UNIT III- METAL DEFORMATION

HOT WORKING AND COLD WORKING OF METALS

Bulk Deformation or Metal forming processes are broadly classified into two categories:

 Hot Working
 Cold working

The metal working processes that are carried out above recrystallisation temperature are
Hot working processes whereas those below are Cold working processes.

When atoms reach a certain high energy level under the action of heat of and the force new crystals
start forming. This is termed as Recrystallisation.

Hot working does not mean that the working of a metal is at elevated temperature. Lead and tin
have a recrystallisation temperature below the room temperature, and hence working of these
metals at room temperature is always hot working. But steel has recrystallisation temperature in the
range of 600-700 o C, hence working temperature below that temperature is still Cold working.

Hot Working
Advantages:

 No Strain hardening takes place since working is carried out above recrystallisation
temperature.
 Material should have high ductility at high temperature. Brittle materials can also be hot
worked easily.
 Shear stress will decrease when temperature increases and hence force required to
achieve the necessary deformation is very less compared to cold working.
 Better mechanical properties will be achieved by controlling the temperature.

Limitations:

 At high working temperature scaling will be formed on the surface of the

material which leads to loss of materials and poor surface finish.
 Due to thermal expansion of metals, dimensional accuracy is difficult to
achieve.
 It is very difficult to handle and maintain hot metals.

Cold working
Advantages:
 High dimensional accuracy.
 Surface defects are removed.
 Thin gauge sheets are produced.
 Since cold working done at room temperature or low temperature no
oxidation and scaling of the work piece occurs.
COMPARISON OF HOT WORKING WITH COLD WORKING

The comparison of hot working with cold working is given in Table

 FORGING

Forging is one of the bulk deformation processes in which compressive force is applied to
manipulate the metal in such a way that the required final shape is obtained. Forging normally a hot
working operation though cold forging is also used at times. The forging operation can create parts
that are stronger than those manufactured by any other methods of production.
In forging, the grains remain unbroken ( refer figures given below) and assume the contour
of the part. Here it is easy to see that the grains, not only remain unbroken, but have formed a
tough, fibrous structure conforming to the outline of the part.

(a) Casting (b) Machined (c) Forged

Grains flow in Crankshaft in three different manufacturing methods

Forgings are always used where reliability and human safety are critical. They are commonly used
components like assembled items in air planes, automobiles, tractors, ships, oil rigs, engines,
missiles and all critical equipment’s.

Cold-working Processes Classifications of Squeezing Processes

 Squeezing  Rolling
 Bending  Swaging or Cold forging
 Shearing  Staking
 Drawing  Sizing
 Special forming processes  Coining
 Burnishing
 Cold rolling
 Knurling
 Riveting
 Thread Rolling
Classifications of Drawing Processes Classifications of Shearing Processes
 Blank Drawing  Punching
 Tube Drawing  Blanking
 Wire Drawing  Cutting off
 Embossing  Trimming
 Metal Spinning  Perforating
 Stretch forming  Notching
Classifications of Bending Processes  Slitting
 Angle bending  Lancing
 Roll forming  Shaving
 Plate bending
 Seaming
 Curling
 FORGING OPERATIONS

Upsetting

It is the process of increasing the cross-sectional area of the bar at the expense of its height. When a
bar is compressed by the open dies the material is squeezed and upsetting takes place. Figure given
shows the upsetting.

Punching

Punching is the process of producing holes in workpiece as shown in figure below. Normally, the
hole produced is cylindrical. Punching can be done by placing the workpiece over a hole in the
anvil or over a cylindrical die of required size or over a correct size in a swage block. The hot
punch is placed on the workpiece and hammered.

Fullering

In this operation, the cross-section area of workpiece is reduced and lengthened for further
operation like drawing like drawing down or setting down.

Drawing Down

It is the processes of increasing the length of the bar. When the length increases, either the
thickness or width decreases or both. The metal is heated to a required temperature, held with tongs
and placed between two fillers over the anvil. The bottom fuller is positioned in a square hole
(Hardie hole) in the anvil. The top fuller is placed over the job directly above the fuller using a
sledge hammer.
 Drawing Down

Ironing
Sometimes, it is desirable to produce drawn parts with considerable variation in thickness of metal.
The drawing process used to accomplish this is known as Ironing.

Welding

It is a process of joining two metal pieces by applying pressure after heating them to a higher
temperature is known as forge welding .Wrought iron and mild steel are most suited for forge
welding.

Welding
Cutting

This process is used to cut the large pieces into small pieces for further operations. Cutting can be
done by using either cold or hot chisel.
 Trimming
It is the operation of removing flash material from the forged components.

