Transcript Slayt 1

ME 333
PRODUCTION PROCESSES II
CHAPTER 6
SHEET METAL WORKING PROCESSES
6.1 INTRODUCTION
Sheet metalworking includes cutting and forming operations performed on
relatively thin sheets of metal (0.4-6 mm).
The tooling used to perform sheet metalwork is called punch and die. Most sheet
metal operations are performed on machine tools called presses.
The term stamping press is used to distinguish these presses from forging and
extrusion presses. The sheet metal products are called stampings.
The commercial importance of sheet metalworking is significant.
The number of consumer and industrial products that include sheet metal parts:
automobile and truck bodies, airplanes, railway cars and locomotives, farm
and construction equipment, small and large appliances, office furniture,
computers and office equipment, and more. Sheet metal parts are generally
characterized by high strength, good dimensional tolerances, good surface finish,
and relatively low cost.
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Sheet-metal processing is usually performed at room temperatures (cold working).
The exemptions are when the stock is thick, the metal is brittle, or the deformation
is significant.
These are usually cases of warm working rather than hot working.
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The three major categories of sheet-metal processes:
(1) cutting (shearing, blanking, piercing)
(2) bending
(3) drawing.
Cutting is used to separate large sheets into smaller pieces, to cut out a part
perimeter, or to make holes in a part.
Bending and drawing are used to form sheet metal parts into their required
shapes.
Piercing and Blanking
Cut off
Lancing
scrap
blank
scrap
blanking
piercing
Final shape required
Fig.6.1 Some cutting operations
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Classification of Sheet
Metalworking Processes
Fig.6.2 Basic sheet
metalworking
operations:
(a) bending,
(b) drawing, and
(c) shearing;
(1) as punch first
contacts sheet and
(2) after cutting.
Force and relative
motion are indicated
by F and v
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Classification of Sheet Metalworking Processes
Fig.6.2 Basic processes involved in forming sheet metal components. (a) Processes involving
local deformation.
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6.2. PIERCING AND BLANKING
A commonly used piercing-blanking die set and related terms are shown in the
following figure.
Fig.6.3 Components of a punch and die
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Blanking and punching
Blanking and punching are similar sheet metal cutting operations that involve
cutting the sheet metal along a closed outline.
If the part that is cut out is the desired product, the operation is called blanking and
the product is called blank.
If the remaining stock is the desired part, the operation is called punching. Both
operations are illustrated on the example of producing a washer:
Starting stock produced
by shearing operation
from a big metal sheet
Fig.6.4 Steps
in production
of washer
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The cutting of metal between die components is a shearing process in which the
metal is stressed in shear between two cutting edges to the point of fracture, or
beyond its ultimate strength.
The metal is subjected to both tensile and compressive stresses; stretching beyond
the elastic limit occurs; then plastic deformation, reduction in area, and, finally,
fracturing starts and becomes complete.
Blanking punch diameter= Db-2c
Blanking die diameter= Db
Hole punch diameter= Dh
Hole die diameter= Dh+2c
Fig.6.5 Shearing of sheet metal
between punch and die
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The cutting of metal between die components is a shearing process in which the
metal is stressed in shear between two cutting edges to the point of fracture, or
beyond its ultimate strength. The metal is subjected to both tensile and
compressive stresses; stretching beyond the elastic limit occurs; then plastic
deformation, reduction in area, and, finally, fracturing starts and becomes complete.
Fig.6.5 Shearing of sheet metal between punch and die
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Engineering analysis of metal cutting:
Cutting of sheet metal is accomplished by a shearing action between two sharp edges. The
shearing action is illustrated in the figure:
Fig. 6.6. Shearing of sheet metal
between two cutting edges:
(1) just before the punch contacts
work;
(2) punch begins to push into
work, causing plastic
deformation;
(3) punch compresses and
penetrates into work, causing a
smooth cut surface; and
(4) fracture is initiated at the
opposing cutting edges that
separate the sheet.
Symbols v and F indicate motion
and applied force, respectively.
Fig.6.3 Shearing
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At the top of the cut surface is a region
called the rollover. This corresponds to
the depression made by the punch in the
work prior to cutting. It is where initial
plastic deformation occured in the work.
Just below the rollover is a relatively small
