© 2008 pearson education south asia pte ltd chapter 7: sheet-metal forming processes manufacturing...

© 2008 Pearson Education South Asia Pte Ltd© 2008 Pearson Education South Asia Pte Ltd

Chapter 7: Sheet-Metal Forming ProcessesChapter 7: Sheet-Metal Forming ProcessesManufacturing Processes for Engineering Materials Manufacturing Processes for Engineering Materials

Manufacturing Processes Manufacturing Processes for Engineering Materials for Engineering Materials

(5(5thth Edition in SI Units) Edition in SI Units)

Chapter 7: Sheet Metal Forming ProcessesChapter 7: Sheet Metal Forming Processes

•Shearing-Blanking•Bending•Spinning•Deep Drawing



Sheet-Metal Characteristics• Forming of sheet metals is carried out by tensile forces

in the plane of the sheet.• Influencing parameters include:1. Elongation2. Yield-point elongation3. Anisotropy4. Grain size5. Residual stresses6. Springback7. Wrinkling



Elongation

• Tension undergoes uniform elongation till UTS and begins to neck.

• True-stress–true-strain curve represented by

• Necking takes place at an angle to the direction of tension.

nK n

strain at which necking begins



Elongation

• Necking is localized depends on the strain-rate sensitivity, m, of the material.

• Parameters that affect the sheet-metal forming process:

1. Yield-point elongation• Yield-point elongation involving upper and lower yield

points

mC



Elongation

2. Anisotropy• Acquired during the thermo-mechanical processing

history of the sheet.3. Grain size• Important for mechanical properties and surface

appearance for materials.4. Residual stresses• Due to non-uniform deformation that

sheet undergoes during forming.



Elongation

5. Springback• Subjected to relatively small strains during forming.6. Wrinkling• Circumferential compressive stresses that develop in

the flange.7. Coated sheet• Zinc is used as a coating on sheet steel to protect sheet

from corrosion.



Shearing

• Cutting sheet metal, plates, bars, and tubing into pieces using punch and die subjecting to shear stress.

• Variables are punch force, speed, edge condition of the sheet, materials, corner radii, punch-die clearance and lubrication.



Shearing

• As clearance increases, edges rougher and deformation zone larger. For tight clearance, die wears quickly.

• Deburring process



Shearing operations

2. Fine blanking• Can produce very smooth and square edges.



Shearing

Punch force• The punch force is product of shear strength of sheet

metal and cross-sectional area being sheared.• Maximum punch force is

• Due to plastic deformation, friction, and cracks, the punch-force vs. stroke curves can take on various shapes.

tLUTSF 7.0max UTS = ultimate tensile strengtht = thicknessL = total length of the sheared edge



Estimate the force required in punching a 25-mm diameter hole through a1.8-mm-thick 5052-O aluminium sheet at room temperature.

SolutionUTS for this alloy is 190 MPa.

Example 7.1Calculation of maximum punch force

kg 1880190258.17.0 F



Shearing operations

• Common shearing operations are:1. Die cutting• Parts produced have various uses:a) Perforatingb) Partingc) Notchingd) Slittinge) Lancing



Shearing operations

3. Slitting.• Carried out with a pair of circular blades.• 2 types of slitting equipment:1. driven type, the blades are powered2. pull-through type, the strip is pulled through idling

blades.



Bending of Sheet and Plate

• Used to form parts and impart stiffness.• Bend allowance is the length of neutral axis in bend

area and used to find blank length for a bent part.• Bend allowance is given as

ktRLb a = bend angleR = bend radiusk = constantt = sheet thickness



Minimum bend radius

• Strains at outer and inner fibers are equal in magnitude.

• Due to shifting of neutral axis towards the inner surface, the length of bend is in inner region.

• Minimum bend radii for various materials have been determined experimentally.

1/2

10

tR

ee i



Minimum bend radius

• Studies shows that there is a relationship between the minimum R/t ratio and a mechanical property of the material.

Factors affecting bendability• Bendability increased by increasing its tensile reduction

of area.• As length increases,

minimum bend radius increases.

160

min rt

R



Springback

• In bending, recovery is called springback.-elastic zone• Relationship for pure bending is

• From this relationship, Ks is defined as

• Approximate formula to estimate springback is (Handbook)

ffii

tR

tR

22allowance Bend

12

12

tR

tRK

f

i

i

fs

1343

Et

YR

Et

YR

R

R ii

f

i



A 20-gage (0.091186 cm) steel sheet is bent to a radius of 1.27 cm. Assumingthat its yield stress is 275.79029 MPa, calculate (a) the radius of the part afterit is bent, and (b) the required bend angle to achieve a 1.57 rad bend afterspringback has occurred.

