Overview of grinding tools

Abrasives are tools used for grinding, grinding and polishing. Most of the abrasive tools are artificial abrasive tools made of abrasives and binders, and there are also natural abrasive tools directly processed from natural mineral rocks. In addition to being widely used in machinery manufacturing and other metal processing industries, abrasive tools are also used in grain processing, papermaking industry, and processing of non-metallic materials such as ceramics, glass, stone, plastics, rubber, and wood.
Overview
The grinding tool structure is roughly divided into three categories: tight, medium and loose. Each category can be further subdivided into numbers, etc., and are distinguished by organization numbers. The larger the structure number of the abrasive tool, the smaller the volume percentage of the abrasive in the abrasive tool, and the wider the gap between the abrasive grains, which means the looser the structure. On the contrary, the smaller the organization number, the tighter the organization. Grinding tools with a looser structure are not easy to passivate during use, generate less heat during the grinding process, and can reduce thermal deformation and burns of the workpiece. The abrasive grains of the abrasive tool with a tight structure are not easy to fall off, which is beneficial to maintaining the geometric shape of the abrasive tool. The structure of the abrasive tool is only controlled according to the abrasive tool formula during manufacturing and is generally not measured. Superabrasive bonded abrasives are mainly abrasives made of diamond, cubic boron nitride, etc. bonded with binders. Due to the high price and good wear resistance of diamond and cubic boron nitride, the bonded abrasive tools made with them are different from ordinary abrasive bonded abrasive tools. In addition to the superhard abrasive layer, there is also a transition layer and a matrix. The superabrasive layer is the part that plays a cutting role and is composed of superabrasive and bonding agents. The substrate plays a supporting role in grinding and is composed of metal, bakelite or ceramics. The transition layer is used to connect the matrix and the superabrasive layer. It is composed of a bonding agent and can sometimes be omitted. Commonly used binders include resin, metal, electroplated metal and ceramics. The manufacturing processes of bonded abrasive tools include: distribution, mixing, molding, heat treatment, processing and inspection, etc. Depending on the binder, the manufacturing process is also different. Ceramic bonded abrasive tools mainly use the pressing method. After weighing the abrasive and binder according to the weight ratio of the formula, they are placed in a mixer and mixed evenly, then put into a metal mold, and the abrasive tool blank is formed on the press. The blank is dried and then put into the kiln for firing. The firing temperature is generally around 1300°C. When using low melting point sinter binder, the firing temperature is lower than 1000℃. Then it is processed accurately according to the specified size and shape, and finally the product is inspected. Resin bonded abrasives are generally formed on a press at room temperature, and some use a hot pressing process that uses heating and pressure under heating conditions. After forming, it is hardened in a hardening furnace. When using phenolic resin as the binder, the hardening temperature is 180~200℃. Rubber bond abrasives are mainly mixed using a pair of roller machines, rolled into thin sheets, and then punched and formed with a punch knife; some also use loose materials, which are put into a metal mold and formed on a press. After molding, it is vulcanized in a vulcanization tank at a temperature of 165~180°C. There are two manufacturing processes for metal bonded grinding tools: powder metallurgy and electroplating, which are mainly used for superhard abrasive bonded grinding tools. The powder metallurgy method uses bronze and other materials as a binding agent. After mixing, it is hot pressed or pressed at room temperature to form, and then sintered. The electroplating method uses nickel or nickel-cobalt alloy as electroplating metal, and the abrasives are bonded to the substrate according to the electroplating process to make abrasive tools. Special types of abrasives include sintered corundum abrasives and fiber abrasives. Sintered corundum grinding tools are made by mixing and shaping aluminum oxide powder and an appropriate amount of chromium oxide, and sintering at about 1800°C. This kind of abrasive tool has a compact structure and high strength, and is mainly used for processing parts such as clocks and instruments. Fiber abrasives are made of fiber filaments (such as nylon filaments) containing or adhering to abrasives. They have good elasticity and are mainly used for polishing metal materials and their products.
Usage
During the use of the abrasive tool, when the abrasive grains are blunted, due to the partial fragmentation of the abrasive grains themselves or the fracture of the bond, the abrasive grains fall off partially or completely from the abrasive tool, and new cuttings continue to appear on the abrasive working surface of the abrasive tool. The cutting edge, or constantly exposing new sharp abrasive grains, enables the abrasive tool to maintain cutting performance within a certain period of time. This self-sharpening property of abrasive tools is an outstanding feature compared with ordinary cutting tools.
