press tool, jigs & fixtures, gear and thread manufacturing,manufacturing processes,Production Technology

Topic to be covered:
• Classification of presses,
• Classification of dies,
• Cutting actions in dies,
• Clearance,
• Cutting forces,
• Methods of reducing cutting forces,
• Minimum Diameter of Piercing Center of Pressure,
• Blanking,
• Piercing,
• Drawing,
• Bending and Progressive Die design,
• Scrap reduction,
• Strip layout.

Introduction:
• Press working has been defined as a chipless manufacturing
process by which various components are made from sheet
metal.
• Press working operations are carried out with the help of a
metal forming machine called press which shears or forms the
component by applying force.
• The main features of the press include a frame which supports a
ram or a slide, a bed and source of power with a mechanism for
operating the ram inline or at right angles to the bed.
• The ram is equipped with a punch or a set of punches which
have the shape of the job to be produced while a die block is
attached to the bed.

Introduction:
• Workpieces are produced or formed as the punch descends
onto the die block.
• Processed sheet metal components find extensive industrial and
household applications such as in manufacture of automobile
and aircraft bodies, office appliances, kitchen ware, storage
cabinets, furniture items, mechanical toys etc.
• Press tool operation of sheet metal is by far the cheapest and
fastest method of complete manufacture of components.

Advantages of Press Working :
Use of presses for sheet metal operations has the following
advantages.
1. High productivity.
2. Better accuracy.
3. Uniformity of parts produced.
4. Possibility of using lesser skilled labour.
5. Low scrap losses.
6. Predictable strength characteristics of the components.
7. Reduction of weight.
8. Considerable cost reduction.
9. Possibility of automotion.

Cutting Operations:
• In cutting operations, the work piece is stressed beyond its
ultimate shear strength and cut off into pieces.
• The stresses involved in the process are shearing stresses
• The common cutting operations using dies and punches
include the following :
1. Blanking 2. Punching or piercing
3. Trimming 4. Shaving
5. Notching 6. Lancing
7. Nibbling 8. Perforating
9. Parting 10. Shearing

Cutting Operations: contd…
1.Blanking :
• Blanking is the operation of cutting out flat intermediate
shapes called blanks which are to be further processed to
produce desired parts.
• The sheet metal from which the blank is to be cut is
properly located over the die and a punch lowered into it to
produce the required cut.
• Neglecting the small amount of spring-back that occurs
when the shearing force is removed, the size of the hole
made in the sheet is the size of the punch while the cut out
piece has a size equal to the die opening.

Blanking set-up
Blanking set-up

1.Blanking :
• The die opening will be made of the required size of the
blank while the punch will be made smaller by an amount
equal to twice the clearance between the punch and die.
• The punch and die should be made of hard wear resistant
materials such as hardened high carbon steel, high
chromium die steel or tungsten carbide.
• The die is provided with a die relief angle or draft about 3 
to 4 so that the cut piece falls freely through the die
opening. The actual value of the draft used depends on the
material thickness and shape.

1.Blanking :
• A stripper plate is provided to release the metal from the
sides of the punch as it rises after completing the stroke.
The sheet metal sticks on the punch due to the spring-back.
• While planning a blanking operation it is necessary to
provide suitable clearances between the adjacent blanks
and the edge of the blank and the edge of the strip.
• When a number of blanks are required to be cut from a
given strip of metal a proper layout of such blanks may be
helpful in determining the maximum utilization of sheet
area.

2.Punching or piercing :
• Piercing or punching is similar to blanking except that the
metal pieces cut out in piercing are scrap. The interest in
piercing is to produce a hole of a desired shape in the
workpiece.
• The size of the punch used in piercing is equal to the
required size of the hole while the die opening is made
larger by twice the desired clearance between the punch
and die.

2.Punching or piercing :

3.Trimming :
• Trimming is the process of removing excess metal which
remains around parting lines after operations like die
casting, forging and sheet metal drawing.
• Trimming is done by forcing the parts to be trimmed
through trimming dies with a suitable punch. The operation
is similar to a blanking operation.
• The presses used for trimming normally have a larger
bolster plate area compared to those used for blanking.

