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Chapter 16Sheet-Metal Forming Processes

Alexandra Schönning, Ph.D.Mechanical Engineering

University of North Florida

Figures by Manufacturing Engineering and Technology

Kalpakijan and Schmid

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Introduction

What is Sheet-Metal Forming ProcessesForming products from sheet metal

Sheet metal forming products includeFile cabinets, car bodies, metal desks

AdvantagesLow weight Versatile shape

MaterialsMost common is low carbon steel• Good strength and formability

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Shearing

Cut the sheet by subjecting it to shear stresses

ScissorsA blank is created by shearing it from a larger sheet (coil)Shearing geometry

Sheared edges are not smooth, nor perpendicular to the plane of the sheetShearing starts by formation of cracks at A, B, C, and DThe cracks meet each other, resulting in separation• Results in a rough fracture

surfaceBurnish depth has a smooth surface resulting from rubbing against the walls of the punch and die

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Shearing (cont.)

ParametersShape and material of the punch and dieSpeed of punchingLubrication Clearance• Increased clearance rougher

edges, and increased zone of deformation

• Secondary operations may be necessary

(Burnished area) / (rough area) • Increases with ductility of sheet metal• Decreases with thickness of sheet metal

Edge quality increases with punch speed • up to 12 m/s

BurrA thin edge or ridge• Increases with clearance, ductility and

dull tools

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Punch Force

Force required Product of the shear strength and the area sheared

T= thicknessL = length sheared (perimeter of a hole)Sut = ultimate shearing strength

F 0.7 T⋅ L⋅ Sut⋅

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Shearing OperationsPunching

You discard the punched areaSimilar to using a hole punch on paper

BlankingYou keep the punched area and discard the area surrounding the hole

PerforatingPunching a number of holes in a sheet

PartingSheering the sheet into two or more pieces

NotchingRemoving pieces from the edges

LancingLeaving a tab without removing any material

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Shearing Operations (Cont.)Fine blanking

A stringer (impingement ring) holds the sheet in place during the shearing operationClearance is finer: 1% of thickness

SlittingA pair of circular blades are used Similar to a can opener

Steel rulesThin strip of hardened steel is bent into the shape of the final product and put on its edge on a flat surface

• Similar to cookie cuttingNibbling

Many overlapping holes are made by moving the punch up and down (using a nibbler) while moving the sheet

ScrapTry to minimize by optimizing how you place the shapes: called nesting

Fine blanking.

Slitting

(b)

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Shearing Operations (Cont.)

Tailor-welded blanksTwo or more flat sheet metal pieces are butt welded togetherReduces the amount of scrap

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Shearing Dies

ClearanceDependent on material type and temper, thickness, size of blank, distance from the edgeSoft materials have lower clearance then hard materialsThe thicker the sheets, the more clearanceSmall holes need more clearance than large holesRough sheared edges are removed by shaving (figure)

Punch and die shapesPunch is often beveled• To reduce the force needed at the

beginning of each stroke• Reduces noise levels

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Shearing Dies (Cont.)Compound dies

Several operations on the same strip are performed in one stroke at one stationSlow processExpensive diesSimple shapes

Progressive diesThe sheet metal is fed through as a coil stripA different operation is performed at the same station with each stroke of a series of punches

Transfer diesDifferent operations at different stationsSheet metal moves forward

Tool and die materialTool steelsCarbides Lubrication reduces tool and die wear and improves edge quality

(c)

Compound die

(d)

Top of a spray can shaped using progressive die

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Sheet Metal CharacteristicsElongation

Uniform region• Desirable elongation

Necking: after the ultimate tensile stress has been reached

Yield Point ElongationCommon with low carbon steelsHave an upper and a lower yield point (material starts yielding in one region before another)Results in Lueder’s bands (stretcher strain marks)

• Reduce effect of these by reducing the thickness of the sheet or by cold rolling

Anisotropy (different properties in different directions)

