12 sheet metal forming.ppt

7/30/2019 12 SHEET METAL FORMING.ppt

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SHEET METAL FORMING

Sheet metal forming consists of three

basic processes; Shearing (cutting) to form a shape (blank)

Forming by bending and stretching

Finishing


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SHEARING

Needed to cut blanks from the large

sheets

A blank is the term for the rough shape

needed to form the final part


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SHEARING PROCESS

Clearance


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Like cutting paper with scissors but using amachine

Shearing starts with cracks developed ontop and bottom of sheet by exceptionallyhigh shear stresses

Top and bottom cracks join to shearslug/blank

Formability of sheared part influenced bythe quality of the edges


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Rough fracture surface due to cracks

Smooth and shiny burnished surface due to rubbingof sheared edge against wall of punch and die

Punched Hole


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SLUG

Sheared edges of the sheet and the slug are

not smooth, nor are they perpendicular to the

plane of the sheet


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Ratio of the burnished to rough areas

with ductility of the sheet metal with material thickness and clearance

Burr height increases with

clearance ductility

Dull-edged tools

Burrs can lead to cracks in subsequentoperations


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CLEARANCE

(a) As clearance, the sheet tends to be pulled intothe die rather than be sheared. Sheared edges

become rougher. Zone of deformation becomes

larger

(b) shows microhardness profile.


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Clearance control is important

Smaller the clearance better is the qualityof the edge

Ranges between 2 to 8% of sheets

thickness

Thicker the sheet, larger the clearance


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SHEARING FORCE

F = 0.7 T L (UTS)where

F force

T workpiece thickness

L total sheared length (e.g. hole perimeter)

UTS Ultimate tensile strength of workpiecematerial

Force required to punch a 25 mm diameterhole through a 3.2 mm thick annealed Ti-6Al-V sheet at RT

0.17 MN


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Process Parameters

Shape and materials for the punch and the

die

Bevel punch (like paper punch) and die

surface for shearing thicker blanks

Speed of punching

Lubrication

Clearance between punch and die


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SHEARING OPERATIONS

Simple shearing

Punching

Blanking


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Case Study: Outer Side Panel of Cars

Side Panel: Including 2 doors/windows

Process

Blanking different parts

Joined by laser welding (why?)

Advantages

Parts with different materials andthickness can be joined

Kalpakjian p. 348


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BENDING

Bend Allowance

(Length ofNeutral axis)

Band angle

Outer surface: Tension; Inner Surface: Compression

Neutral axis is located where e = 0


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BEND ALLOWANCE

Determine blank length

Bend allowance is the length of neutral axis inthe bend area

L = (R + kT)

where is bend angle in radians

R is bend radius

T is thickness

k is constant (ranges from 0.33 { R2T})and is determined by the location of the

neutral axis

If neutral axis is at the center of the sheet, k = 1/2


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MINIMUM BEND RADIUS

As material is bent, the tensile strain at the

outer skin or fiber increases until it cracks

The Minimum Bend Radius (MBR) is the

radius when a crack appears

MBR described in terms of the thickness of

the material


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MINIMUM BEND RADII FOR MATERIALS

Material Condition

Soft HardAluminum alloys 0 6T

Beryllium copper 0 4T

Low Lead brass 0 2T

Magnesium 0 13T

Stainless steels 0.5T 6T

Low carbon ,etc. 0.5T 6T

Titanium 0.6T 3T

Titanium alloys 2.6T 4T

FFT: Relation between MBR and ductility?


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MAXIMUM BENDING FORCE

P = k Y L T2

/ W , excluding frictionwhere

P is the bending force

Y is the yield strength of the material L is the length of the bend

T is the material thickness

W is the width of the die opening

k is a constant that depends on the

die shape (0.3 0.7)


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SPRINGBACK


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Caused by the elastic behavior of workpiece

material

Ri / Rf = 4(Ri Y / E T)3 - 3(Ri Y / E T) + 1

where

Ri is the required bend radius

Rf is the actual bend radius

Y is the yield stress

E is elastic modulus

T is the thickness


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Compensate springback by

Overbending

Applying compressive stress to bend zone

Stretch bending/forming (part in tension

during bending)

Raising temperature

Springback decreases as yield stresses

See Kalpakjian


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BENDING OPERATIONS

Tube Bending?

12 sheet metal forming.ppt

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