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Manufacturing Engineering Technology Sheet-Metal Forming Processes and Equipment


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Title: Manufacturing Engineering Technology Sheet-Metal Forming Processes and Equipment

Manufacturing Engineering Technology Sheet-Metal
Forming Processes and Equipment
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Manufacturing Engineering Technology????
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Chapter Outline
  1. Introduction
  2. Shearing
  3. Sheet-metal Characteristics and Formability
  4. Formability Tests for Sheet Metals
  5. Bending Sheets, Plates, and Tubes
  6. Miscellaneous Bending and Related Operations
  7. Deep Drawing
  8. Rubber Forming and Hydroforming
  9. Spinning
  10. Superplastic Forming
  11. Specialized Forming Processes
  12. Manufacturing of Metal Honeycomb Structures
  13. Design Considerations in Sheet-metal Forming
  14. Equipment for Sheetmetal Forming
  15. Economics of Sheetforming Operations

  • Products made of sheet metals are common
  • Pressworking or press forming is used for general
    sheet-forming operations, as they are performed
    on presses using a set of dies

  • A sheet-metal part produced in presses is called
    a stamping
  • Low-carbon steel has low cost and good strength
    and formability characteristics
  • Manufacturing processes involving sheet metal are
    performed at room temperature

Sheet Metalworking Defined
  • Cutting and forming operations performed on
    relatively thin sheets of metal
  • Thickness of sheet metal 0.4 mm (1/64 in) to 6
    mm (1/4 in)
  • Thickness of plate stock gt 6 mm
  • Operations usually performed as cold working

Sheet and Plate Metal Products
  • Sheet and plate metal parts for consumer and
    industrial products such as
  • ?Automobiles and trucks
  • ?Airplanes
  • ?Railway cars and locomotives
  • ?Farm and construction equipment
  • ?Small and large appliances
  • ?Office furniture
  • ?Computers and office equipment

Advantages of Sheet Metal Parts
  • High strength
  • Good dimensional accuracy
  • Good surface finish
  • Relatively low cost
  • Economical mass production for large quantities

Sheet Metalworking Terminology
  • Punch-and-Die - tooling to perform cutting,
    bending, and drawing
  • Stamping press - machine tool that performs most
    sheet metal operations
  • Stampings - sheet metal products

Three Basic Types of Sheet Metal Processes
  • 1.Cutting (Shearing)
  • Shearing to separate large sheets
  • Blanking to cut part perimeters out of sheet
  • Punching to make holes in sheet metal
  • 2.Bending
  • Straining sheet around a straight axis
  • 3.Drawing
  • Forming of sheet into convex or concave shapes

Sheet Metal Cutting (Shearing)
  • (1) Just before punch contacts work
  • (2) punch pushes into work, causing plastic
    deformation (3) punch penetrates into work
    causing a smooth cut surface and
  • (4) fracture is initiated at opposing cutting
    edges to separate the sheet

  • Before a sheet-metal part is made, a blank is
    removed from a large sheet by shearing
  • The edges are not smooth and perpendicular to the
    plane of the sheet

Punch and Die Sizes
  • Die size determines blank size Db
  • Punch size determines hole size Dh
  • c clearance

  • Processing parameters in shearing are
  • The shape of the punch and die
  • The speed of punching
  • Lubrication
  • The clearance, c, between the punch and the die
  • When clearance increases, the zone of deformation
    becomes larger and the sheared edge becomes
  • Extent of the deformation zone depends on the
    punch speed
  • Height, shape, and size of the burr affect
    forming operations

Clearance in Sheet Metal Cutting
  • Distance between punch cutting edge and die
    cutting edge
  • Typical values range between 4 and 8 of stock
  • If too small, fracture lines pass each other,
    causing double burnishing and larger force
  • If too large, metal is pinched between cutting
    edges and excessive burr results

Clearance in Sheet Metal Cutting
  • Recommended clearance is calculated by
  • c at
  • where c clearance
  • a allowance
  • and t stock thickness
  • Allowance a is determined according to type of

Sheet Metal Groups Allowances
  • Metal group
  • 1100S and 5052S aluminum alloys, all tempers
  • 2024ST and 6061ST aluminum alloys brass,
  • soft cold rolled steel, soft stainless steel
  • Cold rolled steel, half hard stainless steel,
  • half hard and full hard

Shearing Clearance
  • Clearance control determine quality of its
  • edges which influence formability of the
    sheared part
  • Appropriate clearance depends on
  • 1. Type of material and temper
  • 2. Thickness and size of the blank
  • 3. Proximity to the edges of other sheared edges
  • When sheared edge is rough it can be subjected to
  • process called shaving

