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Introduction
Sheet metal working generally incorporates cutting and forming operations on thin sheets of
metal. Typically, the thickness of sheet metal is between 1/4 inch and 1/64 inch. There are
different types of metal forming operations: Bending, Shearing, Blanking, Punching, Stamping
etc. This lab provides an exposure to BENDING operation. Bending is a process by which metal
can be deformed by plastically deforming the material and changing its shape. The material is
stressed beyond the yield strength but below the ultimate tensile strength. The surface area of the
material does not change much. Bending usually refers to deformation about one axis. Bending is
a flexible process by which many different shapes can be produced. Standard die sets are used to
produce a wide variety of shapes. The material is placed on the die, and positioned in place with
stops and/or gages. It is held in place with hold-downs. The upper part of the press, the ram with
the appropriately shaped punch descends and forms the v-shaped bend.
Bending is done using Press Brakes. Press Brakes normally have a capacity of 20 to 200 tons to
accommodate stock from 1m to 4.5m. Larger and smaller presses are used for specialized
applications. Programmable back gages, and multiple die sets available currently can make for a
very economical process.
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Process
In press brake forming, a work piece is positioned over the die block and the die block presses
the sheet to form a shape. Usually bending has to overcome both tensile stresses as well as
compressive stresses. When bending is done, the residual stresses cause the material to spring
back towards its original position, so the sheet must be over-bent to achieve the proper bend
angle.
The amount of springback is dependent on the material, and the type of forming. When sheet
metal is bent, it stretches in length. The bend deduction is the amount the sheet metal will stretch
when bent as measured from the outside edges of the bend. The bend radius refers to the inside
radius. The formed bend radius is dependent upon the dies used, the material properties, and the
material thickness.
Types
There are three basic types of bending on a press brake, each is defined by the relationship of the
end tool position to the thickness of the material. These three are Air Bending, Bottoming and
Coining. The configuration of the tools for these three types of bending are nearly identical. A
die with a long rail form tool with a radius used tip that locates the inside profile of the bend is
called a punch. Punches are usually attached to the ram of the machine by clamps and move toproduce the bending force.
A die with a long rail form tool that has concave or V shaped lengthwise channel that locate the
outside profile of the form is called a die. Dies are usually stationary and located under the
material on the bed of the machine. Note that some locations do not differentiate between the
two different kinds of dies (punches and dies.) The other types of bending listed use specially
designed tools or machines to perform the work.
http://en.wikipedia.org/wiki/Tensile_stresshttp://en.wikipedia.org/wiki/Compressive_stresshttp://en.wikipedia.org/wiki/Compressive_stresshttp://en.wikipedia.org/wiki/Tensile_stress8/2/2019 Report Bending Saufi
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Material Selection:
Steel was chosen for the machine parts for the following reasons:
1 - Low cost.
2 - Easy to machine, in comparison to Aluminum or other materials.
3 - High strength that can withstand the loads.
4 - Long expected life-durable.
Springback in bending
When the bending stress is removed at the end of the deformation process, elastic energy remains
in the bent part causing it to partially recover to its original shape. In bending, this elastic
recovery is called springback. It increases with decreasing the modulus of elasticity, E, and
increasing the yield strength, Y, of a material.
Springback is defined as the increase in included angle of the bent part relative to the included
angle of the forming tool after the tool is removed.
After springback:
The bend angle will decrease (the included angle will increase)
The bend radius will increase
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Objectives
1. To understand the basic bending operation
2. To study the effects of material properties (ductility, types, strength) on the bend radius,
springback and bending force.
3. To study whether or not the material cracked when pressure was applied.
4. To compare the behavior of the different sheet metal.
Apparatus Needed
1. Tooling dies with 0 bend radius, (1.5*thickness) bend radius and (3*thickness) bend
radius
2. Arbor press
3. Work pieces
a. Half Hardened C26000 brass: 1/8 and 1/16 thicknesses
b. Precipitation hardened 6061 Al alloy: 1/8 and 1/16 thicknesses
c. Mild Steel 1018 1/8 and 1/16 thicknesses
4. Protractor
5. Calipers/Micrometer
Procedure
1. Insert the 0 bend radius die into the arbor press.
2. Insert the work piece into the holder.
3. Pull down the handle on the press firmly.
4. Check for material failure (cracking at the bend). Measure the bend radius.
Record results on data sheet provided.