4. Shaving :
• Shaving is similar to trimming except that the amount of
metal removed is much less around 10% of the metal
thickness.
• Shaving is done on blanked or pierced work pieces to
obtain smooth, square edges with closer dimensional
tolerances.
• Shaving dies use much lower clearances than blanking or
punching dies.

5. Notching :
• Notching is the operation of making indentations at the
edge of the work piece as shown in Fig.
• A die and a punch of a suitable shape are used to make the
notch.

6. Lancing :
• In lancing as shown in Fig. a hole is partially cut in a sheet
and the cut metal is bent on one side.

7. Nibbling :
• Nibbling is the process of cutting irregular shapes from
sheet metal without using a die and punch of the
corresponding shape.
• When the number of pieces of a given shape to be
produced are small and the possibility of a repeat
production run is remote it becomes very uneconomical to
use dies and punches of the corresponding shape.
• Such jobs are handled on a nibbler which uses a small
round or triangular punch that reciprocates vertically in and
out of a corresponding die producing a series of
overlapping holes.

7. Nibbling :
• The sheet metal is guided through the machine along the
path desired.
• The process is slow and the cut edges may need to be
further finished but it eliminates the need for costly special
dies.

8. Perforating :
• Perforating is the process of making a large number of very
small holes close to each other in flat work pieces.

9. Parting :
• Parting is the operation in which metal is cut
simultaneously along two parallel lines or contours in such
a say that the side thrust is neutralised.

10. Shearing:
• Shearing is a general purpose shearing operation along
straight lines mostly for cutting small pieces from large
sheet stock.
• General purpose shearing operations use straight line
shearing for cutting.
• The machines used for the purpose can be divided into
1.Square shear
2.Rotary shear

10. Shearing:
• The square shearing operation is shown schematically in
Fig.
• The work piece is sheared between two blades. The lower
blade is fixed to the machine while the upper blade moves
up and down with the ram of the machine.

10. Shearing:
• Rotary shearing is shown in Fig. In this case shearing is
done with rotating disc blades.
• The forces involved are lower in this case and the length of
the sheet does not pose any problem.

Bending Operations :
• The operations for bending can be divided into four
categories:
1.Angle bends – single, double or straight
2.Roll bending
3.Roll forming
4.Seaming

Bending Operations: contd…
1. Angle bending :
• Angle bending is the process in which the sheet is bent
through an angle.
• If the angle of bend is less than 90°, the bend is called
single bending.
• For 90° angle the bend is called straight bending.
• If an angle less than 90° is produced at two places, the
operation as called double bending.

2. Roll bending :
• Roll bending is the process in which sheet is bent in the
form of rolled edges.

3. Roll forming :
• Roll forming is the process of bending the sheet between
rolls.
• The process may also be used to form any impression in the
form of a bend.

4. Seaming :
• Seaming is the process of providing a mechanical lock
between the edges of two different workpiece.
• The edges of the two workpiece are bent in appropriate
direction, assembled and pressed to from the seal.

Die Bending :
• Die bending is used for long straight line bends on press
brakes.
• It may be further divided into-
1. V-bending and
2. Edge bending.

1. V-bending:
• V-bending is done with a wedge shaped punch pressing the
sheet into a V-shaped die as shown in Fig.
• The die used may be of the air bend type or a bottoming
die.
• The process is used to produce sharper and more accurate
bends in thinner sheets. V-bending can be used for
producing included angles which are acute, 90° or obtuse.

2. Edge Bending :
• Edge bending shown in Fig. is a cantilever type of bending
operation in which the punch forces the sheet metal over
the edge of a supporting die.
• A spring loaded pad clamps the sheet against the die block
before the punch contacts it to prevent is from being lifted
during the punch travel.

