Hex nuts and bolts, screws with the use of mechanical fastening the connecting role. Type a hex nut which was the most widely used, C-level nut for rough surface on the machinery, equipment or structures less precision; A grade and B grade nut for surface is smooth, requiring high precision machinery, equipment or structure. Type 2 hex nut thickness M thick, often used for assembly and disassembly of the occasion is often necessary. Hexagon thin nut M thin thickness, used to connect parts of the surface space is limited occasions.