region called the burnish. This results
from penetration of the punch into the
work before fracture began.
Beneath the burnish is the fractured
zone, a relatively rough surface of the cut
edge where continued downward
movement of the punch caused fracture of
the metal.
Finally, at the bottom of the edge is a
burr, a sharp corner on the edge caused
by elongation of the metal during final
seperation of the two pieces.
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6.2.1. Engineering Analysis_CLEARANCE
Process parameters in sheet metal cutting are clearence between punch and die,
stock thickness, type of metal and its strength and length of the cut
Clearance c in a shearing operation is the space between the mating members of
a die set (e.g.punch and die).
For optimum finish of cut edge, proper clearance is necessary and is a function
of the kind, thickness, and hardness of the work material.
In an ideal cutting operation the punch penetrates the material to a depth equal to
about 1/3 of its thickness before fracture occurs, and forces an equal portion of
the material into the die opening.
Common die clearances (linear clearance) are 2-5% of the material thickness.
Angular clearance is gradient given to the hole in the die such that cut material will
easily be removed. Angular clearance is usually ground from 0.25⁰ to 1.5⁰ per side.
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The correct clearance depends on sheet-metal type and thickness t:
c = a*t
where a is the allowance (a = 0.075 for steels and 0.060 for aluminum alloys).
If the clearance is not set correctly, either an excessive force or an oversized burr
can occur:
Fig.6.7 Effect of clearance:
(Left) clearance too small
causes less than optimal
fracture
and
excessive
forces, and (Right) clearance
too large causes oversized
burr.
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Figure (a) Effect of the clearance, c, between punch and die on the
deformation zone in shearing.
As the clearance increases, the material tends to be pulled into the die rather
than be sheared. In practice, clearances usually range between 2% and 10%
of the thickness of the sheet. (b) Microhardness (HV) contours for a 6.4-mm
(0.25-in) thick AISI 1020 hot-rolled steel in the sheared region.
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The calculated clearance value must be;
- substracted from the die punch diameter for blanking operations or
- added to die hole diameter for punching:
Fig.6.8
Die diameter is enlarged with clearance c in punching.
In blanking, the punch diameter is decreased to account for clearance.
D is the nominal size of the final product.
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An angular clearance must be provided for the die hole to allow parts to drop
through it:
Fig.6.9 Angular clearance
for the die opening in
punching and blanking.
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6.2.2. CUTTING FORCE
The pressure (or stress) required to cut (shear) work material is;
P  DtS
(for round holes)
P  SLt
(for any contours)
where;
S= shear strength of material, kg/mm2
D= hole diameter, mm
L= shear length, mm
t= material thickness, mm
For example to produce a hole of 20mmX20mm in a material 2mm in
thickness with 40 kg/mm2 shear strength:
P= 40 kg/mm2x(2x20+2x20)mmx2mm
P= 40x160 kg= 6400 kg force is required.
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6.2.3 TOOLS AND DIES FOR CUTTING OPERATIONS
Simple dies
When the die is designed to perform a single operation (for example, cutting,
blanking, or punching) with each stroke of the press, it is referred to as a simple
die:
Fig.6.10 The basic components of the simple blanking and punching dies
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Multi-operational dies
More complicated pressworking dies include:
• compound die to perform two or more operations at a single position of the
metal strip
• progressive die to perform two or more operations at two or more positions of
the metal strip
Fig.6.11 Method of making a simple washer in a compound blanking and punching die
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Multi-operational dies
Schematic illustrations: (a) before and (b) after
blanking a common washer in a compound die.
Note the separate movements of the die (for
blanking) and the punch (for punching the hole in
the washer). (c) Schematic illustration of making a
washer in a progressive die. (d) Forming of the top
piece of an aerosol spray can in a progressive die.
Note that the part is attached to the strip until the
last operation is completed.
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6.2.4 CENTRE OF PRESSURE
Sheet metal part that to be blanked is of irregular shape the summation of
shearing forces on one side of the center of the ram may greatly exceed the
forces on the other side. This result in bending and undesirable deflections might
happen. Center of pressure is a point, which the summation of shearing forces
will be symmetrical. This point is the center of gravity of the line that is the
perimeter of the blank. It is not the center of gravity of the area.
y
2r

