Shearing and Blanking
Shearing is the separation of metal by two blades moving. A narrow strip of the metal is severely plastically deformed to the point where it fractures at the surfaces in contact with the blades. The fracture then propagates inward to provide complete separation.
The clearance between the blades is an important variable in shearing operations. With the proper clearance the cracks that initiate at the edge of the blades will propagate through the metal and meet near the center of the thickness to provide a clean fracture surface.
Insufficient clearance will produce a ragged fracture and also will require more energy to shear the metal than when there is proper clearance. With the excessive clearance there is a greater distortion of the edge and more energy is required because more metal must plastically deform before it fractures.
Too large clearance, burr or sharp projections are likely to form on the sheared edge. A dull cutting edge also increases the tendency for the formation of burrs.
The height of burr increases with increasing clearance and increasing ductility of the metal.
Clearances generally range between 2-10% of the thickness of the sheet; the thicker the sheet the larger the clearance.
Neglecting friction, the force require to shear a metal sheet is the product of the length cut, the sheet thickness, and the shearing strength of the metal.
Blanking: the same concept of shearing
When the metal inside the contour is desired part: call blanking.