Chapter 17 Sheet Forming Processes (Part 2) Drawing

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Chapter 17Sheet Forming Processes(Part
2)Drawing Stretching, Alternative Methods,
Pipe Welding, and PressesEIN 3390
Manufacturing ProcessesSpring 2012
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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.

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17.4 Drawing and Stretching Processes

• Types of Drawing and Stretching
• 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

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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

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Spinning
Figure 17-34 (Above) Progressive stages in the
spinning of a sheet metal product.
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Spinning
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Spinning
Figure 17-35 (Left) Two stages in the spinning of
a metal reflector. (Courtesy of Spincraft, Inc.
New Berlin, WI.)
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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
  mass-produce 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  that can be directly withdrawn from a one-piece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.

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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 81 are
  possible, but the limit is usually set at about
  51, or 80

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Shearing Forming
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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.
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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 con- cave, convex, or conical parts
  without a matching form block.

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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.
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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
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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.
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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 deep-drawing
process.
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Deep Drawing and Shallow Drawing

• Key variables
• Blank and punch diameter
• Punch and die radius
• Clearance
• Thickness of the blank
• Lubrication
• Hold-down pressure

Figure 17-4 Flow of material during deep drawing.
Note the circumferential compression as the
radius is pulled inward
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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.
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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
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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.
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Deep Drawing
As cup depth increases or material is thin, there
is an increased tendency for forming the defects.
Thin
Thick
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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
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Defects in Drawing Parts
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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

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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.
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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.
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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

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Sheet Hydroforming
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Sheet Hydroforming
Figure 17-50 (Above) One form of sheet
hydroforming.
Figure 17-51 Two-sheet hydroforming, or pillow
forming.
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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.
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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

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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)
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Explosive Forming Processes
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Ironing Processes
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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

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Embossing Process
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Properties of Sheet Material

• 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

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Engineering Analysis of Drawing
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Engineering Analysis of Drawing
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Engineering Analysis of Drawing
It is important to assess the limitation of the
amount of drawing that can be accomplished. Measu
res 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).
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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 lt 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.
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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 lt2.0 r (Db - Dp )/Db (5.5 3.0)/5.5
45.45 lt 50 t/Db (3/32)/5.5 0.017 gt
1 So the drawing operation is
feasible.
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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
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Engineering Analysis of Drawing
Holding Force Fh 0.015YpDb2 (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.
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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
0.015YpDb2 (Dp 2.2t 2Rd)2
0.015(40,000)p5.52 3.0 2.2(3/32)
2(0.25)2 1,885 (30.25 13.74)
31,121 lb
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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)
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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.
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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.
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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.
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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

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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).

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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.
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17.7 Presses
Figure 17-58 Schematic representation of the
various types of press drive mechanisms.
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Types of Press Frame
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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.)
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Types of Press Frame
Figure 17-61 A 200-ton (1800-kN) straight-sided
press. (Courtesy of Rousselle Corporation, West
Chicago, IL.)
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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

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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.)
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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.)
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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.)
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Summary

• Sheet forming processes can be grouped in several
  broad categories
• 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