Shearing Characteristics and Type of Shearing
  • Punch and Die Shape
  • Punch force increases rapidly during shearing
  • Location of sheared regions can be controlled by
    beveling the punch and die surfaces

  • Punch Force
  • Maximum punch force, F, can be estimated from
  • Friction between the punch and the workpiece can
    increase punch force

T sheet thicknessL total length sheared UTS
ultimate tensile strength of the material
  • EXAMPLE 16.1
  • Calculation of Punch Force
  • Estimate the force required for punching a 25-mm
    diameter hole through a 3.2-mm thick annealed
    titanium- alloy Ti-6Al-4V sheet at room
  • Solution
  • UTS for this alloy is 1000 MPa, thus

Blanking and Punching
  • Blanking - sheet metal cutting to separate piece
    (called a blank) from surrounding stock
  • Punching - similar to blanking except cut piece
    is scrap, called a slug

Shearing Shearing Operations
  • Punching is where the sheared slug is scrap
  • Blanking is where the slug is the part to be used
    and the rest is scrap
  • Die Cutting
  • Shearing operation consists of
  • Perforating punching holes in a sheet
  • Parting shearing sheet into pieces
  • Notching removing pieces from the edges
  • Lancing leaving a tab without removing any

Shearing Fine Blanking
  • smaller clearance used
  • pressure pad w/ v-shaped stringer locks the sheet
  • movement of punch, pressure pad, cushion are
    controlled by triple-action hydraulic presses

Shearing Operations Slitting
  • Shearing operations are through a pair of
    circular blades, follow either a straight line, a
    circular path, or a curved path

Shearing Operations Nibbling
  • A contour is cut by a series of overlapping slits
    or notches
  • Simple tools can be used to produce complex shape

Shearing Tailor-welded Blanks
  • Laser-beam butt welding involves two or more
    pieces of sheet metal with different shapes and
  • The strips are welded to obtain a locally thicker
    sheet and then coiled
  • The welded assembly is then formed into a final
  • Resulting in
  • Reduction in scrap
  • Elimination of the need for subsequent spot
  • Better control of dimensions
  • Improved productivity

Shearing Tailor-welded Blanks
  • EXAMPLE 16.2
  • Tailor-welded Sheet Metal for Automotive
  • Production of an outer side panel of a car body
    is by laser butt welding and stamping

Shearing Tailor-welded Blanks
  • EXAMPLE 16.2
  • Tailor-welded Sheet Metal for Automotive
  • Some of the examples of laser butt-welded and
    stamped automotive-body components.

Characteristics and Type of Shearing Dies
  • Transfer Dies
  • Sheet metal undergoes different operations
    arranged along a straight line or a circular path
  • Tool and Die Materials
  • Tool and die materials for shearing are tool
    steels and carbides
  • Lubrication is needed for reducing tool and die
    wear, and improving edge quality

Characteristics and Type of Shearing Dies
  • Progressive Dies
  • a different operation is performed at the same
    station with each stroke of a series of punches

Characteristics and Type of Shearing Dies
  • For high product production rates
  • The part shown below is the small round piece
    that supports the plastic tip in spray cans

Characteristics and Type of Shearing Dies
  • Compound Dies
  • Operations on the same sheet may be performed in
    one stroke with a compound die
  • Limited to simple shapes due to
  • Process is slow
  • Complex dies is more expensive

Shearing Miscellaneous Methods of Cutting Sheet
  • Other methods of cutting sheets
  • Laser-beam cutting
  • Water-jet cutting
  • Cutting with a band saw
  • Friction sawing
  • Flame cutting

Sheet-metal Characteristics and Formability
  • Yield-point elongation having both upper and
    lower yield points.
  • This behaviour results in Luders bands
  • Typically observed with mild-steel sheets
  • Results in depressions on the sheet surface
  • Can be eliminated by temple rolling (but sheet
    must be formed within a certain time after

Sheet-metal Characteristics and Formability
  • Grain Size
  • Affects mechanical properties and surface
  • Smaller the grain size, stronger is the metal
  • Dent Resistance of Sheet Metals
  • Dents caused by dynamic forces from moving
    objects that hit the sheet metal
  • Dynamic yield stress, instead of static yield
    stress, should be the significant strength

Formability Tests for Sheet Metals
  • Sheet-metal formability is the ability of the
    sheet metal to undergo the desired shape change
    without failure
  • Sheet metals may undergo 2 basic modes of
    deformation (1) stretching and (2) drawing
  • Cupping Tests
  • In the Erichsen test, the sheet specimen is
    clamped and round punch is forced into the sheet
    until a crack appears
  • The punch depth is a measure of formability of
    the sheet
  • Easy to perform, but does not simulate exact
    conditions of actual forming, and not reliable
    for complex parts.