Press Working Terminology : contd…
1. Bed :
• Bed is the lowest part of the
press. It acts as a table on
which the bolster plats is
mounted.

2. Bolster plate :
• It is a thick plate attached to
the bed of the press.
• It is used for locating and
supporting the die
assembly. It is usually 50 to
150 mm thick.

3. Die holder :
• Also called die retainer or
die shoe it is used to hold
the die block in proper
alignment with the punch.
• It is mounted on the bolster
plate.

4. Die block :
• It is the female part of the
die set having an opening to
suit the punch. It is made of
a hard wear resistant
material such as hardened
high carbon steel, high
chromium die steel or
tungsten carbide.
• The die is provided with a
die relief angle or draft
about 0.75° so that the cut
pieces fall freely through
the die opening.

5. Stripper:
• Strippers are used to
prevent the sheet metal
from coming up with the
punch when it returns after
the working stroke.
• Strippers may be fixed or
spring loaded and may also
guide the sheet.

6. Stock guides and packing :
• These are used to ensure
that during feeding between
each operation the stock is
guided properly to ensure
accurate location for each
successive component.
• Stock stops and pilots are
often used to ensure that
after each operation the
strip is moved through
correct distance and located
properly for the next
operation.

7. Punch :
• Punch is the male member
of the die set which forces
the sheet strip down to
pierce or form it.
• Like the die it is also made
of a hard wear resistant
material.
• It is attached to the ram
through suitable punch
holders and fastening
devices depending upon its
design.

8. Punch retainer or punch pad :
• Punch retainer or punch pad is
used for holding the punch in
its proper relative position.
• It may be bolted to the punch
holder.

9. Pressure plate :
• A pressure plate or backup
plate is usually provided
between the punch and the
punch holder to provide some
cushioning effect.
• It distributes the pressure over
a wider area of the punch
holder reducing any possibility
of its getting crushed under
heavy load.

10. Punch holder :
• Punch holder is provided to
hold the punch and connect it
to the ram of the press.
• On its top it ends up in a shank
that exactly fits into the ram
opening for proper positioning
and aligning of the punch.

11. Guide posts :
• Guide posts are used to
provide proper alignment
between the die shoe and the
punch holder.
• Guide bushes are provided at
the top of the guide posts.
• These bushes are mounted on
the punch holder and slide
over the guide posts.

Types of Presses :
1.Based on source of power
• Manual Presses: Manually operated presses are the hand,
ball or fly presses.
• Power presses: Power presses can be further classified into
mechanical or hydraulic presses.

Types of Presses : contd…
2.Based on method of actuation of slides :
• Based on the devices used in mechanical presses for
converting rotary motion of the main shaft to linear motion
of the crank the presses can be divided into
1.Crank operated press
2.Cam press
3.Eccentric press
4.Screw press
5.Rack and pinion press
6.Knuckle joint press
7.Toggle joint press.

3.Based on number of slides :
1.Single action press
2.Double action press
3.Triple action press

4.Based on Type of frame used :
1.Back inclinable C Frame
2.Gap bed press
3.Adjustable bed press
4.Horn Press
5.Straight side press
6.Arch press
7.Tie rod type press
8.Pillar type press

5.Based on End use :
• Most machines are capable of doing a variety of work.
• Only some machines are designed for a particular type of
job.
• These machines are known by their end use

Fly Press:
• Typical example of a mechanical
press.
• This press is operated by hand
and can be used for performing
most of the sheet metal
operations on a small scale.
• The press essentially consists of
a screw operating in a nut
formed in the upper part of a
rigid F-type frame.

Types of press frame

Ejecting Mechanisms :
• Ejectors are the devices used for ejecting the components
from die after completing the operation.
• Simple components can be ejected easily but heavy
components need external help for ejection.
• Automatic ejection mechanisms may employ air or oil
pressure or mechanical apparatus to remove the
completed part from a press.
• Ejection mechanisms may be interlocked with the
operating controls to prevent operation until part ejection
is accomplished.

