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Sheet Metal Forming- Part I, Cup Drawing

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ISE 311
Sheet Metal Forming Lab
Cup Drawing
in conjunction with
Section 20.3 in the text book
“Fundamentals of Modern Manufacturing”
Third Edition
Mikell P. Groover
February 4th, 2008
Outline
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Introduction
Mechanics of Drawing
Engineering Analysis of Drawing
Defects in Drawing
Objectives of the Lab
Sheet Metal Forming (Material and Equipment)
Sheet Metal Forming (FE simulations)
Summary
2
Introduction
Basic Principles of Drawing
Drawing is a sheet metal forming operation used to
make cup-shaped, box-shaped, or other complex-curved,
hollow-shaped parts. It is performed by placing a piece
of sheet metal over a die cavity and then pushing the
sheet into the opening with a punch. The blank is held
down flat against the die by a blankholder.
3
Mechanics of Drawing
A blank of diameter Db is drawn into the die by means of a punch
of diameter Dp. The punch and die have corner radii Rp and Rd,
respectively. The sides of the die and punch are separated by a
clearance, c, which is about 10% greater than the sheet thickness.
The punch applies a downward force, F, to deform the metal
while the downward holding force, Fh, is applied by the
blankholder.
4
Mechanics of Drawing
As the punch proceeds towards its final
position, the workpiece experiences a
complex sequence of stresses and strains
as it is formed into its final shape.
In step 1, the blankholder force, Fh, is
applied and the punch begins to move
towards the sheet material.
In step 2, the sheet material is subjected
to a bending operation. The sheet is bent
over the corner of the punch and the
corner of the die.
In step 3, as the punch continues moving
down, a straightening action occurs in the
metal that was previously bent over the
die radius. Metal from the flange is
drawn into the die opening to form the
cylinder wall.
5
Mechanics of Drawing
In step 4, As the metal in the flange moves toward
the center, it is subjected to the following state of
stress:
1- Compression in the circumferential direction
(the outer perimeter becomes smaller)
2- Tension in the radial direction
3- A relatively small compression in the thickness
direction
Since the volume of metal remains constant, and
because the circumferential stress is relatively
large, the sheet will thicken as it moves in the
flange area. (this is why the clearance between the
punch and die is higher than the sheet thickness by
about 10%)
In order for the material to be drawn:
1- Friction between the sheet material and surfaces
of the blankholder and die must be overcome.
2- Deformation energy should be provided.
6
Mechanics of Drawing
The downward motion of the
punch results in a continuation of
the metal flow caused by drawing
and compression. Some thinning
at the cylinder walls occurs as
well. Step 5 shows the completed
drawing process.
7
Deep Drawing Tooling
The next slide illustrates the deep drawing tooling used in the lab.
The blank is placed between the die and the blank holder. To
center the blank, the student should make sure that it is touching
the two pins. The blank holder is attached to three pneumatic
cylinders which move it up and down and also provide the
required holding force (by controlling the air pressure inside the
cylinders). The punch is directly attached to the ram of the
mechanical press. Three guides are used to make sure the tools
remain concentric during the process.
8
Deep Drawing Tooling
Blank holder
Pneumatic cylinder (3)
Pins (2)
Punch
Die
Guide (3)
Blank holder
9
Engineering Analysis of Drawing
The drawing ratio, DR, gives an indication of the severity of the
drawing operation: the higher the ratio, the greater the severity.
The drawing ratio is defined as:
DR пЂЅ
Db
Dp
Where, Db = blank diameter and Dp = punch diameter. This
value is dependant upon punch and die corner radii, friction
conditions, draw depth, and material properties of the sheet
metal.
10
Engineering Analysis of Drawing
The drawing force required to perform a drawing operation can
be roughly estimated by the following formula:
пѓ¦ Db
пѓ¶
F пЂЅ пЃ° D p t пЂЁTS пЂ©пѓ§
пЂ­ 0 .7 пѓ· ;
пѓ§D
пѓ·
p
пѓЁ
пѓё
where
F = Drawing force
t = Original blank thickness
TS = Tensile strength
Db = Blank diameter
Dp = Punch diameter
Increasing the DR will increase the punch force, and this will result in
excessive thinning or even fracture in the cup wall.