Formability Tests for Sheet Metals
  • Forming-limit Diagrams
  • To develop a forming-limit diagram, the major and
    minor engineering strains are obtained
  • Major axis of the ellipse represents the major
    direction and magnitude of stretching
  • Major strain is the engineering strain and is
    always positive
  • Minor strain can be positive or negative
  • Curves represent the boundaries between failure
    and safe zones

Formability Tests for Sheet Metals
  • Forming-limit Diagrams
  • Forming-limit diagrams is to determine the
    formability of sheet metals

Forming-Limit Diagram (FLD)
  • Blank stretched over a punch, deformation
    observed and measured in the region where failure
    has occurred
  • The curves represent the boundaries between
    failure and safe zones
  • Different mat'ls have different FLDs, and the
    higher the curve, the better is the formability
  • Compressive minor strain is associated with a
    higher major strain than a tensile minor strain
    of the same magnitude
  • The effect of sheet-metal thickness is to raise
    the curves
  • Friction, lubrication at punch/sheet-metal
    interface, and surface scratches, are important

Bending Sheets
, Plates, and Tubes
  • Bending is a common industrial forming operation
  • Bending imparts stiffness to the part by
    increasing its moment of inertia
  • Outer fibers are in tension, while the inner in
  • Poisson effect cause the width to be smaller in
    the outer region and larger in the inner region

Bending Sheets, Plates, and Tubes
  • Approximate bend allowance is
  • For ideal case, k 0.5,
  • Minimum Bend Radius
  • Engineering strain during bending is
  • Minimum bend radius, R, is

Bending Sheets, Plates, and Tubes
  • Minimum Bend Radius
  • Increase the bendability by increase their
    tensile reduction of area
  • Bendability also depends on the edge condition of
    the sheet
  • Improve resistance to edge cracking by removing
    the cold-worked regions
  • Cold rolling results in anisotropy by preferred
    orientation or mechanical fibering

Bending Sheets, Plates, and Tubes
  • Springback
  • Plastic deformation is followed by elastic
    recovery when the load is removed, called
  • Springback can be calculated by

Bending Sheets, Plates, and Tubes
  • Compensation for Springback
  • Springback is compensated for by overbending the
  • One method is stretch bending where the part is
    subjected to tension while being bent
  • Bending Force
  • Excluding friction, the maximum bending force, P,
  • For a V-die, it is modified to

Miscellaneous Bending Operations
  • Examples of various bending operations
  • Roll Bending
  • Plates are bent using a set of rolls.
  • Curvatures can be obtained by adjusting the
    distance between the three rolls

V-Bending and Edge Bending
  • V-Bending
  • ?Low production
  • ?Performed on a press brake
  • ?V-dies are simple and inexpensive
  • Edge-Bending
  • ?High production
  • ?Pressure pad required
  • ?Dies are more complicated and costly

Miscellaneous Bending and Related Operations
  • Beading
  • Periphery of the sheet metal is bent into the
    cavity of a die
  • The bead imparts stiffness to the part by
    increasing the moment of inertia of that section

Miscellaneous Bending and Related Operations
  • Flanging
  • In shrink flanging, the flange is subjected to
    compressive hoop stresses and cause the flange
    periphery to wrinkle

Bending Operations
  • most common forming operation
  • paper clip, file cabinet etc

Press Brakes Bending Equipment
  • Sheet metal or plate can be bent easily with
    simple fixtures using a press
  • complex bends
  • with long narrow bed and short adjustable strokes
  • metal bent between interchangeable dies

Roll Forming
  • Also called contour-roll forming or cold-roll
  • Used for forming continuous lengths of sheet
    metal and for large production runs
  • Dimensional tolerances, springback, tearing and
    buckling of the strip have to be considered

Bending of Tube Stock
  • Stretch bending of tube (1) start of process and
    (2) during bending

Tube Bending
  • Large dia. thin wall, with small bend radius tend
    to cause wrinkle at inner side of the tube
  • Oldest method of bending a tube is to first pack
    its inside with loose particles and then bend it
    into a suitable fixture
  • Thick tube can be formed to a large bend radius
    without the use of fillers or plugs

Tube Forming
  • tubular part placed in a split-female die and
    expanded with a polyurethane or rubber plug
  • punch retracted plug returns to its original
    shape and removed by knockout rod
  • finished part removed by opening the split die
    (water pitcher)
  • production of fitting for plumbing

