What is pre-bending?
Plate rolls need a place to grab the material during forming. That’s why, in any plate rolling situation, you have a narrow unbent flat section on the plate’s leading and trailing edges. An operation known as pre-bending reduces the length of these unbent flat sections.
You perform a prebend to the plate’s leading edge, then the trailing edge, and then perform the actual rolling. In production bending with double-pinch, four-roll systems, it’s possible to prebend the leading plate edge, perform the rolling, and then the prebend (in this case it could be called the post bend) to the trailing edge. But in most common setups, pre-bending is performed before rolling the cylinder.
WHY NEED PRE-BENDING？
Pre-bending is to reduce the flat part in the bending process.
WHAT IS THE FLAT PART OF A COLD ROLLING BENDING?
After cold roll bending the position of the rollers on the workpiece inevitably remains a flat part at the ends of the metal profile. This flat part is approximately half the length of the gap between the centers of the lower rollers.
HOW TO REDUCE IT?
Although the appearance of a straight part is considered to be a flaw of asymmetrical roller with 3 rollers, their simple structure, comfortable use, and low price still make them the most wanted model of roll metal section bending machines.
In addition, there are methods which we can use in order to easily eliminate this problem. You can find some of them below:
This solution involves bending the edges of the workpiece on a press brake before the roll bending process.
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.
Regarding the asymmetrical models of roll bending machines with 3 rollers, the position of the rollers is better adapted to eliminate the flat edges of the sheet, in which case the lower rollers can also be mounted vertically.
Thus, in a model with an asymmetric position of the rollers, the lower rotating roller can be lifted up to the middle of the gap between the two rollers, while the other can be positioned in the appropriate position so that the starting part of the metal profile can be folded to form a circle. The flat edge can be eliminated on both sides by turning the rollers in half of the bending process by folding the back of the profile. Additionally, the workpiece can also be turned around after completion of one bending cycle and then the back part will become the front and both flat edges will be eliminated.
3 roll bending machines
The plate roller 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.
While bending, the workpiece is positioned between the lower and upper rollers. After placing the sheet between the rollers, the upper roller is lowered to secure the workpiece, and at the same time, it has achieved sufficient pressure on the sheet, generating plastic bending. When rotating the roller there is friction between the sheet metal and the roller, so the sheet moves in its longitudinal direction. At the same time, the upper roller adds pressure to the workpiece.
When the roller crosses the lower limit of the upper roller (the roller deformation zone) in one turn and the pressure exceeds the bending limit, plastic deformation occurs and the sheet is folded to its desired dimensions by its entire length. By adjusting the relative position between the lower and upper rolls, the sheet can be folded to a diameter that is not less than the diameter of the upper roller. Here you can see the principle of the symmetrical circular bending machine with 3 rollers:
Rollers 2 and 3 move in the same direction and at the same speed. Because of the friction between the sheet metal and the rollers, the workpiece moves in advance. By adjusting the position of the upper roller, the desired bending angle can be achieved. If the workpiece does not achieve the proper curvature after the first bending, it can be adjusted and started the process in the opposite direction until the desired shape is achieved.
After bending due to the position of the rollers on the workpiece inevitably remains a flat part at the ends of the sheet. This flat part is approximately half the length of the gap between the centers of the lower rollers. Although the appearance of a straight part is considered to be a flaw of asymmetrical roller with 3 rollers, their simple structure, comfortable use, and low price still make them the most wanted model of roll sheet metal bending machines.
– this solution involves bending the edges of the workpiece on a press brake before the roll bending process.
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 appropriate allowance plate ends before the bending, you can, after the bending, cut off those parts into a desired bending shape.
Regarding the asymmetrical models of roll bending machines with 3 rollers, the position of the rollers is better adapted to eliminate the flat edges of the sheet, in which case the lower rollers can also be mounted vertically. Thus, in a model with an asymmetric position of the rollers, the lower rotating roller can be lifted up to the middle of the gap between the two rollers, while the other can be positioned in the appropriate position so that the starting part of the sheet can be folded to form a circle. The flat edge can be eliminated on both sides by turning the rollers in half of the bending process by folding the back of the sheet. Additionally, the workpiece can also be turned around after completion of one bending cycle and then the back part will become the front and both flat edges will be eliminated.
4 roll bending machines
In addition to machine tools for roll bending with 3 rollers, there are on the market machines for roll bending with 4 rollers. They offer quicker and easier management, but they also have a higher price.
The principle of operation of the rolling machine with 4 rolls is similar to the machine with 3 rollers, with the main difference being that the additional roller is used for better input of the material and execution of the process of pre-bending. During bending, the upper and lower rollers are static, the material is firmly positioned between them, and the side rollers move and bend the material. In order to eliminate the flat edges of the material, the lower roller moves upward to hold the material firmly, and the side roller moves and occupies the position for pre-bending. Unlike a roll bending machine with 3 rollers, where during the pre-bending it is assumed that the material is lowered when entering the machine, in the rolling machine with 4 rollers, the material is positioned horizontally, allowing the use of motorized horizontal material holders when inserting materials in certain models. In addition, the input of materials can be carried out on both sides, so the machine can be placed next to the wall and save space.
