Port Dickson Polytechnic

CHAPTER 3

Performed on sheet metal with thickness between 1/64 in (0.4 mm) to 1/4 in. (4.0 mm)

Commercially important where it is used in many products.

Sheet metal forming processes are the primary ones for manufacture of automobile body

Most sheet metal forming processes are cold working.

Basic categories:

• Cutting

• Bending

• Drawing

Tooling usually referred to as punch and die

Machine tools are presses and referred to as stamping presses

Products are stampings.

Press may be grouped in two categories :

1. Cutting Operation.

ex : blanking, punching, nothching, lancing etc

2. Forming Operation.

ex : bending, drawing,

PRESS

• Presses are the machine tools for metal

forming operations

• Presses are used to apply a large force

on the piece to conduct the desired

operation; cutting or forming.

• Classification of Presses:

1. Based on the drive type:

• Mechanical

• Hydraulic or

• Manual

2. Based on Frame type

• Arch

• Gap

• Straight-sided

3. Method of slide actuation; This

classification relate to mechanical

system with flywheel • Crank shaft • Cams

• Screws • Rack and pinion

• Knuckles • Toggle

4. Based on number of slides (Actions) • Single-action Press

• Double-action Press

• Triple-Action Press

Open Back Inclinable (OBI) Press – It is also called a gap-frame press

– Most common

– It has C-shape frame that allows

access to its working space

– The frame can be inclined at an

angle to the base, allowing for the

disposal of the finished parts and

scrap by gravity

– Open back to permit strip feeding

from front to back or ejection of

finished parts out the back

– Main use is for blanking and

piercing operations in small

workpiece

• Basic Components of a Press : – Press bed: Supports the bolster plate, open in the center to allow finished parts or scrap to fall by gravity – Bolster plate: A flat steel or cast iron plate fastened to the top of the bed. The die set is usually fastened to it. Bolster plates have a standard dimensions and openings set forth by Joint Industrial Council (JIC). Standard Dimensions are given in Table 1. – Knockouts: Used for ejection of works or blanks, usually operates on the up-stroke

Table 1

Ram or Slide : Provides the motion to the punch holder through its stroke. The length of ram stroke is function of the machine design

or machine size.

Pitman Arm : The connecting link between the ram and the

crank or main shaft.

Power supply : Mechanical; Flywheel, or Hydraulic power

Cushion: An accessory located beneath or within a bolster plate

for producing upward motion or force. It is actuated by air, oil,

rubber, springs or high pressure

• Press Tonnage : The force in tonnes that a press can apply safely on work. • Stroke : The reciprocating motion of the slide. it is the distances travelled

by ram of press during it downward or upword motion. It is expressed in centimetres.

• Shut Height : It is the distance from the die shoe to die holder of the punch

holder when the die is in its closed position. Distance H indicate shut height.

• Die Space : It is the area available for mounting dies in the press. • Standard Press Identification : Developed by JIC as

S2-450-36-28 : S for single action, D for double action, T for triple

action, 2 indicates two-point suspension, 450 press rate capacity 450-ton, and 36 and 28 are the bed size in inches; left to right and front to back.

Common component of a simple die:

- A die is a tool to cut or shape thin metal.

• Right half the die shown in

closed position

• Left half shows the die in

open position.

• Pilots are used to guide the

punch.

• Note: Blank holder and

stripper are not shown

Process plan

– Punch or pierce the two

holes in one stage

– Make Blank in the second

stage, use pilots to register

the work piece for blanking.

A. Wide strip layout B. Narrow strip layout • Selection of layout is function of: – the available sizes of the sheet strips, and – the feed; the length of the travel of the strip between stations.

• Punch holder and die holder

for a Die Set.

• Punch plate: Block of steel to

retains the punches with their

head against the punch holder

• Blanking punch and piercing

punches

• Pilots

• Stripper plate: Free punches from

the scrap strip after operation

• Finger stops: For locating strip at

the first station

• Automatic stop: Automatically

locates the strip at the second

station

• Back gage and front space: Guide

the strip during travel

• Fasteners: For holding various

components of the die

• Bushings

• Dowel

• etc.

Dies are classified by the types of operation perform and by type of construction.