Manufacturing of Bellows
  • (a) Bulged tube
  • Tube bulged at several equidistant locations
  • (b) Compressed tube
  • Bulged tube compressed axially to collapse bulged
    regions, thus forming bellows

Stretch Forming
  • Sheet metal is gripped by two sets of jaws
  • The jaws stretch the metal sheet and wrap it
    around a form block (die)
  • Most of deformation is induced by tensile
    stretching, and forces on the form block are far
  • Very little springback results
  • Form block often made of wood or low-melting
  • Produce large parts in low or limited quantity

Stretch Forming
  • Sheet metal is clamped along its edges and then
    stretched over a male die
  • Die moves upward, downward, or sideways
  • Used to make aircraft wing-skin panels,
    fuselages, and boat hulls

Deep Drawing
  • Sheet metal forming to make cup-shaped,
    box-shaped, or other complex-curved,
    hollow-shaped parts
  • Sheet metal blank is positioned over die cavity
    and then punch pushes metal into opening
  • Products beverage cans, ammunition shells,
    automobile body panels
  • Also known as deep drawing (to distinguish it
    from wire and bar drawing)

Deep Drawing
  • - a punch forces a flat sheet metal into a die
  • - depth greater than diameter

Deep Drawing
  • Material beneath punch remains unaffected and
    becomes cup bottom
  • Cup wall is formed by pulling the remainder of
    disk inward the radius of die
  • - Hoop stress tends to cause buckling or
  • ? - Pressure ring is used to suppress wrinkle

Deep Drawing
  • Wrinkling can be reduced if a blankholder is
    loaded by maximum punch force
  • The force increases with increasing blank
    diameter, thickness, strength and the ratio

  • Cup becomes longer as it is redrawn to smaller
    diameters since volume of the metal is constant
  • If the shape change is too severe, more than one
    drawing step is required.
  • The second drawing step, and any further drawing
    step is referred as redrawing

  • If the clearance between the punch and the die is
    large, the drawn cup will have thicker walls
  • Thickness of the cup wall can be controlled by
    ironing, where drawn cup is pushed through one or
    more ironing rings

Deep Drawing Deep Drawability
  • Earing
  • In deep drawing, the edges of cups may become
    wavy and the phenomenon is called earing
  • Earing is caused by the planar anisotropy
  • Planar anisotropy of the sheet is indicated by

Deep-drawing Practice
  • Earing
  • Too high a blankholder force increases the punch
    force and causes the cup wall to tear
  • Draw beads are needed to control the flow of the
    blank into the die cavity and reduce the
    blankholder forces

Deep-drawing Practice
  • CASE STUDY 16.1
  • Manufacturing of Food and Beverage Cans
  • Aluminum beverage cans has excellent surface
  • Detail of the can lid is shown

Rubber Forming
  • Dies are made of solid materials, such as steels
    and carbides
  • The dies in rubber forming is made of a flexible
    material (polyurethane membrane)
  • In the bending and embossing of sheet metal, the
    female die is replaced with a rubber pad

  • In the hydroform, or fluid-forming process, the
    pressure over the rubber membrane is controlled
    throughout the forming cycle
  • Control of frictional conditions in rubber
    forming is a factor in making parts successfully

Rubber Forming and Hydroforming
  • In tube hydroforming metal tubing is formed in a
    die and pressurized internally by a fluid,
    usually water
  • Rubber-forming and hydroforming processes have
    the advantages of
  • Capability to form complex shapes
  • Flexibility and ease of operation
  • Low tooling cost

Tube Hydroforming
  • CASE STUDY 16.2
  • Tube Hydroforming of an Automotive Radiator
  • Figure shows a hydroformed automotive radiator
  • Sequence of operations (1) tube as cut to
    length (2)
  • afterbending (3) after hydroforming

  • Spinning is a process that involves the forming
    of axisymmetric parts over a mandrel
  • A circular blank of flat sheet metal is held
    against a mandrel (form block of desired shape)
    and rotated while a rigid tool deforms and shapes
    the material over the mandrel Disk of sheet
    metal progressively shaped by localized pressure
    with small roller
  • Suitable for conical and curvilinear shapes

Shear Spinning (Forming)
  • A simplified version of the spinning process in
    which each element of the blank maintains its
    distance from the axis of rotation.
  • Metal flow is entirely in shear and no radial
    stretch has to take place to compensate for the
    circumferencial shrinkage

Shear Spinning
  • Also known as power spinning, flow turning,
    hydrospinning, and spin forging
  • Use to produce an axisymmetric conical or
    curvilinear shape while reducing the sheets
    thickness and maintaining its maximum (blank)

