Introduction to Grinding Tools: Types and Characteristics

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Abrasive tools are tools used for grinding, polishing, and finishing. Most abrasive tools are artificial, made by combining abrasives with binders. However, natural minerals can also be directly processed into natural abrasive tools. These tools find extensive use not only in mechanical manufacturing and metalworking industries but also in food processing, paper making, ceramics, glass, stone, plastic, rubber, wood, and other non-metal materials processing. One outstanding feature that sets abrasive tools apart from general cutting tools is their self-sharpening ability. As the abrasive grains become dull during usage, some of them fracture or the bond between the abrasive grains and the binder breaks, causing the worn-out grains to dislodge partially or entirely from the tool's surface. New cutting edges continually emerge, or fresh sharp abrasive grains are exposed, ensuring the abrasive tool maintains its cutting performance for a certain period.

As early as the Neolithic period, humans began using natural grinding stones to process tools like stone knives, stone axes, bone tools, horn tools, and tooth tools. In 1872, ceramic grinding wheels made by combining natural abrasives with clay were introduced in the United States. Around 1900, artificial abrasives emerged, and various abrasive tools made from artificial abrasives were produced successively, which created conditions for the rapid development of grinding and grinding machines. Since then, the proportion of natural abrasives in abrasive tools has gradually decreased.

There are two main types of abrasive tools: natural abrasive tools and artificial abrasive tools. The commonly used natural abrasive tool in the mechanical industry is oilstone. Artificial abrasive tools can be classified into five categories based on their basic shape and structural characteristics: grinding wheels, grinding heads, oilstones, grinding bricks (collectively known as bonded abrasive tools), and coated abrasives. Additionally, grinding agents are also considered a type of abrasive tool. Bonded abrasive tools can be further divided into conventional abrasive bonded tools and super-hard abrasive bonded tools. The former uses ordinary abrasives like corundum and silicon carbide, while the latter uses super-hard abrasives like diamond and cubic boron nitride. Special varieties like sintered corundum tools also exist.

Conventional abrasive bonded tools are made by combining abrasives and binders to form tools with specific shapes and a certain strength. The three essential elements of bonded abrasive tools are abrasives, binders, and pores. The abrasives perform the cutting action during grinding. Binders solidify the loose abrasives into the tool material and can be either inorganic or organic. Common inorganic binders include ceramics, and sodium silicate, while organic binders include resins, rubber, and shellac. The most commonly used binders are ceramics, resins, and rubber. Pores in abrasive tools play a role in chip storage, chip removal, and cooling during grinding, which helps dissipate grinding heat. Sometimes, pores are impregnated with certain fillers like sulfur and paraffin to improve the tool's performance. These filters are also referred to as the fourth element of abrasive tools. The characteristics of conventional abrasive bonded tools include shape, size of abrasives, grit size, hardness, structure, and binder type. The hardness of the abrasive tool refers to the ease with which the abrasive grains detach from the tool surface under external forces and reflects the strength of the binder holding the abrasive grains. The hardness of the tool depends mainly on the amount of added binder and the tool's density. A lower hardness indicates easier detachment of the abrasive grains, while a higher hardness suggests better adhesion. Hardness is divided into seven major levels: extra soft, soft, medium soft, medium, medium-hard, hard, and extra hard, with several sub-levels within each major level. Common methods to determine the hardness of abrasive tools include the hand cone method, mechanical cone method, Rockwell hardness tester, and sandblasting hardness tester.

