砂轮结构(GRINDING WHEEL CONSTRUCTION)

(A)GRINDING WHEEL CONSTRUCTION
A grinding wheel is composed of millions of extremely hard particles of abrasive grit or grain, bonded together with a suitable material. The orientation of the individual grains is completely random, which explains why they may be exposed at the surface of the wheel with virtually any effective rake angle. The abrasive material can be either manufactured or natural but the latter has very limited use and the vast majority of wheels in use are made with manufactured abrasives. The natural abrasives include sandstone such as this . Corundum and emery are oxides of aluminium (AL2O3), which have to be crushed into granules, but sandstone exists as granules of mainly quartz and silica(SiO2)
The discontinuance of these natural abrasives is not due to any lack of abrasive ability, but to the presence of impurities and non-uniformity of cutting action. In fact the two most common abrasives in general use are manufactured from the two naturally occurring compounds . Manufactured abrasives can be given the most desirable crystalline structure and grain shape,which make them superior for modern grinding applications.
Aluminium-oxide abrasive is produced in electric-arc furnaces by fusing bauxite with finely crushed coke and iron particles. Bauxite is a fine clay composed of almost pure aluminium oxide.
when the product has cooled and solidified,it forms a crystalline abrasive which can be crushed to the size range required for grinding wheel constuction. These are then graded into the respective grain sizes used. Aluminium-oxide abrasives are manufactured and marketed under the names "Aloxite","Alundum"and"Bauxilite".Silicon-carbide abrasive is produced in electrical-resistance furnaces from the chemical reaction of silica and finely crushed coke in the presence of sawdust and sodium chloride. The coke supplies the carbon needed to combine with the silica to from the compound,the two other constituents help to bring about the reaction. The heated mixture gives off a vapour which crystallise around the conductor in the furance. After the process is finished the crystals are easily removed and can then be crushed and graded into the respective grain sizes. Silicon carbide is manufactured and marketed under the trade names "Crystolon" and "Unirundum"
The grading operation entails passing the grains through a series of mesh screens,each subsequent sreen having a finer mesh than the previous one. The grains which do not pass through the mesh are of a particular grain size. The very small grains which pass through the final mesh are then graded by a floation technique in hydraulic tanks.The grains are then ready for moulding into a grinding wheel using a bonding material or binder.
The bonding material must have three characteristics for it to perform
efficiently:
It must be capable of holding the grains in the moulded wheel shape.
It must be able to support the grains during the grinding operation.
It must release the grains when they have become dulled,before the pressure becomes too great.
The bonding material is is classed as soft,medium or hard,which is a measure of how easily it will release the grains. Soft binders will release the grain more readily than hard ones, which retain them for a longer period of operation.
About 80 per cent of all grinding wheels are bonded with vitrified glass as this can be controlled to give a wide range of strengths. The grains are moulded under pressure to the required shape after being thoroughly mixed with the correct amount of semi-dry ceramic clay. The wheels are then dried prior to being fired over a long period, during which time the clay fuses to form the vitrified glass bond. Silicate-bonded wheels are made in the same way.。