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Chapter 7. Sheet-Metal Forming Processes

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(1)

Chapter 7. Sheet-Metal Forming Processes

Processes

Sung-Hoon Ahn

School of Mechanical and Aerospace Engineering

Seoul National University

(2)

Sheet-Metal Characteristics

ƒ Press working / stamping / shearing

ƒ Forming of sheet metals generally is carried out by tensile forces in the plane of the sheet.

ƒ Application of compressive forces could lead to buckling, folding, and wrinkling of the sheet.

wrinkling of the sheet.

ƒ Mechanics of all sheet forming basically consists of the processes of t t hi d b di

stretching and bending.

ƒ Factors influencing the operation.

: elongation, yield-point elongation, anisotropy, grain size, residual stresses,

springback, and wrinkling.

(3)

Elongation (연신율) g ( )

ƒ High uniform elongation is desirable for good formability. (large n, m)

K ε n

σ = C m

n ε σ

ε

&

=

=

Ref.

S. Kalpakjian, "Manufacturing Processes for Engineering Materials", 3rd/4th ed. Addison Wesley

(4)

Yield Point Elongation (항복점 신장) g ( )

ƒ Low-carbon steels exhibit yield-point elongation.

ƒ Lueder’s bands(stretcher strain marks 신장변형마크)

ƒ Lueder s bands(stretcher strain marks, 신장변형마크)

Ref.

S. Kalpakjian, "Manufacturing Processes for Engineering Materials", 3rd/4th ed. Addison Wesley

(5)

stresses (잔류응력) / Springback (스프링백) / Wrinkling (주름)( ) g ( ) g ( )

ƒ Anisotropy

ƒ Acquired during thermo mechanical-processing history. Acquired during thermo mechanical processing history.

ƒ Crystallographic anisotropy(결정립의 방향성)

ƒ Mechanical fibering( 기 계 적 섬 유 화 ) : alignment of impurities(불순물) inclusions(개재물) and voids(공극) impurities(불순물), inclusions(개재물) and voids(공극)

ƒ Coarse grain(조대 결정립) → orange peel

Th t t f id l t li i i

ƒ The temperature of residual stresses relieving is lowered to reduce the energy which enlarges the grain.

ƒ Residual stresses

ƒ Tensile residual stresses on surfaces

→ stress-corrosion cracking(응력-부식 균열)

ƒ Springback : high bend radius-to-thickness ratio.

ƒ Wrinkling : occurs where the compressive g p stresses are developed.

Ref.

S. Kalpakjian, "Manufacturing Processes for Engineering Materials", 3rd/4th ed. Addison Wesley

(6)

Shearing Process (전단작업) (1) g ( ) ( )

ƒ The shearing process involves cutting sheet metal by subjecting cutting sheet metal by subjecting it to shear stresses, usually between a punch and a die.

ƒ Shearing process variables

: Punch force, punch speed, edge p p g condition of the sheet, lubrication, corner radii of the punch and die, and clearance between the punch and die. p

(6~10% of the sheet thickness : for big sheets)

(2~8% of the sheet thickness : for general (2 8% of the sheet thickness : for general sheets)

Ref.

S. Kalpakjian, "Manufacturing Processes for Engineering Materials", 3rd/4th ed. Addison Wesley

(7)

Shearing Process (전단작업) (2) g ( ) ( )

Ref.

S. Kalpakjian, "Manufacturing Processes for Engineering Materials", 3rd/4th ed. Addison Wesley

(8)

Shearing Process (전단작업) (3) g ( ) ( )

L: total length of the sheared d (전단면의 총 길이)

) )(

)(

( 7 .

max 0 UTS t L

F =

edge(전단면의 총 길이)

) )(

)(

max (

ƒ Example – calculation of max. punch force

: d=1in t=1/8in annealed titanium alloy(Ti 6Al 4V)

lb P = 0 . 7 ( 1 / 8 )( π )( 140 , 000 ) = 38 , 500

: d=1in., t=1/8in., annealed titanium-alloy(Ti-6Al-4V)

MN tons 0 . 17

25 .

19 =

=

d

MN tons 0 . 17

25 . 19

Ref.

