12 sheet metal forming.ppt
TRANSCRIPT
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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?