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the bend radius is measured to the inner surface of the bent part

Bending

there is a plane which separates the tension and compression zones. This plane is called neutral axis.

The position of neutral axis depends on the radius and angle of bend. the width of the part L in the outer region is smaller, and in the inner region it is larger, than the initial original width.

Metal on inside of neutral plane is compressed, while metal on outside of neutral plane is stretched

BEND ALLOWANCE

It is the length of the neutral axis in the bend This determines the blank length needed for a bent part

Lb = α ( R + kt )

where, Lb = bend allowance (mm)

a = bend angle (radian)

R = bend radius (mm)

t = thickness of sheet (mm), and

k = constant, whose value may be taken as 1/3 when R < 2 t, and as 1/2

when R 2t .

Example A 20 mm wide and 4 mm thick C 20 steel sheet is required to be bent at 600 at bend radius 10 mm. Determine the bend allowance

R = 10 mm , t = 4 mm

R > 2t, k = 0.5

Bend allowance

MINIMUM BEND RADIUS

As the ratio of the bend radius to the thickness of sheet (R / t) decreases, the tensile strain on the outer fibres of sheet increases. If R / t decreases beyond a certain limit, cracks start appearing on the surface of material. This limit is called Minimum Bend Radius for the material

Minimum bend radius is generally expressed in terms of the thickness of material, such as 2t, 3t, 4t, etc

160

rt

RMinimum

True strain at fracture is

r: percent area reduction

7.65 Calculate the minimum tensile true fracture strain that a sheet metal should have in order to be bent to the following R/t ratios: (a) 0.5,(b) 2, and (c) 4. (See Table 7.2.)

160

rt

RMinimum

r

6015.0 51.0f

8

Types of Sheetmetal Bending

• V-bending - performed with a V-shaped die– Bottom-bending

• Workpiece is in complete contact with punch on one side, and the with the die on the other side

• Angle is set by the form of the tooling (punch & die)

– Air-bending• A form of three-point bending

• Angle is continuously variable* - set by a stop

• Edge bending - performed with a wiping die

V - Bending

• For low production rates

• Performed on a press brake

• V-dies are simple and inexpensive

10

Edge - Bending

• For high production rates

• Pressure pad required

• Dies are more complicated and costly

Spring back :

• In Bending ,after plastic deformation there is an elastic recovery

this recovery is called spring back.

• Spring back can be calculated approximately in terms if radii Ri

and Rf

• Ri/Rf = 4 ( Ri Y / Et )3 – 3 (Ri Y /Et) + 1

Spring back Increases as (R/t ratio & yield stress of material )

increases as elastic modulus E decreases

The part tends to recover elastically after ending,and its bend radius becomes larger.

Under certain conditions,it is possible for the final bend angle to be smaller than the original angle(negative spring)

Compensation for spring back

• Over bending of part

• Bottoming the punch – coin the bend area by subjecting it to high localized compressive between the technique tip of the punch and the die surface.

• Stretch bending – Part is subjected to tension while being bent.In order to reduce spring back bending may also be carried to reduce spring back bending may also be carried out at elevated temperatures

14

Springback and Compensation

•Compensation:

Overbending Bottoming

Local plastic

deformation

Bending force :

K – constant ranges from 0.3(wiping die) – 0.7(u-die)-1.3(V-die)

Y – yield stress

L- length of the bend

T- thickness of sheet

For a V-die

Max bending force, F = (UTS)Lt 2

WUTS – Ultimate tensile strength

Bending Force

Maximum bending force estimated as follows:

Where:

F = bending force;

UTS = tensile strength of sheet metal;

t = stock thickness; and

W = die opening dimension.

For V - bending, K =1.2 to 1.33;

For wiping die (edge bending) K = 0.3-0.34

W

LtUTSkF

2)(

Bending Force : There are two general types of die bending : V – die bending and wiping die bending. V – die bending is used expensively in brake die operations and stamping die operations..

7.66 Estimate the maximum bending force required for a 1 8 -in. thick and 12-in. wide Ti-5Al-2.5Sn titanium alloy in a V -die with a width of 6 in.

UTS= 125,000 psi

W

LtUTSkF

2)(

6

)8

1)(12)(125000( 2

kF

For V - bending, K =1.2 to 1.33

Press brake forming

– Used for sheets 7M(20ft) or longer and other narrow pieces

– Long dies in a mechanical or hydraulic press for small production runs

– Die material range from hardwood to carbides.

Common bending operations:

Roll bending :– Plates are bent using a set if rolls,various curvatures can

be obtained by adjusting the distance between three rolls

Common die-bending operations, Showing the

die-opening dimensions, W, used in

calculating bending forces

Beading :

• The periphery if the sheet metal is bent into the

cavity of a die

(a) Bead forming with a single die (b) Bead forming with two dies,in a press brake

Flanging :

• Flanging is a process of bending the edges of sheet metals to 90o

• Shrink flanging – subjected to compressive hoop stress.

• Stretch flanging –subjected to tensile stresses

Various flanging operations (a)

Flanges on a flat sheet. (b)

Dimpling. (c) The piercing of

the sheet metal to form a

flange.In this operation,a hole

does not have to be

prepunched before the bunch

descends .Note however,the

rough edges along the

circumference of the flange.

(d) The flanging of a tube;

note the thinning of the edges

of the flange

Bending Operations

Straight flanging Stretch flanging Shrink flanging

Hemming seaming curling

Dimpling :

• First hole is punched and expanded into a flange

• Flanges can be produced by piercing with shaped

punch

• When bend angle < 90 degrees as in fitting conical

ends its called flanging

Hemming :• The edge of the sheet is folded over itself

• This increases stiffness of the part

The metal strip is bent in stages by passing it through a series of

rolls

Seaming :• Joining two edges of sheet metal by hemming specifically

shaped rollers used for watertight and airtight joints

• Roll forming is used for continuous lengths of sheet metal

• Used for large production runs

Fig: Schematic illustration of

the roll-forming process

Roll forming :

Tube bending and forming

• Special tooling required to avoid buckling and folding

Methods of bending tubes.Internal mandrels,or the filling of tubes with particulate materials such as sand,areoften necessary to prevent collapse of the tubes during bending .Solid rods and structural shapes can also be bent by these techniques

Bulging :

• Process involves placing tabular,conical or curvilinear part into a split-

female die and expanding it

:Segmented die

• Individuals are placed inside the parts and mechanically

expanded in radial direction and finally retracted.

Stretch forming• Sheet metal clamped along its edges and stretched over a

die or form block in required directions.

Bending and Forming Tubes

Using internal mandrels, or filling tubeswith particulate materials such as sand,prevents the tubes from collapsing duringbending. Solid rods and structural shapesare also bent by these techniques

A method of forming a tube with sharp angles, using an axial compressive force. Compressive stresses are beneficial in forming operations because they delay fracture. Note that the tube is supported internally with rubber or fluid to avoid collapsing during forming

Stretch forming

Fig: Schematic illustration of a stretch forming process. Aluminum skins for aircraft can be made by this process