Modification Of An Existing Metal Sheet-bending Machine

CHAPTER TWO
2.0    BENDING TECHNOLOGY
2.1    Principles of Bending
From the layman point of view the term bending is when a material curves at an angle other from its original shape. To define bending, it is a definite forming operation in which metal is deflected from its original form.
In bending operation the material is formed around a straight axis which extends completely across the material at a bending line. A bending operation produces a plane surface which is at an angle to the original plane of the work piece.
The illustrations in fig 2.1 are basic examples of bending parts showing some basic fixtures in a bend.
The following observations are made from figure2.1 view (a).(I) (i) The bend axis is located at the center of the bend radius.
(ii) The bend radius is tangential to the inner plane surface of the sheet.
(iii) The bend line occurs at the tangent of the bend radius with the inner plane surfaces.
(iv) The bend radius is perpendicular to the inner plane surfaces.
View (b)
The bend radius is equal to zero because the inner corner of the bend is sharp. In a situation like this the bend and the bend lines coincide and are located at the intersection of the inner plane surface of the sheet metal.
2.2 THE MECHANICS OF BENDING
At the application of bending force gradually onto a work piece under free bending, holding to the fact that the compressive and tensile stresses developed on opposite faces of the material are not sufficiently high to exceed the yield strength of the material Upon removal of the applied force, the work piece returns to its original shape.

As the bending force is gradually increasing, the stress produced in the outermost layer (on both the compression and tension sides) of the material eventually exceeds the yield strength and thus lose its ability to return to its original position when force is removed, it thereby assume a new position.
When bending a sheet metal to an angle, the inner fibres of the  bend are compressed, thus experiencing Compression Stress and the outer fibres of the bend are stretched and thereby experience Tensile Stress.
The neutral plane, however lies between the stressed zones which are in opposition to each other and does not change in length. This boundary Le. neutral plane is termed the “Neutral Line’. fig. 2.4 (a) and (b) shows the resulting stress pattern indicated y directional arrows.