MAE Course 3344
Lecture 8
Sheet Metal Shaping and Forming
Professor John J. Mills
Mechanical and Aerospace
Engineering
The University of Texas at Arlington
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
Material Transformation Processes
SLS
Powders
Special
Continuous
Casting/Rolling
Rolling
Ingot
casting
Sheet metal
forming
Forging/
Press forming
Extruding
Molten
Material
Casting
Shapes
Single crystal
pulling
Current lecture
Increasing level of detail
Finishing
Stamping
Machining
Raw Material
Injection
Molding
Firing/
Sintering
Blow
molding
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
Assembly
Pressing
Overview of Sheet Metal Forming
• Overview
• Shearing to make blanks
• Fundamentals of forming sheet metal
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
Sheet Metal Forming History
• Very old process - back to 5000 BC
• Original sheet obtained by hammering
over a stone anvil
• Cut to shape with a knife
• Formed over stone or wooden dies by
hammering
• Now sheet produced by sheet mills
• Cutting to shape and forming is by
machines
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
General Practices
• Most common commercial material is carbon
steel
• Most common aircraft and aerospace
materials are aluminum and titanium
• Aluminum increasingly found in automobiles
• Sheet metal forming consists of three basic
processes;
– Cutting to form a shape (blank)
– Forming by bending and stretching
– Finishing
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
Sheet Metal Forming Processes
Punching
Blanking
Fine Blanking
Stamping
Embossing
Making blanks
Deburring
Cleaning
Coating
Blank
Sheet, Plate
Shearing
Slitting
Cutting
Sawing
Bending
Roll forming
Stretch forming
Deep drawing
Rubber forming
Spinning
Peen forming
Superplastic forming
Explosive forming
Magnetic pulse forming
forming
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
finishing
Sheet Metal Advantages and
Disadvantages
• Advantages
– light weight,
– versatile shapes,
– low cost
• Disadvantages
– tooling costs (for high production runs)
– sheet metal may not be appropriate to
design function
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
Shearing
• Needed to cut rough blanks from the
large sheets
• A blank is the term for the rough shape
needed to form the final part
• Rectangular blanks created by shears,
saws, rotary cutters
• These blanks can
– be further sheared into more complex
shapes
– be further formed (bent, deep drawn, etc)
into more complex shapes
– also be the final product
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
The Basic Shearing Process
• Like cutting paper with scissors but using
a machine
• Shearing starts with cracks developed on
top and bottom of sheet by exceptionally
high shear stresses
– A fracture process
• The Punch is typically the moving part
• The Die is the stationary part.
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
The Basic Shearing Process
Results
• Typically creates rough fracture surfaces
• Smoothing of this surface occurs by rubbing
on the shear blades or the die
• Shears, the machine for cutting metal can
operate up to thickness of several inches
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
Effect of die clearance on
deformation zone
• Smaller the clearance, the better the edge
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
Simple Shearing Advantages and
Disadvantages
• Advantages
– Simple
– Minimal tooling
• Stops for dimensions
• Disadvantages
– Only simple shapes (rectangles)
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
Shearing Operations for more
complex shapes
• Punching
– More complex shapes than simple shearing
– Made by punch and die set
– Internal part (slug) discarded
• Blanking
– Same basic process as Punching but
– Internal part (slug) retained
– Fine blanking - a specialized kind of blanking
• Other operations include
– Parting
Stamping
Notching
Embossing
– Lancing
Perforating Slitting
Nibbling
– Shaving
Steel rules (soft materials only)
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
Other shearing processes
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
Punching
• Circular blanks created by punch and die
Punch
Workpiece
Die
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
A Punched Hole
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
Process variables in shearing with
a punch and die and punch force
F = 0.7 T L (UTS)
where
F
T
L
UTS
force
workpiece thickness
total sheared length (the circumference in this case)
Ultimate tensile strength of workpiece material
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
Major Processing Factors in
Shearing Die Design
• Punch shape
– Bevel
• Reduces shear forces and noise
– Double bevel
• Reduces lateral forces of bevel shear
– Convex shear
• All produce at least one part
(e.g. the blank) which is bent.
