Principles of Rotary Draw Bending

What is rotary bending

pipe bending machine

Rotary bending perhaps is one of the most popular and effective ways of creating a precision bend. Rotary benders, have many advantages over conventional wipe bending methods.

Rotary Bending is accomplished by bending the Pipe or Tube on a fixed die set machined to fit the exact outside diameter. A mandrel is inserted inside the Pipe or Tube to prevent collapse during the bending process.

Rotary or rocker benders consist of a foundation block, often referred to as the saddle. The saddle has a spring-loaded V-shape component called the rocker. This rocker rotates about its centerline and performs the bending action. It acts as both a holding pad and the bending mechanism.

Although this type of bender can be installed in almost any direction with respect to the ram travel, it most commonly is fastened to the upper die shoe. As the bender moves down, the rocker makes contact with the sheet metal. One contact point acts as a holding pad, while the opposite contact point rotates, creating the bending action. After the bend is completed and on the press’s returnstroke, the spring forces the rocker to return back to its original or idle position.

Advantages

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

Disadvantages

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.

Principles of Rotary Draw Bending

Bending Forces

As a tube or pipe is being bent, the outer wall at the point of the bend begins to stretch and thin out. Simultaneously, the corresponding inner wall of the workpiece becomes thicker and more compressed. Controlling these degrees of physical deformation is important for creating a smooth rounded bend. Thick walled tubes bent at a wide radius are likely to have a relatively low degree of deformation, but thinner walled tubes will not. Unison typically measure the wall factor, which is the ratio of a tube’s wall thickness to its external diameter, and the “D” of bend, which tells us if the bend is on a large radius or a small radius, to determine whether bend difficulty.

Bending Radius

A similar comparison is made between the centerline radius and the tube’s external diameter to determine if a bending radius is tight or wide. The combination of the bending radius and the wall factor is used to designate the complexity of the bend. Under parameters in which the inner and outer walls would not be seriously comprised, a standard bending procedure can be performed with a basic die set, such as a bend, clamp, and pressure die array. The clamp die holds the tube in position, while the pressure die forces it against the bend die to curve into the desired shape.

Mandrel Bending

In many cases, the tubing workpiece does not fit the ideal criteria and cannot be properly shaped using a basic die set. As the wall factor measurement grows larger from the external wall thinning, the bend radius also grows tighter and increases the chances of producing a flattened bend. This usually occurs if the wall is too thin to maintain its integrity at the angle of the bend. A mandrel is often used to compensate for this weakness. The mandrel is a device that can be affixed to the interior of the tube at the point of the bend to provide support throughout the operation. It can be designed as a single plug or a sequence of balls that flex and adjust according to the bend. Aside from providing internal support for thin tubes, a plug mandrel can also be used to exert additional bending force on thicker tubes that are more difficult to shape.

Wiper Die

Under more severe bending conditions, like those involving thin tubing undergoing a tight bend radius, internal wall compression may develop unevenly, resulting in a wrinkle defect. A wiper die may be necessary in order to reduce the risk of wrinkling on the workpiece. This wiper is designed to be wedged into the groove between the tube and the bending die, and it has a thin tip that reaches to the point where the tube will start to bend. The wiper completes the gap between the bending die and the tube, leaving the tube constricted and removing any space for a wrinkle to develop. Wipers are often used in conjunction with a mandrel to further reduce the potential for deformation.

Elongation

Elongation is the degree to which a tube can stretch before undergoing structural failure or cracking. Given that material stretching occurs in essentially all tube and pipe bending procedures, elongation can be an important concern for manufacturers. In general, as a bending radius grows tighter, the material will stretch more. In some cases, material selection is dictated by the expected level of elongation. For example, stainless steel has a higher maximum elongation than other grades of steel, making it easier to bend without fracturing along a tight radius

Springback

After bending a tube as with any material, when the pressure comes off the material doesn’t stay, it always springs back a little. Depending on the material the spring back can be so severe that it takes 189 degrees of bend to achieve the 180-degree hairpin.

Radial Growth

Due to the material springback, compensating by over bending and allowing the tube to springback means that the tube does not stay tight to the inside circumference of the former. Instead the radius grows for example if you are bending a tube to 90 degrees with a 95mm CLR former, the true radius after bending may be 95.95mm depending on the springback.

Material Variation

After bending one tube to 180 you may find that the next tube might come out to 179 or 181 degrees. If it’s welded tube, it has a weld seam somewhere on its circumference, and the weld seam’s properties are different from those of the parent material. Changing the weld seam’s location from one bend to the next is a sure way to change the amount of springback from one tube to the next.

If the tube has all of the bends in the same plane, the best orientation for the weld seam is along the neutral axis. It shouldn’t be under compression (on the intrados, or inside of the bend) or tension (on the extrados, or the outside of the bend). If the component has bends in several planes, the weld seam should be oriented so it’s along the neutral axis of the most severe bend.

Variations from one tube order to the next also play a role in springback. The steel itself varies a bit from heat to heat; the rolling process that turns the slab of steel into sheet also plays a role, as does the forming process in the tube or pipe mill.

For particularly tricky bending applications, specifying steel’s chemistry and ordering tube or pipe from the same supplier repeatedly helps to reduce these variations. Over the long run, consistent tube is usually less expensive than low-priced tube.

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 draw machines can perform both push bending and rotary bending with a single tooling setup.

Rotary Draw Bending for Pipe Bending
Source: Chicago Metal Rolled Products

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 about 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.