Transcript Calculating Clearance and Force

Calculating Clearance and Force
Round disk of 5.0” dia. is to be blanked from 1100S
aluminum alloy sheet of 1/4” with shear strength = 7,000
lb/in2. Determine (a) punch and die diameters, and (b)
blanking force.
Clearance: C = at
Cutting forces: F = StL
Calculation for Sheet-metal Bending
Metal to be bent with a modulus of elasticity E = 30x106 lb/in2., yield
strength Y = 40,000lb/in2 , and tensile strength TS = 65,000 lb/in2.
Determine (a) starting blank size, and (b) bending force if V-die
will be used with a die opening dimension D = 1.0in.
(a) Bending Allowance: BA = 2pA(R + Kbat)/360
Kba - factor to estimate stretching. If R < 2t, Kba = 0.33; and if
R>=2t, Kba =0.5.
(b)
Bending force:
F = (KbfTSwt2)/D
Kbf – a constant that counts for differences in an
actual bending processes. For V-bending Kbf
=1.33, and for edge bending Kbf =0.33
Chapter 17
Sheet Forming Processes
(Part 2)
Drawing & Stretching, Alternative
Methods, Pipe Welding, and Presses
EIN 3390
Manufacturing Processes
Spring 2012
17.4 Drawing and Stretching
Processes
Drawing refers to the family of
operations where plastic flow occurs
over a curved axis and the flat sheet is
formed into a three-dimensional part
with a depth more than several times
the thickness of the metal
 Application: a wide range of shapes, from
cups to large automobile and aerospace
panels.

17.4 Drawing and Stretching
Processes
Types of Drawing and Stretching
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Spinning
Shear forming or flow turning
Stretch forming
Deep drawing and shallow drawing
Rubber-tool forming
Sheet hydroforming
Tube hydroforming
Hot drawing
High-energy-rate forming
Ironing
Embossing
Superplastic sheet forming
17.4 Spinning
Spinning is a cold forming operation
◦ Sheet metal is rotated and progressively
shaped over a male form, or mandrel
◦ Produces rotationally symmetrical shapes
 Cones, spheres, hemispheres, cylinders,
bells, and parabolas
Spinning
Figure 17-34
(Above)
Progressive
stages in the
spinning of a
sheet metal
product.
Spinning
Spinning
Figure 17-35
(Left) Two stages
in the spinning of
a metal reflector.
(Courtesy of
Spincraft, Inc.
New Berlin, WI.)
Spinning
Tooling cost can be extremely low. The form block can
often be made of hardwood or even plastic because of
localized compression from metal.
With automation, spinning can also be used to massproduce high-volume items such as lamp reflectors,
cooking utensils, bowls, and bells.
Spinning is usually considered for simple shapes that can
be directly withdrawn from a one-piece form. More
complex shapes, such as those with reentrant angles, can
be spun over multipiece or offset forms.
Shear Forming
Shear forming is a version of spinning
 A modification of the spinning process in which
each element of the blank maintains its
distance from the axis of rotation.
 No circumferential shrinkage
 Wall thickness of product, tc will vary with the
angle of the particular region:
tc = tb(sin a)
where tb is the thickness of the starting blank.
• Reductions in wall thickness as high as 8:1
are possible, but the limit is usually set at
about 5:1, or 80%

Shearing Forming
Direct Shear Forming
Material being
formed
moves in the
same
direction as
the roller
Figure 17-36
Schematic
representation
of the basic
shear-forming
process.
Reverse Shear Forming
•
Material being formed
moves in the opposite
direction as the roller
•
By controlling the position
and feed of the forming
roller, the reverse process
can be used to shape concave, convex, or conical
parts without a matching
form block.
Stretch Forming
An attractive means of producing large sheet
metal parts in low or limited quantities.
A sheet of metal is gripped by two or more sets
of jaws with stretching or wrapping around a
single form block.
Figure 17-39 Schematic of a stretch-forming operation.
Stretch Forming
Most deformation is induced by the tensile
stretching, so the forces on the form block are
far less than those encouraged in bending or
forming.
Very little springback and the workpiece
conforms very closed to the shape of the tool.
Wrinkles are pulled out before they occur since
stretching accompanies bending or wrapping
Stretch Forming
Form blocks can be made of wood, low-melting-point
metal, or even plastic because forces on form block are low.
Quite popular in the aircraft industry to form aluminum,
stainless steel into cowling, wing tip, scoop, and other
large panels.
Low-carbon steel can be stretched to produce large
panels for automotive and truck industry.
If mating male and female dies are used to shape the metal
while it is being stretched, the process is known as
stretch-draw forming.
Deep Drawing and Shallow
Drawing



