Working of Metals.pdf .

Department of Mechanical & Manufacturing Engineering, MIT, Manipal 1 of 29
MANUFACTURING TECHNOLOGY
CHAPTER 4
MECHANICAL WORKING OF METALS

Sheet Metal Operation
1) METAL
CUTTING
OPERATION
2) METAL
FORMING
OPERATION
The sheet metal operation done on a
press may be grouped in to two
category viz.
Shearing, Lancing,
Blanking, Piercing,
notching, Slitting,
Sheaving etc..
Bending,
drawing,
redrawing
etc.
Department of Mechanical & Manufacturing Engineering, MIT, Manipal

Shearing
• Shearing is a general
name for most
sheet-metal cutting.
• Shearing action has
three basic stages:
viz.
1. Plastic deformation
2. Fracture
(small cracks)
3. Shear

Lancing:
• This is cutting
operation in which a
hole is partially cut
and then one side is
bent down to form a
slot of tab.
• No scarp is produced.

Blanking
• A process in which the
punch removes a
portion of material
from the sheet.
• The removed portion
is called as “Blank”
• And the remaining
metal strip is scraped.

Piercing (Punching)
• The piercing is making
various shaped holes
in a sheet metal.
• It is identical to
blanking except of the
fact that the punched
out portion coming out
through the die in
piercing is scrap.

Washer manufacturing by
blanking and piercing.

• Notching:
This is cutting operation by which metal pieces are
cut from the edge of a sheet, strip or blank
• Perforating:
This is a process by which multiple holes that are
very small and close together are cut in a sheet.
• Slitting:
It refers to the operation of making incomplete
holes in a workpiece

Trimming
• This operation
consists of
cutting unwanted
excess material
from the
periphery of a
previously formed
component

Shaving:
• Accurate
dimensioning of
the part are
obtained by
removing a thin
strip of metal
along the edges.

Nibbling:
• Nibbling is removing
the metal in small
increments. When a
specific contour is to
be cut in a sheet
metal, a small punch is
used to punch
repeatedly along the
necessary contour,
generating the
required profile.

Bending:
• Bending is the plastic deformation of metals
about a linear axis with little or no change in
the surface area.
• Bending causes the metal on the outside to
be stretched (Tensile), while on the inner
side it is compressed (compression). The
location in the metal, which is neither
stretched nor compressed, is known as
Neutral axis of the bend.

Spring back effect in bending

In Bending, after plastic deformation there is an
elastic recovery this recovery is called spring back
» In the stress strain
diagram, a stress OA is
applied on the material so
that it reaches the point P
and has a strain of OB
under the load.
» When the load is
removed, the metal
springs back to the
position C, finally with a
permanent deformation of
OC only. The amount CB
is the amount of spring
back

• Methods to overcome or prevent or
minimize Spring back
1.Over bending: the sheet metal is
over-bent so that when it springs back
will have the proper angle.
2. Bottoming: It consists of striking the
metal severely at the radius area to
induce high compressive strains to
minimize compressive yield strength.

3. Stretch forming: It consists of
stretching the sheet metal, so that
yield strength of the material increases
before bending.
4. Ironing: It consists of thinning the
metal near the bending area.

Stretch Forming:
• In the process, the sheet
metal is held in the jaws
of hydraulic cylinders
and is stretched beyond
elastic limit.
• Then the sheet is
brought into contact with
the die as shown in the
Fig. so as to give it the
shape of the die.

Stretch Forming
Advantage
1. Stretch forming is
comparatively simple
and inexpensive,
because it uses a
single die.
2. The uniform
stretching of the
metal generally
improves the
physical properties of
the metal.
Disadvantage
1. complicated shapes
cannot be obtained
2. If the component is
having any holes
then, holes are likely
to be enlarged

Bulging:

Embossing

Coining
• Coining is essentially a
cold forging operation
except for the fact that
the flow of the metal
occurs only at the top
layers and not the
entire volume
• The coining die
consists of the
punch and die,
which are engraved
with the necessary
details required on
both sides of the
final object.

Coining

Curling
• The operation of
forming edges of a
component into a
roll or a curl by
bending the sheet
metal in order to
strengthen the
edges and to
provide smoothness
to its surface is
known as curling.

• Curling is a sheet metal forming process used to form the
edges into a hollow ring. Curling can be performed to
eliminate sharp edges and increase the moment of
inertia near the curled end. Other parts are curled to
perform their primary function, such as door hinges.

Wiring:
• Wiring is nothing
but the curling or
forming of a curl
edge on a
component with a
wire or rod
inserted with in
the curled edge

Roll forming
• Roll forming, also spelled roll-forming or rollforming, is a type
of rolling involving the continuous bending of a long strip of sheet
metal (typically coiled steel) into a desired cross-section. The strip
passes through sets of rolls mounted on consecutive stands, each
set performing only an incremental part of the bend, until the desired
cross-section (profile) is obtained. Roll forming is ideal for producing
constant-profile parts with long lengths and in large quantities.

• Roll-formed sections may have advantages over extrusions of a similar
shapes. Roll formed parts may be much lighter, with thinner walls
possible than in the extrusion process, and stronger, having been work
hardened in a cold state. Parts can be made having a finish or already
painted. In addition, the roll forming process is more rapid and takes less
energy than extrusion.

• Passing the metal
strip between two
rollers to form the
desired shape is
called roll-forming.

Roll bending
• A roll bender is a mechanical jig having three rollers used to bend a
metal bar into a circular arc. The rollers freely rotate about three
parallel axes, which are arranged with uniform horizontal spacing. Two
outer rollers, usually immobile, cradle the bottom of the material while
the inner roller, whose position is adjustable, presses on the topside of
the material.

• After the bar is initially inserted into the jig, the middle roller is manually
lowered and forced against the bar with a screw arrangement. This
causes the bar to undergo both plastic and elastic deformation. The
rollers are then rotated moving the bar along with them. For each new
position, the portion of the bar between the rollers takes on the shape
of a cubic modified by the end conditions imposed by the adjacent
sections of the bar. When either end of the bar is reached, the force
applied to the center roller is incrementally increased, the roller rotation
is reversed and as the rolling process proceeds, the bar shape
becomes a better approximation to a circular arc, gradually, for the
number of passes required to bring the arc of the bar to the desired
radius.

• It is employed to bend or curve sheets and plate in to
cylindrical shape with the help of the three long big rolls.

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