Ultimate Guide to Roll Bending

Estimated reading time: 22 minutes

Through the rotating roller, the method of bending the sheet under the action and friction of the roller is called roll bending. Roll bending is one of the important processing forms of sheet metal bending. Roll bending is done using roll bending equipment (usually a plate bending machine). The bending machine is mainly a piece of forming equipment for continuous three-point bending of sheet metal. The sheet metal can be bent into a cylinder on the bending machine or single-curvature parts such as cones, or hyperboloid parts with a larger radius, such as large-diameter spherical lobes. If the bending machine is equipped with appropriate process equipment, it can also be used for bending section steel.

Roll bending equipment and its operation

According to the number of rolls of the plate rolling machine, there are three shafts and four shafts; according to the arrangement of the shaft rollers, there are symmetrical and asymmetrical types. Among them, the three-axis symmetrical plate rolling machine is the most commonly used in production. Figure 1-1 shows the structure of a three-axis plate symmetrical rolling machine.

The basic principle of roll bending

The basic principle of roll bending is shown in Figure 1-2. If the blank is resting on the lower roller shaft, its lower surface will be in contact with the highest points b and c of the lower roller shaft, and the upper surface will coincide with the lowest point of the upper roller shaft. At this time, the vertical distance between the upper and lower rollers is exactly equal to the thickness of the material. When the lower roller shaft does not move and the upper roller shaft descends, or the upper roller shaft does not move and the lower roller shaft rises, the distance is less than the material thickness. If the two roller shafts are continuously rolled, the blank will be formed within the full range of the roll. The two ends of the blank are still straight for the smooth curved surface due to the lack of rollers and must be eliminated when forming parts.

The curvature of the blank after roll bending depends on the relative position of the roller shaft, the thickness of the sheet, and the mechanical properties. The relationship between them can be approximately expressed by the following formula, as shown in Figure 1-3.

Where d1, d2——the diameter of the roller shaft, mm

          t——the thickness of the sheet, mm

          R——The radius of curvature of the part, mm

The relative distances H and B between the rollers are adjustable to meet the needs of the curvature of the parts. Since it is more convenient to change H than to change B, you generally get different curvatures by changing H. Since it is difficult to calculate and determine the rebound amount of the sheet material in advance, the above relational expression cannot accurately mark the required H value, which is only for reference during the initial roll. In actual production, the test method is mostly adopted, that is, after the position of the upper roller is roughly adjusted based on experience, the paper is gradually tested until the required curvature is reached.

Operating steps of the rolling machine

The steps of the three-axis bending machine are as follows: first, operate to raise the upper roller shaft, it can be seen that the blank thickness adjustment shaft lower roller, the adjustment of the lower roller shaft is basically small when the bending force of the upper roller shaft allows because the adjustment is troublesome, generally, It can be seen that the thickness of the blank is reasonably fixed, the thickness is below 4mm, the two rolls are compressed by 90-100mm, the thickness is 4-6mm, and the shaft shrinks to 110-120mm. , Than descend according to the requirements of roll bending radius, press the blank shaft, and then turn on the rolling bed to rotate the roll shaft, the blank is automatically fed and bent, the upper roll shaft is raised, and the parts are finally removed

Roll bending operation of cylinder

The unrolled material of the cylinder is rectangular, and its surface lines are parallel to each other and equal in length. When rolling on the plate rolling machine, the motion trajectory of the plate is straight and the forward speed is equal. Therefore, the seams at both ends can be just right after rolling. On the top, because the prime lines are equal, the top roller pressure is always the same.

To ensure the quality of roll bending parts, the operator must master the steps and methods of the round roll bending operation during the round roll bending operation. At the same time, they should also have a certain ability to analyze and solve the defects of the cylinder roll bending.

Operation steps and methods of cylindrical roll bending 

The unrolled material of the cylinder is rectangular, and its surface lines are parallel to each other and equal in length. When rolling on the plate rolling machine, the forward speed of the two ends of the plate must be equal. Straight line, the pressure of the upper shaft roller is always equal in any roll. The roll bending operation is roughly composed of five steps: pre-bending, feeding and roll bending, seam welding, and rounding. Figure 1-4 shows the operation process of the cylinder.

• Pre-bending. It can be seen from Figure 1-4 that when the steel plate is bent on the bending machine, due to the arrangement of the shaft rollers, there is a certain distance between the two lower shaft rollers so that there is always a section between the two ends of the steel plate that cannot be rolled, that is, there is Straight edge. Therefore, the straight edges must be pre-bent first.