Forging can be classified as:

1. Smith forging or open-die forging or upset forging

Hand Forging
Power Forging
Hammer Forging
Press Forging
2. Impression Forging or Closed Die Forging
Drop Forging
Press Forging
Machine Forging
Seamless Rolled ring Forging
Cold Forging

OPEN DIE FORGING

When a solid work piece is placed between two flat dies for deformation is called Open –Die
Forging.There is no external restriction of for the material to flow.

Shaft forging (a) Starting Stock held by manipulator

(b) Open-die forging (c) Progressive forging (d) Lathe turning to near net shape

This process progressively works the starting stock into the desired shape, only between flat –faced dies.
 Open die forging comprises of many process variations, permitting an extremely large size to be
produced.

Open-Die forging can produce Round, square, rectangular hexagonal bars and other shapes.
b

Hollow Sleeve Type Forging

Ring Forging
(a) mounted on Saddle/Mandrel
(b) Preform Metal displacement reduces perform wall thickness to increase
diameter (c)Progressive reduction of wall thickness to producer ring dimensions
(d) Matching to near net shape

Hand Forging

Hand forging are made with repeated blows in an open die or an anvil when heated to the proper
temperature, where the operator manipulates the work piece. This is an old manufacturing
process and what a traditional blacksmith does.
 Various operations are drawing down, upsetting punching, bending setting down and welding.
Hand forging is used for making simple shapes such as chains, hooks, shackles, and agriculture
equipment and tools.

Applications
Open-die processes can produce:

1. Step shafts, solid shafts (spindles or rotors) whose diameter increases or decreases at
multiple locations along the longitudinal axis.
2. Hollow cylindrical shapes, usually with length much greater than the diameter of the part
Length, wall thickness, internal and outer diameter can be varied as needed.
3. Ring-like parts can resemble washers or approach hollow cylinders in shape, depending on the
height/wall thickness ratio.
4. Contour-formed metal shells like pressure vessels, which may incorporate extruded
nozzles and other design features.

IMPRESSION DIE FORGING OR CLOSED DIE FORGING

When a solid work piece is placed between two shaped dies, two dies are brought together and
the work piece undergoes plastic deformation until it enlarges sides touches the side wall of die.

Then a small amount of material begins to flow outside the die called flashing.

Closed Die Forging

The flash cools quickly and presents increased resistance to deformation.

It also builds up pressure inside the bulk of the work piece that aids material flow outside icavities in
the die.

Most Engineering metals and alloys can be forged like carbon and alloys steels, aluminum
and copper alloys.
Applications

1. Part geometry’s range from some of the easiest to forge simple spherical shapes, block- like
rectangular solids, and disc-like configurations to the most intricate components with thin and
long sections that incorporate thin webs and relatively high vertical projections like ribs and
bosses.

2. Although many parts are generally symmetrical, others incorporate all sorts of design
elements (flanges, protrusions, holes, cavities, pockets, etc.) that combine to make the forging
very non-symmetrical.

3. In addition, parts can be bent or curved in one or several planes, whether they are
basically longitudinal, equi dimensional or flat.

Machine Forging

The chief difference between hand forging and machine forging is that in the latter technique
various types of machine powered hammers or presses are used instead of hand sledges. The power
hammer can be mechanical or pneumatic type. The stroke of the hammer varies from 350 mm to
1000 mm and corresponding speeds range form 200 to 800 blows per minute. These machines
enable the operator to strike heavy blows with great rapidity and thus to produce forgings of large
size and high quality as swiftly as required by modern production-line methods. Another advantage
of machine forging is that the heavier the blows struck during forging, the greater the improvement
in the quality of metallic structure.

Machine Forging or Upsetting

Fine grain size in the forging, which is particularly desirable for maximum impact resistance, is
obtained by working the entire piece. With large, hand-forged metal, only the surface is deformed,
whereas the machine hammer or press will deform the metal throughout the entire piece.

Drop Forging

Drop forging is the most common forging procedure that is in use.

It gets its name from the fact that the upper half of the die drops onto the lower half.
This process involves several steps.
 The first two steps are called fullerring and edging. Here cross-sectional area of tmetal is
reduced in some areas and gathering in other areas.

Steps in Drop Forging

The third step is blocking and the shape of the part is not pronounced.

In step three flash, begins to appear, this is a thin fin (0.04 mm) of metal that is queezed
between the dies.

The fourth step is finishing operation which gives the final shape.
The final step is called trimming operation where holes are cleared and flash is trimmed.

Press Forging

Press forging is a process similar to heading, where a slow-continuous pressure is applied to the
area to be forged. The pressure will extend deep into the material and can be completed either cold
or hot. A cold press forging is used on a thin, annealed material, and a hot press forging is done on
large work such as armor plating, locomotives and heavy machinery. In this type, only one blow is
given as compared with number of blows in drop forging.

In press forging number of stages are used and only in last stage die cavity is used to get finished
forging. Dies may have less draft, and the forging comes nearer to the desired sizes. Press forging
are shaped at each impression with a single smooth stroke and they stick to the die impression more
rigidly. Unless some provision is made, the escape of air and excess die lubricant may be difficult.
Thus, press-forging dies require a mechanical means for ejecting the forging.