DR < LDR ;
LDR = the Limiting Drawing Ratio
where
11
Defects in Drawing
A number of defects in drawing can occur, which include:
(a) Wrinkling in the flange occurs due to compressive buckling in the circumferential
direction (blank holding force should be sufficient to prevent buckling from occurring).
(b) Wrinkling in the wall takes place when a wrinkled flange is drawn into the cup or if the
clearance is very large, resulting in a large suspended (unsupported) region.
(c) Tearing occurs because of high tensile stresses that cause thinning and failure of the
metal in the cup wall. Tearing can also occur in a drawing process if the die has a sharp
corner radius.
(d) Earring occurs when the material is anisotropic, i.e. has varying properties in different
directions.
(e) Surface scratches can be seen on the drawn part if the punch and die are not smooth or
if the lubrication of the process is poor.
12
Objectives
This lab has the following objectives:
• Become familiarized with the basic processes used in
sheet metal forming.
• Analyze a cup drawing operation and attempt to select
the best process parameters.
13
Sheet Metal Forming – Cup Drawing
• Test Materials and Equipment
– Robinson (Open Back Inclinable- OBI) press
• Model A3
• 25-ton capacity
• Safety Equipment and Instructions
– Wear safety glasses.
– Conduct the test as directed by the instructor.
– Do not use hands to put or remove specimens on the die –
use the supplied tongs.
– Turn off the OBI press whenever you need to adjust its
setting.
14
Sheet Metal Forming – Cup Drawing
Procedure:
• Obtain specimens to be
tested and record the
material data onto your
data sheet.
• Choose one sample and
place it in the tooling
using the tongs.
• For the first specimen, set
the air pressure for the
blankholder cylinders to
approximately 30 psi.
• Step on the foot pedal to
cycle the press one time.
15
Sheet Metal Forming – Cup Drawing
Procedure (continued):
• If the part does not fall out of the bottom on its own, try the following:
Case 1: The part is completely formed but stuck in the die cavity
* Step the pedal again. If the part did not fall, then
* Put a small wooden cylinder in the die cavity and step the pedal.
Case 2: The part is torn (you can see the flange on surface of the die)
* Use a wooden stick to raise the sample up and remove it through the gap
between the die and blank holder
Case 3: The part is stuck around the punch
Ask the lab instructor for help
Note: BE SURE the flywheel has completely stopped and the holding
pressure is zero before attempting to retrieve the part and NEVER put
your hands between the die and the blank holder (always use the supplied
wooden stick)
16
Sheet Metal Forming – Cup Drawing
Procedure (continued):
• Inspect the formed part and record any observed defects. If the part is not
ideal, adjust the blank holding pressure (or other process parameters) to
improve the part quality.
• Select another specimen of the same material and try your new process
parameters. Again, record any defects and speculate appropriate adjustments
to the process parameters.
• Adjust the parameters again and form your third specimen. Inspect the
material and record any observed defects.
• Repeat steps 4-9 for the remaining materials.
17
Finite Element Analysis (FEA) and
Simulations
With FEA it is possible to emulate the metal flow (the
cup shape, the thinning in the cup wall, the stresses and
strains in the deforming cup material) during deep
drawing.
The next few slides illustrate the simulation of a cup
drawing process generated by FEA that simulates the
actual deformation of a steel sheet specimen.
18
Cup Drawing Simulation
Steel 1010: (BHF: 1,000 Lbs)
Stage A (before drawing)
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Cup Drawing Simulation
Steel 1010: (BHF: 1,000 Lbs)
Stage B
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Cup Drawing Simulation
Steel 1010: (BHF: 1,000 Lbs)
Stage C
21
Cup Drawing Simulation
Steel 1010: (BHF: 1,000 Lbs)
Stage D
22
Cup Drawing Simulation
Steel 1010: (BHF: 1,000 Lbs)
Stage E
23
Cup Drawing Simulation
Steel 1010: (BHF: 1,000 LBS)
Load-Stroke Curve
Each point on the curve represents one of the stages from A to E
D
E
C
B
A
24
Summary – Sheet Metal Forming Lab
This lab preparation material introduced:
• The basic principles of shearing, bending and deep
drawing, as well as terminology used
• The objectives of and the expected outcomes from the
evaluation of experimental trials
• The testing equipment and the test procedure
25
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