Grinding: - Grinding is a metal removal process with the help of various powder tools and machines. It is a slow metal removal process. The abrasive grain strike on metal and cut a small chips from the work piece via deformation.
Grinding wheels:- Grinding wheel consists of hard abrasive grains called grits, which perform the cutting or material removal, held in the weak bonding matrix. A grinding wheel commonly identified by the type of the abrasive material used. The conventional wheels include aluminum oxide and silicon carbide wheels while diamond and CBN (cubic boron nitride) wheels fall in the category of super abrasive wheel.
Selection of grinding wheels : - Selection of grinding wheel means selection of composition of the grinding wheel and this depends upon the following factors:
1) Physical and chemical characteristics of the work material
2) Grinding conditions
3) Type of grinding (stock removal grinding or form finish grinding)
Type of abrasives :-
1) Aluminums oxide :- Aluminums oxide may have variation in properties arising out of differences in chemical composition and structure associated with the manufacturing process. Pure Al2O3 grit with defect structure like voids leads to unusually sharp free cutting action with low strength and is advantageous in fine tool grinding operation, and heat sensitive operations on hard, ferrous materials. Monocrystalline Al2O3 grits make a balance between hardness and toughness and are efficient in medium pressure heat sensitive operation on ferrous materials
2) Silicon carbide :- Silicon carbide is harder than alumina but less tough. Silicon carbide is also inferior to Al2O3 because of its chemical reactivity with iron and steel. Black carbide containing at least 95% SiC is less hard but tougher than green SiC and is efficient for grinding soft nonferrous materials. Green silicon carbide contains at least 97% SiC. It is harder than black variety and is used for grinding cemented carbide.
3) Diamond : - Diamond grit is best suited for grinding cemented carbides, glass, sapphire, stone, granite, marble, concrete, oxide, non-oxide ceramic, fiber reinforced plastics, ferrite, graphite. Natural diamond grit is characterized by its random shape, very sharp cutting edge and free cutting action and is exclusively used in metallic, electroplated and brazed bond. Monocrystalline diamond grits are known for their strength and designed for particularly demanding application. These are also used in metallic, galvanic and brazed bond.
Grit size : - The grain size affects material removal rate and the surface quality of work piece in grinding.
Large grit- big grinding capacity, rough work piece surface.
Fine grit- small grinding capacity, smooth work piece surface.
Truing and dressing of grinding wheel :- Truing is the act of regenerating the required geometry on the grinding wheel, whether the geometry is a special form or flat profile. Therefore, truing produces the macro-geometry of the grinding wheel. Truing is also required on a new conventional wheel to ensure concentricity with specific mounting system. In practice the effective macro-geometry of a grinding wheel is of vital importance and accuracy of the finished work piece is directly related to effective wheel geometry.
Dressing :- Dressing is the conditioning of the wheel surface which ensures that grit cutting edges are exposed from the bond and thus able to penetrate into the work piece material. Also, in dressing attempts are made to splinter the abrasive grains to make them sharp and free cutting and also to remove any residue left by material being ground. Dressing therefore produces micro-geometry. The structure of micro geometry of grinding wheel determine its cutting ability with a wheel of given composition. Dressing can substantially influence the condition of the grinding tool. Truing and dressing are commonly combined into one operation for conventional abrasive grinding wheels, but are usually two distinctly separate operation for super abrasive wheel.