# Design of bending dies

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Information about Design of bending dies
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Published on February 25, 2014

Author: vishalsatsangi

Source: slideshare.net

* Design of Bending Dies

*  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 tanks etc.  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. V-Bending  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.

Edge Bending  In edge bending, the punch forces the metal against the supporting die.  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 punch.  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) T-sheet thickness R-bend radius 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. 2-5t 0.35t Al 1t SS 0.3-0.5t Bend radius for different materials- C.Steel Brass 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 cracks.  Ratio at which the crack appears on outer surface is minimum bend radius.

* Maximum bending force, P = KYLT2 W 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 W 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 Rf. *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 amount.

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