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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