S. Kalpakjian, "Manufacturing Processes for Engineering Materials", 3rd/4th ed. Addison Wesley

(9)

Shearing Operations (전단공정) g ( )

ƒ Punching(천공)

ƒ sheared slug is discarded. g

ƒ Blanking(블랭킹)

ƒ slug is the part itself, and the rest is scrap.

ƒ Die cutting

ƒ Perforation(연속천공)

ƒ Parting(분리)

ƒ Notching(노칭)

ƒ Lancing(랜싱)

ƒ Fine blanking

ƒ Very smooth and square edges can be produced.

ƒ Clearance on the order of 1% of the h t thi k

sheet thickness.

ƒ Slitting(분단) : similar to can opener

ƒ Steel rules(강척다이) Nibbli (니블링)

ƒ Nibbling(니블링)

Ref.

S. Kalpakjian, "Manufacturing Processes for Engineering Materials", 3rd/4th ed. Addison Wesley

(10)

Blanking (블랭킹) / Slitting (분단) g ( ) g ( )

Ref.

S. Kalpakjian, "Manufacturing Processes for Engineering Materials", 3rd/4th ed. Addison Wesley

(11)

Bending (굽힘) g ( )

ƒ One of the most common metalworking operations.

ƒ To form parts as certain shapes and impart stiffness to a part by

ƒ To form parts as certain shapes and impart stiffness to a part by increasing its moment of inertia.

ƒ Outer surface is in tension and inner surface is in compression. p

ƒ : shifting of the neutral axis toward the inner surface.

1

i

o e

e >

2 ⎟ + 1

⎜ ⎞

= ⎛

=

t e R

e o i

Ref.

S. Kalpakjian, "Manufacturing Processes for Engineering Materials", 3rd/4th ed. Addison Wesley

(12)

Minimum bend radius (최소굽힘반경) (1) ( ) ( )

ƒ The radius at which crack appears on the outer surface of the bend.

ƒ Minimum bend radii for various materials ref textbook table 7 2

ƒ Minimum bend radii for various materials – ref. textbook table 7.2

ƒ Such as 2T, 3T, 4T.

ƒ The R/t ratio approaches zero at a tensile reduction of area of 50% - the pp material can be folded over itself.

ƒ In bending of sheets, appropriate direction of the sheets should be considered due to the anisotropy of the cold rolled sheets.

ln 100

ln ⎟

⎜ ⎞

= ⎛

⎟ ⎟

⎜ ⎜

= ⎛ A

o

ε

f

) 2 / ln (

1 ) / 2 ( 1 1 ln ) 1

ln(

ln 100 ln

⎟⎟ ⎠

⎜⎜ ⎞

⎛ +

= +

⎟⎟ ⎠

⎜⎜ ⎞

⎛ + +

= +

=

⎟ ⎠

⎜ ⎝ −

⎟ ⎠

⎜ ⎝

t R

t R t

e R

r A

o o

f f

ε ε

50 1

. min

) 2 / ( 1

) / 2 (

=

⎝ +

⎝ +

r t

R

t R t

R

60 1

.

min = −

r t

R : Experimental data

(13)

Minimum bend radius (최소굽힘반경) (2) ( ) ( )

Ref.

S. Kalpakjian, "Manufacturing Processes for Engineering Materials", 3rd/4th ed. Addison Wesley

(14)

Minimum bend radius (최소굽힘반경) (3) ( ) ( )

Ref.

S. Kalpakjian, "Manufacturing Processes for Engineering Materials", 3rd/4th ed. Addison Wesley

(15)

Springback (스프링백) (1) g ( ) ( )

ƒ Because all materials have a finite modulus of elasticity, plastic deformation is followed by elastic recovery upon removal of the deformation is followed by elastic recovery upon removal of the load; in bending this recovery is known as springback.

ƒ Elastic recovery increases with the stress level and with decreasing elastic modulus.

ƒ Negative springback(역 스프링 백)

ƒ Compensation of springback

: In practice, springback is usually compensated for by using p p g y p y g

overbending(과도굽힘) .