There are many pre-bending methods, which can be selected according to the actual situation of production and processing. Figure 1-5 (a) shows the method of pre-bending a section on the liner: a thick steel plate is first bent to the required curvature, and then the steel plate is placed on it to pre-bend the bend. Figure 1-5 (b) shows the method of pre-bending the end of the steel plate on the press with a pressing die. Figures 1-5 (c) and (d) are the manual pre-bending methods suitable for small and narrow pieces of thin plates, and the batch is small. Figure 1-5 (c) is the pre-bending method with a sledgehammer on the bending machine. When pre-bending the elbow, the upper shaft roller should be properly compressed to prevent bounce. The hammering sequence is from the outside to the inward section to prevent the straight section; Figure 1-5 (d) is the pre-bending using section steel. The section steel can be channel steel or I-beam; for the pre-bending of the end of the steel plate, a card-shaped template should be used to check whether it meets the curvature requirements of the workpiece. Otherwise, it is difficult or impossible to modify the end of the workpiece after it is formed.

• Feeding. After pre-bending the two ends of the material, place it between the upper and lower shaft rollers. Adjust the upper shaft roller downward so that it gently presses the steel plate. Then align it.

There are many ways to align, you can use the plate end to align the axial engraved line on the lower shaft roller, as shown in Figure 1-6 (a), or you can draw several lines parallel to the plate on the steel plate. The curved element line at the end, when loading, makes the curved element line coincide with or parallel to the engraved line on the shaft rod. During the roll bending process, the alignment of the steel plate can be checked at any time, as shown in Figure 1-6(b). Using the axial engraved line on the shaft roller as a reference, measuring the two sides of the steel plate with a 90° square is also a method of alignment, as shown in Figure 1-6 (c). In addition, a special baffle can be set up on the bending machine to check the alignment of the blank, as shown in Figure 1-6(d).

The above alignment method can be used alone, or both methods can be applied at the same time to make the alignment effect more accurate.

• Roll bending. After the steel plate is aligned, the plate rolling machine can be started for roll bending. Roll bending should be carried out by a gradual method, that is, the upper shaft roller should be adjusted down by an appropriate amount each time, and then adjusted down after rolling it once, repeating it, and gradually reaching the required curvature. The use of progressive roll bending can reduce the cold work hardening of the material and avoid possible cracks or fractures that may occur due to excessive overall stress of the material. The parts with severe cold work hardening can be eliminated by annealing.
• Seam welding. After the roll bending is completed, if the seam quality is good, the subsequent seam welding can be carried out. The welding of the seam welding is often divided into two steps: first, check the quality of the seam, and then fix it after being qualified; and finally welding. The joint fixing is generally done on the coiling machine, and the welding may be done on the coiling machine or off-site depending on the structure of the processed radius part. In fact, there are often various defects before the joints are fixed, such as wrong edges, gaps, and uneven gaps. Such defects are usually completed directly on the rolling machine using a crane. The commonly used methods are mainly as follows:

Handling of the wrong side and slight wrong mouth. First, turn the opposite longitudinal seam to a horizontal position for easy operation. For misaligned defects, the following methods can be used alone or several methods can be used in combination, mainly including: slightly raising the upper shaft roller, lifting the pressure with F-shaped round steel, lifting the crane, and moving it to the right, and turning the opposite seam down; For slight misalignment defects, you can use long pads along with the lower shaft roller where the convex misalignment is located or move the crane backward. While moving, use F-shaped round steel to apply torsion to make slight misalignments. The mouth is corrected, see Figure 1-7.

Dealing with large mistakes. The reason for this type of defect is that the position of the board is not aligned at the time of initial rolling. The method of alignment is to make the broken edge of the board parallel to the contour line of a certain shaft roller. The processing method is {as shown in Figures 1-8(a) and 1-8(b), for the convenience of description, only the case of no wrong edges is assumed here}; firstly, a long angle is wound on the upper edge of the long-angled lower shaft roller. For the slats, the misalignment is basically corrected at this time. Depending on the situation, flat steel with a hook or a chain can be used for fine adjustment.