Drawing is typically used
to form solid-bottom
cylindrical or
rectangular containers
from sheet metal.
When depth of the
product is greater than its
diameter, it is known
“Deep drawing”.
When depth of the
product is less than its
diameter, it is known
“shallow drawing”.
Figure 17-40 Schematic of the deepdrawing process.
Deep Drawing and Shallow
Drawing

Key variables:
1) Blank and punch
diameter
2) Punch and die radius
3) Clearance
4) Thickness of the
blank
5) Lubrication
6) Hold-down
pressure
Figure 17-4 Flow of material during deep drawing. Note
the circumferential compression as the radius is pulled
inward
Deep Drawing and Shallow Drawing
During drawing, the material is pulled inward, so its
circumference decrease. Since the volume of material
must be the same,
V0 = Vf
the decrease in circumferential dimension must be
compensated by a increase in another dimension,
such as thickness or radial length.
Since the material is thin, an alternative is to relieve the
circumferential compression by bulking or wrinkling.
The wrinkling formation can be suppressed by compressing
the sheet between die and blankholder service.
Deep Drawing and Shallow Drawing
The hold-down force
is independent of
the punch position.
The restraining force
can be varied
during the drawing
operation.
Multi-action presses
are usually
specified for the
drawing of more
complex parts.
Figure 17-42 Drawing on a double-action press, where
blankholder uses the second press action
Deep Drawing and Shallow Drawing
Once a drawing process has been designed and
the tooling manufactured, the primary variable
for process adjustment is hold-down pressure
or blankhoder force.
If the force is too low, wrinkling may occur at
the start of the stroke. If it is too high, there is
too much restrain, and the descending punch
will tear the disk or some portion of the
already-formed cup wall.
Thick
Thin
Deep Drawing
As cup depth increases
or material is thin,
there is an increased
tendency for forming
the defects.
Limitations of Deep Drawing
Wrinkling and tearing are typical limits
to drawing operations
 Trimming may be used to reach final
dimensions

Figure 17-45 Pierced blanked, and drawn part before and after
trimming
Defects in Drawing Parts
Forming with Rubber Tooling or
Fluid Pressure
Blanking and drawing operations
usually require mating male and female
die sets
 Processes have been developed that seek
to

◦ Reduce tooling cost
◦ Decrease setup time and expense
◦ Extend the amount of deformation for a
single set of tools
Alternative Forming Operations

Several forming
operations replace
one of the dies with
rubber or fluid
pressure
◦ Guerin process

Other forming
operations use fluid
or rubber to
transmit the
pressure required
to expand a metal
blank
◦ Bulging
Figure 17-47 Method of blanking sheet metal using
the Guerin process.
Figure 17-48 Method of bulging tubes with rubber
tooling.
Guerin Process (Rubber-die forming)
Guerin process was developed by aircraft industry for
small number of duplicate parts. The sheet materials
can be aluminum up to (1/8”) thick and stainless
steel up to 1/16”. Magnesium sheet can also be
formed if it is heated and shaped over heated form
block.
Sheet Hydroforming

Sheet hydroforming is a family of processes in
which a rubber bladder backed by fluid
pressure replaces either the solid punch or
female die set

Advantages
◦ Reduced cost of tooling
◦ Deeper parts can be formed without
fracture
◦ Excellent surface finish
◦ Accurate part dimensions
Sheet Hydroforming
Sheet Hydroforming
Figure 17-50 (Above) One form of sheet hydroforming.
Figure 17-51 Two-sheet hydroforming, or pillow forming.
Tube Hydroforming
Process for manufacturing strong, lightweight, tubular
components
 Frequently used process for automotive industry
 Advantages

◦ Lightweight, high-strength materials
◦ Designs with varying thickness or varying cross section can be made
◦ Welded assemblies can be replaced by one-piece components

Disadvantages
◦ Long cycle time
◦ Relatively high tooling cost and process setup
Figure 17-52 Tube hydroforming. (a) Process schematic.
Additional Drawing Operations

Hot-drawing
◦ Sheet metal has a large surface area and small
thickness, so it cools rapidly
◦ Most sheet forming is done at mildly elevated
temperatures

High-Energy Rate Forming (HERF)
◦ Large amounts of energy in a very short time
◦ Underwater explosions, underwater spark
discharge, pneumatic-mechanical means, internal
combustion of gaseous mixtures, rapidly formed
magnetic fields

