Basic Conditions of Abrasives
| Hardness (Hardness of Abrasive materials) | ・Harder than workpiece. ・Fracture (abrasive break apart and creat new cutting edge) against external force during grinding. |
| Toughness, shatter resistance . | ・Resistant to deformation and fracture which received from external force during operation. ・Microfracture |
| Fire resistance | ・Abrasives have a moderate fire resistance which prevents blunting the edge due to grinding heat. |
| Inertness | ・Chemical inertness (no ability to react with workpiece). |
Hardness and Toughness are conflicting properties. When hardness is high and *self-sharpening is performed well, whereas low toughness grains increase wasteful consumption and it will effect finishing.
* Abrasives are fallen off when these are worn down and lost sharpness, then these expose new cutting edges constantly.
Type of Abrasive materials
There are many kinds of abrasive materials.
The major types of grain and its application are as follows.
| Name | Code | Color tone | Applications |
| Brown Alumina | A | Brown | Hand-free grinding of carbon steels, Precision grinding of carbon steels. |
| Mono-crystalline fused alumina | HA | Ash gray | Precision grinding of alloy steels, tool steels and hardened steels |
| Pink Alumina | PA | Pink | Precision grinding of alloy steel, tool steel and hardened steel |
| White Alumina | WA | White | Precision grinding of alloy steels, tool steels and hardened steels |
| Zirconia alumina | AZ | Gray | Deseaming, deburring and cutting of steels |
| Black silicon carbide | C | Black | Grinding, precision grinding of nonferrous metals and nonmetal materials |
| Green silicon carbide | GC | Green | Grinding of superalloy |
About Grit
Size of abrasive particles called “grit”, and it’s selected based on finishing accuracy of grinding surface.
Grit is specified in JIS R6001, it’s denoted by “F + number”.
(Example: when grain size is 36, it will be denoted as “F36”) For micropowder of abrasive material, it is defined with European standard FEPA and Japanese Standard JIS.
The lower numbers mean large spacing of abrasives.
Abrasives play the most important roles in the raw materials as described above. It’s important to know that material hardness is different from abrasive tools hardness grade. The hardness grade of abrasive tools is related to the grinding feeling which is caused by different self-sharpening ability of grain.
Extra Information – History of Abrasive materials-
Humans have polished and ground wood, born and stone with various materials from the very beginning of history.
Polishing and grinding are very familiar to the way we working today.
Various abrasives of sand stone (silica stones), emery and garnet, alumina came into use in the Stone Age, Copper Age and Iron Age respectively but all abrasives are natural stones.
In 1881, American chemist Edward Goodrich Atcheson worked on development of methods to produce artificial diamond. He heated a mixture of clay and coke in arc furnace and found some hard, big crystals. He presumed that it was a compound of carbon and corundum and called it “carborundum”, the result of chemical analysis showed that it was silicon carbide (SiC).
This was the first synthetic abrasive. The technology were implemented by the Carborundum Company and later spread around the world.
In 1897, C. B. Jacobs developed a method to produce artificial corundum by heating a mixture of bauxite (aluminum) and coke with arc furnace. This substance was named “alundum” and was commercialized by the Norton Company. Later alundum was produced worldwide.
In Japan, silicon carbide and artificial corundum was respectively first produced in 1917 and 1918 by Kagoshima tramway Co., Ltd. Although the production was suspended temporarily, many company resumed the manufacture after 1930.
Since silicon carbide grain and aluminum oxide grain were established, with the development of the industry, many countries have improved and produced a variety of artificial abrasives.
There are many kinds of alumina abrasives (WA, PA,SA,MA and PW etc.), AZ is a crystalline structured abrasive that is produced by fusing alumina and zirconia, is a groundbreaking abrasive which has a higher toughness combined with desirable microfracture property.
Artificial abrasives have made a great progress due to practical application of cubic boron nitride (CBN) and artificial diamond, commencement of night operation in the precision machining industry.
Nowadays, some companies have developed new abrasive material called ceramic abrasive which has microcrystal structure. It manufactured by chemical syntheses and not by fusion method, and has already been put to practical use in some industries.
The zirconia and ceramic sand are both high level abrasive material for abrasive tools. With good self-sharpening ability, the grain will get new sharp edges after consume former grains. So they are both more popular in many applications.