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
The Blank
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
Major Processing Factors in
Independent parameters
Shearing Dependent parameters
• Punch and Die
– Shape
– Material
• Clearance between punch and die
Rougher edge
– Increased clearance
Larger deformation zone
Increased burr height
• Workpiece ductility and thickness
Greater ratio of burnished to
– Increased ductility
rough areas
Decrease max. punch force
– Decreased thickness
• Dulled tools
• Speed of punch/shear
– Decreased speed
• Increased Lubrication
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
Major Processing Factors in
Shearing Die Design
• Clearances
– Depends on
•
•
•
•
Workpiece material
Thickness
Size of hole
Proximity of hole to sheet edge
– Small holes required larger clearances
than large holes
– Typically range form 2-8% of sheet
thickness
– Can range from 1%(Fine Blanking) to 30%
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
Fine Blanking
• A device called a V-shaped Stinger locks the
sheet in place
• Prevents distortion at sheared edges
• Very tight (<1%) clearances)
• Therefore tight tolerances possible
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
Other Methods of Cutting Sheet
Metal
• Band saw
– Very versatile but not very precise
– Used a lot in job shops
• Flame cutting
– Used mostly on thick steel sheet
– Can cut quite complex shapes but is not
precise
– Leaves a very rough edge and often a heat
affected zone
• Laser-beam cutting
– Very popular since it can be readily
programmed to cut complex shapes
– Leaves a fine heat affected zone (much
smaller than flame cutting)
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
Other Methods of Cutting Sheet
Metal
• Friction sawing
– Cut-off saw
– Uses abrasive disk
– Versatile but inaccurate
• Water jet
– Uses high pressure jet of water to cut
– Leaves nice finished edge
– Limited in materials that can be cut
• Abrasive water jet
–
–
–
–
Like water jet but with abrasives contained in jet
Cuts anything
Leaves nice edge and is precise
Programmable and can cut almost any shape
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
• Shears
Equipment
– A long stationary blade (lower) and a
moveable top blade with a table to support
the material. Upper blade can be at an angle
to reduce forces but this gives a curved
blank
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
Equipment
• Saws
– Band
• A continuous blade that moves at high speeed
through a hole in the table which supports the work
piece. The material is moved around while the blade
is stationary
– Cut-off
• Can be band type or a circular rotating blade. The
material is clamped to a table and the weight of the
blade holder forces the moving blade through the
material
• Punch presses
– Like forging machines but can provide high
repetition rates
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
Equipment
• Presses
–
–
–
–
Used for shaped punches and dies
Precision
Fast acting
Often combine forming operations as well
• CNC nibblers
– Can create many shapes using nibbling tools
• Automated punch presses
– moves large sheet around to position a
specific location over a punch and die which is
automatically changed to deliver a variety of
shapes and diameters of holes
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
Equipment
• Flame, laser and water jet cutting
systems
– Typically are robots that have the cutting
device on the end of the robot arm (the
end effector)
– The robots are programmed to cut a shape
– The robot can be a simple as a linear
mechanism to move the end effector over
a straight line to cut large slabs
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
Fundamentals of Sheet Metal
Forming
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
Sheet Metal Forming
• The sheet metal forming process includes bending,
stretching, drawing and otherwise deforming sheet
with tools and machines to create a product or
component.
• To form sheet metal it must have a yield point and
exhibit plastic flow
– Brittle materials such as ceramics and carbides cannot
be formed this by these processes
• We must understand the mechanical properties of the
sheet before deforming it
– Yield stress, elongation, anisotropy,surface finish, grain
size, edge conditions
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
Basic modes of deformation
• Bending
– Folding the sheet
– The most common operation
– The only deformation occurs at the bend
• Stretching
– Characterized by uniaxial or biaxial
uniform strain
– Typically the material is grasped by the
edges and pulled over a die
• Drawing
– Characterized by deforming the sheet into
a die with a punch or by other means.
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
Sheet Metal Characteristics for
Forming
• Important properties
– Elongation
• Need high uniform
elongation
– True strain at
which necking
occurs (= strain
hardening coeff.)