Ironing
◦ Process that thins the walls of a drawn cylinder by
passing it between a punch and a die
Hot-Drawing Processes
Figure 17-5 Methods of hot drawing a cup-shaped part. (Up left) First draw. (Up
right) Redraw operation. (Lower) Multi-die draw. (Courtesy of United States Steel
Corp., Pittsburgh, PA)
Explosive Forming Processes
Ironing Processes
Additional Drawing Operations

Embossing
◦ Pressworking process in which raised lettering
or other designs are impressed in sheet
material

Superplastic sheet forming
◦ Materials that can elongate in the range of
2000 to 3000% can be used to form large,
complex-shaped parts with ultra-fine grain
size and performing the deformation at low
strain rates and elevated temperature.
◦ Superplastic forming techniques are similar to
that of thermoplastics
Embossing Process
Properties of Sheet Material
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Tensile strength of the material is important in
determining which forming operations are
appropriate.
Sheet metal is often anisotropic- properties
vary with direction or orientation. A metal with
low-yield, high-tensile, and high-uniform
elongation has a good mechanical property for
sheet-forming operations.
Majority of failures during forming occur due to
thinning or fracture
Strain analysis can be used to determine the
best orientation for forming
Engineering Analysis of Drawing
Engineering Analysis of Drawing
Engineering Analysis of Drawing
It is important to assess the limitation of the
amount of drawing that can be accomplished.
Measures of Drawing:
1) Drawing ratio (cylinder) DR = Db/Dp
Where Db – blank diameter, Dp – punch diameter
The greater the ratio, the more severe is the
drawing.
An approximate upper limit on the drawing ratio
is a value of 2.0. The actual limiting value for a
given drawing depends on punch and die
corner radii (Dp and Dd), friction conditions, depth
of draw, and characteristics of the sheet metal
(ductility, degree of directinality of strength in the metal).
Engineering Analysis of Drawing
2) Reduction r (another way to characterize a
given drawing)
r = (Db - Dp )/Db
It is very closely related to drawing ratio.
Consistent with Dr <= 2.0, the value of r should be
less than 0.5.
3) Thickness-to-diameter ratio:
t/Db
Where t – thickness of the starting blank, Db – blank
diameter.
The ratio t/Db is greater than 1%. As t/Db decreases,
tendency for wrinkling increases. If DR , r, t/Db are
exceeded by the design, blank must be draw in two or
more steps, sometimes with annealing between steps.
Engineering Analysis of Drawing
Example: Cup Drawing
For a cylindrical cup with inside diameter = 3.0” and height
= 2.0”, its starting blank size Db = 5.5”, and its thickness
t = 3/32”, please indicate its manufacturing feasibility.
Solution:
DR = Db/Dp = 5.5/3.0 = 1.833 <2.0
r = (Db - Dp )/Db = (5.5 – 3.0)/5.5 = 45.45% < 50%
t/Db = (3/32)/5.5 = 0.017 > 1%
So the drawing operation is feasible.
Engineering Analysis of Drawing
Drawing Force
F = pDpt(TS)(Db/Dp – 0.7)
Where F – drawing force, lb(N); t – thickness of blank, in.
(mm); TS - tensile strength, ib/in2 (Mpa); Db and D p –
starting blank diameter and punch diameter, in. (mm).
0.7 – a correction factor for friction. The equation is the
estimation of the maximum force in the drawing.
The drawing force varies throughout the downward
movement of the punch, usually reaching its maximum
value at about one-third the length of the punch stroke.
Clearance c: about 10% than the stock thickness (t)
c = 1.1 t
Engineering Analysis of Drawing
Holding Force
Fh = 0.015Yp[Db2 – (Dp + 2.2t + 2Rd)2]
Where Fh – holding force in drawing, ib (N); Y – yield
strength of the sheet metal, lb/in2 (Mpa); t – starting
stock thickness, in. (mm); Rd – die conner radius, in.
(mm). The holding force is usually about one-third the
drawing force [1].
[1]: Wick, C., et al., “Tool and Manufacturing Engineers, 4th ed. Vol. II.
Engineering Analysis of Drawing
Example Forces in Drawing
Determine the (a) drawing force, and (2) holding force for the case
in previous example for feasibility, where tensile strength of the
metal = 70,000 lb/in 2 and yield strength = 40,000 lb/in 2 , the die
corner radius = 0.25”.
Solution:
(a)
F = pDpt(TS)(Db/Dp – 0.7)
=p(3.0)(3/32)(70,000)(5.5/3.0 – 0.7)
=70,097 lb
(b)
Fh = 0.015Yp[Db2 – (Dp + 2.2t + 2Rd)2]
= 0.015(40,000)p{5.52 – [3.0 + 2.2(3/32) + 2(0.25)]2}
= 1,885 (30.25 – 13.74)
= 31,121 lb
Engineering Analysis of Drawing
Blank Size Determination
Assume that the volume of the final product is the same
as the that of the starting sheet-metal blank and the
thinning of the part wall is negligible.
For a cup with its height H and the same diameters Dp in
the bottom and top:
pDb2/4 = pDp2/4 + pDp H, and
Db = SQRT(Dp2 + 4Dp H)
Design Aids for Sheet Metal Forming
Figure 17-57 (Left) Typical pattern for sheet metal deformation analysis; (right) forming limit
diagram used to determine whether a metal can be shaped without risk of fracture. Fracture
is expected when strains fall above the lines.
Design Aids for Sheet Metal Forming
A pattern is placed on the surface of a sheet.
Circles have diameters between 2.4 and 5 mm
(0.1 – 0.2”).
During deformation, the circles convert into
ellipses.
Regions where the enclosed area has expanded
are locations of sheet thinning and possible
failure.
Regions where the area has contracted have undergone
sheet thickening and may be sites of buckling or
wrinkles.
Design Aids for Sheet Metal Forming
Using the ellipses on the deformed pattern, the
major strains (strain in the direction of the
largest radius) and the associated minor strain
(strain 900 from the major) can be determined for a
variety of locations.
If both major and minor strains are positive,
the deformation are stretching, and the sheet
metal will decrease in thickness.
If the minor strain is negative, this contraction may
partially or whole compensate any positive stretching
in the major direction. The combination of tension
and compression is known as drawing, and the
thickness may decrease, increase, or stay the same,
depending on relative magnitude of the two strains.
17.5 Alternative Methods of
Producing Sheet-Type Products