Tube Spinning
  • Tube Spinning
  • The thickness of hollow, cylindrical blanks is
    reduced by spinning them on a solid, round
    mandrel using rollers
  • Can be carried out externally or internally
  • Various external and internal profiles can be
    produced from cylindrical blanks with constant
    wall thickness

  • Incremental Forming
  • Simplest version is incremental stretch expanding
  • A rotating blank is deformed by a steel rod with
    a smooth hemispherical tip to produce
    axisymmetric parts
  • CNC incremental forming uses a CNC machine tool
    to follow contours at different depths across the
    sheet-metal surface
  • Advantages are low tooling costs and high
    flexibility in the product shapes

Explosive Forming
  • Use of explosive charge to form sheet (or plate)
    metal into a die cavity
  • Explosive charge causes a shock wave whose energy
    is transmitted to force part into cavity
  • Applications large parts, typical of aerospace
  • (1) Setup, (2) explosive is detonated, and
  • (3) shock wave forms part and plume escapes water

Explosive Forming
  • The peak pressure, p, is given by
  • The mechanical properties of parts similar to
    those made by conventional forming methods
  • The dies may be made of aluminum alloys, steel,
    ductile iron or zinc alloys

p pressure, psi K constant that depends on
the type of explosive
Explosively Formed Part
  • Explosive used as a source of energy
  • Rapid conversion of explosive charge into gas
    generates a shock wave
  • Pressure of shock wave is sufficient to form
    sheet metal
  • no limit to the size of the workpiece (suitable
    for low quantity of large parts, ie aerospace

Electromagnetic Forming
  • coil current rapidly discharged from capacitor
  • eddy current generated in the tube (workpiece)
  • repelling force between the coil and the tube
  • forces generated collapse the tube
  • Higher the electrical conductivity of the
    workpiece, the higher the magnetic forces

Peen Forming
  • Used to produce curvatures on thin sheet metals
    by shot peening one surface of the sheet
  • Surface of the sheet is subjected to compressive
  • The process also induces compressive surface
    residual stresses, which improve the fatigue
    strength of the sheet

Laser Forming
  • Involves the application of laser beams as a heat
    source in specific regions of the sheet metal
  • Process produce thermal stresses, which can cause
    localized plastic deformation of the sheet
  • In laser-assisted forming, the laser acts as a
    localized heat source, thus reducing the strength
    of the sheet metal at specific locations
  • Improve formability and increasing process

Superplastic Forming
  • The behavior of superplastic are where tensile
    elongations were obtained within certain
    temperature ranges
  • Superplastic alloys can be formed into complex
    shapes by superplastic forming
  • Have high ductility but low strength
  • Advantages
  • Complex shapes can be formed
  • Weight and material savings
  • Little residual stresses
  • Tooling costs are lower

Superplastic Forming
  • Limitations of superplastic forming
  • Part will undergo shape changes
  • Must be formed at sufficiently low strain rates
  • Diffusion Bonding/Superplastic Forming
  • Fabricating of complex sheet-metal structures by
    combining diffusion bonding with superplastic
    forming (SPF/DB)
  • Application for aerospace industry
  • Improves productivity and produces parts with
    good dimensional accuracy and low residual

Specialized Forming Processes
  • CASE STUDY 16.3
  • Cymbal Manufacture

Manufacturing of Metal Honeycomb Structures
  • A honeycomb structure has light weight and high
    resistance to bending forces, used for aircraft
    and aerospace components
  • 2 methods of manufacturing honeycomb materials
  • Expansion process
  • Corrugation process

Manufacturing of Metal Honeycomb Structures
  • A honeycomb structure consists of a core of
    honeycomb bonded to two thin outer skins
  • Has a high stiffness-to-weight ratio and is used
    in packaging for shipping consumer and industrial

Design Considerations in Sheet-metal Forming
  • Blank Design
  • Poorly designed parts will not nest properly
  • Blanks should be designed to reduce scrap to a

Equipment for Sheet-metal Forming
  • Press selection for sheet-metal forming
    operations depends on
  • Type of forming operation
  • Size and shape of workpieces
  • Number of slides
  • Maximum force required
  • Type of mechanical, hydraulic, and computer
  • Features for changing dies
  • Safety features

CNC Turret Press Parts
  • Sheet metal parts produced on a turret press,
    showing variety of hole shapes possible (photo
    courtesy of Strippet Inc.)

Economics of Sheet-forming Operations
  • Sheet-forming operations are versatile and can
    produce the same part
  • The costs involved depend on die and equipment
    costs and labor
  • For small and simple sheet-metal parts, die costs
    and lead times to make the dies are low
  • Deep drawing requires expensive dies and tooling
  • Equipment costs depend on the complexity of the
    forming operation