Solutiona. The appropriate formula is

andHence, b. Required bend angle is

Example 7.2Estimating springback

942.010192.030192.04 3 f

i

R

R

0192.0

091186.096.199947

79029.27527.1

Et

YRi

cm 348.1942.0

27.1fR

rad 665.1

12

12

tR

tR

i

ffi



Springback

Negative springback• Bend angle becomes larger after the bend is completed

and load removed.

Compensation for springback1. Overbending2. Coining3. Stretch bending4. Carried out at elevated

temperatures



Forces

• Bending force is a function of the material’s strength, length and thickness of the part.

• General expression for the maximum bending force is

Common bending operations1. Press-brake forming2. Other bending operations3. Beading

W

LtUTSkF

2

max



Tube bending

• Tubes can be plugged with various flexible internal mandrels.



Miscellaneous Forming Processes

Stretch forming• Sheet metal is clamped and stretched over a die or

form block.• Make aircraft-wing skin panels, automobile door panels,

and window frames.• Cannot produce parts with sharp contours or re-entrant

corners.• Used for low-volume production,

it is versatile and economical.



A 38.1-cm-long sheet with a cross sectional area of 3.2258 cm2 isstretched with a force, F, until α = 0.35 rad. The material has a true-stress–truestrain curve σ = 100,000 e0.3. (a) Find the total work done, ignoring end effectsand bending. (b) What is αmax before necking begins?

Solutiona) The true strain is

Work done per unit volume is

Since volume of workpiece is

The work done is

Example 7.4Work done in stretch forming

3114.0

0

3.05114.0

0

kg/cm-cm 3028.32110 ddu

114.01.38

67.42lnln

0

L

L f

3cm 90298.1222258.31.38 V

kg-cm 071.489,39 VuWork



Solutionb) The necking limit for uniaxial tension is

From similar triangles, we obtain

For necking to begin,

Example 7.4Work done in stretch forming

cm83.20 and cm 48.30 Thus

87.483or 7.124.25 222222

ba

baba

cm 308.511.38 3.00max nLL

rad 59.0833.048.30

4.25cos max



Miscellaneous Forming Processes

Bulging• Process involves placing a tubular part in a split female

die and expanding it with a rubber or polyurethane plug• Formability enhanced by compressive stresses

longitudinal to the parts.



Spinning

Conventional spinning• A circular sheet metal is held against a rotating mandrel

where the tool deforms and shapes it over the mandrel.• Tooling costs are low and economical for relatively

small production runs only.Shear spinning• Axisymmetric conical is

generated where the diameter of the part remains constant.



Spinning

Shear spinning• 2 rollers are preferred to balance the radial forces and

maintain dimensional accuracy.• The thickness of the spun part is• For ideal case in shear spinning of a cone,

• True strain is

• Max spinning reduction in thickness is

sin0tt

sin0 futFt Ft = tangential forceu = specific energy of deformation

3

cot

3

%1000

0 t

tt f



Spinning

Shear spinning• For an ideal case in shear spinning of a cone,

• u is the area under the true-stress–true-strain curve,

• As the thickness decreases, the max spinning reduction in thickness is

sin0 futFt

3

cot

3

%100reductionMax 0

0

t

tt f



Spinning

Tube spinning• Tubes or pipes reduce thickness by spinning them on a

cylindrical mandrel using rollers.• Ideal tangential forward force is

• Friction will double the actual force using the equation above.

fttYFt 0



High-Energy-Rate Forming

Explosive forming• The peak pressure generated in water is given by the

expression

• Compressibility of the energy-transmitting medium and acoustic impedance is important for peak pressure.

a

R

WKp

3

p = peak pressure K = constant for type of explosive W = weight of the explosiveR = distance of explosive from workpiece a = constant



Calculate the peak pressure in water for 0.04536 kg of TNT at a standoff of0.3048 m. Is this pressure sufficiently high for forming sheet metals?

SolutionPeak pressure is

This pressure is sufficiently high to form sheet metals.

Example 7.5Peak pressure in explosive forming

MPa 75.6153048.0

04536.0514

15.13

p



Deep Drawing

• Flat sheet-metal blank is pressed, using punch, into the die cavity.

• Bank is held in place with a blankholder under a certain force.



Deep DrawingVariables in deep drawing are:1. Properties of the sheet metal.2. Ratio of the blank diameter to the punch diameter.3. Sheet thickness.4. Clearance between the punch and the die.5. Corner radii of the punch and die.6. Blankholder force.7. Speed of the punch.8. Friction at the punch, die and workpiece interfaces.



