Bending is the metal working process by which a straight length
is transformed into a curved length. It is very common forming
process, for changing sheet and plate into channels, drums and
During the bending operation, the outer surface of the material
is in tension and the inside surface is in compression.
The strain in the bent material increases with decreasing the
radius of curvature.
The stretching of the bend causes the neutral axis to move
toward the inner surface. In most cases, the distance of the
neutral axis from the inside of the bend is 0.3 t to 0.5 t , where
“t” is thickness of the part.
Two methods are commonly used for Bending Operation.
In v-bending , a wedge shaped punch forces the metal sheet
into a wedge shaped die cavity.
The bend angle may be acute, 90°, or obtuse.
In edge bending, the punch forces the metal against the
The workpiece is clamped to the die block by a spring
loaded pad, before the punch contacts the workpiece to
prevent its movement during downward travel of the
The bend axis is parallel to the edge of the die.
The length of curved section before bending is called bend allowance.
Bending allowance is given by-
Ba= θ ( R + KT)
θ -bend angle (radians)
K-constant; 0.33when R is less than 2t & is 0.50 when R is more than 2t.
Bend allowance for ideal caseBa= θ(R+(T/2)); k=0.5
An inside radius of bend equal to three times stock thickness is preferable.
Greater the thickness, hardness or rigidity of the material, the greater the
radius required to avoid fracturing.
Bend radius for different materials-
Minimum bend radius
Engineering strain on a sheet during bending
E = 1 / (2R/T)+1
As R/T decreases, tensile strain at outer fiber increases and material
Ratio at which the crack appears on outer surface is minimum bend
Maximum bending force, P = KYLT2
K – Die opening factor
Y – yield stress
L- length of the bend
T- thickness of sheet
For a V-die
Max bending force, P = KSLT 2
S – Ultimate tensile strength
*In Bending ,after plastic deformation there is an elastic recovery this
recovery is called spring back.
*Spring back can be calculated approximately in terms of radii Ri and
*Ri/Rf = 4 ( Ri Y / ET )3 – 3 (Ri Y /ET) + 1
*Spring back Increases as elastic modulus E decreases.
To avoid spring back bend angle
should be increased by a certain