(16)

Springback (스프링백) (2) g ( ) ( )

2 allowance 2

Bend ⎟

⎜ ⎞

⎛ +

⎟ =

⎜ ⎞

⎝ ⎛ +

= t

t R

R

i

α

i f

α

f

( )

( 2 2 / / ) 1 1

2 2

+

= +

=

t R

t K R

f i i

f s

f f

i i

α

α ( )

1 3

4

3

⎟ +

⎜ ⎞

− ⎛

⎟ ⎠

⎜ ⎞

= ⎛

Et Y R Et

Y R R

R

i i

f i

f

Ref.

S. Kalpakjian, "Manufacturing Processes for Engineering Materials", 3rd/4th ed. Addison Wesley

(17)

Springback (스프링백) (3) g ( ) ( )

ƒ Negative springback(역스프링백)

Ref.

S. Kalpakjian, "Manufacturing Processes for Engineering Materials", 3rd/4th ed. Addison Wesley

(18)

Springback (스프링백) (4) g ( ) ( )

ƒ Compensation of springback

ƒ Overbending the part in the die

ƒ Overbending the part in the die.

ƒ High localized compressive stresses between the tip of the punch and the die surface.

ƒ Subjected to tension while being bent; stretch bending.

ƒ Bending may be carried out at elevated temperature.

Ref.

S. Kalpakjian, "Manufacturing Processes for Engineering Materials", 3rd/4th ed. Addison Wesley

(19)

3-D shape Laminate Composite Fabrication (1) ( )

ƒƒ Bending Bending mold mold and and laminate laminate composite composite

ƒƒ Hybrid Hybrid 33 D D shape shape laminate laminate composite composite (glass (glass 88 piles piles + + carbon carbon 88 plies) plies)

ƒƒ Hybrid Hybrid 33--D D shape shape laminate laminate composite composite (glass (glass 88 piles piles + + carbon carbon 88 plies) plies)

Mold

200

Composite

200 200

199 199

R100

Composite

R100 R100 199

199

(mm) (mm)

35 - 24

(mm)

(mm)

(20)

3-D shape Laminate Composite Fabrication (2)

ƒƒ The The angle angle of of ‘C’ ‘C’ shape shape mold mold and and composite composite springback springback

ƒƒ θ θ :: angle angle of of mold mold bending bending ● ● θ θ :: angle angle of of composite composite springback springback

ƒƒ θ θ

MM

:: angle angle of of mold mold bending bending ● ● θ θ

PP

:: angle angle of of composite composite springback springback

θ θ

x x x

x θ θ M M

θ θ P P

Deformation

Deformation

yyyy yy

θ

θ = tan = tan - -1 1 (x / y) (x / y)

35 - 29

(21)

3-D shape Laminate Composite Fabrication (3)

ƒƒ Springback Springback compensation compensation by by changing changing bending bending angle angle of of the the mold mold :: ‘C’ ‘C’ shape shape

2

0

H-C

C-C

k angle ( θ k angle ( θ

PP

))

-4 -2

te springbacte springbac G-C

-6

Composi t Composi t

G

G- -C shape C shape C C- -C shape C shape H H- -C shape C shape

-8

-2 -1 0 1 2 3 4

Mold bending angle (θ Mold bending angle (θ

MM

))

θ

θ

M M

( ( °° )) θ θ

P P

( ( °° )) θ θ

M M

( ( °° )) θ θ

P P

( ( °° )) θ θ

M M

( ( °° )) θ θ

P P

( ( °° )) 0

0°° - -5.71 5.71°° 0 0°° - -2.29 2.29°° 0 0°° - -0.29 0.29°°

2.86

2.86°° 1.43 1.43°° 2.86 2.86°° 0 0°° - -1.43 1.43°° 0 0°°

1.43

1.43°° 0 0°° 2.86 2.86°° 0 0°° - -2.86 2.86°° 1.43 1.43°°

35 - 31

¾ ¾ G : Glass, C : Carbon, H : Hybrid G : Glass, C : Carbon, H : Hybrid

(22)

Springback (스프링백) (5) g ( ) ( )

0359 0

10 29 000

40 5

0 in Y psi E

6

psi t in

R ×

ƒ Example – estimating springback

0192 .

) 0 0359 .

0 )(

10 29 (

) 000 , 40 )(

5 . 0 (

. 0359 .

