Sheet Metal Forming

1
Sheet Metal Forming

• 2.810 Fall 2002
• Professor Tim Gutowski

Minoan gold pendant of bees encircling the Sun,
showing the use of granulation, from a tomb at
Mallia, 17th century BC. In the Archaeological
Museum, Iráklion, Crete.
2
Historical Note
Sheet metal stamping was developed as a mass
production technology for the production of
bicycles around the 1890s. This technology
played an important role in making the system of
interchangeable parts economical (perhaps for the
first time).
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Steps in making Hub
Steps in Sprocket making
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Stress Strain diagram materials selection
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Basic Sheet Forming Processes(from
http//www.menet.umn.edu/klamecki/Forming/mainfor
ming.html)
Shearing
Drawing
Bending
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Shear and corner press
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Brake press
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Finger press
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Shearing Operation Force Requirement
F 0.7 T L (UTS)
T Sheet Thickness L Total length Sheared UTS
Ultimate Tensile Strength of material
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Yield Criteria
t max (1/2) Y
t max (2/3)1/2 Y
Tresca
Mises
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Schematic of a Blanked Edge
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Bending Force Requirement
Force
Punch
T Sheet Thickness W Total Width Sheared
(into the page) L Span length UTS Ultimate
Tensile Strength of material
T
L
Engineering Strain during Bending e
1/((2R/T) 1) R Bend radius
Minimum Bend radius R T ((50/r) 1)
r tensile area reduction in percent
13
Stress distribution through the thickness of the
part
Y
Y
yY
s
s
s
h
-Y
-Y
Elastic
Elastic-plastic
Fully plastic
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Springback

• Over-bend
• Bottom
• Stretch

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Pure Bending
tension
compression
Bending Stretching
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Stretch Forming
Loading
Pre-stretching
Wrapping
Release
source http//www.cyrilbath.com/sheet_process.h
tml
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Stretch Forming
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Stretch forming
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Stretch Forming Force Requirement
F (YS UTS)/2 A
F stretch forming force (lbs) YS material
yield strength (psi) UTS ultimate tensile
strength of the material (psi) A
Cross-sectional area of the workpiece (in2)

• Example of Force Calculation
• Calculate the force required to stretch form a
  wing span having a cross-sectional area of
  .50X120 made from 2219 aluminum alloy having a
  yield strength of 36,000 psi and a UTS of 52,000
  psi
• F 88000/2 60 2,640,000 lbs 1320 tons
• Calculate the force required to shear a 10
  diameter, 1/8 thick blank from mild steel with a
  UTS of 45,000 psi
• F 0.7 (.125)(p)(10) 45,000 62 tons

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Auto body panels

• 10 - 11 panels
• 3 to 5 dies each
• 0.5M each
• 20M investment

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Tooling for Automotive Stamping
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Machines
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Material Selection
Material selection is critical in both product
and process design. Formability is the central
material property. This property must be balanced
with other product and process considerations
such as strength, weight, cost, and corrosion
resistance.
Auto vs. Aerospace Example Auto Body
Panel Airplane Body Panel Progressive
stamping stretch forming 1010 Steel,
cold-rolled 2024 Aluminum, T3 temper .04 sheet,
custom order .08 sheet, oversize Double-sided
Zinc clad mechanically polished Cost
.35-.45/lb Cost 4.0/lb UTS 300 MPa UTS
470 MPa YS 185 MPa YS 325 MPa Elongation
42 Elongation 20 n .26 n .16
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Comparison of representative Parts Aero and Auto
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Aerospace Stretch Forming Body Panel Process
Parts Received
Mylar Protection Applied
Burr Edges in tension
Stretch Forming
Index to Block
Burr Edges and Inspect
Hand Trim
Chemical Milling
Clad and Prime Surfaces
Process Flow for Automobile Door Stamping
Operation
Raw material
Blank material starting dimensions
Drawing
Pierce
Flange
Restrike
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Design Stretch Forming vs. Stamping

• Stretch Forming Advantages over Stamping
• Tighter tolerances are possible as tight as
  .0005 inches on large aircraft parts
• Little problem with either wrinkling or spring
  back
• Large, gently contoured parts from thin sheets
• Stretch forming Disadvantages over Stamping
• Complex or sharply cornered shapes are difficult
  or impossible to form
• Material removal blanking, punching, or
  trimming requires secondary operations
• Requires special preparation of the free edges
  prior to forming

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Springback
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Elastic Springback Analysis

• Assume plane sections remain plane
• ey - y/r (1)
• Assume elastic-plastic behavior for material