α
rSin 

xy
2r

x
ab
3
y
h
3
Fig.6.14 Center of pressure for some shapes
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Procedure to find center of pressure:
1.
2.
3.
4.
Divide cutting edges into line elements, 1,2,3, ...
Find the lengths l1, l2, l3, ...
Find the center of gravity of each element as x1, x2, x3, ..., y1, y2, y3, ...
Calculate the center of pressure from:
l1x1  l2 x2  l3 x3  ....  lx
x

l1  l2  l3  ....
l
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l1 y1  l2 y2  l3 y3  ....  ly
y

l1  l2  l3  ....
l
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EXAMPLE
Find the center of pressure and the required cutting force of the following blank
(S=40 kg/mm2 and t=2mm).
Element
l
X
Y
(l)(x)
(l)(y)
1
4.00
0.00
6.25
0.00
25.00
2
4.71
1.50
9.20
7.05
43.33
3
3.20
4.00
7.00
12.80
22.40
4
2.50
4.00
5.00
10.00
12.50
5
3.00
1.50
4.25
4.50
12.75
6
1.57
1.00
0.00
1.57
0.00
2
3
1
4
115.98
5
and the cutting force is;
P= LtS 189.8mmx2mmx40kg/mm2 =15184 kg
6
TOTAL
x
18.98
35.92
 1.89cm
18.98
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35.92
y
115 .98
 6.10 cm
18.98
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6.2.5 REDUCING CUTTING FORCES
Since cutting operations are characterized by very high forces exerted over very
short periods of time, it is some times desirable to reduce the force and spread it
over a longer portion of the ram stroke.
Two methods are frequently used to reduce cutting forces and to smooth out the
heavy loads.
1. Step the punch lengths; the load may thus be reduced approx. 50%.
2. Tapering the punch; grind the face of the punch or die at a small shear angle
with the horizontal. This has the effect of reducing the area in shear at any time,
and may reduce cutting force as much as 50%. The angle chosen should
provide a change in punch length of about 1.5 times of material thickness. It is
usually preferable to a double cut to prevent setup of lateral force components.
Fig.6.15 Different configurations
for reducing the cutting force
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0.25+t
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Fig.6.– Effect of different clearances when punching hard and soft alloys
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6.3 SCRAP-STRIP LAYOUT FOR BLANKING
In designing parts to be blanked from strip material, economical strip utilization is of
high importance. The goal should be at least 75% utilization.
where;
t : thickness of the stock,
W: width of the stock,
B: space between part and edge (1.5t),
C: lead of the die (L+B),
L&H: dimensions of the work piece.
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Locating the work piece for maximum economy is very important.
Scrap
% Scrap 
X 100
Total
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Util .
%Util . 
X 100
Total
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HOMEWORK:
If two strips (250 mm and 125 mm width) are available for the production of 100
mm blanks, which one have to be preferred for maximum material utilization?
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6.4 BENDING
Bending is defined as the straining of the sheet metal around a straight edge:
Fig.6.15 Bending of sheet metal
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Bending operations involve the processes of V-bending and edge bending:
Fig.6.16 (Left) V-bending, and (Right) edge bending; (1) before and (2) after bending
•V-bending—sheet metal is bent along a straight line between a V-shape punch and die.
•Edge bending—bending of the cantilever part of the sheet around the die edge.
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Bending is the process by which a straight length is transformed into a curved
length. It is a very common forming process for changing sheet and plate into
channel, tanks, etc.
For a given bending operation the bend radius can not be made smaller than a
certain value, otherwise the metal will crack on the outer tensile surface. Minimum
bend radius is usually expressed in multiples of the sheets thickness. It varies
considerable between different metal and always increases with cold working. Bend
radius is not less than 1 mm and for high strength sheet alloys the minimum bend
radius may be 5t or higher.
R - bend radius
BA - bend allowance
 - bend angle
L0 - original length
t - sheet thickness
Lf  L0
Lf=L1+L2+BA
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This is the stretching length that occurs
during bending. It must be accounted to
determine the length of the blank,
where Lb is the length of the blank, L are
the lengths of the straight parts of the
blank, BA is the bend allowance,
Fig.6.17 Calculation of bend allowance
where A is the bend angle; t is the sheet thickness;
R is the bend radius; Kba is a factor to estimate stretching,
defined as follows:
Kba = 0.33
for R < 2t
Kba = 0.50
for R ≥ 2t
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The minimum bend radius for a given thickness of sheet can be predicted fairly
accurately from the reduction of area measured in tension test, Ar.
Rmin
1