The hardness of the abrasive tool corresponds to its dynamic elastic modulus, which allows the use of acoustic methods to determine the tool's dynamic elastic modulus and represent its hardness. In grinding processes, if the workpiece material is hard, abrasive tools with lower hardness are typically chosen, while for softer workpieces, abrasive tools with higher hardness are preferred. The structure of the abrasive tool can be roughly divided into three categories: dense, medium, and loose. Each category can be further subdivided into several levels based on their structure number. A higher structure number indicates a larger volume percentage of abrasives in the tool and wider gaps between abrasive grains, suggesting a looser structure. Conversely, a lower structure number indicates a tighter structure with less space between abrasive grains. Tools with looser structures are less likely to become dull during usage, generate less heat during grinding, and reduce workpiece thermal deformation and burn. On the other hand, tools with tighter structures have better abrasive grain retention, which helps maintain the tool's geometric shape during grinding. The structure of the abrasive tool is usually controlled during the manufacturing process based on the tool's formulation and is not typically measured directly.

Super-hard abrasive bonded tools are primarily made by combining super-hard abrasives like diamond and cubic boron nitride with binders. Due to the high cost and excellent wear resistance of diamond and cubic boron nitride, the manufacturing process for super-hard abrasive tools differs from that of conventional abrasive bonded tools. Besides the super-hard abrasive layer, there are transition layers and substrates in super-hard abrasive bonded tools. The super-hard abrasive layer is the part responsible for cutting and is composed of super-hard abrasive grains and binders. The substrate provides support during grinding and can be made from materials like metal, electroplated wood, or ceramics. The transition layer connects the substrate and the super-hard abrasive layer, formed mainly from binders, and is sometimes omitted. Commonly used binders include resins, metals, electroplated metals, and ceramics. The manufacturing process of bonded abrasive tools includes several steps, such as material allocation, mixing, molding, heat treatment, processing, and inspection. The specific process depends on the type of binder used.

Ceramic-bonded tools mainly use a pressing method. The abrasive and binder are mixed uniformly in a mixer according to the weight ratio specified in the formulation. The mixture is then placed in a metal mold and shaped using a hydraulic press to create the tool blank. The blank is then dried and fired in a kiln at temperatures around 1300°C to complete the sintering process. Ceramic-bonded tools are known for their dense structure and high strength, making them suitable for precision processing of components like timepieces and instruments. Resin-bonded tools are generally shaped at room temperature using a hydraulic press. Alternatively, a heat pressing process is used by heating during pressing. After shaping, the tools are cured in a hardening furnace. When the phenolic resin is used as the binder, the curing temperature is around 180-200°C. Rubber-bonded tools are made using a calendering process, where the abrasive-impregnated fibers, such as nylon fibers, are rolled into thin sheets and then cut into shape using a die. Alternatively, loose material is placed in a metal mold and shaped with a hydraulic press. The tools are then vulcanized in a vulcanization tank at temperatures of 165-180°C. Metal-bonded tools are manufactured using two methods: powder metallurgy and electroplating, mainly for super-hard abrasive bonded tools. In the powder metallurgy method, materials like bronze are used as the binder. After mixing, the mixture is pressed under heat, using either hot pressing or cold pressing at room temperature, and then subjected to sintering and processing. In the electroplating method, materials like nickel or cobalt-based alloys are electroplated to solidify the abrasive grains onto the substrate, creating the tool.

In addition to the mentioned types, there are also special varieties of abrasive tools, such as sintered corundum tools and fiber-based tools. Sintered corundum tools are made by mixing aluminum oxide powder with a suitable amount of chromium oxide, shaping the mixture, and sintering it at around 1800°C. These tools have a dense structure and high strength, making them ideal for processing precision components like timepieces and instruments. Fiber-based tools, on the other hand, are made using fibers containing or adhering to abrasives, such as nylon fibers. These tools have excellent elasticity and are mainly used for polishing metal materials and their products.

In summary, abrasive tools are essential in various industries for grinding, polishing, and finishing tasks. They come in two main categories: natural and artificial abrasive tools, with the latter being more commonly used due to their versatility and better performance. Within the artificial abrasive tools, there are conventional and super-hard abrasive bonded tools, each with distinct characteristics and applications. The selection of the right abrasive tool depends on the hardness of the workpiece material and the specific requirements of the grinding process.