Press forging are generally more accurate dimensionally than drop forging. The cost of the process
is three to four times than that in drop forging but with press forging, unskilled labour can be used
and production rate is higher. The working conditions with the press are better as there is no noise
and vibrations.
Cold Forging

Forging is mostly done by hot work, at temperature up to 1370° C.A variation of impression die
forging is cold forging. Cold forging encompasses –Bending, cold working cold heading, coining,
extrusions and more to yield a diverse range of part shapes.

Upsetting and Piercing

 ROLLING

Rolling is a deformation process in which the thickness of the work piece is reduced by
compressive forces exerted by two opposing rolls. Rolling is categorized into two types, Hot rolling
and Cold rolling. In hot rolling, the metal is heated to just below its melting point before being fed
into the rollers. It is very useful for brittle materials like Cast iron, the hot rolled steel cools down
with finer grains in the crystalline micro-structure, and is stronger and less brittle, e,g. wrought iron.
A structural change which occurs in the direction of rolling and the velocity of material at exit is
higher than that at the entry. After crossing the stress zone, grains starts refining in the case of hot
rolling. In cold rolling, grains retain the shape acquired by them during rolling.

Rolling process and Deformation of grains in Rolling

Rolling Operations
Main use rolling is in plants where the metal is made like steel- making plants, liquid iron is formed in a blast
furnace by reducing the iron oxide. thus further processing of the liquid metal, including conversion from
iron to steel, it is cast into raw stock shapes by a process called continuous casting. Large pieces of steel
(several tonnes each) `with typical cross-sections including Bloom (square section 6x6 inches or larger) Slab
(rectangular cross- section 10 x 1.5 inches ), Round bars (circular cross-section ), or Beams ( I- sections).

TYPES OF ROLLING MILLS

A rolling mill consists of one or more roll stands, motor drive, reduction gears, and flywheel and
coupling gears between units. The roll stand is the main part of the mill, where the rolling process is
performed. It basically consists of housings in which bearings are fitted, which are used for
mounting the rolls. Depending upon the profile of the rolled product, the body of the roll may be
either flat for rolling sheets (plates or strips) or grooved for making structural members (channel, I-
beam, rail).

Rolling mills are classified according to the number and arrangement of rolls in a stand. They are
classified as:
(A) Hot rolling of metals (Two-high rolling mill, Three-high rolling mill)
(B) Cold rolling of metals (Four high rolling mill, Cluster rolling mill)

(1) Two-high rolling mill: It is basically of two types i.e., non-reversing and reversing rolling mill.
The two high non-reversing rolling stand arrangements is the most common arrangement. In this
the rolls always move in only one direction, while in a two-high reversing rolling.

(2) Three-high rolling mill: It is used for rolling of two continuous passes in a rolling sequence
without reversing the drives. After all the metal has passed through the bottom roll set, the end of
the metal is entered into the other set of the rolls for the next pass. For this purpose, a table-tilting
arrangement is required to bring the metal to the level with the rolls. Such type of arrangement is
used for making plates or sections.

(3) Four-high rolling mill: It is generally a two-high rolling mill, but with small sized rolls. The
other two rolls are the backup rolls for providing the necessary rigidity to the small rolls. It is used
for both hot and cold rolling of wide plates and sheets.

(4) Cluster rolling mill: It uses backup rolls to support the smaller work rolls. In this type of mill,
the roll in contact with the work can be as small as 1/4 in. in diameter. Foil is always rolled on
cluster mills since the small thickness requires small-diameter rolls.

Applications of Rolling

Rolling is used to produce components having constant cross-section throughout its length. The
whole range of rolled products can be divided into the following types:
(a) Structural shapes or sections: This includes sections like round, square, hexagonal
bars, channels, H and I beams and special sections like rail section.
(b) Plates and sheets: These are produced of varying thickness
(c) Special purpose rolled products: These include rings, balls, wheels and ribbed tubes.
 Two types of thread –rolling processes
(a) Dual –roller dies (b) Reciprocating flat dies

Defects in Rolling

Zipper cracks are usually caused by low ductility.

Barreling is caused by interfaces, which records the free flow of the material.

Edge cracks are occurs in plates and slabs because of either limited ductility of metal or uneven
deformation especially at the edges.