• Need large strain
hardening exponent
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
Sheet Metal Characteristics for
Forming
– Yield point elongation (important for low
carbon steels and Al/Mg alloys)
• Non-uniform elongation
– Restricts the amount of deformation possible during
forming
– Some parts yield while others do not
– Leuders bands
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
Sheet Metal Characteristics for
Forming
• Important properties
– Anisotropy
• Produces non-uniform deformation
– Gives ears during deformation
– Two kinds
» Planar
» Normal
– Grain size
• Influences strength of product
• Influences surface finish
– Large grains give mottled appearance
– State of the sheared edges
• Rough edges cause premature failure during forming
– State of the sheet surface
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
Failure mechanisms include:
– Necking
• As occurs at the ultimate tensile stress
– Tearing
• As it sounds
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
Formability
• The term "Formability" integrates the
important properties into one word
• Definition
– The ability of sheet to undergo the
required shape change or deformation
without failure
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
Generic Formability Tests
• Tests to measure the formability of the metal
– Tensile testing
• Universal test method - stress and strain to failure
under uniaxial stress
– Biaxial tensile testing
• More generic and representative of forming
conditions
• Very difficult and hence expensive to do properly
– Cupping
• A simple generic test for all forms of sheet metal
forming
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
Cupping Test and Formability
Diagram
• The Cupping test
– Push a round steel punch into firmly held
sheet until a crack appears
– Metric is the amount of deformation when
crack appears measures the formability
– Use Cupping test on various widths to
change the strain conditions to provide
data on forming limits
• Narrow widths undergo simple uniaxial tension
• Large widths undergo equal biaxial stretching
– The forming limits as a function of major
and minor strain is the Forming Limit
Diagram
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
Cupping test
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
Forming Limit Diagram
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
Forming Limit Diagram
• Give the limits of major and minor stress
for cracking and tearing
• Carbon steel and brass have higher limits
than high strength steel and aluminum
alloys and are more formable
• Increased thickness raises the curves
BUT
– Thicker material difficult to bend around
tight radii - see later
• Note that having a compressive
(negative) minor strain is advantageous
– need special tooling
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
Other forming tests
• Depend on the specific forming method
– Bending
– Stretching
– Drawing. etc
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
Summary
Characteristic
Importance
Elongation
Determines the capability of the sheet metal to
stretch without necking and failure; high strainhardening exponent (n) and strain-rate sensitivity
exponent (m) desirable.
Observed with mild-steel sheets; also called
Lueder’s bands and stretcher strains; causes
flame like depressions on the sheets surfaces;
can be eliminated by temper rolling, but sheet
must be formed within a certain time after rolling.
Exhibits different behavior in different planar
directions; present in cold-rolled sheets because
of preferred orientation or mechanical fibering;
causes earing in drawing; can be reduced or
eliminated by annealing but at lowered strength.
Determines thinning behavior of sheet metals
during stretching; important in deep-drawing
operations.
Determines surface roughness on stretched sheet
metal; the coarser the grain, the rougher the
appearance (orange peel).
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Yield-point elongation
Anisotropy (planar)
Anisotropy (normal)
Grain size
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
Summary
Characteristic
Importance
Residual stresses
Caused by nonuniform deformation during
forming; causes part distortion when sectioned
and can lead to stress-corrosion cracking;
reduced or eliminated by stress relieving.
Springback
Caused by elastic recovery of the plastically
deformed sheet after unloading; causes
distortion of part and loss of dimensional
accuracy; can be controlled by techniques such
as overbending and bottoming of the punch.
Quality of sheared edges Depends on process used; edges can be rough,
not square, and contain cracks, residual
stresses, and a work-hardened layer, which are
all detrimental to the formability of the sheet;
quality can be improved by control of clearance,
tool and die design, fine blanking, shaving, and
lubrication.
Surface condition of sheet Depends on rolling practice; important in sheet
forming as it can cause tearing and poor surface
quality; see also Section 13.3.
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
Sheet Metal Forming Processes
Punching
Blanking
Fine Blanking
Stamping
Embossing
Deburring
Cleaning
Coating
Blank
Sheet, Plate
Shearing
Slitting
Cutting
Sawing
Bending
Roll forming
Stretch forming
Deep drawing
Rubber forming
Spinning
Peen forming
Superplastic forming
Explosive forming
Magnetic pulse forming
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
Summary
• Shearing of sheet to form flat shapes
• The fundamentals of changing that flat shape
into a three dimensional one
• Next lecture discusses different sheet metal
forming processes
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Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366