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Shearing type Operations

Shearing is a cutting operation used to remove a blank of required dimensions from a large sheet. To understand the shearing mechanism, consider a metal being sheared between a punch and a die, Fig 1 Typical features of the sheet and the slug are also shown in this figure. As can be seen that cut edges are neither smooth nor perpendicular to the plane of the sheet.

Fig 1 (a) Shearing with a punch and die (b) features of a punched hole and (c) features of the slug.

Shearing starts as the punch presses against the sheet metal. At first, cracks form in the sheet on both the top and bottom edges (marked T and T', in the figure). As the punch descends further, these cracks grow and eventually meet each other and the slug separates from the sheet. A close look at the fractured surfaces will reveal that these are quite rough and shiny; rough because of the cracks formed earlier, and shiny because of the contact and rubbing of the sheared edge against the walls of the die.

The clearance between the punch and the die plays an important role in the determination of the shape and quality of the sheared edge. There is an optimum range for the clearance, which is 2 to 10% of the sheet thickness, for the best results. If the clearance increases beyond this, the material tends to be pulled into the die and the edges of the sheared zone become rougher. The ratio of the shining (burnished) area to the rough area on the sheared edge decreases with increasing clearance and sheet thickness. The quality of sheared edge is also affected by punch speed; greater the punch speed better the edge quality.

Shearing Operations

For general purpose shearing work, straight line shears are used. As shown in Fig 2, small pieces (A, B, C, D……….) may be cut from a large sheet.


Shearing may also be done between a punch and die, as shown in Fig 1. The shearing operations make which use of a die; include punching, blanking, piercing, notching, trimming, and nibbling.

Punching/Blanking: Punching or blanking is a process in which the punch removes a portion of material from the larger piece or a strip of sheet metal. If the small removed piece is discarded, the operation is called punching, whereas if the small removed piece is the useful part and the rest is scrap, the operation is called blanking, see Fig 3.

In punching, the metal inside the part is removed; in blanking, the metal around the part is removed.

A typical setup used for blanking is shown in Fig 4.


The clearance between the die and punch can be determined as c = 0.003 t. t where t is the sheet thickness and t is the shear strength of sheet material. For blanking operation, dies size = blank size, and the punch is made smaller, by considering the clearance.

The maximum force, P required to be exerted by the punch to shear out a blank from the sheet can be estimated as

P = t. L. t

where t is the sheet thickness, L is the total length sheared (such as the perimeter of hole), and t is the shear strength of the sheet material.

Stripping force: Two actions take place in the punching process – punching and stripping. Stripping means extracting the punch. A stripping force develops due to the spring back (or resiliency) of the punched material that grips the punch. This force is generally expressed as a percentage of the force required to punch the hole, although it varies with the type of material being punched and the amount of clearance between the cutting edges. The following simple empirical relation can be used to find this force

SF = 0.02 L.t

where SF = stripping force, kN

L = length of cut, mm

t = thickness of material, mm

Piercing: It is a process by which a hole is cut (or torn) in metal. It is different from punching in that piercing does not generate a slug. Instead, the metal is pushed back to form a jagged flange on the back side of the hole.

A pierced hole looks somewhat like a bullet hole in a sheet of metal.

Trimming: When parts are produced by die casting or drop forging, a small amount of extra metal gets spread out at the parting plane. This extra metal, called flash, is cut – off before the part is used, by an operation called trimming. The operation is very similar to blanking and the dies used are also similar to blanking dies. The presses used for trimming have, however, relatively larger table.

Notching: It is an operation in which a specified small amount of metal is cut from a blank. It is different from punching in the sense that in notching cutting line of the slug formed must touch one edge of the blank or strip. A notch can be made in any shape. The purpose of notching is generally to release metal for fitting up.

Nibbling: Nibbling is variation of notching, with overlapping notches being cut into the metal. The operation may be resorted to produce any desired shape, for example flanges, collars, etc.

Perforating: Perforating is an operation is which a number of uniformly spaced holes are punched in a sheet of metal. The holes may be of any size or shape. They usually cover the entire sheet of metal.