Types of bending processes

AIR BENDING

Air Bending is a bending process in which the punch touches the work piece and the Work piece does not bottom in the lower cavity. As the punch is released, the work Piece springs back a little and ends up with less bend than that on the punch (greater Included angle). This is called spring-back.

BOTTOMING Bottoming is a bending process where the punch and the work piece bottom on the die. This makes for a controlled angle with very little spring back. The tonnage required on this type of press is more than in air bending. The inner radius of the work piece should be a minimum of 1 material thickness.

Introduction

Definition:-Bending is defined as the straining of metal around a straight axis. During this process, the metal on the inside of the neutral axis is compressed, while the metal on the outside of the neutral axis is stretched.

α = bend angle w = width of sheetR = bend radiust = sheet thicknessα′ = 180° - α, “included” angle

Bending is a flexible process by which a variety of different shapes can be produced though the use of standard die sets or bend brakes. The material is placed on the die, and positioned in place with stops and/or gages. It is held in place with hold-downs. The upper part of the press, the ram with the appropriately shaped punch descends and forms the v-shaped bend. Bending is done using Press Brakes. Press Brakes can normally have a capacity of 20 to 200 tons to accommodate stock from 1m to 4.5m (3 feet to 15 feet). Larger and smaller presses are used for diverse specialized applications. Programmable back gages, and multiple die sets currently available can make bending a very economical process.

COINING Coining is a bending process in which the punch and the work piece bottom on the die and compressive stress is applied to the bending region to increase the amount of plastic deformation. This reduces the amount of spring-back. The inner radius of the work piece should be up to 0.75 of the material thickness.

V Bending In V-bending, the clearance between punch and die is constant (equal to the thickness of sheet blank). It is used widely. The thickness of the sheet ranges from approximately 0.5 mm to 25 mm.

U DIE BENDING U-die bending is performed when two parallel bending axes are produced in the Same operation. A backing pad is used to force the sheet contacting with the punch bottom. It requires about 30% of the bending force for the pad to press the sheet contacting the punch.

WIPING DIE BENDING Wiping die bending is also known as flanging. One edge of the sheet is bent to 90 while the other end is restrained by the material itself and by the force of blank-holder and pad. The flange length can be easily changed and the bend angle can be controlled by the stroke position of the punch.

punch

pad

Spring back in bending

When the bending stress is removed at the end of the deformation process, elastic energy remains in the bent part causing it to partially recover to its original shape. In bending, this elastic recovery is called springback. It increases with decreasing the modulus of elasticity, E, and increasing the yield strength, Y, of a material.

Springback is defined as the increase in included angle of the bent part relative to the included angle of the forming tool after the tool is removed.

After springback:• The bend angle will decrease (the included angle will increase)• The bend radius will increase

Following is a schematic illustration of spring back in bending:

Manufacturing processes by S. Kalpakjian and S. Schmid

αi: bend angle before springback αf: bend angle after springback Ri: bend radius before springback Rf: bend radius after springback