5. Repeat steps 1-4 for each type of material.
6. Repeat steps 1-5 for each bend radius
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Experiment Method
No Task Parameters Drawing
1
2
Marking Process
Create the centerline at thesurface side of four specimen
and punch and die set by
using scribble.Then, punchthe center line by using center
punch. The centerline is used
to accurate the position ofpunch and die set and
specimen before bending
process and make it fixed and
securely.
Bending process
Place the specimen on the die
at the press hydraulic
machine.The drawing view
show the right position of thespecimen.Ensure the punch
tip is touch to the specimen.
Next, take the reading of
pressure indicated on pressuregauge on the press machine
before bending process. Pressthe specimen untill obtain the
90 bend.
Take the maximum pressure
to develop the complete
bending.
Minors final pressure with the
initial pressure to get the
actual pressure.
When the pressure gauge
show maximum pressure, the
punch is start bend the platemetal.
Thera are 4 specimens used in
this bending process.
Specimen 1= 0.0027 mm
P initial = 65 bar
P final = 145 barP =Pi - Pfinal
= 145 bar65 bar
= 80 bar= 8 x 106 Pa.
Diameter Bore = 0.07 m.
Specimen 2= 0.0027 mmP initial = 60 bar
P final = 270 bar
P =Pi - Pfinal
= 270 bar60 bar= 210 bar
= 21 x 106
Pa.
Find the UTS:
F average =75.0405 Nt = 0.0027 m
w = 0.04 m
k = 1.33
= 0.01492 m
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Repeat the same process to
other four specimens and start
to make calculation required
in this process such as force.
After completed all process,our group go to the metrologylab and measured the actual
bend radius and angle by
manual observation.
Then, compared all the result
that get from projector
file(manual observation) withby calculation result.
Make a conclusion based onthe result.
Find the spring back radius :
Rb = 0.012 m
t = 0.0027 mE = 200 GPa
Rf = Spring Back Radius
By manual observations the angle
is for spring back is 85.1The radius by manual observation
is 23 mm.
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Specimen 3= 0.0019 mm
P initial = 60 bar
P final = 190 bar
P =Pi - Pfinal= 190 bar60 bar
= 130 bar= 13 x 106
Pa.
Diameter Bore = 0.07 m.
Specimen 4 = 0.0019 mm
P initial = 60 bar
P final = 230 barP =Pi - Pfinal
= 230 bar60 bar
= 170 bar
= 17 x 106
Pa.
Diameter Bore = 0.07 m.
Find the UTS:
F average=57.72 kNt=0.0019 m
w=0.04 m
k=1.33
=0.01492 m
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Find the spring back radius :Rb = 0.012 m
t = 0.0019 m
E = 200 GPa
Rf = Spring Back Radius
By manual observations the angle
is for spring back is 83.5The radius by manual observation
is 18 mm.
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Observations
a. The specimen must be place securely and parallel with punch and die position on the
press hydraulic machine.
b. The centerline at punch, die and specimen must be clear and tidy.
c. The movement of bore press machine is not smooth and fluent during bending process.
d. When bending process, the force to bend thin specimen is lower than bend the thick
specimen. The thick specimen need more force than the thin specimen.
e. We need to do bleeding process at the hose pipe to get the smooth movement.
Results
a. The force and spring back radius is different depend on the thickness of specimen.
Difficulties faced
a) During bending process, the bore cylinder is not move smooth and fluent to bend the
specimen. There are bubbles in the hose of the press hydraulic machine.
b) It is hard to ensure the centerline of the specimen position accurately according to the
centerline of the punch and die position.
c) The machine is poor and the lever is hard to press.
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Graph R (minimum bend radius) VS t (material thickness)
Effect of ductility on minimum bend radius
In the production of parts from sheet blanks by bending with a decrease in the bend radius there
is an increase in the stiffness, a decrease in cross sectional area of the parts. The minimum
allowable bend radius is determined by the ductility of the blank material and the method of
bending. In order to decrease the minimum bend radius from the viewpoint of failure of the
material is it necessary to increase the ductility of the material and decrease the intensity of
deformations on the convex surface of the blank, which is where crack formation is normally
found.