As early as the Neolithic Age, humans have begun to use natural grindstones to process tools such as stone knives, stone axes, bone tools, horn tools, and tooth tools; in the 19th century, ceramics fired by combining natural abrasives with clay appeared in the United States. Grinding wheel; around 1900, artificial abrasives came out, and various abrasive tools made of artificial abrasives were produced one after another, creating conditions for the rapid development of grinding and grinding machines. Since then, the proportion of natural abrasives in abrasives has gradually decreased.
Abrasives can be divided into two types according to their raw material sources: natural abrasives and artificial abrasives. The only natural abrasive tools commonly used in the machinery industry are oilstones. Artificial abrasives are classified according to their basic shapes and structural characteristics, including grinding wheels, grinding heads, whetstones, and sand tiles, which are collectively referred to as five categories: bonded abrasives and coated abrasives. In addition, abrasives are also customarily classified as abrasive tools.
Classification
According to the different abrasives used, bonded abrasives can be divided into ordinary abrasive bonded abrasives and superabrasive bonded abrasives. 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 grinding tools.
Ordinary abrasive bonded abrasives are abrasives that consolidate ordinary abrasives into a certain shape and have a certain strength by a bonding agent. Generally composed of abrasives, binders and pores, these three parts are often called the three elements of bonded abrasives.
Abrasives play a cutting role in grinding tools. Binder is a material that consolidates loose abrasives into abrasive tools. There are two types: inorganic and organic. Inorganic binders include ceramics, amaranth and sodium silicate, etc.; organic binders include resin, rubber and shellac, etc. The most commonly used are ceramic, resin and rubber bonding.
The pores play a role in containing and discharging wear chips during grinding, and can accommodate coolant, helping to dissipate grinding heat. In order to meet certain special processing requirements, the pores can also be impregnated with certain fillers, such as sulfur and paraffin, to improve the performance of the grinding tool. This filler is also called the fourth element of the grinding tool.
Items that represent the characteristics of ordinary abrasive bonded abrasives include: shape, size, particle size, hardness, structure and bonding agent. Abrasive tool hardness refers to the ease with which abrasive particles fall off the surface of the abrasive tool under the action of external force. It reflects the strength of the bond to hold the abrasive particles.
Abrasive grain size
Particle size refers to the size of abrasive particles, and its size is expressed by the particle size number.
The national standard stipulates two particle sizes: abrasive and micro powder.
Particle size refers to the size of the abrasive particles. Particle size is divided into two categories: abrasive grains and micro powder. Abrasives with large particle sizes larger than 40 μm are called abrasive grains. Classified by the screening method, the particle size number is expressed by the number of holes per inch on the screen through which the abrasive particles pass. For example, a 60# abrasive grain means that its size is just enough to pass through a screen with 60 holes per inch. Abrasives with particle sizes less than 40 μm are called micropowder. Use microscopic measurement method to classify, use W and the number after it to represent the particle size number, and the value after W represents the actual size of the micropowder. For example, W20 indicates that the actual size of the micropowder is 20μm. The abrasive particle sizes of various particle sizes are shown in Table 1.
Table 1 Abrasive particle size number and particle size
The function of the bonding agent is to bind the abrasive into a grinding wheel with a certain strength and shape. The strength, impact resistance, heat resistance and corrosion resistance of the grinding wheel mainly depend on the performance of the bond.
Commonly used bonding agents include ceramic bonding agent (V), resin bonding agent (B), rubber bonding agent (R) and metal bonding agent (M).
Ceramic bond: the most widely used, suitable for outer circle, inner circle, plane, centerless grinding and form grinding grinding wheels, etc.;
Resin bond: suitable for cutting and grooving wafer grinding wheels and high-speed grinding wheels;
Rubber bond: suitable for centerless grinding guide wheels and polishing wheels;
Metal bond: suitable for diamond grinding wheels, etc.
Grinding tool hardness
The hardness of the abrasive tool mainly depends on the amount of binder added and the density of the abrasive tool. If the abrasive grains fall off easily, it means the hardness of the abrasive tool is low; otherwise, it means the hardness is high. The hardness levels are generally divided into seven levels: extra soft, soft, medium soft, medium, medium hard, hard and extra hard. These levels can be further subdivided into several smaller levels. The most commonly used methods for measuring the hardness of abrasive tools include the hand cone method, the mechanical cone method, the Rockwell hardness tester method and the sandblasting hardness tester method.
The hardness of the abrasive tool has a corresponding relationship with its dynamic elastic modulus, which is conducive to using the audio method to measure the dynamic elastic modulus of the abrasive tool to express the hardness of the abrasive tool. In grinding processing, if the material of the workpiece to be ground has high hardness, abrasive tools with low hardness are generally used; otherwise, abrasive tools with high hardness are used.