Shearing Process

SHEARING PROCESS

Shearing is the process of cutting a portion from the sheet metal strip. Figure 4.1 shows the shearing action takes place in a sheet metal. Cutting die and punches are produced with sharp edges. The sheet metal to be sheared is well-seated on the die block over die opening.

When the punch contacts the sheet metal, the cutting pressure will start to build up. During further movement of the punch, the material starts deforming elastically. When the elastic limit of the material exceeds, the material will undergo plastic deformation and it will flow into the clearance between punch and die.

During further movement of the punch, the fracture of material begins in the line of cutting edge. Further descending of punch completely removes the slug from the parent sheet metal. At the end of cutting operation, a small portion of deformed material is projected out as tensile burrs at the top of the slug and bottom of the parent sheet metal. Burs can be controlled by regulating the die clearance.

Phases in Shearing

The process of cutting a straight line across a strip, sheet or bar is known as shearing process. This action has three important basic stages.

1) Plastic deformation

2) Shear and

3) Fracture.

Figure 4.2 shows the three stages of the shearing process.

(i) Plastic deformation:

When blades begin to apply pressure, the workpiece will start to undergo elastic deformation. As the pressure increases, the workpiece will undergo a plastic deformation.

(ii) Shear:

The blades begin to penetrate either side which produces the shear on workpiece along a vertical line. Work hardening occurs at the centre.

(C) Fracture

Fracture lines originating from the point of each blade meet at the centre of the workpiece causing separation before the blades have fully penetrated. To reduce the presence of burrs, it is important that the blades are sharp. Also, it is important that they do not deflect under pressure and that they are not set too far apart (offset). If it happens, the fracture lines may not meet cleanly and the material will be dragged down the in gap between the blades.