1
t
2 Ar
for Ar< 0.2,
Rmin ( 1  Ar )2

2
t
2 Ar  Ar
for Ar> 0.2,
Ar 
Ao  A f
Ao
Another common problem is springback. It is the dimensional change of the formed
part after pressure of the forming tool has been removed. It results from the change
in strain produced by elastic recovery.
Springbackratio 
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 f Ro  t/ 2

o R f  t/ 2
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The commonest method of compensating for springback is to bend the part to a
smaller radius of curvature than is desired so that after springback the part has
the proper radius.
Springback is the elastic recovery leading to the increase of the included angle
when the bending pressure is removed.
To compensate for springback two methods are commonly used:
1. Overbending—the punch angle and radius are smaller than the final ones.
2. Bottoming—squeezing the part at the end of the stroke.
Fig.6.18 Springback in bending
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Fig.6.19 Compensation of springback by:
(a) and (b) overbending; (c) and (d) bottoming
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The force required bending a length L about a radius R may be estimated from;
P
 o Lt 2
2( R  t/ 2)
tan

2
Bending forces
The maximum bending force is estimated as
where Kbf is the constant that depends on the process, Kbf = 1.33 for V-bending
and Kbf = 0.33 for edge bending; w is the width of bending; D is the die opening
dimension as shown in the figure:
Fig.6.20 Die opening dimension D,
(a) V-bending, (b) edge bending
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Equipment for bending operations
Fig.6.21 Press brake with CNC gauging system
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Fig.6.22 Dies and stages in the press brake
forming of a roll bead
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6.5 DEEP DRAWING (Derin Çekme)
Deep drawing is the metal working
process used for shaping flat sheets
into cup-shaped articles such as
bathtubs,
shell
cases,
and
automobile fenders. Generally a hold
down or pressure pad is required to
press the blank against the die to
prevent wrinkling. Optional pressure
pad from the bottom may also be
used.
Fig.6.23 Drawing of a cup shaped part
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Deep drawing of a cup-shaped part
Fig.6.24 Deep drawing of a cup-shaped part: (Left) start of the
operation before punch contacts blank, and (Right) end of stroke
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In the deep drawing of a cup the metal
is subjected to three different types of
deformations. In the flange part, as it is
drawn in, the outer circumference must
continuously decrease from that of the
original blank
Do
to that of the finish
cup Dp. This means that it is
subjected to a compressive strain in the
hoop (tangential) direction and a tensile
strain in the radial direction. As a result
of these principal strains, there is a
continual increase in the thickness as
the metal moves inward. However, as
the metal pass over the die radius, it is
first bend and then straightened while
at the same time being subjected to a
tensile stress. This plastic bending
under tension results in considerable
thinning. Punch region is under very
little stress.
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Fig.6.23 Types of deformations in different region
during deep drawing of a cup shaped part
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Clearance
Clearance c is the distance between the punch and die and is about 10% greater
than the stock thickness:
c = 1.1t
Holding force
The improper application of the holding force can cause severe defects in the
drawn parts such as (a) flange wrinkling or (b) wall wrinkling if the holding force is
too small, and (c) tearing if the folding force is overestimated.
Fig.6.25 Defects in deep drawing of a cup-shaped part
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The force on the punch required to produce a cup is the summation of the ideal
force of deformation, the frictional forces, and the force required to produce
ironing. Mathematical calculation of the drawing force is very complex. Following
approximate equation is developed:
D
d    / 2