Alligatoring is a complex phenomenon that results from inhomogeneous deformation of the

material during rolling or from defects in the original cast ingot, such as piping. The work piece
splits along a horizontal plane on exit from the rolls.
 ROD, WIRE AND TUBE DRAWING

To manufacture long slender products (wire, tube), material is drawn through a die. The material is
deformed by compression, but the deformation force is supplied by pulling on the deformed end of
the wire or rod. This is termed ‘indirect compression’. Most drawing is done cold. Wire drawing is
an operation to produce wire of various sizes within certain specific tolerances. The process
involves reducing the diameter of rods or wires by passing them through a series of wire drawing
dies with each successive die having smaller bore diameter than the one preceding it. The drawing
force must not exceed the strength of the drawn wire. Typically this means that the maximum
reduction (as area, not diameter) attainable is less than 50%. In practice reduction is usually limited
to 20-30% to avoid frequent breakage. The final wire size is reached as the wire passes through the
last die in the series.

Schematic diagram of drawing process

Rod drawing

The principle behind in drawing of bar rod and wire are one and the same, though the equipments
used are different for different sized products. Rods and tubes cannot be produced on drawing
benches. A long draw bench is used drawing rod in straight lengths. It consists of a die, a gripper
and lever for pulling the rod and a chain used to transmit the power to drive the cold drawing
trolley. Before the rod is drawn, its surfaces must be cleaned. The Non-ferrous alloys, the surface
resulting from the previous hot rolling or extrusion processes is adequate in this respect. Pointing
rod is required for easy insertion and holding in the gripper jaws.

Draw Bench
 Wire drawing

To manufacture long slender products (wire, tube), material is drawn through a die. The material is
deformed by compression, but the deformation force is supplied by pulling on the deformed end of
the wire or rod. This is termed ‘indirect compression’. Most drawing is done cold. Wire drawing is
an operation to produce wire of various sizes within certain specific tolerances. The process
involves reducing the diameter of rods or wires by passing them through a series of wire drawing
dies with each successive die having smaller bore diameter than the one preceding it. The drawing
force must not exceed the strength of the drawn wire. Typically this means that the maximum
reduction (as area, not diameter) attainable is less than 50%. In practice reduction is usually limited
to 20-30% to avoid frequent breakage. The final wire size is reached as the wire passes through the
last die in the series.

Wire Drawing

Wire Drawing Die

Defects in Wire Drawing

Defects occur in wire drawing because of ploughing by hard particles and local breakdown of the
lubricating film. Some common defects are:
1. Bulge formation: This occurs in front of the die due to low reduction and high die angle.
2. Internal cracks (Centre burst or centre-cracking): The tendency of cracking increases with
increasing die angle, with decreasing reduction per pass, with friction and with the presence of
inclusions in the material.
3. Seams: These appear as longitudinal scratches or folds in the material. Such defects can open up
during subsequent forming operations by upsetting, heading, thread rolling or bending of the rod or
wire.
4. Surface defects: Various types of surface defects can also result due to improper selection of
process parameters and lubrication.

Tube drawing

Tube drawing normally makes tubes to size from hollow ‘tube shells’ produced by extrusion. They
are then cold drawn to size by a succession of passes, with inter stage anneals as required and
supplied neither in straight lengths or coil. shows a typical tube drawing process with a floating
mandrel.

Tube Drawing

The common methods of tube drawing are: Tube sinking, Tube drawing with a plug or stationary
mandrel and Tube drawing with a moving mandrel.

1. Tube sinking: This method is generally not preferred since no support is provided on the
inner surface of the tube and as a result wall thickness may slightly increase.
Tube drawing with a plug: In this method tubes of greater dimensional accuracy are obtained
because of the proper support provided both at the inner and outer surfaces of the tube. The plug
used may be of cylindrical or conical shape and is of either fixed or floating type. In a fixed plug,
friction is more as compared to a floating plug. For the same reduction in area, the drawing load
will be less with floating plug than with a fixed plug.

Tube drawing with a moving mandrel: This method is similar to that of a plug drawing except
the difference that in this case a movable mandrel is used. Because of the movable mandrel, friction
is minimized but the mandrel has to be removed by rolling, hence there is a slight increase in the
diameter of tube. This results in reduction of dimensional tolerances.

Defects

Internal defects in the rod and wire include cracks due to seam or pipe in the hot rolled starting
material and a defect known as cupping. Cupping is the rupturing of the centre of the wire when it
is subjected to tensile force is identified by necking during drawing or by cup and cone type
fracture when wire is broken.
Surface discoloration and ground in oxide result from improper cleaning of hot rolled bar and the
rod.
 EXTRUSION

Extrusion differs from drawing in that the metal is pushed, rather than pulled under tension.
Extrusion processes can be carried on hot or cold.

Cold extrusion: Cold extrusion is the process done at room temperature or slightly elevated
temperatures. This process can be used for most materials subject to designing robust enough
tooling that can withstand the stresses created by extrusion. Cold extrusion can be used with any
material that possesses adequate cold work ability–e.g., lead, tin, aluminum alloys, copper,
titanium, molybdenum, vanadium, steel. Typical parts which are cold extruded are collapsible
tubes, aluminum cans, cylinders, gear blanks.

