Abrasives introduction

Time:2019-07-22 09:18:07

Abrasives are tools used for grinding, grinding and polishing. Most of the abrasive tools are artificial abrasives made of abrasive plus binder, and natural abrasives directly processed from natural mineral rock. In addition to being widely used in machinery manufacturing and other metal processing industries, abrasive tools are also used in the processing of food processing, paper industry and non-metallic materials such as ceramics, glass, stone, plastics, rubber, and wood.

During the use of the abrasive tool, when the abrasive particles are blunt, the abrasive grains are partially or completely detached from the abrasive tool due to partial fragmentation of the abrasive particles or the bond breakage, and the abrasives on the working surface of the abrasive tool are constantly emerging. The cutting edge, or constantly revealing new sharp abrasive particles, allows the grinding tool to maintain cutting performance for a certain period of time. This is the self-sharpness of the abrasive tool. This self-sharpness of the abrasive tool is a prominent feature of the abrasive tool compared with the general tool.

Abrasives are classified according to their raw materials, including natural abrasives and artificial abrasives. Natural abrasives commonly used in the machinery industry are only oilstone. Artificial abrasives are distinguished according to their basic shapes and structural features. There are five types of grinding wheels, grinding heads, oilstones, sand tiles (collectively referred to as consolidated abrasives) and coated abrasives. In addition, it is customary to classify abrasives as a type of abrasive.

Consolidated abrasives can be divided into ordinary abrasive bonded abrasives and superhard abrasive bonded abrasives according to the abrasive used. The former uses ordinary abrasives such as corundum and silicon carbide, and the latter is made of superabrasives such as diamond and cubic boron nitride. In addition, there are some special varieties, such as sintered corundum abrasives.

Ordinary abrasive bonded abrasives are abrasive tools in which a fixed abrasive is consolidated into a certain shape by a bonding agent and has a certain strength. Generally composed of abrasives, binders and pores, these three parts are often referred to as the three elements of bonded abrasives.

The abrasive plays a cutting role in the abrasive tool. Bonding agents are materials that consolidate loose abrasives into abrasive tools, both inorganic and organic. Inorganic binders include ceramics, rhombohedral and sodium silicate; organic resins, rubber and shellac. The most commonly used are ceramic, resin and rubber binders.

The air hole acts as a chip and chip on the wear debris during grinding, and can accommodate the coolant to help dissipate the heat. In order to meet certain special processing requirements, some fillers such as sulfur and paraffin may be impregnated in the pores to improve the performance of the abrasive tool. This filler, also known as the fourth element of the abrasive tool.

Items that represent the characteristics of conventional abrasive bonded abrasives are: shape, size abrasive, particle size, hardness, texture, and bonding agent. Abrasive hardness refers to the difficulty of the abrasive particles falling off the surface of the abrasive under the action of external force. It reflects the strength of the abrasive particles held by the binder.

The hardness of the abrasive tool mainly depends on the amount of the binder added and the density of the abrasive. The abrasive particles are easy to fall off, indicating that the hardness of the abrasive is low; otherwise, the hardness is high. The hardness grades are generally divided into seven levels: super soft, soft, medium soft, medium, medium hard, hard and super hard. From these levels, several small levels can be subdivided. The method for determining the hardness of the grinding tool is more commonly used by a hand cone method, a mechanical cone method, a Rockwell hardness tester method, and a sand blast hardness tester.

The hardness of the abrasive tool has a corresponding relationship with its dynamic elastic modulus, which is advantageous for determining the dynamic modulus of the abrasive tool by the audio method to indicate the hardness of the abrasive tool. In the grinding process, if the material of the workpiece to be ground is high in hardness, a grinding tool having a low hardness is generally used; and vice versa, a grinding tool having a high hardness is used.

Abrasives are roughly classified into three types: compact, medium, and loose. Each class can be subdivided, etc., and is distinguished by the organization number. The larger the abrasive tissue number, the smaller the volume fraction of the abrasive in the abrasive tool, and the wider the gap between the abrasive grains, indicating that the tissue is looser. Conversely, a smaller organization number indicates a tighter organization. The abrasive material of the looser structure is not easy to passivate when used, and the heat is less during the grinding process, which can reduce the heat deformation and burn of the workpiece. The abrasive particles of the tighter structure are not easy to fall off, which is beneficial to maintain the geometry of the abrasive tool. The organization of the abrasive tool is controlled only by the abrasive formula at the time of manufacture, and is generally not measured.

The super-hard abrasive bonded abrasive is mainly an abrasive tool consolidated with a bonding agent such as diamond or cubic boron nitride. Due to the high price of diamond and cubic boron nitride and good wear resistance, the fixed abrasives manufactured by them are different from ordinary abrasive bonded abrasives. In addition to the superabrasive layer, there are transition layers and substrates.

The superabrasive layer is part of the cutting action and consists of a superabrasive and a binder. The matrix is supported by grinding and consists of materials such as metal, bakelite or ceramic. The transition layer is used to join the matrix and the superabrasive layer, consisting of a bonding agent, and sometimes it can be omitted. Commonly used binders are resins, metals, plated metals and ceramics.

The manufacturing process of the bonded abrasives includes: distribution, mixing, molding, heat treatment, processing, and inspection. The manufacturing process is different depending on the binder. The ceramic bond abrasive tool mainly adopts the pressing method, and the abrasive and the bonding agent are weighed according to the weight ratio of the formula, and then uniformly mixed in the mixing machine, and put into the metal mold, and the abrasive blank is formed on the press. The blank is dried and then calcined in a kiln, and the firing temperature is generally about 1300 °C. When a low melting point fusion bonding agent is used, the firing temperature is lower than 1000 °C. Then accurately process according to the specified size and shape, and finally check the product.

The resin bond abrasive is generally formed on a press at room temperature, and is also subjected to a hot press process which is heated while being heated under heating. After molding, it hardens in the hardening furnace. When the phenol resin is used as a binder, the curing temperature is 180 to 200 °C.

The rubber bond abrasives are mainly mixed with a roller machine, rolled into a thin sheet, and then punched and formed by a punching knife; and some are loosened and put into a metal mold to form on a press. After molding, it is vulcanized in a vulcanization tank at a temperature of 165 to 180 °C.

The metal bond abrasives are manufactured by powder metallurgy and electroplating, and are mainly used for superabrasive bonded abrasives. The powder metallurgy method uses bronze or the like as a binder, and after the mixture is mixed, it is formed by hot pressing or under room temperature conditions, and then sintered. The electroplating method uses nickel or nickel-cobalt alloy as the electroplating metal, and the abrasive is consolidated on the substrate by an electroplating process to prepare an abrasive article.

Special varieties of grinding have sintered corundum abrasives and fiber abrasives. The sintered corundum abrasive is made by mixing and molding aluminum oxide powder and an appropriate amount of chromium oxide, and sintering at about 1800 °C. This type of abrasive tool is compact and has high strength, mainly used for processing.