Dr.R.Narayanasamy, B.E.,M.Tech.,M.Engg.,Ph.D.,(D.Sc) Professor,

Department of Production Engineering ,National Institute of Technology,

Tiruchirappalli - 620015,Tamil Nadu,India.

Deep Drawing

By

Introduction

Deep drawing is a sheet metal forming operation.

Punch forces a flat sheet metal into a deep die cavity

Cylindrical, Conical, Rectangular shaped components can be manufactured.

Typical Tool Setup for Deep Drawing

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

If h/d > 0.5, it is called Deep drawing.If h/d <0.5, it is called Shallow drawing.

Introduction Cont….

Components

Cylindrical cupsUtensilsTube light starter

Square or RectangularFan regulator boxTube light soak cover

Conical & HemisphericalRocket cone nose

Calculation of blank diameter

Assumptions: No change in thickness

Where D = Diameter of the blank before forming

dhdD

dhdD

dhdD

4

4

4/

2

22

22

Stress Pattern

Metal is subjected to three different types of deformation in different regions namelyFlange regionWall regionPunch bottom region.

Stress Pattern During Drawing

Flange region In the radial direction it is subjected to Tensile stress.

The outer circumferences continuously decreases. This means the circumference of the blank is

subjected to Compressive Hoop stress. It is a Plane stress deformation. Because, only two stresses are acting on the flange

region. Thickening takes place in the flange region.

Wall region

Here the sheet is subjected to Bi-axial Tensile stress.

In the wall region the metal is bent and then straightened

Punch Bottom

Here the metal is getting Thinned down, at the bottom of the punch.

In the punch nose region sheet metal is subjected to plane strain deformation.

Total Punch Force

Total punch force can be divided into three components.

They are :Force due to idle deformation (Fideal)Force due to friction (Ff )Force due to ironing ( Fironing )

Punch Force Continued…. Force due to idle deformation:

Increases linearly with increase in stroke. Force due to friction:

Frictional force peaks early and decreases with increasing ram travel.

Force due to Ironing: Ironing occurs in the later stage of the drawing process.

Ironing takes place when the clearance between the die and the punch is not sufficient.

Punch force vs. stroke for Deep drawing

Clearance

Clearance > Sheet thickness.Less clearance leads to ironing.

Total Punch Force

Punch force = 1st Term +2nd Term + 3rd Term1st Term : Force due to idle deformation2nd Term: Force due to friction and blank holder

pressure.3rd Term: Force required to bend and straighten the

blank.

BeDD

HDD

hDPo

p

p

oop

2/)2(ln)1.1(

Notation

P =Total punch load σo = Average flow stress

Dp = Diameter of punch Do = Blank diameter H = Blank hold on pressure h = Wall thickness μ =Co-efficient of friction B = Force required to bend and straighten the blank

BeDD

HDDhDP

o

p

p

oop

2/)2(ln)1.1(

Fracture at the cup bottom Punch load is applied to the bottom of the cup. Punch load is transmitted to side wall of the cup. Usually failure occurs over the punch nose region

(Bent region of the cup). Metal in the wall is subjected to Tensile stress. It is plane strain stretching and thinning. If drawing stress ≥ tensile strength of the material,

fracture takes place. Su=Ultimate tensile strengthhDS pu

32

maxP

DrawabilityIt is the ratio of the initial blank diameter to

the diameter of the cup drawn from the blank.For a given material, there is a limiting draw

ratio(LDR) LDR = Limiting draw ratio

Do = Blank diameter LDR = Dp = Diameter of the punch

max

p

o

DD

Factors affecting drawability Die radius: should be about 10 times of sheet

thickness Punch radius : A sharp radius leads to tearing and

thinning Clearance between punch and die: 20 to 40 %

greater than sheet thick ness Hold down pressure: about 2 % of average of σys

and σts

Lubricate die side to reduce friction in drawing

Redrawing

Reducing the cup part to smaller diameter and increasing height is known as Redrawing

Methods of redrawing :Direct redrawing Reverse redrawing

Direct redrawing

The metal subjected to severe strain hardening .

This is due to bending and unbending at punch and die radii.

Reverse redrawing Cup is turned inside out, So the out side of the drawn cup

becomes the inside surface of the redrawn shell. The bending is always in the same direction. Less strain hardening of walls. Better control over wrinkling. Residual stresses are nullified. Increased drawability compared to direct redrawing. Greater reduction is possible if the metal is annealed in

between re draws. Most of the metal permit to a total reduction of 50 – 80 %

before annealing.

Methods to improve drawability

Cup nose wall region must be strengthened:This can be done by using rubber pads in the tool

set up.Drawability can be increased by roughening the

punch.With hold the lubrication to the punch improves

drawability.Heating the flange region may shift the failure.By controlling the crystallographic texture,

drawability can be increased.

Texture

• The correct texture gives the proper orientation of the slip systems.

• So that the strength in the thick ness direction is greater than that in the plan of the sheet.

• Resistance to thick ness thinning is measured by plastic strain ratio (R) R = = width strain / thickness strain.

)/ln()/ln(ttWWo

o

Defects in formed components

Bottom fracture Wrinkling Orange peeling Stretcher strain Earring

Bottom fracture

This is due to thinning near the punch radius. It can be minimized by :

Reducing the thinning by using a larger punch radius

Decreasing the punch load required for drawing operations

Wrinkling

This is due to high compressive circumferential stresses.

To prevent this defect, it is necessary to use sufficient hold down pressure to suppress the buckling.

Orange peeling

This occurs in sheet metal of relatively large grain size.

The individual grains tend to deform independently.

This can be minimized by using finer grain size sheet metal

Stretcher strain It is commonly found in low carbon steels. This defect shows up a flame like pattern of depressions, on

the surfaces. As the deformation continues , they spread and join

together to produce uniform rough surface. The depressions appear along planes of maximum shear

stress. This is non uniform deformation which results from yield

point elongation. To avoid this problem , cold reduction of 1-2 % deformation

is given in rolling after annealing operation

Earring

It is the formation of a wavy edge on the top of drawn cup .

This formation is due to directional property of sheet metal.

Earring can be correlated with the planar anisotropy( R)

R = (R0 + R90 – 2R45)/2

Plot of Blank Holder force Vs Depth of Shell

The Effect of Steel Grade on Depth of Shell

Construction of Tractrix Die

TRACTRIX Profile

Typical TRACTRIX Die

Conical Die

Tooling for combined cupping & Ironing

Two Different Punches

Reference

Dr.R.Narayanasamy, Ph.D. thesis on “Drawability of sheet metals”, 1992, National Institute of Technology, Tiruchirappalli – 620 015, Tamil Nadu, India.

THANK YOU

Dr.R.Narayanasamy, B.E.,M.Tech.,M.Engg.,Ph.D.,(D.Sc) Professor, Department of Production Engineering ,

National Institute of Technology, Tiruchirappalli - 620015,Tamil Nadu,India.

Email id’s: narayan@nitt.edu&

narayan10455@yahoo.co.in