The advantages of cold extrusion are:

1. No oxidation takes place.
2. Good mechanical properties due to severe cold working as long as the temperature created are
below the recrystallization temperature.
3. Good surface finish with the use of proper lubricants.

Hot extrusion: Hot extrusion is basically a hot working process. It is done at fairly high
temperatures, approximately 50 to 75% of the melting point of the metal. The pressures can range
from 35-700 MPa. Due to the high temperatures and pressures and its detrimental effect on the die
life as well as other components, good lubrication is necessary.

The principal variables, which influence the force required to cause extrusion, are:
(1) The type of extrusion
(2) The extrusion ratio
(3) The working temperature
(4) The speed of deformation, and
(5) The frictional conditions at the die and container wall.

Typical parts produced by hot extrusion are trim parts used in automotive and construction
applications, window frame members, railings, aircraft structural parts.

Types of Extrusion

Extrusion process is classified as

1. Direct or forward extrusion

2. Indirect or backward extrusion
3.Other extrusion processes
 Tube extrusion
 Hydrostatic extrusion
 Impact extrusion
 Cold extrusion forging
 Side extrusion
 Typical cross-section in extrusion

Direct or Forward Extrusion

In this method, the heated metal billet is placed in to the die chamber and the pressure is applied
through ram. The metal is extruded through die opening in the forward direction,
i.e. the same as that of the ram. In forward extrusion, the problem of friction is prevalent because of
the relative motion between the heated metal billet and the cylinder walls. To reduce such friction,
lubricants are to be commonly used. At lower temperatures, a mixture of oil and graphite is
generally used. The problem of lubrication gets compounded at the higher operating temperatures.
Molten glass is generally used for extruding steels.
 Direct or Forward Extrusion

Indirect or Backward Hot Extrusion

In indirect extrusion, the billet remains stationary while the die moves into the billet by the hollow
ram (or punch), through which the backward extrusion takes place. Since, there is no friction force
between the billet and the container wall, therefore, less force is required by this method. However
this process is not widely used because of the difficulty occurred in providing support for the
extruded part.

Backward Extrusion

Tube extrusion

This process is an extension of direct extrusion process where additional mandrel is needed to
restrict flow of metal for production of seamless tubes. Alumminium based toothpaste and
medicated tubes are produced using this process.
 Hollow products extruded with (a) Fixed (b) Piercing mandrels and
(c) Bridge or spider type dies

Hydrostatic extrusion

In this process, the chamber is filled with a fluid that transmits the pressure to the billet, which is
then extruded through the die. There is no friction along the walls of the container. Because the
billet is subjected to uniform hydrostatic pressure, it does not upset to fill the bore of the container
as it would in conventional extrusion. This means that the billet may have a large length to diameter
ratio (even coils of wires can be extruded) or it may have an irregular cross section. Because of the
pressurized fluid, lubrication is very effective, and the extruded product has good surface finish and
dimensional accuracy. Sin e friction is nearly absent, it is possible to use dies with very low semi-
cone angle which greatly minimizes the redundant deformation. The only limitation with this
process is the practical limit of fluid pressure that may be used because of the constraint involving
the strength of the container and the requirement that the fluid does not solidify at high pressure.

Hydrostatic extrusion

Impact Extrusion

It is a form of indirect extrusion and is particularly suitable for hollow shapes. It is usually
performed on a high-speed mechanical press. The punch descends at a high speed and strikes the
blank, extruding it upwards.. The thickness of the extruded tubular section is a function of the
clearance between the punch and the die cavity. Although the process is
performed cold, considerable heating results from the high-speed deformation. Impact extrusion is
restricted to softer metals such as lead, tin, aluminum and copper.

Impact Extrusion

Extrusion defects

Piping

Piping is due to surface materials drawn into the centre as shown in figure. Severe surface
material’s serration, called the fir-tree defect, results from momentary sticking of extrusion in the
die area.

Shape Error

Shape error occurs because of the different extrusion speeds at different points of the vertical
section of the componenet.It can be avoided by providing uniform speed throughout the vertical
section.
Internal Cracking

It is due to improper die angle, extrusion ratio and friction.Chervon cracking occurs due to
hydrostatic tension(outer layer in compression and inner layer in tension, if entire part is not
plastic)It can be eliminated by reducing the friction

Advantages of Extrusion

 The tooling cost is low, as well as the cost due to material

 Intricate cross sectional shapes, hollow shapes and shapes with undercuts can be produced.
 The hardness and the yield strength of the material are increased.
 In most applications, no further machining.

Limitations of Extrusion

 High tolerances are difficult to achieve.

 The process is limited to ductile materials.
 Extruded products might suffer from surface cracking. It might occur when the surface
 Temperature rise significantly due to high extrusion temperature, friction, or extrusion
speed.
 SHEET METAL OPERATIONS

Sheet metal work is generally regarded as the working of metal from 16 gauge to 30 gauge, with hand
tools and simple machines into various forms of cutting, forming into shapes and joining. In most of the
cases the manufacturing of sheet metal components is a cold working process because, the metal, when
heated, has a lower resistance to deformation.

