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Chapter 17 Sheet Forming Processes (Part 2:Drawing

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Title: Chapter 17 Sheet Forming Processes (Part 2:Drawing


1
Chapter 17Sheet Forming Processes(Part
2Drawing Stretching) (Review)EIN 3390
Manufacturing ProcessesSummer A, 2012
2
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.

3
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

4
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

5
Spinning
Figure 17-34 (Above) Progressive stages in the
spinning of a sheet metal product.
6
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    that can be directly withdrawn from a one-piece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.

7
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

8
Shearing Forming
9
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.
10
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.

11
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.
12
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
13
Deep Drawing
During drawing, the material is pulled inward, so
its circumference decrease. Since the volume of
material must be the same, V0 V f 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.
14
Deep 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.
Drawing on a double-action press, where
blankholder uses the second press action
15
Deep 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.
16
Deep Drawing
As cup depth increases or material is thin, there
is an increased tendency for forming the defects.
Thin
Thick
17
Defects in Drawing Parts
18
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

19
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

20
Engineering Analysis of Drawing
21
Engineering Analysis of Drawing
22
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).
23
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.
24
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 5.5/3.3
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.
25
Engineering Analysis of Drawing
Drawing Force F pDpt(TS)(Db/D p
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
26
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.
27
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/D p 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
28
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)
29
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.
30
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.
31
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.
32
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.
33
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
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