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Recommended Reading:

 Manufacturing Engineering and Technology

  Machinery's Handbook

  Marks' Standard Handbook for Mechanical Engineers

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4.1 SHEARING PROCESS

Operations that cut sheet metal using press tools use the shearing process

Fig 4.1.1.

Fig 4.1.1. shows the four stages of the shearing process: -

1. The blades begin to apply pressure, the workpiece starts to undergo elastic deformation.
2. As pressure increases the workpiece undergoes plastic deformation.
3. The blades begin to penetrate either side. Work hardening occurs in the centre.
4. Fracture lines originating from the point of each blade meet in 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 this happens the fracture lines may not meet cleanly and the material will be dragged down the gap between the blades.

Both effects cause excessive burrs and sharpness on the finished part.

4.2. PIERCING

Piercing is a method of producing a hole in a component.

Fig 4.2.1.

The part to be pierced rests on a die and the punch shears the slug from the part. The clearance angle in the die allows the scrap slug to fall clear.

4.3. BLANKING.

Blanking is the same process as piercing, i.e. both use a punch and die, but in this case it is the slug that is required.

Blanked parts usually come from strip material fed through the press tool between guides. To blank large areas very high forces are required, requiring the use of a powerful press machine.

To improve productivity it is quite normal to combine a piercing and blanking operation in one press tool, Fig 4.3.1. shows a typical follow on pierce and blank press tool to produce a plain washer.

Fig 4.3.1.

After each action of the press the strip is fed forward to stop against the previously blanked hole. The pilot on the blanking punch locates in the hole already pierced, prior to blanking the part from the strip.

4.4 CROPPING

Cropping is the term applied when shearing across the full width of a strip. The cropping tool may be of any shape and produce a part of any length – Fig 4.4.1

Fig 4.4.1.

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