0
5
10
15
20
25
0 0.0005 0.001 0.0015 0.002 0.0025 0.003
Series1
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Effect of ductility on spring back
Metals have a property which many other materials such as wood, glass or ceramic do not
possess, this property is known as ductility. According to the Merriam Webster Online
Dictionary, ductility is defined as capable of being fashioned into a new form . Ductility is a
mechanical property which illustrates the degree to which a solid material can be plastically
deformed without cracking. Therefore, when a material has the characteristic of being ductile, it
can be bent and formed into an unlimited number of shapes.
In bending, residual stresses cause the material to spring back slightly after the bending
operation. Due to this elastic recovery, it is necessary to over-bend a precise amount to achieve
the desired bend radius and bend angle. The final bend radius will be greater than initiallyformed and the final bend angle will be smaller. The ratio of the final bend angle to the initial
bend angle is defined as the springback factor, Ks. The amount of springback depends upon
several factors, including the material, bending operation, and the initial bend angle and bend
radius. From a molecular point of view, it is the metals ductility characteristic which allows the
bending to occur. The ductility of a metal is influenced by many factors such as the hardness and
composition of the metal. For example, stainless steel is much harder to bend than copper, a soft
metal which can be bent into never-ending shapes.
http://glossary.last.linkclick%28%27bend%20radius%27%29/http://glossary.last.linkclick%28%27bend%20angle%27%29/http://glossary.last.linkclick%28%27bend%20angle%27%29/http://glossary.last.linkclick%28%27bend%20radius%27%29/8/2/2019 Report Bending Saufi
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Tabulate the springback vs. thickness for the different materials and explain the trend based on
their youngs modulus.
Springback, like bend allowance, depends on several factors, such as material properties, initial
sheet thickness, bend radius, and stress/strain distribution across the sheet thickness. Since the
springbackis also affected by the material properties, such as Youngs modulus and initial yield
stress. The effect of thickness on spring back is increasing of sheet thickness, spring backincreases .
Compare the bending forces required for hardened brass vs. annealed brass for different
thicknesses and die opening.
While using a press brake and standard die sets, there are still a variety of techniques that
can be used to bend the sheet. The most common method is known as V-bending, in which the
punch and die are "V" shaped. The punch pushes the sheet into the "V" shaped groove in the V-
die, causing it to bend. If the punch does not force the sheet to the bottom of the die cavity,
leaving space or air underneath, it is called "air bending". As a result, the V-groove must have a
sharper angle than the angle being formed in the sheet. If the punch forces the sheet to the
bottom of the die cavity, it is called "bottoming". This technique allows for more control over the
angle because there is less springback. However, a higher tonnage press is required. In both
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techniques, the width of the "V" shaped groove, or die opening, is typically 6 to 18 times the
sheet thickness. This value is referred to as the die ratio and is equal to the die opening divided
by the sheet thickness.
Result, Discussion and Conclusion
Two common bending methods are:
- V-bending
- Edge or wipe bending.
In V-bending the sheet metal blank is bent between a V-shaped punch and die. The
figure below shows a front view and isometric view of a V-bending setup with the arrows
indicating the direction of the applied force:
When bending a piece of sheet metal, the residual stresses in the material will cause the
sheet to springback slightly after the bending operation. Due to this elastic recovery, it is
necessary to over-bend the sheet a precise amount to achieve the desired bend radius and bend
angle. The final bend radius will be greater than initially formed and the final bend angle will be
smaller. The ratio of the final bend angle to the initial bend angle is defined as the springback
factor, KS. The amount of springback depends upon several factors, including the material,
bending operation, and the initial bend angle and bend radius.
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The act of bending results in both tension and compression in the sheet metal. The
outside portion of the sheet will undergo tension and stretch to a greater length, while the inside
portion experiences compression and shortens. The neutral axis is the boundary line inside the
sheet metal, along which no tension or compression forces are present. As a result, the length of
this axis remains constant. The changes in length to the outside and inside surfaces can be related
to the original flat length by two parameters, the bend allowance and bend deduction, which are
defined below.
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Appendix
Table 1: Elastic properties of the given materials
Formulae
R = Minimum bend radiust = Material thickness
%A = Tensile reduction in area%E = % of elongation at break
References
1. M.P. Groover, Fundamentals of modern manufacturing, 3rd edition, (2007).
2. S. Kalpakjian, S.R. Schmid, Manufacturing procedure for engineering materials, 2nd
edition, p. 350-351.
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