P  dt 11
.  o n   2 H  e
B

d
D 

where;
P = total punch load,
d = punch diameter,
H = hold drawn force,
t = wall thickness,
 = efficiency
o=
D =
B =
=
average flow stress,
blank diameter,
force required to bend,
coefficient of friction,
Drawing force may be calculated for practical purposes by:
P   odt
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when LDR  2 (Limiting Drawing Ratio)
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The drawability of a metal is measured by the ratio of the blank diameter to the
diameter of the cup drawn from the blank (usually accepted as punch diameter). For
a given material there is a Limiting Drawing Ratio (LDR), representing the largest
blank that can be drawn through a die without tearing.
D 
LDR     e
d 
Where,  is an efficiency term to account for frictional losses. If =1, then LDR=2.7
while =0.7, LDR2 which is used in most practical applications.
Some of the practical considerations which affect drawability:
Rd 10t
Rp should be big enough to prevent tearing.
Clearance between punch and die; 20 to 40% greater than “t”.
Hold-down pressure; 2% of o and lubricate die walls
The diameter of blank required to draw a given cup may be obtained approximately
by equating surface areas.
D2 d 2

 dh and
D  d 2  4dh
4
4
where; h is height of cup.

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6.5.1 REDRAWING
If the shape change required by the part design is too severe (limiting drawing ratio is
too high, or LDR is not sufficient to form a desired cup), complete forming of the part
require more than one drawing step. The second drawing step and any further
drawing steps if needed, are referred to as redrawing. Throat angle is 10-15.
Redrawing is generally done in decreasing ratios as given below:
(D/d)= 1.43, 1.33, 1.25, 1.19, 1.14 and 1.11.
If these redrawing steps are not enough to reach required cup diameter, annealing
have to be performed and then redrawing can be performed.
1st draw
last draw
Fig.6.26 Redrawing of a cup
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6.5.2 EXAMPLE:
A  200 mm blank is to be drawn to a 50 mm cup. Estimate the minimum number of
draws required using the drawing ratios given below:
Draw
1st
2nd
3rd
4th
5th
6th
Ratio
1.43
1.33
1.25
1.19
1.14
1.10
Solution:
LDR=2
D/d = 200/50 = 4 > 2
So that redrawing is necessary.
1.
2.
3.
D
200
 1.43  D1 
 139.86
D1
1.43
D1
139.86
 1.33  D2 
 105.16
D2
1.33
D2
105.16
 1.25  D3 
 84.13
D3
1.25
4.
5.
6.
D3
84.13
 1.19  D4 
 70.69
D4
1.19
D4
70.69
 1.14  D5 
 62.01
D5
1.14
D5
62.01
 1.1  D6 
 56.38
D6
1.1
56.38>50
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Therefore annealing should be applied. But it might be better to anneal the blank
before 6th draw to reduce number of redraws. We know that LDR=2. So that if
annealing is performed after 3rd draw where D3 = 84.13 mm, than ratio to reach
required cup diameter is:
84.13
 1.68 < 2
50
Therefore, after 3rd draw, blank is annealed and then redraw with a ratio of 1.68 to
obtain required cup diameter. The required number of drawing is then 4.
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6.6 OTHER SHEET-METAL FORMING OPERATIONS
The Guerin process
The Guerin process involves the use of a thick rubber pad to form sheet metal
over a positive form block:
Fig.6.27 The Guerin process: (Left) start of the operation before
rubber pad contacts sheet, and (Right) end of stroke
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Examples of equipment and products manufactured by the Guerin process:
Fig.6.28 Rubber pad press
forming tools on the press table
showing
Advantages:
Limitations:
Area of application:
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Fig.6.29 A large number of different components
can be made simultaneously during one press cycle
with rubber pad presses
small cost of tooling
for relatively shallow shapes
small-quantity production
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Hydroforming