Various types of operation dies are :

1. Simple Dies : Perform only one operation such as blanking, piercing, notching, trimming.

2. Multi Operation Dies : perform several operations in stroke of ram.

Further Classified ;

a) Compound Dies

b) Progressive Dies

c) Combination Dies

Compound dies – two or more cutting operations such as blanking and piercing can be perform in a single stroke at single station.

Combination dies – cutting and forming are combine and carried out in single operation.

Progressive dies – workpiece moves from one station to another, with separate operations being performed at each station.

• Designed to perform several sheet metalworking processes • Can be designed to blank, draw, pierce, etc. in the same time • Figure shows the following processes: – Blank at A – Drawing at B – Trimming at C – Shearing of the center at D

A progressive has a series of

stations.

At each station, an operation

is performed on a workpiece.

• First station for piercing three holes • Second station idle • Third station for piercing two notches • Fourth station idle • Fifth and sixth station perform forming • Seventh station idle • Eighth final forming process and part ejection

PUNCH CHARACTERISTIC : i. Punches should not defect during use

ii. Punches should be of proper hardness

iii. Should be strong enough to withstand force

iv. Should not rotate as a result of cutting action

Types of punches – Numerous types to be discussed

– Basic types :

• Plain punch

• Pedestal punch

• Perforator punch

• Punches mounted in plates

1. Plain Punch • Block of hardened tool steel

shaped to conform to the

cutting contour.

• Either mounted directly to the

punch holder, or onto a flat

punch plate, an extra length is

needed.

• Secured by screws and

dowels.

• Easy and economical to

construct.

2. Pedestal Punches (Flanged Punches)

• Constructed by machining with a flange. • Base area larger than cutting force. • Stability is an advantage. • Suitable for heavy cutting loads.

3. Perforated-Type Punches

• Punches with cutting

face diameter less than

or equal to 1”.

• Commercially available

punches or inserts.

• There are many methods

for mounting perforator

type punches as shown

in the figure.

4. Punches mounted in punch plates Odd shaped punches are not as easy as the perforators to mount

on punch plates.

• Fig. a: mounting to increase

rigidity.

• Fig. b: A stephead punch

mounted in the punch plate.

• Interference fit used between

punch and punch holder to

ensure adequate location.

• Fig. c: Shows bevel head punch

similar to a step head punch.

• Square or rectangular punches

are difficult to mount, because of

requirement to machine corners.

Piercing punches should not be smaller in diameter than the thickness of the strip.

If it is necessary, then punch shank diameter should be at least 2 times the hole diameter.

The maximum allowable punch length (L) can be calculated using the following formula :

where:

d = punch diameter,

t = strip thickness,

E= Modulus of elasticity, and

Ss/Fs= unit shear strength of stock.

The punch should have sufficent compressive strength so it can apply necessary force required during punching :

Let d = Minimum diameter of hole (mm)

t = material thickness (mm)

fs = Shear strength of work material

fc = compressive strength

P = Cutting force = πdtfs

Fc = Compressive strength of punch = πd fc

4

2

1. Stepped Punches

2. Single Shear on die or punch

3. Double Shear on die and punch

The amount of shear to be ground on the punch or die depends on the reduction in punch force required

Function of a pilot is to position the workpiece or stock strip accurately

Pilots are made of steel heat treated for maximum toughness and to a hardness of rockwell HRC 56 to HRC 60

Pilots are two types :

1. Acorn @ Spherical Types 2. Flattened End Type

Pilots are be fitted to the punch by following methods :

i. Press fit pilot (pic. a)

ii. Threaded shank pilots (pic. b)

iii. Socket set screw pilots (pic. d)

Cutting sheet metal usually done by shearing action between sharp edges.

Figure (Groover) depicts the stages of shearing actions in sheet metal.

1. Just before the punch contacts the work

2. Punch begins to punch into work,

causing plastic deformation

3. Punch penetration into work;

“about one-third of the sheet thickness”,

appearing as smooth cut surface

4. Fracture initiation at opposing cutting edge

and leads to separation

Symbols: V = motion, F = force

The characteristics of sheared edge

are:

• Rollover at the top as a result of

plastic deformation

• Burnish is a result of punch

penetration, and relatively

smooth

• Fractured zone, rough surface

of the cut edge, due fracture of

the sheet.

