Plate Bending Techniques and Roll Machine Mechanics

Fundamentals of Plate Bending

Bending a sheet involves shaping it, in part or in whole, into a cylindrical or conical form. The bending process can be classified as either open or closed. The bending of sheets is achieved by a bending force caused by pressure applied manually, by machine, or through impact shocks.

The Neutral Fiber Concept in Bending

To understand the development of a bent piece, consider a sheet 10 mm thick and 628 mm long, with edges perfectly perpendicular to the faces. When forming a cylinder of revolution with this plate, we observe the following:

• A) The thickness does not change during the bending process.
• B) The two extreme edges match perfectly.

If a sheet is bent and its length is compared to the original, we see that the inside of the curve has decreased while the outside has increased. Fibers are subjected to traction on one side and compression on the other. This force is greater the closer the fibers are to the outer and inner surfaces. However, the fibers in the middle are not subject to any stress and retain their original length. These fibers are known as the neutral axis.

Rule for Calculating Neutral Fiber

To find the required length of a sheet to be curved, the calculation must be based on the diameter or radius of the neutral fiber.

Types of Plate Bending Machines

These machines are often referred to as roll bending machines. These rolls can be of considerable length and possess great resistance to bending.

Operation of Roll Bending Machines

A translational motion is applied to the plate through two mechanically driven drive rolls. Additionally, it is continually subjected to the action of a bending roller, causing an uninterrupted series of bends that result in a smooth curve.

Classification of Bending Machines

Depending on the arrangement of the rollers, four distinct types of sheet metal bending machines are recognized:

1. 3-roller pyramid (horizontal or vertical).
2. Machines with 3 or 4 horizontal rollers.
3. Bending edge machines with center horizontal rollers.
4. Two-horizontal-roller machines.

3-Roll Pyramid Bending Machines

Horizontal and Vertical Roller Systems

Horizontal Roller Machines: The two drive rollers are fixed without translation and are positioned in the same horizontal plane, rotating in the same direction. The bending roller rotates freely (idle) and is mobile in vertical translation.

Horizontal Roller Machines for Fine and Medium Sheets: The rollers are powered by a crank or a motor. To remove the curved plate, one side of the machine's enclosure is completely opened.

Horizontal Roller Machines for Medium and Large Plates: These are powered directly by an electric motor.

Vertical Roller Machines: These are very robust and are exclusively used for bending thick plates. The bending roller moves in a horizontal plane.

Advantages:

• a) They maintain the cylinder generatrix perpendicular to the edge of the plate as it rests on the horizontal crossbar of the machine.
• b) They eliminate the inconvenience of irregular curves caused by the weight of the sheet, which tends to open during bending on horizontal machines.