The 3-roll plate bending machine with 3 rollers usually has two fixed rollers, while the upper vertical roller is positioned accordingly in order to complete the desired bending. By changing the relative position and rotating movement of the working roller, continuous plastic deformation is achieved in order for the workpiece to receive a predetermined shape.
The working principle of the 3-roll plate rolling machine
When roll bending the steel plate, the symmetrical 3-roll plate rolling machine places the steel plate between the upper and lower rollers, and the three cutting points exposed by the three rollers on the metal sheet can bend the steel plate into an arc or a closed circle. Therefore, the sheet bending process of sheet metal can be regarded as a continuous three-point bending process by a 3-roll plate rolling machine.
During the processing, one end of the metal plate is fed into the three-roll plate roller between the upper and lower rollers, and the upper roller moves downward on the metal plate, causing the plate below it to undergo a certain plastic bending deformation due to pressure. As the drum rotates, there is friction between the plate and the rollers, so as the rollers rotate, the plate also moves longitudinally. The lower roll rotates alternately forward and reverses, the rolling plate moves back and forth, the two lower rolls rotate, and the friction drives the movement of the plate and the roll. At the same time, the upper roller continues to exert downward pressure on the roller, and the upper roller moves back and forth on the printing plate. When the sheet passes through the bottom of the upper roll (roll deformation zone) in turn, when the stress exceeds the yield limit, plastic deformation will occur, and the sheet will obtain plastic bending deformation along the entire length and be processed into the desired shape.
By properly adjusting the relative positions of the upper and lower rollers, the sheet can be bent to a radius not less than the radius of the upper roller.
Working principle of symmetrical 3-roll plate rolling machine
The unbent flat section of the plate rolling process
In the plate rolling process, you have the unbent flat section at the leading and trailing edges. It’s usually barely noticeable, especially in sheet metal and plates rolled to a large diameter. But they’re there, and they’re unavoidable because the pinch rolls need a place to hold the material.
As can be seen from the figure, same as the section bending machine, the three rolls of the symmetrical 3-roll plate rolling machine are arranged in an isosceles triangle, so during the rolling process, an unbent flat section must be left at both ends of the workpiece.
The length of the unbent flat section is about half the distance between the centers of the two lower rolls. This part of the straight line is where the plate roller cannot roll, which is the biggest disadvantage of the symmetrical three-roll plate bending machine.
Although the symmetrical three-roll plate bending machine has many shortcomings, it has been widely used because of its simple structure, convenient operation, and low cost.
How to eliminate the unbent flat section?
As for the problem of eliminating the unbent flat section, different methods can be used in combination with specific situations.
Adding a “base” for the workpiece during the pre-bending
This method involves the installation of a “base” together with a workpiece designed for bending and performing the edge pre-bending process on the circular bending machine itself. The application of this method should take into account the possibilities of the circular bending machine and calculate the strength of the applied force on the “base” and the workpiece.
Leaving allowance at the ends – if you add an appropriate allowance at plate ends before the bending, you can, after the bending, cut off those parts into a desired bending shape.
An operation called pre-bending minimizes the flat sections on the plate’s leading and trailing edges. In a typical setup, the operator performs the prebend to the leading and trailing edge, usually leaving an unbent flat 1.5 to 2.5x the material thickness, depending on the application and material.
For critical-dimension cylinder rolling, an operation might opt to roll the cylinder, weld the longitudinal seam, grind it down, then reroll to eliminate the unbent flat. But in most cases, that small unbent flat section remains.
3 methods were applied to the symmetrical plate bending machine
Elbow Pre-bending: This method uses a die to prebend both ends of the steel plate at the press to achieve the desired curvature.
Retention Allowance: Add the appropriate board allowance to the board ends, and cut off the remainder (aka the unbend flat).
Add the bottom plate for pre-bending: This method is carried out on the plate bending machine, but when using this method, the capacity of the plate bending machine should be fully considered, that is, the sum of the bending force of the workpiece and the required gasket should be less than the bending capacity of the plate bending machine.
Asymmetric three-roll plate bending machine
For another asymmetric three-roll plate roller, the arrangement of the axle rolls is designed to eliminate unbending flat on the rolled workpiece. The feature of this rolling machine is that the two lower rollers can be adjusted vertically.
Any one of the lower rollers can be adjusted to the center distance of the upper roller, and the other roller can be raised to a suitable position. The starting end of the steel sheet can be bent and rolled. After half rolling, change the alignment of the two lower rolls and continue rolling to eliminate the straight section at the end of the workpiece. The workpiece can also be turned over, the rear end becomes the front end for rolling, and the unbend flat can also be eliminated.
The unbend flat of an asymmetrical bend is approximately 1/6 to 1/10 of the symmetrical bend.