• Burr, sharp edge due to the

elongation of the metal during

the final separation stage.

Figure 15-36 (DeGarmo) shows

a photograph for a blanking

edge without treatment.

The basic components include:

– Punch is the cutting tool

attached to the ram of the

press. Punch face is usually

ground normal to the axis of

motion or to an angle called

shear. The shear helps in

reducing the cutting force.

– Die is attached to the bolster

plate of the press.

– Stripper plate to prevent

material from riding upward

on the punch as it moves

upward.

CUTTING OPERATIONS

Basic Component of Blanking Die

►SHEARING – It is the cutting operation along a

straight edge

– It is performed on a large sheet using

power shears (Figure 15.40 DeGarmo)

Shearing, Blanking and Punching

► BLANKING

– cut out part from the material strip (blank)

is the required component/product.

The hole and metal left behind as waste.

– Blanking is performed

by a set of a die and a punch.

PRESS OPERATIONS

BLANKING

►PUNCHING / PIERCING

- Shaped hole are made in sheet metal.

Cut out from sheet metal is waste,

hole left in the strip being required.

CUTTING OPERATIONS

Punching/Piercing

PUNCHING/PIERCING

NOTCHING

This operation is used to remove

small amount of metal from the

edges of the metal strip (sheet metal).

LANCING

Combined bending and cutting

operation.

CUTTING OPERATIONS

CUTTING OFF

separate the work material along

a straight line in a single line cut

PARTING

separate the work material but

in a double line cut way.

CUTTING OPERATIONS

CUTTING OPERATIONS

Clearance, c • Typical value 4% to 8% of sheet metal thickness t,

Figure 22.5 (Groover).

• Small clearance tends to cause double burnishing

and larger cutting force

• Large clearance leads to heavy burrs.

• In special operations requiring straight edge such as

shaving and fineblanking clearance = 1% of stock

thickness.

• clearance:

c = at

where c = clearance in in. (mm),

a = allowance,

t = stock thickness, in. (mm)

The clearance can be used to

calculate the size of the punch

and die. Figure 22.6 shows the

die arrangement.

The dimensions depends on the

die punch function; blanking or

punching.

For round blank of Diameter Db:

Blanking punch diameter = Db - 2c

Blanking die diameter = Db

CUTTING OPERATIONS

For round hole “punching” of

diameter Dh:

– Hole punch diameter = Dh

– Hole die diameter = Dh + 2c

Die opening should have angular

clearance of 0.25° to 1.5° on each

side to allow the blank or slug to

drop out (see figure beside).

CUTTING OPERATIONS

Angular Clearance

Cutting force is a function of the area of the cut edge being

sheared at any instant and the shearing strength of the

workpiece material.

CUTTING OPERATIONS

• CUTTING FORCE

P = Cutting force (tonnes)

= π D t τs (for round hole)

= L t τs ( for other contours)

Where:

• τs = shear strength of the sheet metal, lb/in2 (Mpa), if τs is unknown,

τs = 0.7 to 0.8 TS.

• t = stock thickness, in. (mm);

• L = length of the cut edge, in. (mm)

• D = hole diameter (mm)

CUTTING OPERATIONS

CUTTING OPERATIONS

Compound dies Progressive dies

1. More than one

operation at one time

at one station.

2. Tonnage press is

more because need

double action or

tripple action press.

3. More accurately

4. More expensive to

construct and repair

1. Perform one

operation at a time at

a station.

2. Simpler in

construction

3. Economical to repair

CUTTING OPERATIONS

Economical stock utilization is of high importance. The goal should beat at least 75 % utilization.

In preparing layout should consider the distance between nearest two point of blanks and between the blank and the edge of strip should be less than the sheet thickness

• The part could run using any of, say, four layouts: • Wide run positioning – Requires wider die and

– High waste of material

• Narrow run positioning – Useful when bend is required, however, it is

high waste of material.

• Double raw layout – Less waste of material, but may lead to

expensive difficult to run dies

• Nesting – Can be optimized for material utilization

– Can be arranged for optimal run and die

design.