Two types• Crystallographic anisotropy

○ Preferred orientation of the grains• Mechanical fibering

○ Alignment of impurities and voidsGrain Size

Grain size effects the mechanical properties and surface appearanceThe coarser the grain, the coarser the surface finish

Yield point elongation

Stretcher strains at the bottom of a steel can for household products

(a)

(c)

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Bending Sheet and Plate

Bending: one of the most common forming operations

Where do you see it?• File cabinets, paper clips

Why do you bend?• Form flanges, seams and

corrugations• Increase stiffness by modifying

the moment of inertiaTerminology• Length: width of the part

○ L is smaller at the outer radius than at the inner radius (due to Poisson’s ratio)

♦ Observe by bending a rubber eraser

• Bend radius• Setback• Bend radius• Bevel angle• Bend angle

• Bend allowance (Lb): length of the neutral axes in the bend

○ T: thickness, R: bend radius, α: bend angle, k: constant related to location of the neutral axes

♦ Ideal case k = 0.5

♦ k: from 0.33 (for R<2T) to 0.5 (for R > 2T)

Lb α R k T⋅+( )⋅

Lb α RT2

+

⋅

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Minimum Bend Radius

Engineering Strain

As R/T decreases, the tensile stress increases on the outer surface leading to cracks

Minimum bend radiusThe ratio at which cracks start to appear2T, 3T, 4T….

Bendability decreases with edge roughnesscold working

• Annealing or machining can helpinclusions

Anisotropy affects bendabilityCut the sheet in an advantageous direction

e1

2RT⋅

1+

Narrowing due to Poisson’s effect

(a) (b)

(c)

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Springback

When the load is applied, Elastic deformation beginsPlastic deformation follows

As load is removedSome elastic recovery occurs• Called “springback”

SpringbackAngle after springback is smallerBend radius will be a little larger

Ri, Rf as shownY: yield stressE: elastic modulusSpringback increases with• R/T• Y

Decreases w/ E

RiRf

4Ri Y⋅ET

3⋅ 3

Ri Y⋅E T⋅

⋅− 1+

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Springback (cont.)

Compensation for springback

Overbending (fig a,b)Bottoming the punch• Apply high compressive

stresses between the punch tip and the die (fig c, d)

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Maximum bending force

P: maximum bending forcek: factor dependent on the type of die

Typical values from 0.3 – 1.3Y: yield strengthT: thicknessW: die opening dimension

Pk Y⋅ L⋅ T2

⋅

W

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Common bending operations

Press brake formingLong dies are used in a press (mechanical or hydraulic)

Figure 16.23 (a) through (e) Schematic illustrations of various bending operations in a press brake. (f) Schematic illustration of a press brake. Source: Verson Allsteel Company.

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Common bending operations (cont.)

Roll-bendingPlates are bent using a set of rollsVarious curvatures can be obtained by adjusting the position of the rolls

Bending in a 4-slide machine

Bending of short pieces

BeadingThe periphery of the sheet is bent into the cavity of the die

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Common bending operations (cont.)

FlangingProcess of bending the edge of a sheetFigure (a)

DimplingA hole is punched and then expanded into the flangeFigure (b)

Figure 16.25 Various flanging operations. (a) Flanges on a flat sheet. (b) Dimpling. (c) The piercing of sheet metal to form a flange. In this operation, a hole does not have to be prepunched before the bunch descends. Note, however, the rough edges along the circumference of the flange. (d) The flanging of a tube; note the thinning of the edges of the flange.

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Common bending operations (cont.)

HemmingAlso called flatteningEdge of sheet is folded over itselfIncrease stiffness, improve appearance

SeamingJoining two edges of sheet metal by hemming

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Common bending operations (cont.)

Roll FormingAlso called contour roll forming, cold roll formingMetal is bent in stages by passing it through a series of rollsUsed to make• Gutters, door and picture

frames, pipes and tubing with lock seams

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Tube bending and forming

How do you avoid the tube from buckling when you bend it?

