report bending saufi

8/2/2019 Report Bending Saufi

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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_stress


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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.


Specimen 4 = 0.0019 mm

P initial = 60 bar

P final = 230 barP =Pi - Pfinal

= 230 bar60 bar

= 170 bar

= 17 x 106

Pa.


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.
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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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