The advantage of a rolling machine with 4 rollers has a unique complete bending process. The machine accepts the material in one stroke, performs the pre-bending, and continues to bend the sheet. This system does not require an operator to remove, turn and then straighten the material after pre-bending, as is the case with 3-rollers bending machines. The material remaining in the machine makes the machine with 4 rollers up to 50% more efficient compared to 3-roller models and allows the material to bend at the desired diameter immediately after pre-bending. The bending of the flat edge starts after bending the material in one direction.
The side rollers are positioned to the left and to the right of the bottom roller and have their own axes. The independent axis of each roller allows perfect bending. The “rear” side roller (located on the other side of the material insertion) also has a rear end stop function and aligns the material. This feature allows the operator to work without the assistance of other people. Bending in the shape of the coupe is also possible on the machine with 4 rollers. The side rollers can be positioned in the corner for making the coupling and the bottom roller can be lifted to hold and position the material.
3 methods of plate roller machine rolling metal cylinder
Reeling method for pre-bent sheets
- First adjust the upper roll balance mechanism of the Plate Rolling Machine so that the upper roll is in a horizontal position, and raise the two lower rolls to a certain degree
- Feed the metal sheet between the upper and lower work rolls, and then stand the sheet centering mechanism. After operating the sheet material against the sheet, align the sheet centering mechanism to make the sheet The end is parallel to the work roll axis.
- Raise the two lower rollers at the same height, clamp the sheet material, and pay attention to whether the sheet material is placed properly, otherwise adjust it.
- After confirming that the sheet is placed correctly, continue to raise the two lower rollers to press the sheet alternately. The amount of boost is determined by the diameter of the drum and the proficiency of the operation.
- The work roll rotates in both directions, causing the sheet to bend and deform.
- Repeat the above steps 4 and 5 according to the diameter of the roll until it meets the requirements.
- Lower the lower rollers to the lower limit position alternately.
- On the return side of the dumping side hydraulic cylinder, the inverted body is dumped, and the workpiece is unloaded.
- The inverted body is reset, and the two lower rollers are raised alternately to complete the whole rolling process.
The process of bending the end pre-bent sheet into a cylindrical workpiece on a W11H series three-roll rolling machine.As shown in Figure 1
Figure 1 The reeling process of the pre-bent sheet at the end
Load the pre-bent end of the sheet at one time and round it
- First, according to steps 1 and 2 of the A.1 roll method, level the upper roll of the rolling machine to center the sheet.
- Lift the lower plate roller to clamp the sheet with the upper spokes. The lower roller on the other side plays a supporting role. At this time, the three work rollers become an asymmetrical arrangement.
- The work roll is rotated in both directions to make the sheet bend and deform. Then check with the template. In order to get a good pre-bend at the end of the sheet, the upper and lower rollers should clamp the edge of the sheet as much as possible. , You can also appropriately raise the lower roll at the pre-bent end of the sheet to increase the amount of pressure increase until one end of the sheet reaches the required bending radius.
- Change the position of the lower roll and pre-bend the other end of the sheet in the same way. Step: Lower the lower rolls on the pre-bent forming side of the sheet, and raise the lower rolls on the pre-bent sheet side, alternately. And make sure that the lower and upper rollers have the same lifting amount to prevent the sheet from falling off, and then rotate the work roller in both directions, until the pre-bending of the other end is completed.
- Using the method of alternately lifting the two lower rollers, adjust the two lower rollers to a symmetrical position with the upper rollers.
- Operate according to steps 4-9 in A.1 roll method to complete the entire bending process.
The process of feeding the pre-bent end of the sheet once and rolling it into a cylindrical workpiece is shown in Figure 2:
Figure 2 — The process of rounding the ends of the secondary pre-bent sheet
Feed the pre-bent end of the sheet at one time and roll it into a cone
The rolling process of conical workpieces is basically the same as the rolling process of cylindrical workpieces. For a specific operation, refer to the method described in A.2, but note the following points:
- The ability to roll conical workpieces is 50%-60% of the ability to roll cylindrical workpieces, which can be approximately understood as the maximum thickness and width of rolled conical workpieces. Only the maximum thickness of rolled cylindrical workpieces is allowed. And 1/2 to 3/5 of the width.
- The initial sheet of the conical workpiece is fan-shaped. When feeding the sheet, the small end of the fan must be close to the friction block in the conical workpiece device to reduce the speed of the small end.
- When the fan-shaped sheet runs back and forth in the work roll, the roller pressing line should always coincide with the bus bar of the fan-shaped blank.
- It is required that the taper of the rolled conical workpiece is generally less than 30 degrees, and the equipment should be specially designed when the taper is large.
What are the steps for rounding
After the sheet material is rolled into a cylindrical shape or a cone shape by the above three methods, it is then welded. However, it is necessary to perform the rounding according to the following three steps to achieve the ideal workpiece。
- Loading: According to experience or calculation, adjust the roller to the position of the required maximum correction curvature。
- Rounding: Roll the roller 1 ~ 2 times under the corrected curvature (to roll the weld zone) to make the curvature of the whole circle uniform。
- Unloading: gradually remove the load, so that the workpiece can be rolled multiple times under the condition of gradually reducing load。