Post on 24-Dec-2015
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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MAE Course 3344Lecture 8
Sheet Metal Shaping and Forming
Professor John J. MillsMechanical and Aerospace
EngineeringThe University of Texas at Arlington
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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Material Transformation Processes
Ass
embl
y
Ingotcasting
MoltenMaterial
Powders
CastingShapes
RollingForging/
Press forming
Stamping
Pressing
Sheet metalforming
ContinuousCasting/Rolling
InjectionMolding
Mac
hini
ng
Fin
ishi
ng
Raw
Mat
eria
l
Special
Extruding
Single crystalpulling
Firing/Sintering
SLS
Increasing level of detail
Blowmolding
Current lecture
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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Overview of Sheet Metal Forming
• Overview• Shearing to make blanks• Fundamentals of forming sheet metal
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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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
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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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
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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Sheet Metal Forming Processes
ShearingSlittingCuttingSawing
PunchingBlanking
Fine BlankingStamping
Embossing
BendingRoll forming
Stretch formingDeep drawing
Rubber formingSpinning
Peen formingSuperplastic forming
Explosive formingMagnetic pulse forming
DeburringCleaningCoating
Sheet, P
late
Blank
Making blanks forming finishing
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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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
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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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
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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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.
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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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
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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Effect of die clearance on deformation zone
• Smaller the clearance, the better the edge
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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Simple Shearing Advantages and Disadvantages
• Advantages– Simple– Minimal tooling
• Stops for dimensions
• Disadvantages– Only simple shapes (rectangles)
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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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)
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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Other shearing processes
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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Punching• Circular blanks created by punch and die
Punch
Die
Workpiece
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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A Punched Hole
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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Process variables in shearing with a punch and die and punch force
F = 0.7 T L (UTS)where
F forceT workpiece thicknessL total sheared length (the circumference in this case)UTS Ultimate tensile strength of workpiece material
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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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.
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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The Blank
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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• Punch and Die– Shape– Material
• Clearance between punch and die– Increased clearance
• Workpiece ductility and thickness– Increased ductility
– Decreased thickness
• Dulled tools • Speed of punch/shear
– Decreased speed
• Increased Lubrication
Major Processing Factors in Shearing
Rougher edgeLarger deformation zoneIncreased burr height
Greater ratio of burnished to rough areasDecrease max. punch force
Independent parametersDependent parameters
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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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%
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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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
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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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)
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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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
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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Equipment• Shears
– 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
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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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
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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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
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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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
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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Fundamentals of Sheet Metal Forming
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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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
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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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.
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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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
Sheet Metal Characteristics for Forming
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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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
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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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
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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Failure mechanisms include:
– Necking• As occurs at the ultimate tensile stress
– Tearing• As it sounds
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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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
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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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
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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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
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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Cupping test
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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Forming Limit Diagram
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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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
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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Other forming tests
• Depend on the specific forming method– Bending– Stretching– Drawing. etc
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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SummaryCharacteristic ImportanceElongation Determines the capability of the sheet metal to
stretch without necking and failure; high strain-hardening exponent (n) and strain-rate sensitivity exponent (m) desirable.
Yield-point elongation 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.
Anisotropy (planar) 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.
Anisotropy (normal) Determines thinning behavior of sheet metals during stretching; important in deep-drawing operations.
Grain size Determines surface roughness on stretched sheet metal; the coarser the grain, the rougher the appearance (orange peel).
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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SummaryCharacteristic ImportanceResidual 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.
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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Sheet Metal Forming Processes
ShearingSlittingCuttingSawing
PunchingBlanking
Fine BlankingStamping
Embossing
BendingRoll forming
Stretch formingDeep drawing
Rubber formingSpinning
Peen formingSuperplastic forming
Explosive formingMagnetic pulse forming
DeburringCleaningCoating
Sheet, P
late
Blank
Professor John J. Mills: Email: jmills@arri.uta.edu; Tel (817) 272-7366
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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