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Press working may be defined as, a
manufacturing process by which various
components are made from sheet metal. This
process is also termed as cold stamping. The
machine used for press working is called a
press.
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Press operations are broadly classified in to two
groups:-
• Cutting operations,
• Non cutting operations (Forming operations)
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1: Blanking:
Blanking is the operation of
cutting a flat shape from sheet metal.
The product punched out is called the
“blank” and the required product of
the operation the hole and the metal
left behind is discarded as waste.
2: Punching or Piercing:
It is a cutting operation by
which various shaped holes are made
in sheet metal. Punching is similar to
blanking except that in punching, the
hole is the desired product. The
material punched out from the hole
being waste.
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3: Trimming:
This operation consists of cutting
unwanted excess of material from the
periphery of a previously formed
component.
4: Shaving:
The edge of a blanked part are
generally rough, uneven and un square.
Accurate dimensions of the part are
obtained by removing a thin strip of metal
along the edges.
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5: Notching:
This is cutting operation by which metal pieces are cut from the edge
of the sheet , strip or blank.
6: Perforating:
This is a process by which multiple holes are very small and close
together are cut in a flat sheet metal.
7: Slitting:
It refers to the operation of making incomplete holes in a work piece.
8: Lancing:
This is a cutting operation in which a hole is partially cut and then
one side is bent down to form a sort of tab. Since no metal is actually removed
and there will be no scrap.
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1: Bending:
In this operation; the material in
the form of flat sheet or strip is uniformly
strained around a linear axis which lies in
the neutral plane and perpendicular it’s the
length wise direction of the sheet or metal.
2: Drawing:
This is a process of forming a flat
work piece into a hollow shape by means
of a punch which cause the blank into a
die cavity.
3: Squeezing:
Under the operation, the metal is
caused to flow to all portions of a die
cavity under the action of compressive
forces.
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1: Bed:
The bed is the lower part of the press frame that serves as a table to which a
Bolster plate is mounted.
2: Bolster Plate:
This is a thick plate secured to the press bed , which is used for locating
and supporting the die assembly. It is usually 5 to 12.5 cm thick.
3: Die Set:
It is unit assembly which incorporates a lower and upper shoe, two or more
guide parts and guide part bushings.
4: Die Block:
It is a block or a plate which contains a die cavity.
5: Lower Shoe:
The lower shoe of the a die set is generally mounted on the bolster plate of
a press. The die block is mounted on the lower shoe, also the guide post are mounted
on it.
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6: Punch :
This is male component of a die assembly, which is directly or indirectly
moved by and fastened to the press ram or slide.
7: Upper Shoe:
This is the upper part of the die set which contains guide post bushings.
8: Punch Plate :
The punch plate or punch retainer fits closely over the body of the punch
and holds it in proper relative position.
9: Back up Plate:
Back up plate or pressure plate is placed so that intensity of pressure
does not become excessive on punch holder. The plate distributes the pressure
over a wide area and the intensity of pressure on the punch holder is reduced to
avoid crushing.
10: Stripper:
It is a plate which is used to strip the metal strip from cutting a non-
cutting Punch or die. It may also guide the sheet.
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The travel for the penetration of the punch, progresses in
three zones:
1. Compression
2. Shearing
3. Breakage
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If ‘t’ is the total travel of the punch in the sheet thickness,
then ‘t’ can be expressed as –
Where,
a= Punch progression responsible towards compression;
around 10 – 30 % of ‘t’,
b= Punch progression responsible towards true shearing;
around 40 – 80 % of ‘t’,
c= Punch progression responsible towards breakage;
around 10 – 30 % of ‘t’,
t = a+b+c
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The ratio b/t is known as “Percentage Penetration” and denoted by ‘k’.
Then shear force requirement:-
Shear Force = Failure stress X Area under cutting
= Ultimate shear stress X Peripheral length of punch X Thickness
of material X % Penetration
Shear Force = u.s.s. X π.d X t X k (For a circular punch)
This expression can be used to calculate press capacity.
The factor k depends on:
1. Work piece material
2. Thickness of stock
3. Clearance between die and punch
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Punching Blanking
The difference in dimensions between the mating members of a die set is
called clearance. This clearance is applied in following manner:
1: when the hole has to be held to size i.e. the hole in the sheet metal is to be
accurate (punching operation) , and slug is to be discarded. The punch is made to
the size of the hole and the die opening size is obtained by adding clearance to the
punch size.
2: In blanking operation , where the slug or blank is the desired part and has to be
held to size, the die opening size equals the blank size and the punch size is
obtained by subtract.
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Types of Die-Set:-
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Strip Layout:
Percentage utilisation of raw material = (Component area / w x p) X 100
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