In grinding wheel following abrasives are used
The natural abrasives are emery and corrundum,These are impure forms of aluminium oxide.
Artificial abrasives are silicon carbide and aluminium oxide.
Spot facing is a machining operation for producing a flat seat for bolt head, washer or nut at the opening of a drilled hole. The tool is called a spot facer or a spot facing tool. Spot facing is similar to counter-boring, except that it is shallower. Tools that are used for counter-boring can be used for spot facing as well.