Deep drawing :

Punch forces a flat sheet metal into a deep die

cavity

Round sheet metal block is placed over a

circular die opening and held in a place with

blank holder & punch forces down into the die

cavity

Deep drawing process :

Wrinkling occurs at the edges

Process variables in deep drawing. Except for the punch force,F,all the parameters indicated in the figure are independent variables.

Ex: beverage cans (soda).

How are they made?

Phase 1: a round blank is clamped between a blank holder and

a die with the punch initially placed as seen in the figure: the

punch then pushes the blank into the die cavity.

t

Blank

holder

die

punch

RD

RP

ri

r0

P B

ri

RP

Variables:

1. Geometry: Blank thickness, t

blank radius, ro

Cup radius, ri

die shoulder radius, RD

punch shoulder radius, RP

draw ratio,

2. Material: Stress-strain curve

anisotropy

friction coefficient

3. Process: Blank holder force, B

punch force, P

pD

DLDR 0

0

Consider a frictionless drawing of a rigid-perfectly

plastic sheet:

υ

σ

Y σr=0

ri

r’0

rr d

r

d

Summing forces:

r

d

r

rr The equilibrium

equation

We can represent the stress state by Mohr’s circle, stress

normal to the sheet is negligible.

σσrσϴ σt

τ

For Tresca Criterion

Y minmax

Equilibrium equation becomes

r

d

r

Y r

0

ir

f

i

r

rr

r

drYd

i

or

r

rY

iln

Deep Drawing

For ideal drawing:

i

rr

rY 0ln

ir

ri

Calculate force required for drawing

tr

P

i

ri

2

,ln2

2

0

i

i

ir

r

rYtrP

trPi

So, what is the biggest blank I can form for any given size cup?

Ytr

P

HtrYYtrP

i

ii

2

ln22 maxmax

Cancel terms to get:

ln(H)=e=2.718

This assumes perfect plastic material properties

drawn under frictionless conditions

In practice, for the most common metals;

Steel, Hmax = 2.1 to 2.3

Really r0 = 2.1 ri

Hmax = Limit Drawing Ratio, LDR

Example: Consider a sheet with the following properties that will be

deep drawn into a cylindrical cup shape.

t=0.041 in

Y=21000 psi

UTS=385000 psi

K=71000 psi

n=0.27

ri =1.5in

RD =0.375

λ=0.20

μ=0.10

By Experiment, found Pmax = 20000 Ho -20000 LB

Homax =2.2

Flow max stress chosen is: no

n

Hn

K

n

KY ln

11

Ho Pcalc Pexp

1.5 10800 10000

1.75 16400 15000

2.0 22000 20000

2.2 26500 24000

Comparing calc. and experimental

results

Deep drawability :

Deep drawability is expressed in LDR

Limiting drawing ratio (LDR)

LDR – Max blank dia/punch dia =Do/Dp

Drawability of metal is determined by normal anisotropy( R )

or plastic anisotropy.

R = width strain / thickness strain =ɛw /εt=ln(w0/wf)/ln(t0/tf)

Earing or planar anisotropy :

Edges of cups may be wavy this

phenomenon is called Earing

The above condition is called

planar anisotropy

∆ R = (R0 – 2 R45 + R 90 )/ 2

Where ∆ R = 0 => no ears formed

Height of the ears increases as ∆ R

increases.

Deep drawing Practice :

Blank holder pressure – 0.7% -1.0 % of Yield strength + UTS

Clearance usually – 7% -14 % > sheet thickness

.Ironing is a process in which the thickness of a drawn cup is made constant by pushing of the cup through ironing rings.

Redrawing – Containers or shells which are too difficult to draw in one operation undergo redrawing

Draw beads are used to control flow of blank into die cavity

the rate of material flow into die cavity must be controlled so that a better quality is

maintained and defects like wrinkling, tearing and galling are prevented

Generally the restraining force required to control the material flow is provided by

either the blank holder or the drawbeads. The blank holder creates restraining force by

friction between sheet and the tooling.

When a high restraining force is required, higher binder pressure must be applied to

increase the frictional resistance force, which which may cause excessive wear in the

tooling and galling in strip

In some cases the required binder force may exceed the tonnage capacity of the

press.