• E e??? e ?e ?
• ???Y e ?e

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3. We want to construct the following
Bending Moment M vs. curvature 1/r curve
Springback is measured as 1/R0
1/R1 (2) Permanent set is 1/R1
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4. Stress distribution through the thickness of
the beam
Y
Y
yY
s
s
s
h
-Y
-Y
Elastic
Elastic-plastic
Fully plastic
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dA
ds
b
5. M ?A s y dA
y
dy
h
Elastic region
(3)
At the onset of plastic behavior s - y/r E -
h/2r E -Y (4)
This occurs at 1/r 2Y / hE 1/rY (5)
Substitution into eqn (3) gives us the moment at
on-set of yield, MY MY - EI/rY EI 2Y / hE
2IY/h (6)
After this point, the M vs 1/r curve starts to
bend over. Note from M0 to MMY the curve is
linear.
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Y
In the elastic plastic region
yY
s
(7)
Note at yYh/2, you get on-set at yield, M
MY And at yY0, you get fully plastic moment, M
3/2 MY
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To write this in terms of M vs 1/r rather than M
vs yY, note that the yield curvature (1/r)Y can
be written as (see eqn (1))
(8)
Where eY is the strain at yield. Also since the
strain at yY is -eY, we can write
(9)
Combining (8) and (9) gives
(10)
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Substitution into (7) gives the result we seek
(11)
(12)
Elastic unloading curve
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Now, eqns (12) and (13) intersect at 1/r 1/R0
Hence,
Rewriting and using 1/r 2Y / hE, we get
(13)
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(No Transcript)
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New developments

• Tailored blanks
• Binder force control
• Segmented dies
• Quick exchange of dies
• Alternative materials cost issues

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The Shape Control Concept
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Conventional Tooling
Tool
Pallet
Parking Lot
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60 Ton Matched Discrete Die Press(Robinson et al,
1987)
Press Motion
Tool Setup Actuators
Passive Tool
Programmable Tool
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Cylindrical Part Error Reduction
6
0
1
.
6
1
.
4
5
0
M
A
X
1
.
2
R
M
S
4
0
1
MAXIMAL SHAPE ERROR
x0.001 in.
RMS Error x0.001 in.
3
0
0
.
8
0
.
6
2
0
0
.
4
1
0
0
.
2
S
Y
S
T
E
M

E
R
R
O
R

T
H
R
E
S
H
O
L
D
0
0
P
1
P
2
P
3
P
4
P
A
R
T

C
Y
C
L
E
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Large Scale Tool
6 feet
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Stretch Forming with Reconfigurable Tool _at_
Northrop Grumman
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Stamping and TPS Quick Exchange of Dies

• Ref. Shigeo Shingo, A Revolution in
  Manufacturing
• The SMED System Productivity Press. 1985
•  
• Simplify, Organize, Standardize,
• Eliminate Adjustments,
• Convert Internal to External Set-Ups

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Standard fixtures
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Alternative materials for auto body panels
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Comparison Steel Vs SMC

• 0.35/lb
• 0.03 thick
• 7.6 lb
• 40 scrap
• 4.25 matl cost
• 400/hr
• 5 workers
• 18.90/hr (Union)
• 0.24 labor cost
• 5,000,000 equipment
• 900,000 tools
• 7.71 unit cost at 100,000 units

• 0.65/lb
• .0.12 thick
• 7.0 lb
• 6 scrap
• 4.84 matl cost
• 40/hr
• 12.50/hr (non-Union)
• 0.63 labor cost
• 1,200,000 eqipment
• 250,000 tools
• 7.75 unit cost at 100,000 units

Ref John Busch
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Cost comparison between sheet steel and plastics
and composites for automotive panels ref John
Busch
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Environment

• punching Vs machining
• hydraulic fluids and lubricants
• scrap
• energy
• painting, cleaning

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Steel can production at Toyo Seikan
See Appendix D http//itri.loyola.edu/ebm/
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Summary

• Note on Historical Development
• Materials and Basic Mechanics
• Aerospace and Automotive Forming
• New Developments
• Environmental Issues
• Solidworks and Metal Forming your Chassis

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Readings

• Sheet Metal Forming Ch. 16 Kalpakjian (3rd ed.)
• Economic Criteria for Sensible Selection of Body
  Panel Materials John Busch and Jeff Dieffenbach
• Handout from Shigeo Shingo, The SMED System
• Steps to Building a Sheet Metal Chassis for your
  2.810 Car Using Solidworks, by Eddy Reif
• Design for Sheetmetal Working, Ch. 9 Boothroyd,
  Dewhurst and Knight