Nesting

The formula used in calculating scrap strip dimension (over 2mm thickness):

L = Length of part

H = Width of part

W = Width of scrap strip

t = thickness of material

D = Lead of die

B = Allowance between successive blanks

= 1 ¼ t when D is less than 60 mm

= 1 ½ t when D is 60 mm or more

D = L + B

N = Number of blank that can be punch

= S - B

D

S = length of stock

Thickness (less than 2 mm), then B use table below :

Values of stock allowance B (when t > 0.625 mm) :

It is the centre of gravity of the line (the perimeter of the blank).

Centre of pressure determine by : i. Draw the outline of the actual actual cutting edge .

ii. Draw XX and YY axis at right angle in convention position.

iii. Divide the cutting edge into line element such as straight line, arc etc. Find lengths L1,L2,L3,L4, etc

iv. Mark the centre of gravity of these elements say C1,C2,C3,C4 etc

v. Find X1,Y1 the distance of gravity C1 from YY axis and XX axis

vi. Determine the distance X and Y of centre of pressure :

Example :

Calculate the centre of pressure of the component below.

Bending in sheet metal is defined as straining of the metal around a straight axis (see Figure). – Neutral axis

– Outside fibers exposed to

tension and inside fibers

exposed to compression

– Metal goes through plastic

deformation to bend metal

permanently

– Bending operations usually

performed using a punch and

die

BENDING OPERATIONS

i. Edge Bending

ii. V-bending

iii. U-bending

BENDING OPERATION

Fig : Edge Bending Fig : V-bending

Low production, also called air bending

Material bent at one edge with help of punch

Used U-shaped punch

BENDING OPERATION

BENDING OPERATION

1. Material to be bend be ductile and strong. It should not be hard

2. Bending is smooth if the axis of the bend is perpendicular to the direction of grains.

3. Spring back phenomenon should be taken care of

4. Hole pierced before bending will be distorted if they are close to the bend area

5. In most of bending operation lubrication required is very less

BENDING OPERATION

t = thickness of material

S = set back

A = Bend allowance

BENDING OPERATION

Konstant function for K;

For V-bending = 1.33 for (W=8t)

= 1.2 for ( W=16t)

U-bending = 2.66 for (W=8t)

= 2.4 for (W=16t)

Edge bending = 0.67 for (W=8t)

= 0.6 for (W=16t)

BENDING OPERATION

(Mpa)

For edge bending width ( W ) between contact point on die :

W = R1 + R2 + C

where ;

R1 = Die radius t = thickness

R2 = Punch radius

Clearance, C = t

BENDING OPERATION

In order to estimate the required flat work piece length to make a bend.

R = internal radius of bend

t = thickness

K = Constant

= ¼ when R ≤ t

= 1/3 when R ≤ 2t

= ½ when R > 2t

Length of flat blank ;

L = L1 + L2 + A

L1 = length of bend leg 1

L2 = length of bend leg 2

A = Bend allowance

BENDING OPERATION

Metal tries to resume its original position causing a decrease in bend angle

Such a metal movement is calles spring back phenomenon

It is causes by elastic stress remaining in the bend area.

BENDING OPERATION

1. Calculate the blank length to make the part shown in figure 1. Also

determine the bending force required if the ultimate tensile strength of material is 3500 kg/cm2. And die radius is 8 mm.The bend length being is 120 cm.

BENDING OPERATION

Figure 1

DRAWING OPERATION

DRAWING OPERATION

DRWAING OPERATION

Other Drawing Operation

• Redrawing

– If r > 50%, redrawing is required.

– Redrawing of a cup

– Reverse drawing

DRAWING OPERATION

• Drawing without blank holder:

– The limits are:

– Db - Dp < 5t ,

– where Db = blank diameter,

– Dp = punch diameter

– Die should have a funnel or conical

shape

* IRONING

Performed after drawing to

reduce the thickness of the

cup’s walls to the desired value.

• Wrinkling

– Wrinkling happens when t/Db is

less than 1%, wrinkling appears

on the flange

– If drawing continues, wrinkling

appears on the walls.

• Tearing: Occurred due high tensile

stress and thinning of the walls at the

bottom of the cup.

• Earing: Appears when the blank

material is not perfectly isotropic.

• Surface stretches: Can occur if the die

and the punch are not smooth or if the

lubricant is not enough.

DRAWING OPERATION