Transcript Machining processes - Massachusetts Institute of Technology

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.
Historical Note;
Sheet metal stamping was developed as a mass
production technology for the production of bicycles
around the 1890’s. This technology played an important
role in making the system of interchangeable parts
economical (perhaps for the first time).
Steps in making Hub
Steps in Sprocket making
Stress Strain diagram – materials
selection
Basic Sheet Forming Processes
(from http://www.menet.umn.edu/~klamecki/Forming/mainforming.html)
Shearing
Drawing
Bending
Shear and corner press
Brake press
Finger press
Shearing Operation Force
Requirement
Sheet
Punch
D
Die
T
Part or slug
F = 0.7 T L (UTS)
T = Sheet Thickness
L = Total length Sheared
UTS = Ultimate Tensile Strength of material
Yield Criteria
t
Y/2
s
Y
t
max
= (1/2) Y
Tresca
t
max
= (2/3)1/2 Y
Mises
Schematic of a Blanked Edge
Bending Force Requirement
WT 2
F 
(UTS )
L
Force
Punch
Workpiece
T
T = Sheet Thickness
W = Total Width Sheared
(into the page)
L =Span length
UTS = Ultimate Tensile
Strength of material
Die
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
Stress distribution through the
thickness of the part
Y
s
Elastic
s
Y
yY
-Y
Elastic-plastic
s h
-Y
Fully plastic
Springback
•Over-bend
•Bottom
•Stretch
Pure Bending
tension
compression
Bending & Stretching
Stretch Forming
Loading
Wrapping
* source: http://www.cyrilbath.com/sheet_process.html
Pre-stretching
Release
Stretch Forming
Stretch forming
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 crosssectional 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
Auto body panels
10 - 11 panels
•3 to 5 dies each
• ~$0.5M each
• ~$20M investment
Tooling for Automotive Stamping
Machines
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
Auto Body Panel
Progressive stamping
1010 Steel, cold-rolled
.04” sheet, custom order
Double-sided Zinc clad
Cost ~ $.35-.45/lb
UTS ~ 300 MPa
YS ~ 185 MPa
Elongation ~ 42%
n = .26
vs.
Aerospace Example
Airplane Body Panel
stretch forming
2024 Aluminum, T3 temper
.08” sheet, oversize
mechanically polished
Cost ~ $4.0/lb
UTS ~ 470 MPa
YS ~ 325 MPa
Elongation ~ 20%
n = .16
Comparison of representative
Parts: Aero and Auto
Part Description
Forming Process
MATERIAL
Material
Scrap
Material Cost
Per part
LABOR
Set-up Time
Parts/Run
Cycle Time
Total Labor
Labor Rate**
Stretch-Form Labor Cost
FIXED
Equipment
Tools/Dies
TOTAL TRANSFER COST
Auto
Body Panel
54"X54"
Aero
Body Panel
54"X54"
Progressive Stamping
Stretch Forming
1010 Steel, cold-rolled,
.04" sheet, custom order
double-sided Zinc clad
40%
$0.45/lb
$15.75
2024 Aluminum, T3
temper, .08" sheet,
oversize mechanically
polished
20%
$4.00/lb
$105.00
1.5hr
2,000
0.25 min
0.30 min
$20.00/hr
$0.10
1.0hr
30
2.5 min
4.5 min
$20.00/hr
$1.50
$5,000,000
$900,000
$1,000,000
$45,000
(200 manhours labor)
$265
$25
Aerospace Stretch Forming Body Panel Process
Parts
Received
Mylar Protection
Applied
Clad and Prime
Surfaces
‘Burr’ Edges
in tension
Stretch
Forming
‘Burr’ Edges
and Inspect
Hand
Trim
Chemical
Milling
Index to
Block
Process Flow for Automobile Door Stamping Operation
Raw
material
Blank material
starting dimensions
Drawing
Pierce
Restrike
Flange
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
Springback
Elastic Springback Analysis
y
x
h
L
b
r = 1/K
M
M
y
1.
Assume plane sections remain plane:
ey = - y/r
2.
Assume elastic-plastic behavior for material
(1)
s
sY
E
ey
e
s= E e
e e
s sY
e e
3. We want to construct the following
Bending Moment “M” vs. curvature “1/r” curve
M
Loading
MY
EI
EI
1/rY
1/R1
Springback is measured as
Permanent set is
Unloading
1/R0
1/R0 – 1/R1
1/R1
1/r
(2)
4. Stress distribution through the thickness of the beam
Y
s
Elastic
s
Y
yY
-Y
Elastic-plastic
s h
-Y
Fully plastic
ds
5. M = A s y dA
dA
y
b
dy
h
Elastic region
M   sydA   E 
y2
dA  
EI
r
r
At the onset of plastic behavior
s = - y/r E = - h/2r E = -Y
This occurs at
1/r = 2Y / hE = 1/rY
(3)
(4)
Y
s
(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 M=0 to M=MY the curve is linear.
Y
In the elastic – plastic region
M   sybdy  2 
h/2
yY
 2Yb
2 h/2
y
2
yY
2
s
yY
y
Ybydy 2  Ybydy
y
0 Y
3 yY
Y y
b
yY 3
0
h2
2
 Yb(  yY2 )  yY2Yb
4
3
2