Deep Drawing

• During the deep-drawing, workpiece is subjected to the states of stress.

• Important to know how much pure drawing and stretching is taking place.

• Stresses increase with an increasing Do/Dp ratio and can eventually lead to failure.



Deep drawability (limiting drawing ratio)

• LDR is defined as the max ratio of blank diameter to punch diameter that can be drawn without failure, Do/Dp.

• Normal anisotropy of the sheet metal is

• Based on volume constancy, it reduce to

• Average R value is

f

f

t

w

tt

ww

R0

0

ln

ln

00

0

ln

ln

lw

lw

ww

Rff

f

4

2 90450 RRRR

where the subscripts refer to angular orientation



Estimate the limiting drawing ratio (LDR) that you would expect from a sheetmetal that, when stretched by 23% in length, decreases in thickness by 10%.

SolutionFrom volume constancy,

From the information given, we obtain

Hence,

Thus,

Example 7.7Estimating the limiting drawing ratio

903.00

w

w f

1000

000 lrw

ltwltwlrw ffffff

90.0or 10.0 23.1or 23.000

0

00

0

t

t

t

tt

l

l

l

ll ffff

4.2965.0lnln 00

LDR

tt

ww

Rff



Show that, assuming no thickness change of the sheet during drawing, the theoretical limiting drawing ratio is 2.718.

SolutionThe diametral change from a blank to a cup involves a true strain of

Drawing stress is , in the limit

Consequently,

Example 7.8Theoretical limiting drawing ratio

pp D

D

D

D 00max lnln

max Yd

718.20 pD

D

1or maxmax YY




Earing• Planar anisotropy causes ears to form in drawn cups.• The controlling parameters: 1. alloying elements2. processing temperatures3. annealing cycles after processing4. thickness reduction in rolling5. cross (biaxial) rolling of plates to make sheets



A steel sheet has R values of 0.9, 1.3, and 1.9 for the 0 rad, 0.785 rad, and 1.57 raddirections to rolling, respectively. For a round blank 100 mm in diameter, estimatethe smallest cup diameter to which it can be drawn. Will ears form during this operation?

SolutionSubstituting the given values

To determine whether or not earing will occur in this operation,

Ears will form in deep drawing this material.

Example 7.9Estimating cup diameter and earing

35.1

4

9.13.129.0

R

1.0

2

9.13.129.0

R




Maximum punch force• Work consists of deformation, redundant, frictional and

ironing.• Approximate formula for the maximum punch force is

• The punch force is supported by the cup wall.• Tearing occurs when force is excessive.

7.00

0maxp

p D

DUTStDF



Deep-Drawing practiceConsiderations in deep drawing:1. Clearances and radii2. Draw beads3. Blankholder pressure4. Redrawing5. Drawing without a blankholder6. Tooling and equipment7. Lubrication



Formability of Sheet Metals and Modeling3 factors that influence formability:1. properties of sheet metal2. friction and lubrication 3. characteristics of the equipment, tools and dies used

Testing for formability:1. Tension tests2. Cupping tests3. Bulge test4. Forming-limit diagrams5. Limiting dome-height test



A thin-walled spherical shell made of an aluminium alloy is being expanded by internal pressure. If the original shell diameter is 200 mm, what is the maximum diameter to which it can safely be expanded?

SolutionMax allowable engineering strain is about 40%, thus,

Example 7.10Estimating diameter of expansion

mm 280

40.0200

200

0

0

f

ff

D

D

D

DDe



Dent resistance of sheet-metal parts

• A dent is a small, permanent, biaxial deformation.• The factors in dent resistance is yield stress, thickness,

and shape of the panel.• Dent resistance is expressed as• S is the panel stiffness defined as• Denting needs higher energy levels as compared to

static conditions.• Dynamic forces cause more localized dents than static

forces.

S

tY 42

resistanceDent

ShapetES a



Equipment for Sheet-Metal Forming

• Operations consists of mechanical, hydraulic, pneumatic, or pneumatic-hydraulic presses.

Press selection includes the considerations of:1. types of forming operation, 2. size and shape of the parts3. length of stroke of the slides4. number of strokes per minute5. press speed6. shut height



Design Considerations

• Guidelines for design issues of sheet-metal forming operations:

1. Blank design2. Bending 3. Stamping and progressive-die operations4. Deep drawing

© 2008 pearson education south asia pte ltd chapter 7: sheet-metal forming processes manufacturing...

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