Applications of sheet metal forming

1. Almirahs 2. Filing cabinets 3. Fan wings 4. Bodies of automobiles

5. Refrigerator and water coolers 6. Computer desktop bodies

7. Household appliances and utensils 8. beer and soft drink cans etc.

Advantages1. Low cost

2. Light weight

3. High strength

4. Good dimensional accuracy

5. Good surface finish

6. Possibility of large variety of shapes

Note: Sheet metal thickness varies from 0.5 mm to 6 mm. Most commonly used sheet material is mild steel or low carbon steel. In aerospace applications Al and Titanium are the common sheet materials.

Stresses induced Operations

Shear Shearing, blanking and punching, piercing, trimming, shaving, notching, nibbling and perforating

Tension Stretch forming

Compression Coining, sizing, ironing, hobbing

Tension and compression Drawing, spinning, bending, forming, embossing

Classifications of press tool operations

Shearing Action

1. As the punch starts, plastic deformation occurs around the punch (A,B,C,D)

2. With further movement of punch, cracks start forming on both top and bottom edges (T and T’)

3. As punch descends further, these cracks grow and if clearance is correct, cracks eventually meet each other resulting on separation of slug from sheet.

Characteristic of cut in shearing

Factors affecting the edge quality of hole and slug1. Punch force 2. Punch speed 3. Punch and die materials 4. the punch

die clearance and 6. lubrication

Calculation of clearances

0.0032. .C t mmWhere, t = sheet thickness, mm

= material shear stress, MPa

Clearance is the major factor determining the shape and quality of the sheared edge.

As clearance increases, the edge become rougher and zone of deformation becomes larger.

The ratio of burnished area to the rough area on the sheared edge deceases with increasing clearance

Burr height increases with increasing clearance.

Effect of clearance on shearing load and edge characteristics

If the clearance is optimum, two fracture lines from top and bottom of the sheet meets and a clean edge is obtained.

If clearance is small, then fracture lines miss each other and a secondary deformation takes place, resulting in an unclean edge and larger cutting forces.

If it is large, drawing action takes place and quality of edge is again quite poor.

Piercing : 1. production of cylindrical hole in a sheet metal. 2. the material punched out is the waste.

Punching : 1. similar to the piercing operation. 2. hole produced may be any other shape.

Blanking : 1. the metal punched out is the required product. 2. the plate with the hole left as waste.

Nibbling: a small punch is used to punch repeatedly along the necessary contour, generating the required profile.

Slitting: is an operation to cut a coiled sheet metal length wise to produce narrower strips.

Lancing: It makes cuts partway through the metal without producing any scrap.

Notching: material is removed from the side of the sheet metal.

Perforating: it is the operation of punching a number of holes in sheet metal.

Shearing operations

Shearing operations (contd.)

Blanking Punching Nibbling

Slitting LancingNotching

Perforating Trimming

Simple blanking/ piercing die

Importance of angular clearance on die

after the final breaking, the slug will spring back due to release of stored elastic energy.

this will make the blank to cling to the die face unless the die opening is enlarged.

It depends on the material, thickness, and shape of the stock used.

the normal value is from 0.25 to 0.75 deg/side.

Land: after every sharpening of the die the opening increases because of the provision of the angular clearance. So, to maintain the die size as per the design die land/cutting land is provided

Function of Stripper

due to the release of the stored elastic energy in the stock left on the die, the stock tends to grip the punch as the punch moves upward.

this necessitates the use of a stripper to separate the punch from the stock.

depends on the material, size and position of the punched holes.

Punching force calculation

punching force (N), =shear strength of sheet metal (MPa)

length of cut (mm)

Maximum forc )e(

where P

L

P Lt

( )Energy in press work E P punch travel P K t C

K=percentage of penetration required to cause rupture

C=1.16=amount of extra energy required

How to reduce the shearing force on punch

Requirements:

to accommodate a component on a smaller capacity punch press

is to distribute the cutting action over a period of time

shear is relived of the punch or die face so that it contacts the stock over a period of time rather than instantaneously

by providing the shear only reduces the maximum force to be applied but not the total work done in shearing the component

Shear is provided on the die for blanking and on the punch for piercing

Types of Dies

1. Progressive die 2. compound die 3. combination die

A simple progressive die It performers two or more operations simultaneously in a single stroke of punch press.

The place where each operations are carried out called stations.

The stock moves from station to station undergoing particular operations.