Ejecting Mechanisms : contd…
1.Injector pin inserted through a hole in the punch:
• Fig. shows a set up for a curling operation.
• After the operation is completed an ejector pin inserted
through the hole in the punch helps in pushing the
workpiece out of the punch cavity.

2.Knockout pin through the Die:
• In this case die platen is at the top while the punch is at the
bottom constituting an inverted punching operation.
• The descending die first shears the workpiece and then
forces, the platen further down on the pressure plate (or
the bottom ejector).

2.Knockout pin through the Die :
• When the die moves up, the work-plate is knocked up
under the spring pressure.
• The top ejector within the die knocks out the blank held in
the die cavity.

3.Ejection with the help of a lever:
• The lever is operated manually by the worker after the
press work is completed or may be connected mechanically
with the upward motion of the press ram.

4.Ejection with hooks in top tool:
• In this arrangement the top tool of the press is provided
with hinged hooks.
• The hooks slide smoothly over the surface of the workpiece
when the ram is descending but when ram starts going
back the hooks engage in grooves provided for them
thereby ejecting the finished workpiece.

5.Ejection by hydraulic and pneumatic cushions
• Hydraulic or pneumatic cushions using oil/air provide
uniform force throughout the stroke length.
• The force can also be varied suitably by adjusting the oil/air
pressure and the movement is smoothly.
• Pneumatic ejectors however tend to be noisier.

6.Robots :
• Robots are used in many organisations for loading,
unloading, assembly of parts.
• When a robot is required to remove the finished work it is
important to ensure that the movements of the robotic
arm is not obstructed by any part of the set up.

The Metal Shearing Process:
• When the punch is forced down into the sheet metal the
punch first draws the stock in a circular bend over the
cutting edge.
• The grains elongate near the cutting edge A of the punch
and die corner D.

The Metal Shearing Process: contd…
• As the punch descends further this local deformation
increases and when local strain in the surface fibres
reaches the limiting value for the work material the fibres
rupture.
• The crack starts just ahead of the punch corner but with
the advancement of the punch, the inner fibres also get
ruptured and the crack starts advancing from both sides

• If the clearance between the die and punch is correct the
cracks started at the two ends spread towards each other
till they meet causing separation with a clean edge.
• The amount of clearance depends upon the thickness of
the sheet and ductility of the work material. For steels,
this clearance varies from 5 to 8 percent of the thickness
of the sheet. Larger clearances up to 10 percent are
required for aluminium.

• If the clearance between the punch and die is too large or
too small the cracks started at the punch and die do not
meet as shown.
• Additional bands of metal must be sheared in such cases
to complete the separation. This leads to higher cutting
forces and cut edges are generally not smooth.
• Excessive clearance also causes a large radius to be
formed at the corners of the slug and work piece.

• An approximate value of the correct clearance to be used
for a shearing operation on each side may be obtained by
the following relationship.
Where,
C = clearance, mm [on each side]
t = sheet thickness, mm
fs = shear strength of the material, MPa

• Values of shear strength for common engineering
materials

• Clearance values normally used for punching

Effect of Shear Angle :
• Shear is provided on the punch or die to reduce the forces
required on the punch often to accommodate the
operation on a lower capacity machine available in the
shop.
• The purpose of the shear is to distribute the shearing
action over a period of time so that the punch does not
contact the work piece over its entire length at the same
time.

Effect of Shear Angle : contd…
• The provision of shear, as mentioned earlier, does in no way
affect the total work required to complete the shearing
operation.
• The total work done in shearing with or without the shear
is the same.
• The total travel of the punch to complete the operation in
however increased with the provision of shear.

Stripping Force :
• The force required for stripping depends primarily on the
thickness of the sheet, size and number of the holes and
the location of the holes.
• Small holes in the middle of a sheet or holes in thicker
material require more stripping force than holes in a thin
sheet or holes located near the edge of the workpiece.
• A number of punches located close to each other or
punches with rough walls are comparatively more difficult
to strip than single punches with smooth walls.