Treatment of comprehensive defects. For comprehensive defects with wrong edges, wrong edges, or uneven gaps, the lever screw tensioner (or pusher) shown in Figure 1-9 (a) can be used for adjustment. Figure 1-9 (b) is a schematic diagram of the wrong port adjustment method. First, place the lever screw tensioner (or pusher) at the cylinder port, and screw the screw rod 7 to the appropriate position in advance to prepare for expanding or reducing the gap. Tighten the screw rod 6 and the cylinder body as a whole, by rotating the screw rod 7 can reduce or expand the gap between the mouth, by rotating the screw rod 4 can adjust the wrong side of the mouth. The number of lever screw tensioners (or pushers) can be selected according to the rigidity and size of the cylinder to be adjusted.

If there is a wrong opening, two clamps can be placed at both ends of the cylinder, and then a tightening tool such as an inverted chain can be used.

• Rounding. After the welding of the cylinder body is completed, rounding is generally required. In production, in addition to manual correction with a sledgehammer, rounding can also be completed on the plate bending machine by adding pads, which can improve work efficiency and reduce labor intensity.

In the process of operation, the thickness of the backing plate should be 3-8mm and the width 40-60mm, and it should be selected according to the deformation. In addition, during the correction process, the lifting of the upper roller should be flexible and changeable, so as not to damage the non-deformed area, and the pressure of the upper roller should follow the principle of “When it does not enter the deformation area, it can start to press down to overpressure and leave the deformation area. It should be raised to normal pressure before proceeding. If the operation is completed by more than one person, one person should be designated to command and cooperate with each other. The rounding operation methods of various deformations are as follows.

• Rounding operations with internal edges and corners or straight sections appearing in the entire length. When correcting the deformation zone, the upper shaft roller is gradually corrected according to the pressurization sequence of “underpressure-normal pressure-overpressure”. Repeatedly rotate the deformation zone from left to right several times to basically eliminate the peach shape or straight section. The circle can be calibrated by step-by-step pad pressure within the full-length range, as shown in Figure 1-10.

• Rounding operation in which the arc at one end is just right, and the arc at the other end is an inner edge or a straight section appears locally. The operation method is: press the upper roller to normal pressure, and pad the plate at the highest point of the deformed part as it rotates (the thickness of the pad is determined according to the deformation), and it can be corrected by turning it once or several times, as shown in Figure 1-10.
• Rounding operation with external edges and corners or protruding sections in the full length. The operation method is: turn the highest point of the deformed part to above the lower roller, press the upper roller, and then rise to normal pressure; turn the highest point of the deformed part to above the zero roller, then press the upper roller, and then rise to Atmospheric pressure; repeat the above operations to basically eliminate the outer corners or protruding sections, and add a step-by-step pad pressure within the full length to correct it, see Figure 1-11.

• The rounding operation of the arc at one end and the outer corner or local protrusion at the other end. Operation method: pressure was added to the upper roller, which rotates with the plate pad at the highest point of the deformed part, once or several times to correct the rotation, see Figure 1-11.
• The rounding operation of the protrusion of the simple welding seam. This kind of deformed rounding single-use bending machine is powerless and the only way is to use the pad pressing method. Deformation needs to be cushioned section by section to increase its pressure. In order to correct the protruding weld seam, the backing plate should be placed at the weld seam, and only the weld seam is compressed when pressurizing, otherwise straight sections will appear. In Figure 1-12, the dotted line on the right indicates that the pad is placed too early, and the dotted line on the left indicates that the pad is placed too late. The solid line in the figure indicates the correct placement.

• The rounding operation in which the local convex and straight sections exist at the same time. Operation method: press the upper roller to normal pressure, and pad the plate on the inner and outer deformed parts as it rotates, and it can be corrected by turning it once or several times. This method is faster than the separate pad pressure, see Figure 1-13.

Precautions for roll bending operation

In the process of roll bending operation, attention should be paid to the structure of sheet metal processing parts to be used in conjunction with other processing equipment, auxiliary tools, etc. The following is the inner diameter ∅5000mm, the material thickness t is 26mm, and the plate cylinder is rolled on a 30mmX3000mm plate bending machine. As an example, describe its operation precautions.

The cylindrical sheet is not only thicker but also longer (the length of the unfolded material is 15789.6mm). In order to facilitate the movement of the sheet and prevent it from bending, it needs the cooperation of a crane, and also in order to measure and control the size of the rolled cylinder, Also needs to have a measurement template.