When the strip finally leaves the last station, a finished component is ready.

The pilot is used to bring the stock into the correct position for the succeeding blanking or piercing operations.

A compound die

It performers all the necessary operations at a single station, in a single stroke of the ram.

A compound die consists of the necessary sets of punches and dies.

During the part of the stroke, piercing of holes is done in the stock and upon further travel, the blanking operation is done.

Drawing

Making cups, shells etc.

Shallow drawing: cup height is less than half the diameter

Deep drawing: cup height is greater than half the diameter

Clearance in deep drawing is larger than that of blanking or punching operation.

Blank thickness+thickening of sheet d (12% of bluring dr ank thicawin kng ess)C

Flow of metal in deep-drawing

Flange

The deformation of the sheet is mainly in the flange and the cup wall is subjected only to elastic stresses.

The punch transmits the drawing force through the walls of the cup and to the flange being drawn into die cavity.

Material from flange moves radially to the die cavity.

Processes involved in deep-drawing

Drawing of flange under friction between the die and blank holding plate.

Bending of sheet at the die profile radius.

Slipping of sheet over the die profile radius.

Unbending of sheet into the cup wall.

Bending over punch profile radius.

Formation of wrinkling

Any flange section diameter is getting reduced while entering into die cavity. Therefore the flange suffers circumferential compression. Due to this compression, any element on the flange contracts in the hoop direction.

When any element of the flange lying at particular diameter moves towards the die cavity, its thickness increases because it is being reduced in diameter. Therefore, the outer most circumference has the maximum thickness (approx 12% larger than the blank thickness). These causes wrinkling.

Wrinkling

Blank holding in deep drawing

Blank holding plate removes wrinkling of the edges by applying sufficient pressure to the blank.

The excessive pressure will however increase the frictional force which will increase stresses in cup wall (due to increase in punch load) and may lead to early fracture at punch profile radius.

The tendency of wrinkle decreases with increase in the stock thickness.

If the ratio of punch diameter and stock thickness is quite small, tendency of wrinkle is minimized.

Deep drawability/drawing ratio: maximum blank diameter (D)/diameter of the cup drawn (d)------usually taken to be equal to punch diameter.

LDR (limiting drawing ratio): after which the punch will pierce a hole instead of drawing the blank. Maximum drawing ratio is 1.6-2.3

The ratio depends on: (i) type of material, amount of friction present etc.

Draw die design

Size of blank:

2 4 20D d dh when d r

2 4 0.5 15 20D d dh r when r d r

2 4 10 15D d dh r when r d r

2( 2 ) 4 ( ) 2 ( 0.7 ) 10D d r d h r r d r when d r

r=corner radius on the punch, mm

h=height of the shell, mm

d=outer diameter of the shell, mm

D= blank diameter, mm

Percent reduction:

Theoretically, it is possible to get a percentage reduction up to 50, but, practically limited to 40.

100 1d

PD

Height to No. of draws

dia ratio

Percent reduction

First draw Second draw

Third draw Fourth draw

Up to 0.75 1 40 ------- --------

-------

0.75 to 1.50 2 40 25

-------- --------

1.50 to 3.00 3 40 25 15 ---------

3.00 to 4.50 4 40 25 15 10

Reduction in drawing with cup height

1. Radius of draw die:

If the radius is too small, the resistance to the flow of metal increases resulting in cutting or tearing of the metal.

If the radius is too high wrinkles can be formed as the metal is released early by the blank holder.

2. Punch radius:

To avoid cutting or tearing the metal.

An excessive punch radius increases tendency of material to buckle.

When more than one draw is needed to give the final shape to the part, the minimum punch radius should be 4 times the stock thickness.

The center point should be within the cup of the next draw.

4 10dr to the stock thickness

3. Draw clearance:

It prevents from thickening of metal over the die radius when the flat blank is drawn into die cavity.

Otherwise the blank may get jammed in the die cavity

Blank thickness, mm

First draw Redraws

Up to 0.38 1.07t to 1.09 t 1.08t to 1.1t

0.40 to 1.27 1.08t to 1.1 t 1.09t to 1.12t

1.30 to 3.18 1.1t to 1.12t 1.12t to 1.14t

3.20 and up 1.12t to 1.14t 1.15t to 1.2t

Drawing Force

DP dts C

d

P=drawing force, N

T=thickness of the blank material, mm

S=yield strength of the metal, MPa

C=constant to cover friction and bending. Its value is between 0.6 and 0.7