Common examples of sheet metal applications are canisters, guards, covers, hoppers, pipes, hoods, boxes,
etc. Parts made of sheet metal have many attractive qualities such as good accuracy of dimensions,
adequate strength, light weight and a broad range of possible dimensions. Knowledge of geometry,
mensuration, and properties of metal is most essential since nearly all patterns come from the
development of the surfaces of a number of geometrical models such as cylinder, prism, cones and
pyramids.

Deformation in sheet metal occurs mainly due to tension. During a sheet metal operation, plastic flow of
metal to finished size takes place. Thus a sheet metal should posses the following characteristics.

1. Plasticity
2. Malleability
3. Good bending properties
4. Stretchability
5. Good shearing characteristics
6. Formability

Materials used for sheet metal working:

Aluminum, zinc, cooper, steel and lead

Coated Materials and their applications:

1. Tin plate : Used on Steel food cans-tin is non-toxic and corrosion resistant
2. Galvanized sheet metal : Zinc gives excellent corrosion resistance
3. Terme Plate : Lead coating is used on steel to prevent corrosion of petrol tanks.
4. Aluminium : Used on steel exhaust pipes to reduce corrosion
5. Paint and Polymers : Used on many metals-part decorative and part corrosion resistance.
TYPICAL SHEARING OPERATIONS

Cutting Operations

Shearing is the mechanical cutting of materials without the formation of chips or the
use of burning or melting. The process involves cutting of flat material forms such as
sheets and plates using two cutting blades.

The group of sheet metal forming processes that involve cutting or shearing the sheet
metal by subjecting it to shear stress between punch and die, or between the blades of
a shear could be categorized into many processes such as punching, piercing, slitting,
etc.
Operations that make up press work are varied, but ate broadly classified as shearing,
bending and drawing. Press working tools are called punches and dies. The punch of
the assembly is attached to the ram of the press and is forced into die cavity. The die
has the opening to receive the punch.
Die I used in shearing typically have small clearances between the punch (moving part)
and the die (non- moving backing). If the gap is too great, the parts will have rough edges
and excess shear force will be required. Clearances that are too small lead to pre-mature
wear.

Shearing

Shearing is a general description

for most sheet metal cutting, but in a
specific case, it is cut along a straight line
completely across a strip, sheet or bar. As
a punch descends on the metal, the
pressure first causes a plastic deformation
to take place. The metal is highly stressed
adjacent to punch and a die edges, and
fractures start on both sides of the sheet
as the deformation continues. When the
ultimate tensile strength of the material is
reached, the fracture progresses and if the
clearance is correct and both edges of
equal sharpness, the fractures meet at the
centre of the sheet.

Mechanics of Shearing
 Phase 1: Due to the action of the cutting force F, the stress on the material is lower
than the yield stress. This phase is that of elastic deformation. In order to prevent the
movement of material during the cutting operation, the material is held by the
material holder at force Fd.

Phase 2: The stress on the material is higher than the yield stress but lower than the
Ultimate Tensile Stress. The phase is that of plastic deformation.\

Phase 3: In this phase, the stress on the material is equal to the shearing stress. The
material begins to part not at the beginning edge but at the appearance of the first
crack or breakage in the material. Fracture of the material occurs in this phase. As the
applied exceeds the shear strength, the material tears or ruptures through the
remainder of the thickness.

Because of the normal non-homogeneities in a metal and the possibility of non-

uniform clearance between the shear blades, the final shearing does not occur in a
uniform manner. Fracture and tearing begins at the weakest point and proceeds
progressively and intermittently to the next weakest location. The results are usually
rough and ragged edge.

Proper clearance between punch an die, or the shearing blades would result in
sufficiently smooth edge condition which may be used without further finishing
operation.

Blanking and Punching

Cuts an entire piece from sheet metal, but there is stock entirely around the contour
of the part in the work piece sheet metal. The usable part is called the blank. The
remaining is part is often called the skeleton or the waste. If the operation is to cut a
hole, and the material in the hole is a waste, then the operation is called piercing or
punching.The basic tool of blanking and punching are die and punch.
 There are three phases of this process.

1. Elastic 2. Plastic 3. Fracture

Blanking and Punching

Cuts an entire piece from sheet metal, but there is stock entirely around the contour
of the part in the work piece sheet metal. The usable part is called the blank. The
remaining is part is often called the skeleton or the waste. If the operation is to cut a
hole, and the material in the hole is a waste, then the operation is called piercing or
punching.The basic tool of blanking and punching are die and punch.