bh
1  yY  
M
Y 1  
 
4  3  h / 2  
2
(7)
Note at yY=h/2, you get on-set at yield, M = MY
And at yY=0, you get fully plastic moment, M = 3/2 MY
yY
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))
1
rY

eY
h/2
(8)
Where eY is the strain at yield. Also since the strain at yY
is -eY, we can write
1
r

eY
yY
Combining (8) and (9) gives
yY
(1 r )Y

h/2
1r
(9)
(10)
Substitution into (7) gives the result we seek:
 1  (1 r )
3
Y
M  M Y 1  
2
 3  1 r



2



(11)
Eqn(11)
M
Loading
MY
EI
EI
1/rY
1/R1
Elastic unloading curve
Unloading
1/R0
1/r
MY
M
(1 r )Y
1 1 
  
 r R1 
(12)
Now, eqn’s (12) and (13) intersect at 1/r = 1/R0
Hence,
 1  (1 r )
MY  1 1  3
Y




M
1

Y



(1 r )Y  R0 R1  2
 3  1 R0



2



Rewriting and using 1/r = 2Y / hE, we get
1
1
Y
2 Y 


3

4
R

0


hE
 hE 
 R0 R1 
3
(13)
New developments
Tailored blanks
Binder force control
Segmented dies
Quick exchange of dies
Alternative materials; cost issues
The Shape Control Concept
Conventional Tooling
Tool
Pallet
Parking Lot
60 Ton Matched Discrete Die
Press(Robinson et al, 1987)
Tool Setup
Actuators
Press Motion
Passive
Tool
Programmable
Tool
1.6
1.4
50
MAX
1.2
RMS
40
1
30
0.8
0.6
20
0.4
10
0.2
SYSTEM ERROR THRESHOLD
0
0
P1
P2
P3
PART CYCLE
P4
RMS Error [x0.001 in.]
60
[x0.001 in.]
MAXIMAL SHAPE ERROR
Cylindrical Part Error
Reduction
Large Scale Tool
6 feet
Stretch Forming with Reconfigurable
Tool @ Northrop Grumman
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
Standard fixtures
Alternative materials for auto
body panels
Comparison
Steel
Vs
$0.35/lb
0.03 thick
7.6 lb
40% scrap
$4.25 mat’l 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
Ref John Busch
SMC
$0.65/lb
.0.12 thick
7.0 lb
6% scrap
$4.84 mat’l 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
Cost comparison between sheet steel
and plastics and composites for
automotive panels
ref John Busch
Environment
punching Vs machining
hydraulic fluids and lubricants
scrap
energy
painting, cleaning
Steel can production at Toyo Seikan
See Appendix D; http://itri.loyola.edu/ebm/
Summary
Note on Historical Development
Materials and Basic Mechanics
Aerospace and Automotive Forming
New Developments
Environmental Issues
Solidworks and Metal Forming your Chassis
Readings
1. “Sheet Metal Forming” Ch. 16 Kalpakjian (3rd ed.)
2. “Economic Criteria for Sensible Selection of Body
3.
4.
5.
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