0 ,

10 29 ,

000 , 40

., 5 . 0 Et

6

Y R

in t

psi E

psi Y

in R

i i

× =

=

=

×

=

=

=

5 0

942 . 0 1 ) 0192 .

0 ( 3 ) 0192 .

0 (

4

3

R R

f

i

= − + =

W Lt k UTS

F

2 max

)

= (

. 531 . 942 0

. 0

5 .

0 in

R

f

= = W

Ref.

S. Kalpakjian, "Manufacturing Processes for Engineering Materials", 3rd/4th ed. Addison Wesley

(23)

Bending Operations (굽힘가공) (1) g ( ) ( )

ƒ Press-brake forming(프레스-브레이크 성형) : sheet metal or plate can be bent easily with p y

simple fixtures, using press.

ƒ Beading(비딩)

: edge of the sheet metal is bent into the g cavity of a die.

ƒ Flanging(플랜징)

: process of bending the edges of sheet : process of bending the edges of sheet

metals, usually in 90°.

ƒ Shrink flanging : compressive hoop stresses.

ƒ Stretch flanging : tensile hoop stresses. g g p

ƒ Hemming(헤밍)

: edge of the sheet is folded over itself.

ƒ Roll forming(롤성형) g( )

ƒ Tube bending(관재굽힘작업)

: bending and forming tubes and other hollow sections requires special tooling to o o sec o s equ es spec a oo g o

avoid buckling and folding.

Ref.

S. Kalpakjian, "Manufacturing Processes for Engineering Materials", 3rd/4th ed. Addison Wesley

(24)

Bending Operations (굽힘가공) (2) g ( ) ( )

Ref.

S. Kalpakjian, "Manufacturing Processes for Engineering Materials", 3rd/4th ed. Addison Wesley

(25)

Stretch forming (신장성형) g ( )

ƒ The sheet metal is clamped around its edges and stretched over a die or form block which moves upward downward or sideways die or form block, which moves upward, downward, or sideways, depending on the particular machine. (for low-volume production)

Ref.

S. Kalpakjian, "Manufacturing Processes for Engineering Materials", 3rd/4th ed. Addison Wesley

(26)

Bulging

ƒ Bulging involves placing a tubular, conical, or curvilinear hollow part in a split female die and expanding it with a rubber or part in a split female die and expanding it with a rubber or polyurethane plug.

ƒ Example – water pitchers(주전자), beads on drums(드럼통)

Ref.

S. Kalpakjian, "Manufacturing Processes for Engineering Materials", 3rd/4th ed. Addison Wesley

(27)

Rubber forming (고무성형) g ( )

ƒ One of the dies in a set is made of flexible material, such as a rubber or polyurethane membrane

rubber or polyurethane membrane.

ƒ Hydroforming(하이드로폼 가공) or fluid-forming(유체성형가공법)

Ref.

S. Kalpakjian, "Manufacturing Processes for Engineering Materials", 3rd/4th ed. Addison Wesley

(28)

Spinning

ƒ Spinning involves the forming of axisymmetric parts over a rotating mandrel using rigid tools or rollers

mandrel, using rigid tools or rollers.

Ref.

S. Kalpakjian, "Manufacturing Processes for Engineering Materials", 3rd/4th ed. Addison Wesley

(29)

Various Forming Methods (1)

ƒ Peen forming : used to produce curvatures on thin sheet metals by shot peening one surface of the sheet

shot peening one surface of the sheet.

Ref.

S. Kalpakjian, "Manufacturing Processes for Engineering Materials", 3rd/4th ed. Addison Wesley

(30)

Various Forming Methods (2)

ƒ Honeycomb structures

(a) Adhesive(접착제) → curing(경화) → stretching(확장) (a) Adhesive(접착제) → curing(경화) → stretching(확장)

(b) Designed rolls(롤) → corrugated sheets(골판재) → adhisive(접착 제) → curing(경화)

제) g( 화)

Ref.

S. Kalpakjian, "Manufacturing Processes for Engineering Materials", 3rd/4th ed. Addison Wesley

(31)

Deep drawing (1)

ƒ A flat sheet-metal blank is formed into a cylindrical or box-shaped part by means of a punch that passes the blank into the die cavity

means of a punch that passes the blank into the die cavity.

