Modification Of An Existing Metal Sheet-bending Machine

2.3 Plastic Deformation Due to Bending
Bending operation changes the shape of a material but doe it affect the volume. The side, which is at the portion of a bend, becomes distorted.
It’s notable that some distortions are proportionate to the ductility of the material and the severity of the bend axis beyond the bend area. This distortion is called plastic deformation because it is associated with the characteristic plastic flow of the material within the bend area.
The severity of a bend is determined by some major factors such as; size of the bend angle and sheet metal thickness. These factors, singularly or in combination affects severity of bend as follows:
(I)     Increasing the bend angle increases the severity.
(ii)     Increasing the sheet metal thickness increases the severity. More strain hardening is experienced in thicker material than in thinner materials bent to the same in radius.
(iii)     Decreasing the bend radius increases the severity i.e small radius bend cause greater cold deformation than a longer radius bend in a materials of some thickness.
The type of material also H1Iluen plastic deformation which occurs in a bend.
In general, softer mat material do not offer much resistance to plastic flow as harder or less ductile materials. This enables a more ductile material to be bend more radically than the less ductile ones. This bend or plastic deformation occurs before their ultimate strength is reached.
2.4 Spring Back
The elastic recovery of shape is termed spring back. in bending operation, the elastic limit of the metals Iii process is exceeded. Therefore, some of the original elasticity of sheet metal is still present at the peak of the bending cycle. In this case, when the bending force is withdrawn, the material on the compression side tends expand slightly and the metal on the extension side tends to contact. Its combination results in the work piece tending to resume its original shape, causing the bend to spring open. This reaction of the metal is called Spring Bark.
The condition of the sheet, sheet thickness, and type affects spring back. Spring back also varies directly in proportion to the size of the bend radius. The larger the bend radius, the greater the spring back and vice —versa.
2.5 Method for Compensating Spring Back
Holding to their different compositions, mechanical properties of materials, spring back varies on different materials. Due to the composition of certain materials, some undergo more cold-working than others.
When cold-worked, the condition of the material upon which bending operation is carried out has an influence on the amount of spring back likely to result. For inst rice, cold rolled sheet metal usually exhibits greater spring back than the same sheet metal of equal thickness when in annealed condition.
Generally, the extent to which the material will stretch (elongate) on the tension ‘ (out of bend f bend) determines the amount of spring back. The following methods are used to compensate for spring back
i. Air bending (or angular punch relieve)
ii. Coining (or corner setting)
iii. Over bending.
The first two methods are mostly u when using press brake or vee tool in fly press while the last method is applied when using the spring beam bending machine.
In air bending, the bending angle is over closed to compensate for the spring back of the material.
In the case of coining, the steel  mental is bent at a bend area resulting into additional compressive strait within the material which overcomes the spring back of a normal bend. The nose of the vee tool crushes the natural air bending radius on the inside of the bend to effect the coining.