Return to Overview Slide Professor John J. Mills: Email: [email protected]; Tel (817) 272-7366 MAE Course 3344 Lecture 8 Sheet Metal Shaping and Forming.

Professor John J. Mills: Email: [email protected]; Tel (817) 272-7366

Return to Overview Slide

MAE Course 3344Lecture 8

Sheet Metal Shaping and Forming

Professor John J. MillsMechanical and Aerospace

EngineeringThe University of Texas at Arlington



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



Overview of Sheet Metal Forming

• Overview• Shearing to make blanks• Fundamentals of forming sheet metal



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



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



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



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



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



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.



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



Effect of die clearance on deformation zone

• Smaller the clearance, the better the edge



Simple Shearing Advantages and Disadvantages

• Advantages– Simple– Minimal tooling

• Stops for dimensions

• Disadvantages– Only simple shapes (rectangles)



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)



Other shearing processes



Punching• Circular blanks created by punch and die

Punch

Die

Workpiece



A Punched Hole



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



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.



The Blank



• 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



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%



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



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)



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



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



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



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



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



Fundamentals of Sheet Metal Forming



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



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.



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





– 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




• 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



Failure mechanisms include:

– Necking• As occurs at the ultimate tensile stress

– Tearing• As it sounds



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



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



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



Cupping test



Forming Limit Diagram



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



Other forming tests

• Depend on the specific forming method– Bending– Stretching– Drawing. etc



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).



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.



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



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

Return to Overview Slide Professor John J. Mills: Email: [email protected]; Tel (817) 272-7366 MAE Course 3344 Lecture 8 Sheet Metal Shaping and Forming.

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