ƒ Example – cans(캔), kitchen sinks(싱크대), automobile panels(자동차 판)

ƒ Pure drawing(순수 드로잉) : the amount of drawn sheet metal P t t hi (순수 신장) th t f t t h d h t t l

ƒ Pure stretching(순수 신장) : the amount of stretched sheet metal

ƒ Draw beads(드로우 비드) : the blank can be prevented from flowing freely into the die cavity.

ƒ Deformation of the sheet metal takes place mainly under the punch and the sheet begins to stretch, eventually resulting in necking and tearing.

ƒ Ironing(아이어닝) g(아이어닝)

ƒ If the thickness of the sheet as it enters the die cavity is greater than the clearance between the punch and the die, the thickness will be reduced.

ƒ Limiting Drawing Ratio(LDR 한계 드로잉비) Limiting Drawing Ratio(LDR, 한계 드로잉비)

ƒ The maximum ratio of blank diameter to punch diameter that can be drawn without failure, or D

o

/D

p

.

ƒ Earing(귀생김)

ƒ Earing(귀생김)

ƒ Planar anisotropy causes ears to form in drawn cups, producing a wavy edge.

(32)

Deep drawing (2)

Ref.

S. Kalpakjian, "Manufacturing Processes for Engineering Materials", 3rd/4th ed. Addison Wesley

(33)

Deep drawing (3)

Ref.

S. Kalpakjian, "Manufacturing Processes for Engineering Materials", 3rd/4th ed. Addison Wesley

(34)

Deep drawing (4)

Ref.

S. Kalpakjian, "Manufacturing Processes for Engineering Materials", 3rd/4th ed. Addison Wesley

(35)

Deep drawing (5)

Limiting drawing ratio

ln w w

o

⎟ ⎟

⎜ ⎜

⎛ ε

ln t t R w

f o f t

w

⎟ ⎟

⎜ ⎜

⎟ ⎠

⎜ ⎝

=

= ε ε

ln w w

R

f

o f

⎟ ⎟

⎜ ⎜

ln w l l R w

o o

f f

f

⎟⎟ ⎠

⎜⎜ ⎞

⎛ ⎝ ⎠

=

4

2

45 90

0

R R

R = R + +

Ref.

S. Kalpakjian, "Manufacturing Processes for Engineering Materials", 3rd/4th ed. Addison Wesley

(36)

LDR (1)

Ref.

S. Kalpakjian, "Manufacturing Processes for Engineering Materials", 3rd/4th ed. Addison Wesley

(37)

LDR (2)

Maximum punch force Example – estimating the limiting drawing ratio

: stretched by 23% in length decreased in

1 or

=

= w t l

l t l w

t w l t w

o o o

f f f f

f f o o

o

⎟ ⎟

⎜ ⎜

⎛ −

= ( ) 0 . 7

max

p o o

p

D

UTS D t

D

F π

: stretched by 23% in length, decreased in thickness by 10%

23 . 1 or

23 .

0

f

=

− =

t t

t

l l l

l l

o o

o f

o o o

903 . 0

Hence

. 90 . 0 or

10 . 0

=

=

− =

w

t t t

t t

f

o f o

o f

965 107 0

. 1 ln ln

=

⎟ =

⎜ ⎜

= w

w R

w

f o o

isotropy planar

has sheet the

if

965 . 111 0 . 1 ln ln

=

=

=

⎟ ⎟

⎜ ⎜

= ⎛

R R

t t R

f o

4 . 2

Then,

isotropy.

planar has

sheet the

if

=

=

LDR R

R

(38)

Forming-limit Diagram

Ref.

S. Kalpakjian, "Manufacturing Processes for Engineering Materials", 3rd/4th ed. Addison Wesley

(39)

Design Considerations

Ref.

S. Kalpakjian, "Manufacturing Processes for Engineering Materials", 3rd/4th ed. Addison Wesley

(40)

Case study

Ref.

S. Kalpakjian, "Manufacturing Processes for Engineering Materials", 3rd/4th ed. Addison Wesley

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