Shearing Operations: Punching/Blanking, Piercing, Trimming, Notching, Nibbling, Perforating
Shearing
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, As can be seen that cut
edges are neither smooth nor perpendicular to the plane of the sheet.
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 revel
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 may also be done
between a punch and die, 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.
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| Punching & Blanking |
In punching, the metal inside the part is removed; in
blanking, the metal around the part is removed.
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, die 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
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| Blanking & Piercing |
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.
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| Perforating, Notching, Slitting |
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.
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