Electroforming
◦ Directly deposits metal onto preshaped
forms or mandrels
◦ Nickel, iron, copper, or silver can used
◦ A wide variety of sizes and shapes can be
made by electroforming

Spray forming
◦ Spray deposition
◦ Uses powdered material in a plasma torch
◦ Molten metal may also be sprayed
17.6 Pipe Welding

Lap-welded pipe
◦ Skelp has beveled edges and the rolls form the
weld by forcing the lapped edges down against
a supporting mandrel.
◦ The process is used primarily for large sizes of
pipe, with diameters from about 50 mm (2”) to
400 mm (14”). Product length is limited to
about 6 to 7 m (20 to 25 ft).
17.7 Presses
Factor for selection of presses:
type of power (mechanical, hydraulic), number
of slides or drives, type of drive, stroke length
for each drive, type of frame or construction,
and the speed of operation.
17.7 Presses
Figure 17-58
Schematic
representation of the
various types of
press drive
mechanisms.
Types of Press Frame
Types of Press Frame
Figure 17-60
Inclinable gap-frame
press with sliding
bolster to
accommodate two die
sets for rapid change
of tooling. (Courtesy of
Niagara Machine &
Tool Works, Buffalo,
NY.)
Types of Press Frame
Figure 17-61
A 200-ton (1800kN) straight-sided
press. (Courtesy
of Rousselle
Corporation,
West Chicago,
IL.)
Special Types of Presses
Presses have been designed to perform
specific types of operations
 Transfer presses have a long moving
slide that enables multiple operations
to be performed simultaneously in a
single machine
 Four-slide or multislide machines are used
to produce small, intricately shaped parts
from continuously fed wire or coil strip

Figure 17-62 Schematic showing the arrangement of dies and the transfer
mechanism used in transfer presses. (Courtesy of Verson Allsteel Press Company,
Chicago, IL.)
Figure 17-63 Various operations can be performed during the production of stamped
and drawn parts on a transfer press. (Courtesy of U.S. Baird Corporation, Stratford,
CT.)
Figure 17-65 Schematic of the operating mechanism of a multislide machine. The material
enters on the right and progresses toward the left as operations are performed. (Courtesy of
U.S. Baird Corporation, Stratford, CT.)
Summary

Sheet forming processes can be grouped in
several broad categories
◦
◦
◦
◦
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
Shearing
Bending
Drawing
Forming
Basic sheet forming operations involve a
press, punch, or ram and a set of dies
Material properties, geometry of the
starting material, and the geometry of
the desired final product play important roles
in determining the best process