Hence a local mechanism is desired which restrains the material flow

sufficiently at relatively low blank holder pressure.

The draw bead consists of a small groove on the die surface / binder surface

matched by protrusion on the binder surface / die surface.

After the binder closure, the sheet metal is drawn over the drawbead and is

subjected to a bending and a subsequent unbending around the entry groove

shoulder, bead and at the exit groove shoulder.

These bending and unbending deformations together with the frictional force

account for the draw bead restraining force.

cup of inner radius Ri, wall thickness t0, and fairly small height H, is first

produced by deep drawing.

The thickness t0 is usually much less than the radius Ri.

Then, during ironing the cup is forced to flow into a conical die of semicone angle

α and inner radius Rf and is pushed downward at a velocity vf by a punch of

radius Ri over which the cup is mounted.

The gap between the die and the punch (Rf -Ri) is the thickness tf: this is the final

thickness of the cup, and tf < t0.

As the punch advances, the wall of the cup extrudes through the gap and its

thickness decreases from t0 to tf while the length H increases.

The outer radius of the cup decreases from R0 to Rf while the inner radius remains

constant at Ri.

Deep drawn cup; t0 << Ri.

Drawing without blank holder :

Deep drawing without blank holder must be provided

with sheet metal which is sufficiently thick to

prevent wrinkling

Range : Do – Dp < 5t

Lowers forces and increases drawability

commonly used lubricants are mineral oils ,soap

solutions,heavy duty emulsions.

Tooling & equipment for drawing :

Tool & die materials are tool steels cast irons carbides

Equipment is hydraulic press or mechanical press

Lubricants

Rubber forming :

In bending and embossing of sheet metal , the female die is replaced with rubber pad

Hydro-form (or) fluid forming process :

The pressure over rubber membrane is controlled through out the forming cycle ,with max pressure up to 100 Mpi

As a result the friction at the punch-cup interface increases ,this increase reduces the longitudinal tensile stresses in the cup and delays fracture

Bulging :

Process involves placing tabular,conical or curvilinear part into a split-

female die and expanding it

Segmented die :

Individuals are placed inside the parts and mechanically

expanded in radial direction and finally retracted.

Stretch formingSheet metal clamped along its edges and stretched over a

die or form block in required directions.

Stretch forming

Fig: Schematic illustration of a stretch forming process. Aluminum skins for aircraft can be made by this process

Spinning :

Shaping thin sheets by pressing them against a form

with a blunt tool to force the material into a desired

form

Conventional spinning :

A circular blank if flat or performed sheet metal hold

against a mandrel and rotated ,while a rigid metal is

held against a mandrel and rotated ,wile a rigid tool

deforms and shapes the material over the mandrel.

Fig 16.40 (a) Schematic illustration of the conventional spinning process (b) Types of parts conventionally spun.All parts are antisymmetric

Shear Spinning

Shear spinning :

Known as power spinning, flow turning, hydro-spinning, and spin forging

Produces axisymmetric conical or curvilinear shape

Single rollers and two rollers can be used

It has less wastage of material

Typical products are rocket-

motor casing and missile

nose cones.

Tube spinning :

Thickness of cylindrical parts are reduced by spinning them on a cylindrical mandrel rollers

Parts can be spun in either direction

Large tensile elongation up to 2000 % are obtained within certain temperature ranges and at low strain rates.

Super Plastic forming :

Advantages :

Lower strength is required and less tooling costs

Complex shapes with close tolerances can be made

Weight and material savings

Little or no residual stress occurs in the formed parts

Disadvantages :

Materials must not be super elastic at service temperatures

Longer cycle times

Explosive forming :

Explosive energy used s metal forming

Sheet-metal blank is clamped over a die

Assembly is immersed in a tank with water

Rapid conversion of explosive charge into gas generates a shock wave

.the pressure of this wave is sufficient to form sheet metals

Peak pressure (due to explosion):

caused due to explosion , generated in water

P = k( 3sqrt(w) /R)9

P- in psi

K- constant

TNT- trinitrotoluene

W-weight of explosive in pounds

R- the distance of explosive from the work piece

Diffusion Bonding and Superplastic Forming

Fig: Types of structures made by diffusion bonding and super plastic forming of sheet metal.

Such structures have a high stiffness-to-weight ratio.