During the use of abrasive tools, when the abrasive particles become dull, some or all of the abrasive particles fall off from the abrasive tools due to the fragmentation of the abrasive particles themselves or the fracture of the bonding agent, and the abrasive on the working surface of the abrasive tools constantly appears new cutting edges or exposes new sharp abrasive particles, so that the abrasive tools can maintain cutting performance for a certain period of time. This self-sharpening of abrasive tools is a prominent feature of abrasive tools compared with general tools.
As early as the Neolithic Age, humans began to use natural grindstones to process tools such as knives, axes, bones, horns and teeth. 1872, ceramic grinding wheels fired with natural abrasive and clay appeared in the United States. 1900 or so, artificial abrasives came out, and various abrasives made of artificial abrasives came out one after another, which created conditions for the rapid development of grinding and grinding machines. Since then, the proportion of natural abrasives in abrasives has gradually decreased.
Abrasives are divided into natural abrasives and artificial abrasives according to the source of raw materials. The only natural abrasive tool commonly used in machinery industry is oilstone. According to the basic shape and structural characteristics, artificial abrasives can be divided into five categories: grinding wheel, grinding head, oilstone, sand tile (collectively referred to as consolidated abrasive) and coated abrasive. In addition, it is customary to classify abrasives as a kind of abrasive tools.
According to the different abrasives used, consolidated abrasives can be divided into ordinary abrasives and superhard abrasives. The former is made of ordinary abrasives such as corundum and silicon carbide, while the latter is made of superhard abrasives such as diamond and cubic boron nitride. In addition, there are some special varieties, such as sintered corundum abrasive.
Ordinary abrasive bonded abrasive is a kind of abrasive with certain strength, which is solidified into a certain shape by binder. Generally, it consists of abrasives, binders and pores, which are often called the three elements of consolidated abrasives.
Abrasive plays a cutting role in abrasive tools. Binder is a material that solidifies loose abrasives into abrasives. There are two kinds of abrasives: inorganic and organic. Inorganic binders include ceramics, magnesite and sodium silicate. Organic materials include resin, rubber and shellac. Among them, the most commonly used are ceramics, resins and rubber adhesives.
In the process of grinding, air holes play the role of containing and discharging debris, and can contain cooling liquid, which is helpful to the dissipation of grinding heat. In order to meet some special processing requirements, some fillers, such as sulfur and paraffin, can also be impregnated in the pores to improve the performance of abrasive tools. This filler is also called the fourth element of abrasive tools.
The items representing the characteristics of ordinary abrasive bonded abrasives are: shape, abrasive size, particle size, hardness, texture, backing, adhesive and binder. Abrasive hardness refers to the difficulty of abrasive particles falling off the surface of abrasive tool under the action of external force, which reflects the strength of bonding agent to hold abrasive particles.
The hardness of abrasive tools mainly depends on the amount of binder and the density of abrasive tools, and abrasive particles are easy to fall off, indicating that the hardness of abrasive tools is low; On the contrary, it means high hardness. Hardness is generally divided into seven grades: super soft, soft, medium soft, medium hard, hard and super hard, and several sub-grades can be subdivided from these grades. The commonly used methods for measuring the hardness of abrasive tools are hand cone method, mechanical cone method, Rockwell hardness tester and sandblasting hardness tester.
There is a corresponding relationship between the hardness of abrasive tools and their dynamic elastic modulus. Measuring the dynamic elastic modulus of abrasive tools by audio frequency method is helpful to express the hardness of abrasive tools. When grinding, if the material hardness of the workpiece to be ground is high, abrasive tools with lower hardness are generally selected; On the contrary, choose abrasive tools with high hardness.
Abrasive structures can be roughly divided into three categories: dense, medium and loose. Each category can be subdivided into numbers and so on. , distinguished by the organization number. The larger the number of abrasive tissues, the smaller the volume percentage of abrasive in the abrasive and the larger the gap between abrasive particles, indicating that the tissues are looser. Conversely, the smaller the organization number, the tighter the organization. Abrasive tools with loose structure are not easy to be passivated and generate less heat when grinding, which can reduce the thermal deformation and burn of the workpiece. Abrasive particles with compact structure are not easy to fall off, which is beneficial to maintain the geometric shape of abrasive tools. The structure of abrasive tools is only controlled according to the formula of abrasive tools at the time of manufacture, and it is generally not measured.