Other Sheet metal cutting operations:

Shaving: It is a finishing or sizing

operation during which a slight amount of
material (about 10%) of the material
thickness is removed or shaved from the
blanked or pierced edges in order to obtain
edges which are smooth, square and within
closer dimensional tolerances.
Slitting: Slitting is the length wise cutting
of coil or sheet stock into narrower widths.
Slitting is a shearing operation used to cut
wide coils of materials into several coils of
narrow width as the material passes
lengthwise through circular blades.

Lancing: Lancing makes a cut part way

through a blank. Lancing is a combined
shearing and bending operation where a
portion of the periphery of a hole is cut into
the workpiece and the remainder is bent to
the desired shape. No material is removed
from the workpiece.

Perforating: Perforating is the cutting of a

small and evenly spaced grouping of holes.
It is a punching process in which a desired
pattern of holes is cut into the workpiece by
means of multiple punches and dies.

Nibbling: Nibbling is a shearing process

that utilizes a series of overlapping cuts to
make complex shapes from sheet metal. In
nibbling, a machine called a nibbler moves
a straight punch up and down rapidly into
a die.

Notching: It is a cutting operation from the

side of a strip or sheet or blank. It is a
shearing operation by which metal scrap is
removed from the outside edge of a
workpiece by multiple shear blades set at
right angles to each other.

Trimming: It is a cutting operation

performed on a formed part to remove
excess metal and establish size. The term
has the same basic meaning as in forging.
Removing the top portion of a vessel.
Slotting: Punching operation that cuts out
an elongated or rectangular hole.

BENDING

Bending is the metal working process by which a straight length is transformed into a
curved length. It is a very common forming process for changing sheet and plate into
channels, drums, tanks etc. During the bending operation, the outer surface of the
material is in tension and the inside surface is in compression.. The strain in the bent
materials increases with decreasing radius of curvature. The stretching of the bend
causes the neutral axis of the section to move towards the inner surface. In most
cases the distance of the neutral axis is 03 t to 0.5 t where ‘t’ is the thickness of the
part.

Types of Bending Operations:

i. Supported Bending: Any bending where a pad usually spring loaded, is

included as support for the formed part. Ex: Finishing stage of U and V
bending
ii. Unsupported Bending: Similar to the process of stretching, where a flat
piece of metal, retained in die, stretches along with the application of
tool pressure. Ex: Both U and V die bending are considered as
unsupported bending processes at their beginning stages.

V-Die Bending:

During V-die bending, the punch slides down, coming

first to a contact with the unsupported sheet metal. By
progressing further down, it forces the material to follow
along, until finally bottoming on the V-shape of the die.

At the beginning, the process is unsupported, but as the

operational cycle nears its end, the bent up part becomes
totally supported while retained within the space
between the punch and die. Even though the most
inaccurate of all bending processes, it is widely used
because the tooling is simple and may be used for more
than one flange and for more than one part.

U-Die Bending

In this process, the process begins with a sheet metal

positioned over a U- shaped opening or an insert of such
a shape. As the punch progresses, it contact the sheet
material first and pulls it along. On further progress,
forcing it into a U-shaped opening.

U-Die Bending with a Spring Pad

A spring loaded pressure pad is added to the U-Die

bending. The blank, when pulled by the punch into the
die opening, is firmly supported by the pressure pad
already at the beginning of the forming operation. In
this way, the punch cannot stretch the material ,
leaving the bottom of the part flat. When the punch
metal pad sandwich finally bottoms, the formed part
remains the same, with no bulging or distortion of any
kind.

DRAWING

Drawing operation is the process of formation of flat pieces of material into hollow
shape by means of a punch which causes the blank to flow into a die-cavity. The
depth of the draw may be shallow, moderate or deep. If the depth of the formed cup
is upto half of its diameter, the process is called ‘shallow drawing’. If the depth of
the formed cup exceeds the diameter, it is termed as ‘Deep Drawing’. Parts of various
geometries and sizes are made by drawing operation, two extreme examples being
bottle caps and automobile panels.

As the drawing progresses, as the punch forces the blank into the die cavity, the
blank diameter decreases and causes the blank to become thicker at its outer portions.
This is due to the circumferential compressive stresses to which the material element
in the outer portions is subjected. If the stress becomes excessive, the outer portions
of the blank (flange) will have the tendency to buckle or wrinkle. To avoid this,
pressure pads or blank holder is provided. The holding down pressure is obtained by
means of springs, rubber pad, compressed air cylinder or the auxiliary ram on a
double action press.

The portion of the blank between the die wall and the punch surface is subjected to
nearly pure tension and tends to stretch and become thinner. The portion of the
formed cup, which wraps around the punch radius is under tension in the presence of
bending. This part becomes the thinnest portion of the cup. This action is termed as
‘necking’ and in the presence of unsatisfactory drawing operation, is usually the first
place to fracture.