It is similar to Guerin process but instead of rubber pad a rubber diaphragm
filled with fluid is used:
Fig.6.30 Hydroform process:
(1) start-up, no fluid in the cavity;
(2) press closed, cavity pressurized
with hydraulic fluid;
(3) punch pressed into work to form
part.
Symbols:
v - velocity,
F – applied force, and
p - hydraulic pressure
Advantages:
Limitations:
Area of application:
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small cost of tooling
simple shapes
small-quantity production
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Stretch forming
In stretch forming the sheet metal is stretched and bent to achieve the desired shape:
Fig.6.31 Stretch forming: (1) start of the process; (2) form die is pressed into the work
causing it to stretched and bent over the form. Symbols: v - velocity, Fdie - applied force
Advantages:
Limitations:
Area of application:
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small cost of tooling, large parts
simple shapes
small-quantity production
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Spinning
Spinning is a metal forming process in which an axially symmetric part is gradually
shaped over a mandrel by means of a rounded tool or roller:
Fig.6.32 In spinning operation, flat circular blanks are often formed into hollow shapes such
as photographic reflectors. In a lathe, tool is forced against a rotating disk, gradually forcing
the metal over the chuck to conform to its shape. Chucks and follow blocks are usually
made of wood for this operation
Advantages:
Limitations:
Area of application:
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small cost of tooling, large parts (up to 5 m or more)
only axially symmetric parts
small-quantity production
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HIGH-ENERGY-RATE FORMING (HERF)
These are metal forming processes in which large amount of energy is applied in a
very short time. Some of the most important HREF operations include:
Explosive forming
It involves the use of an explosive charge placed in water to form sheet into the die cavity.
Fig.6.33 Explosive forming: (1) set-up, (2) explosive is detonated, and (3) shock wave
forms part
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Explosively formed elliptical dome 3-m in diameter being removed from
the forming die
Fig.6.34 Explosively formed elliptical
dome 3-m in diameter being
removed from the forming die
Advantages:
Limitations:
Area of application:
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small cost of tooling, large parts
skilled and experienced labor
large parts typical of the aerospace industry
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Electrohydraulic forming
This is a HREF process in which a shock wave to deform the work into a die cavity is
generated by the discharge of electrical energy between two electrodes submerged
in water. Similar to explosive forming, but applied only to small part sizes.
Fig.6.35 Setup of electrohydraulic forming
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Electromagnetic forming
The sheet metal is deformed by the mechanical force of an electromagnetic field
induced in the workpiece by a coil:
Fig.6.36
Electromagnetic
forming: (1) set-up in which
coil is inserted into tubular
workpiece surrounded by
die, (2) formed part
Advantages:
Limitations:
Area of application:
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can produce shapes, which cannot be
produced easily by the other processes
suitable for magnetic materials
most widely used HERF process to
form tubular parts
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HOMEWORK:
If two strips (250 mm and 125 mm width) are available for the production of 100
mm blanks, which one have to be preferred for maximum material utilization?
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THE END
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