Stripping Force : contd…
• In general, stripping forces vary from 2.5 to 20 percent of
the punch force but 5 to 10 percent is quite satisfactory in
most cases.
• A formula generally used for finding the stripping force is,
Fs = k · L · t
Where,
Fs = stripping force, kN
L = perimeter of the cut, mm
t = stock thickness
k = stripping constant depending upon the material,
and size and location of the cut

Stripping Force : contd…
• The value of k is given in Table.
• The press selected for the shearing operation must have
enough capacity, not only to provide for maximum punch
force and the stripping force but also to account for friction
and other losses during the process.

Press Working Dies: contd…
(a)Based on type of press operation:
1. Shearing Dies: Dies used for blanking piercing, punching,
notching perforating, trimming, shearing and parting. All
these are known as cutting dies.
2. Bending dies: Dies used for bending, curling, forming etc.
3. Drawing dies: Dies used for curling, flanging, embossing,
bulging.
4. Squeezing dies: Dies used for coining, sizing, swaging,
flattering.

(b)Based on type of die construction:
1. Simple dies
2. Progressive dies
3. Compound dies
4. Combination dies
5. Transfer dies
6. Multiple or gang dies

1. Simple dies :
• Simple dies are dies designed to perform only one specific
operation like blanking, punching, notching, trimming etc.
• The die may be provided with all the usual elements
including stop pins which are used for proper indexing of
the metal strip.

2. Progressive dies:
• When the quantity of pieces to be produced is large, more
complex dies are often used to increase production rate.
Dies included in this category are progressive, compound
and combination dies.
• A progressive die uses two or more punch and die sets
located at different stations to perform a number of
operations simultaneously in one stroke of the ram.
• The sheet is moved progressively from one station to the
other till at the last station a complete job is obtained.

• A simple two-stage progressive
punch and blanking die used for
making washers is in fig.
• The metal strip is introduced
from one end and is advanced
till the first station where a hole
is punched in the first stroke of
the ram.
• The metal is now advanced to
the next stage, the correct
distance being set by the stop
shown.

• As the ram moves down for its
next stroke the pilot on the
blanking punch enters the hole
pierced at the previous station
to locate the sheet.
• The working punch itself moves
down to remove the completed
washer.
• In the mean time a new hole
has been pierced at the first
stage.

• In each subsequent stroke of
the ram two operations are
performed one at each station
and one job is completed at the
last station.
• Straight pieces of metal are fed
manually maintaining a slight
pressure on the strip as it
passes through the die.

3. Compound dies:
• In compound die two or more functions are performed at
the same station of the die.
• Fig. shows a simple compound die for producing washers.
• It combines a blanking and a shearing operation at one
station.

Compound dies

3. Compound dies:
• The piercing punch and the blanking die are bolted to the
ram. The spring loaded stripper for the piercing punch is
located within the blanking die.
• The lower die has cutting edges on both sides. The inner
edge serves as a piercing die for the piercing punch while
the outer edge serves as a blanking punch.

3. Compound dies:
• The blanking punch is so located that as the ram is lowered
the blanking operation is completed earlier than the piercing
operation.
• This ensures that the metal strip is firmly held before the
relatively weaker piercing punch contacts it.
• The spring loaded stripper plates help in knocking out the
washer produced.

4. Combination dies:
• A combination die set up is
shown in Fig.
• It combines blanking and
drawing operations.
• The drawing punch is located
inside the blanking punch while
the die blank has two working
surfaces one for each punch.

4. Combination dies:
• As the blanking punch descends
it cuts a blank of required size
from the metal using the outer
working surface of the die.
• The inner drawing punch now
descends and draws out the
metal to form the required part
while the blanking punch
serves as a pressure pad.
• The drawn cup is then ejected
from the die.

5. Transfer dies :
• Transfer dies are similar to progressive dies in that the stock
is transferred progressively from one station to the next till
at the last station the job is completed.
• Generally pre-cut blanks are used in these dies.