There are two types of deep drawing processes

i. Pure Drawing ii. Ironing

Pure Drawing

Pure drawing is the process done

without reducing the thickness of the
workpiece material. It is a cold
forming process in which a flat blank
of sheet metal is shaped by the action
of a punch, forcing the metal into a
die cavity.

An important aspect of drawing is determining the extent of stretching and the extent
of pure drawing is taking place. Either with a high blank holder force or with the use
of draw beads, the blank can be prevented from flowing freely into the die cavity.
The deformation of the sheet metal takes place mainly under the punch, and the sheet
begins to stretch, eventually resulting in necking an tearing.

Significant independent variables in deep drawing are:

1. Properties of the sheet metal

2. The ratio of blank diameter to the punch diameter
3. The thickness of the sheet
4. The clearance between the punch and the die
5. The corner radii of the punch and the die
6. The blank holder force
7. Friction and lubrication at the punch, die and workpiece interfaces
8. The speed of the punch.

Ironing

Deep drawing with a reduction in thickness

of the workpiece material is called ironing.
If the thickness of the sheet as it enters of
the die cavity is more than the clearance
between the punch and die, the thickness
will have to be reduced. This effect is
known as ironing. Ironing produces a cup
with constant wall thickness. The smaller
the clearance, the greater the amount of
ironing. The ironed cup will be longer than
the cup produced with a larger clearance.

STRETCH FORMING

Stretch forming is a very accurate and precise method for forming metal shapes,
 economically. The level of precision is so high that even intricate multi-components
and snap-together curtain wall components can be formed without loss of section
properties or original design section.

Stretch forming capabilities include portions of circles, ellipses, parabolas and arched
shapes. These shapes can be formed with straight leg sections at one or both ends of
the curve. This eliminates several conventional fabrication steps and welding.
Like rubber-press forming, stretch forming uses only the male die or form block.
Sheet metal is stretched to the yield point in tension, and then wrapped over and
around the form block.

This process involves stretch

forming a metal piece over a male
stretch form block (STFB) using a
pneumatic or hydraulic stretch
press. Stretch forming is widely
used in producing automotive body
panels. Unlike deep drawing, the
sheet is gripped by a blank holder
to prevent it from being drawn into
the die. It is important that the
sheet can deform by elongation and
uniform thinning.

The variety of shapes and cross-

sections that can be stretch formed
are almost unlimited. Window
systems, skylights, store fronts,
signs, flashings, curtain walls,
walkway enclosures and hand
railings can be accurately and
precisely formed to the desired
profiles.

Close and consistent tolerances, no surface marring, no distortion and no surface

misalignment of complex profiles are important benefits in stretch forming.

Four methods of stretch forming are:

1. Stretch draw forming

2. Stretch wrapping
3. Compression forming
Features:-
1. Large parts with shallow contours
2. Suitable for low-quantity production
3. High labour costs and equipment costs
4. High tooling
Advantages :
1. Compared to other forming methods spring-back is either greatly
reduced or completely eliminated, since direct bending stresses are never
introduced. All of the plastic deformation is tensile extension in the
direction of pulling.
2. Low tooling cost as male dies or form blocks may be of wood, masonite, zinc alloys or
cast iron.
METAL SPINNING

Spinning is one of the oldest

methods of sheet metal
forming. Parts that have
circular cross section can be
made by spinning from sheet
metal. The method involves
the forming of a work piece
over a rotating form block or
chuck held in a special lathe.
A smooth, hardened, rotating
or stationary tool is held by
the operator and is pressed
against the blank to
progressively bend the work-
piece to conform to the chuck
or mandrel. The mode of
deformation of the metal
during spinning is bending
and stretching, making
the process most suitable for shaping of hollow parts from ductile metals and alloys.
The thickness of the spun part is nearly the same as the thickness of the undeformed
blank. The thickness of the blank is around 6mm for soft non-ferrous metals and upto
5mm for low carbon steels.

Spinning speeds vary from 1.5m/s for small parts to 25m/sec for large-diameter parts.
Spinning has been used to produce parts more than 3.6m in diameter. Before
spinning, a suitable lubricant should be applied to the surface of the metal. Soap,
beeswax, white lead and linseed oil are commonly used.

Applications: Funnels,
Reflectors, Kitchenware,
Bells, light fixtures, kettles,
radar dishes, rocket motor
cases and musical
instruments.

Advantages

1. Low equipment cost

2. Low tool cost
3. Some
complex parts
like kettles,
pitchers can
be made
economically
Drawbacks

1. Depends to large extent on skill of operator

2. Finished products not uniform and close tolerances cannot be obtained.
Comparison of Spinning and Drawing

1. Due to low tooling and equipment cost, spinning is normally used for low volume
production
2. Drawing is used for mass production
3. Labor costs are higher for manual spinning and production rates are lower.
4. For complex shapes and big sizes of components, spinning becomes more competitive.