• The cooperation of cranes. Figure 1-14 (a) shows the cooperation of the crane at the beginning of the winding, and the rear end is matched with a crane and pipes. Figure 1-14(b) shows the cooperation of the front end crane after the start of the coil. Since the forming arc is not large enough, the hook is on the inside of the cylinder. As the shaft roller continues to rotate, the crane continues to rise and move to the right, the curved surface gradually increases (the curved surface is more rigid than the straight surface, and the curvature is greater than the less curvature). At this time, the rigid plate can be removed from the crane. , If the rigidity is poor, it is still necessary to use a crane to cooperate with one section. With the gradual formation of the curved surface, depending on the rigidity of the curved surface, it is decided to use or not to cooperate with the crane, as shown in Figure 1-14 (c).

• The detection position of the template. After the thicker plate is rolled to one circle, due to the increase in rigidity, there is basically no need for cranes to cooperate. After that, the card template is used to check the curvature. In order to be able to get the card as close to the actual curvature as possible, it must be in a free part without any load. The side is under the action of the falling force of the board, the curvature at this time is greater than the actual curvature, while the right side is shorter and basically in a free state, which can reflect the true curvature, so the left side is wrong, the right side is correct, see Figure 1- 15.

• Handling methods of over-rolling. The roll forming of the cylinder should be gradually completed in stages. Therefore, the downward pressure of the upper shaft roll is also implemented in stages. When the downward pressure of the upper shaft roller is too large, the curvature of the cylinder will be smaller than the design curvature. This phenomenon is called overwinding. There are mainly the following methods for processing over-rolling.

Manpower compression method. The manual pressurization method is often used for the belt plate of the storage tank with large curvature. During operation, use the lower roller as a fulcrum. One or two persons on the remote station apply pressure, pressurize and move the plate backward to achieve the release. The purpose of the arc is shown in Figure 1-16.

Lifting and straightening method. The lifting and straightening method is often used when the curvature is about to reach the design curvature, and the over-winding is formed by excessive pressure due to misoperation. During operation, use a crane to lift the upper end to release the arc, turn one section and place one section until it is completely boarding. Rewind after lifting the upper shaft roller, as shown in Figure 1-17.

Hammer correction method. Figure 1-18 The method of correcting the over-curling of the end arc may be caused by the pre-bending arc, or the arc at the beginning of the winding. During operation, the following shaft rod is the fulcrum and hit with a sledgehammer. If only the tip arcs, just hit the tip, if the proximal tip also arcs, you can move the board out a bit and hammer again until it matches the template.

Backpressure method. Back Pressure applied to the proximal end of the case through the arcs, the arc should be in line with the principle of light to heavy roll back and forth several times, it can achieve the purpose of the arc, and the ability to turn back the rolled plate, Figure 1- 19.

Decompression arc method. After rolling for a certain distance, or after the template is stuck or the rolling is found by eye observation, the rolling should be stopped immediately. In addition to the above-mentioned methods, the decompression rewinding method can also be used to release the arc. The specific operation method is: slightly raise the upper shaft roller, and then rewind the arc that has already been rolled. The over-rolled cylinder can be unified in another curvature under a little lower pressure, so as to achieve the arc release purpose.

• The method to ensure the curvature of the counterpart after welding. In the rolling process, due to the elastic cooperation of the crane, the rolled cylinder may not guarantee the design curvature, but both ends are clamped to the template without restraint, so the ends must be designed with curvature. After aligning, the curvature of the fixed end of the anti-deformation plate can be spot-welded at the same time to ensure that there will be no big errors after welding, as shown in Figure 1-20.

• Welding method and sequence. For large cylinders, in order to facilitate welding and control the amount of welding deformation, attention should be paid to the application of appropriate welding methods and reasonable sequences. After spot welding the anti-deformation plate on the inner side, it should generally be lifted out of the coiling machine with a self-locking rope buckle. , Make the counter seam face down, and use manual arc welding to weld the inner bead. In order not to affect the manual arc welding, the center gap of the anti-deformation plate should be opened larger, which not only prevents deformation, but also does not interfere with welding; After finishing the inner side, turn the welding seam to the appropriate operating position to remove the root, and complete the subsequent welding according to the welding requirements.
• Cleaning the job site. During the entire operation, care should be taken to ensure that the dirt, iron skin, burrs and other dirt on the surface of the shaft and the steel plate are cleaned; during the work, the scale and other debris on the steel plate should be constantly purged to avoid damage to the workpiece surface .