6. Multiple or gang dies :
• In these dies, number of simple die and punch sets are
grouped together in such a way that two or more work
pieces are produced in each stroke.

Sheet Metal Layout :
• An important requirement in planning sheet metal
operations is to prepare a proper layout design.
• A proper stock layout is essential in order to decide how
the blanks will be cut from the sheet stock in the best way.
• While planning a blanking operation, for example, it is
necessary to provide suitable clearances between adjacent
blanks and the edge of the last blank or the edge of the
strip to prevent the metal being drawn between the punch
and the die.

Sheet Metal Layout :
• Fig. shows some of the terms that are used in relation to
strip layout.

Sheet Metal Layout : contd…
• t = stock thickness
• a = distance between edge of strip and the blank called
front and back scrap

• b = distance between the nearest points of the blanks
and also the distance between the edge of the blank
and the edge of the strip, called scrap bridge.

• c = die advance or lead of the die. It is the distance from a
point on one blank to the corresponding point on the
next blank.

• h = height of the blank or diameter in case of circular blank
• W = width of the strip = h + 2a

• L = length of the strip for a given number of blanks

• The distance a is generally given by the equation
a = t + 0.015 h

• Distance b must be large enough to prevent the scrap from
twisting and wedging between the punch and die.
• A rule of thumb is to make it at least equal to 1.5t.

• Other factors that permit it to be thinner are strip
thickness, hardness of the material, type of operation,
shape of blank etc.

• The commonly used values of b are given in Table

Factors Affecting the Strip Layout :
1.Grain direction :
• The strips are usually available in coils.
• An important point that has to be considered is the grain
direction or the direction in which rolling was done.
• The strip should always be fed and processed in the
direction of the grains.

Factors Affecting the Strip Layout : contd…
2. Cost of tooling :
• The tooling needed for presswork is quite costly and takes a
long time to design and manufacture.
• A compromise between reasonable tool cost and optimum
layout is often required. For high production rates,
however, tool cost per piece becomes less important and
more attention should be paid for economy of material and
ease of operation.

3.Ease of operation :
• The strip has to be planned in such a way that actual
feeding into the die is as simple as possible and location of
holes is accurate.
• The strip may get jammed if there are any projecting
contours.
• The strip may also need to be turned over when feeding for
different passes.

4.Availability of standard strip widths :
• Strips are available in standard sizes only.
• As far as possible only these sizes should be used because
they are available easily and faster.
• Uncommon sizes have to be ordered specially and are
usually costlier and have longer delivery periods.

5.Strip or coiled stock :
• The available stock may be in the form of a coil or a strip.
• A strip may be passed through the die any number of times
but the coiled stock is usually passed through the die only
once.
• Coiled stock is used when production volumes are high and
metal thickness is low.
• The feed in this case is usually automatic.
• Strip stock is used with low production volumes and thicker
metal sheets. The stock feed is mostly manual.

6.Direction of burr :
• A burr is produced on the die side of the scrap strip and on
the punch side of the blank.
• Sometimes it may be desirable to keep the burr on a
particular side.
• It is however, not a very important point in most cases.

8.Narrow run or wide run :
• If the length of a component is more than its width it is
necessary to make a choice whether to keep the length side
or width side towards the feeding direction.
• As shown in Fig. if the narrow side is towards feeding
direction it is called a narrow run, otherwise it is the wide
run.

Material Utilization Factor :
• The economy of material used is usually expressed in terms
of Material Utilization Factor (MUF) defined as
• Thus if one blank of dimensions w x h is produced the sheet
used will be (w + b) x (h + 2a) and

Material Utilization Factor : contd…
• Quite often it is possible to improve the material utilization
by rearranging the layout.
• Fig (a). with one punch but it is not the best arrangement
from area utilization point of view.

Centre of Pressure : contd…
• Quite often it is possible to improve the material utilization
by rearranging the layout.
• Fig (a). with one punch but it is not the best arrangement
from area utilization point of view.

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