Pack the inside with loose particles (sand) before bending.Sand is shaken out after bend has been completed

Thick walls with a large bend radius can be bent without filling

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Bulging

Place a tubular, conical, curvilinear part (plug) into a split female dieExpand the plug

Commonly polyurethane plugRetract the plugOpen the die and remove the plugUsed in making

Coffee / water pitchersBarrels

Internal fluid pressure Replaces the plugEnds are sealed mechanicallyExhaust pipes

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Segmented dies

Individual segments are placed inside the tubeExpanded in the radial directionRetracted

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Stretch forming

Sheet metal is clamped along its edges then stretched over a dieUsed in making wing-skin panels for aircraftsMaterial will shrink in width as it is stretchedDisadvantage

Can’t produce parts with • sharp corners• Re-entrant corners

AdvantagesLittle (or no) lubrication is necessaryVersatileEconomical

Used for low production runs

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Deep Drawing

Deep drawing can be used to make cylindrical and box shaped parts using deep drawing

Kitchen sinksProcess

A sheet metal blank is placed over a die opening and is held in place with a blank holder (hold-down ring)A punch forces the blank into the cavity• Forming a cup

Cup wall may be subjected to a tensile stress in order to elongate the wall

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Deep Drawing (cont.)

Important variablesSheet metal propertiesBlank diameter / punch diameterClearancePunch radiusDie corner radiusBlank holder forceFrictionLubrication

Deep drawabilityFailure typically occurs from thinning of the cup walls under high tensile stressesTensile tests are performed to determine the deep drawabilityof the material

EaringEdges of the cups may be wavy called earing

Due to anisotropy of the material

• 2,4,8 ears

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Deep Drawing (cont.)

Draw beadsControls the flow of the blank into the cavity

Ironing• Ensuring the wall thickness

is constant• Cup is pushed through

ironing rings

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Deep Drawing (cont.)

RedrawingWhen the shells are too difficult to draw in one operation

EmbossingShallow or moderate drawsUsed for• Stiffening of flat panels• Decorations• Logo, text

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Steps in Manufacturing an Aluminum

Can

Figure 16.31 The metal-forming processes involved in manufacturing a two-piece aluminum beverage can

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Rubber Forming

One of the dies in a set are made of a flexible material (polyurethane)Female die is replaced with a rubber pad

• Protects the outer surface of the sheet from scratches

Figure 16.38 Examples of the bending and the embossing of sheet metal with a metal punch and with a flexible pad serving as the female die. Source: Polyurethane Products Corporation.

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Hydroform Process

Also called fluid-forming processPressure over the rubber membrane is controlled by a fluidClose control of the part during formingDeeper draws can be obtained with the hydroform process than with conventional deep drawing techniques

• since the pressure around the rubber membrane aids in reducing the longitudinal tensile stresses in the sheet

Figure 16.39 The hydroform (or fluid forming) process. Note that, in contrast to the ordinary deep-drawing process, the pressure in the dome forces the cup walls against the punch. The cup travels with the punch; in this way, deep drawability is improved.

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Spinning

Forming of axisymmetricparts over a mandrel

Conventional• Sheet metal is held against

the mandrel and rotated• Conical and curvilinear

shapesShear • Rollers are used during the

forming process• Conical or curvilinear

shapes• Max diameter is maintained

is maintained• Thickness is reduced

Tube spinning• Thickness is reduced

resulting in thinner tubes

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Dent Resistance

Dynamic forces causes localized dentsStatic forces spreads the dentDent resistance

Increases with • Increased yield strength• Increased thickness

Decreases with • Increased elastic modulus

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Economics of sheet metal forming

A part can often be manufactured using numerous different techniques

Even different sheet metal forming techniques

Need to do a cost analysis before deciding which technique to useSheet metal processes can be considered economical

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