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Three Conventional Profile Bending Processes (2) – Rotary Bending

Rotary bending of 3 conventional profile bending processes
Figure 1: Rotary bending of 3 conventional profile bending processes. 1-Round bending die; 2- chuck; 3- side pressure roller; 4- profile; 5- tension device (guide block)

Rotary bending is a typical and widely used metal profile bending process. The metal profiles produced by the rotary bending method are of high quality. The principle of the test device is shown in Figure 1.

The operation method of the rotary bending process is to clamp one end of the metal profile in the chuck of the bending die, and the side pressure wheel presses it against the bending die and rotates around the bending die during work, and the chuck rotates with the metal profile, so Metal profiles can be bent into desired shapes.

Read More: Rotary Draw Bending: Basics, Benefit, 8 Key Knowledge

Process characteristics

The application of mandrel during bending
Figure 2: The application of mandrel during bending

The bending die is the power device in the rotary bending process. Due to the restriction of the profile by the chuck fixed on the bending die, the deformation zone is the part in contact with the bending die. If the bending die is inclined, the profile can be bent into a three-dimensional article. Compared with other bending processes, the spring-back of the product after bending around the bending die is larger.
Adding an opposite force along the axial direction at the end of the profile can effectively reduce the amount of spring back. It can be seen from Figure 1 that this process has certain requirements for the length of the metal profile because the profile must be sufficiently long when it is fixed, which limits its application to a certain extent.

For the common defects such as wrinkling, collapse, and cross-sectional deformation of hollow profiles during bending, the degree of defects can be reduced by supporting the profile with a core rod whose cross-section is similar to the hollow part of the profile during the bending process, as shown in Figure 2.

Read More: Profile Bending Machine: 5 Things Before Buy, 6 Steps to Bend

Advantages of Rotary Bending

Rotary bending has some advantages over other methods. The most advantageous feature is the simplicity of adjustment. Changes in the bend angle can be made simply by shimming or grinding the height of the assembly. Doing so takes very little time, and time is money.

Rotary benders can bend as much as 120 degrees and are well-suited to bending high-strength material. One company in Sweden has successfully created two 90-degree return bends in steel with a yield strength of 980 megapascals. This translates into steel that by U.S. standards has a yield strength of more than 142,000 pounds per square inch (PSI)—five times stronger than low-carbon steel. Attempting to make such a bend in a conventional wipe-bending operation most certainly would be impossible.

Unlike conventional wipe bending, rotary benders require much lower forces to create the bend. Anywhere from a 40 percent to 80 percent reduction in force can be expected. This makes this method ideal for producing long, heavy-gauge, large parts, such as trucks and semi-frame rails.

You can expect less hole distortion in rotary bending. Consider a hole that is pierced in a flat blank and later bent into a vertical wall. During conventional bending, this hole can be subjected to a great deal of tension, which causes the hole to distort. Because rocker benders fold the metal around the punch, hole distortion is eliminated.

Rotary benders can be used to bend up or down. They also can be placed on cam slides.

Disadvantages of Rotary Bending

Despite the many advantages, rotary benders do have some disadvantages. First, they can be quite expensive; however, consider the advantages of the reduction in downtime and frustration. Overall, they often pay for themselves in a short period of time.

Also consider that you most likely will not need an external pad, which reduces die cost. Often the true cost of designing and building a conventional wipe-bending die is much greater than the rocker bender. Don’t confuse cost with value. In my opinion, rotary benders are worth every penny.

Because these benders have moving parts, there is a risk of galling up and failing to rotate. This can be prevented by periodically cleaning and lubricating them.

Remember that rotary benders can be used for straight-line bending only. Avoid using them to bend special-shaped trim lines that do not allow for simultaneous punch contact. Angled corners are not good candidates for rocker benders.

Rotary Draw Bending For Pipe Bending

One of the most versatile and common methods to bend pipe and tube is rotary draw bending. The radius of such bends is often described as, for example, “2D.” A 2D bend is one whose center-line radius is equal to two times the outside diameter of the pipe to be bent.

Rotary draw bending involves clamping on the outside diameter of a pipe and drawing it over a form whose radius matches the desired bend radius.

Rotary draw bending often employs an internal supporting mandrel and a wiper die to prevent wrinkling on the inside wall of a tight bend. Some rotary drawing machines can perform both push bending and rotary bending with a single tooling setup.

Trouble Shooting Guide

Tube bending is a matter of handling a handful of procedures and variables. The bending process can cause the material to get thicker where the metal is under compression and thinner where it is under tension. Too much thickening can result in wrinkles and too much thinning will ultimately result in failure.

The machine operator needs to look at clamping pressure when bending. If there is not enough pressure, the tube will slip during the bending process. If too much pressure is used, then that will cause the tube to collapse if a mandrel isn’t used and to wrinkle if a mandrel is used. The problem occurs when metal flows into areas where it isn’t supposed to go. Successful bending comes down to proper material containment while at the same time reducing drag. It is critical to know every characteristic of the tubing: shape, size, wall thickness, tolerances, yield strength, tensile strength, and ductility. This information will help assess if the material will be able to be formed to the desired bend radius.