PRINCIPLE OF METAL CUTTING:

The cutting of sheet metal in press work is a shearing process.The punch and die have same shape of the part.The sheet metal is held between punch and die.The punch moves down and presses the metal into the opening of the die.

There is a gap between the punch and die opening.This is called as “Clearance”. The amount of clearance depends upon the type and thickness of the material. The punch touches the metal and travels downward. The material is subjected to both tensile and compressive stresses. By this pressure, the metal is deformed plastically.

The plastic deformation takes place in small area between punch and die cutting edges. So the metal in this area is highly stressed. When the stress exceeds the ultimate strength of the material,fracture takes place. The cutting edge of the punch starts the fracture,in the metal from the bottom.The cutting edge of the die starts the fracture from the top. These fractures meet at center of the plate. As the punch continuous tomove down, the metal under the die is completely cutoff from the sheet metal. The cut out portion of the metal drops down through the die opening.To make the metal to drop down freely,a die relief is given in the die block. If the clearance is too large or too small cracks do not meet and a ragged edge results due to the material being dragged and torn through the die.