness of holes or cylindrical surfaces.

Honing Working Principle

• The honing tool with abrasive stones is inserted into the bore.
• The tool rotates and reciprocates at the same time.
• Abrasive stones remove a very small amount of material.
• Coolant removes heat and chips.
• The required size and surface finish are obtained.

Applications: Engine cylinders, hydraulic cylinders, bearing bores, and gear bores.

The Lapping Process

Lapping is a precision finishing operation in which loose abrasive particles are placed between a lap and the workpiece to obtain high dimensional accuracy and a very smooth surface finish.

Lapping Working Principle

• Abrasive paste (silicon carbide, aluminum oxide, or diamond) is applied on the lap.
• The workpiece is pressed against the lap with light pressure.
• The lap and workpiece move relative to each other.
• Loose abrasive particles remove a very small amount of material.
• The process continues until the required size and surface finish are obtained.

Advantages of Lapping

• Produces excellent surface finish.
• Gives high dimensional accuracy.
• Removes small surface defects.
• Suitable for hard and brittle materials.