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Sheet Metal Forming

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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.

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

Shearing

Drawing

Bending

Sheet

T

Punch

D

Die

Part or slug

F = 0.7 T L (UTS)

T = Sheet Thickness

L = Total length Sheared

UTS = Ultimate Tensile Strength of material

t

Y/2

s

Y

tmax = (1/2) Y

tmax = (2/3)1/2 Y

Tresca

Mises

Workpiece

Die

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

Stress distribution through the

thickness of the part

Y

Y

yY

s

s

s

h

-Y

-Y

Elastic

Elastic-plastic

Fully plastic

Springback

- Over-bend
- Bottom
- Stretch

Pure Bending

tension

compression

Bending & Stretching

Loading

Pre-stretching

Wrapping

Release

* source: http://www.cyrilbath.com/sheet_process.html

Stretch forming

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

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

Machines

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 PanelAirplane Body Panel

Progressive stampingstretch forming

1010 Steel, cold-rolled2024 Aluminum, T3 temper

.04” sheet, custom order.08” sheet, oversize

Double-sided Zinc cladmechanically polished

Cost ~ $.35-.45/lbCost ~ $4.0/lb

UTS ~ 300 MPaUTS ~ 470 MPa

YS ~ 185 MPa YS ~ 325 MPa

Elongation ~ 42%Elongation ~ 20%

n = .26n = .16

Aerospace Stretch Forming Body Panel Process

Parts Received

Mylar Protection Applied

‘Burr’ Edges in tension

Stretch Forming

Index to Block

Clad and Prime Surfaces

Chemical Milling

‘Burr’ Edges and Inspect

Hand Trim

Process Flow for Automobile Door Stamping Operation

Raw material

Blank material starting dimensions

Drawing

Pierce

Restrike

Flange

- 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

y

x

h

L

b

r = 1/K

M

M

y

sY

s

E

e

ey

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

- = E e e e
- Ye e

M

Loading

MY

EI

EI

Unloading

1/r

1/rY

1/R1

1/R0

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

4. Stress distribution through the thickness of the beam

Y

Y

yY

s

s

s

h

-Y

-Y

Elastic

Elastic-plastic

Fully plastic

Y

s

dA

ds

b

5. M = As 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 M=0 to M=MY the curve is linear.

Y

In the elastic – plastic region

yY

s

(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

To write this in terms of M vs 1/rrather than M vs yY, note that the yield curvature (1/r)Ycan 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)

Eqn(11)

M

Loading

MY

EI

EI

Unloading

1/r

1/rY

1/R1

1/R0

Substitution into (7) gives the result we seek:

(11)

(12)

Elastic unloading curve

Now, eqn’s (12) and (13) intersect at 1/r = 1/R0

Hence,

Rewriting and using 1/r = 2Y / hE, we get

(13)

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

Tool

Pallet

Parking Lot

Press Motion

Tool Setup

Actuators

Passive

Tool

Programmable

Tool

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

Large Scale Tool

6 feet

Stretch Forming with Reconfigurable Tool @ Northrop Grumman

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

$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

$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

Ref John Busch

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

See Appendix D; http://itri.loyola.edu/ebm/

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

- “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