Constant radius or variable radius?

Complex bending requires a sophisticated bending technology

The FABRICATOR July 2007
July 10, 2007
By: Sabine Neff

While rotary draw bending is the mainstay of the tube bending industry, variable-radius bending is making some headway. Variable-radius bending offers many advantages, primarily more sophisticated bends and faster cycle times. However, it has limitations, too. A main one is that an initial bend must be a minimum of approximately 15 degrees.

Tube bending

Like many industries, the automotive industry is in a state of perpetual change. Automobile manufacturers continuously strive for higher safety ratings, reduced fuel consumption, smaller environmental impact, and reduced costs. To address these concerns, many automakers work to develop space-frame structures and reduce automobile weight. Space frames and weight reductions are the result of advancements in design, use of new and unusual materials, and developments in bonding and forming technology.

Two forming technologies that have gotten quite a bit of attention during the past few years are bending and hydroforming. While rotary draw bending is one of the most commonly used bending processes, another process, free-form bending, has the potential to be a viable alternative.


Rotary draw bending is the principal method of tube bending. It forms the tube by drawing it around a rotating bend die. A rotary bender clamps the tube between the bend die insert and clamp die and uses the rotary movement of the bend die to bend the tube. The pressure die takes up the resulting force and simultaneously moves forward as the tube is bent. The pressure die's forward motion ensures that there is no movement between the tube and the pressure die.

The bending process compresses the tube on the intrados (inside radius) and stretches it on the extrados (outside radius). This causes the wall thickness to increase on the intrados and decrease on the extrados. Too much compression on the intrados can cause the tube to wrinkle; too much stretching on the extrados can cause it to break.

Rotary draw bending enables tight bending radii, accommodates bending changes, and is suitable for automation. However, it does have several drawbacks. It allows just one fixed bending radius per tool. For many bending machines, changing the bending radius requires changing the tool. The exception is a multistack bender, which can hold up to four tools and therefore can handle four bending radii without a tool change.

Rotary draw bending requires many process steps before bending the tube: open the clamp die, return the bend-die, open the counterpressure device, position the tube for the bend, close the counterpressure device, and close the clamp die.

Another limitation relates to the bending radius: The maximum radius is approximately six times the tube OD. This is a limitation of the stroke length of the counterpressure device in the clamping unit.


Free-form bending is a completely different method of bending tube. First, the machine layout and tooling are different from those of fixed-radius benders. Free-form bending uses a bend die, pressure rolls, and a forming roll (see Figure 1). The bend die does not determine the radius of the bend; the position of the forming roll does this. The bend angle is determined by the length of the free-form transport in conjunction with the booster system, which pushes the tube forward between the pressure rolls and bend die. Note that the boost pressure doesn't have much effect on the radius. The boost pressure is important, though, because as the bend radius decreases, the required boost pressure increases.

Second, free-form bending uses fewer tools, so this is a cost advantage in tooling purchase, maintenance, and replacement. Third is bending speed. The six process steps rotary draw bending requires before bending are reduced to one continuous process in free-form bending. Fourth is versatility. Because it relies on adjustable rolls rather than bend dies, free-form bending can accommodate part changes without a tooling change. Also, the adjustable rolls enable its biggest benefit: the ability to bend tubing with a varying radius. Finally, like rotary draw bending, it is well-suited to automation.

However, free-form bending has several disadvantages. First, it cannot be adjusted by directly setting a bend radius. Second, the smallest possible radius without a mandrel and wiper die is limited to approximately nine times the OD if the wall factor is 30 and five times the OD if the wall factor is 15. With a mandrel or wiper die, the smallest radius is approximately 3.7 times the OD if the wall factor is 30 and approximately 2.7 times the OD if the wall factor is 15.

Free form bending

Figure 2: An actual part, and a candidate for free-form bending, is a roof rail bent in seven separate bends on a rotary draw bender.

A Case in Point

A simple application for free-form bending is furniture. A typical chair requires a change from one small radius to one large radius. Free-form bending can do more than this, and a more challenging part is a roof rail (see Figure 2). This case is based on a roof rail formed with seven small bends made in steel tube 65 mm in diameter with a 2-mm wall thickness (see Figure 3). The bending radius is 480 mm.

Rotary Draw Bends for a Roof Rail
Bend No.Transport (Y) (mm)Rotation (B) (Degrees)Bend (C) (mm)

Figure 3

A straightforward approach would be a matter of copying the values for transporting, rotating, and bending, and simply change the method to free-form bending. However, this is impossible. Free-form bending cannot make small bend angles, such as 3.5 degrees or 6 degrees. Starting a free-form bend is a breakoff process, so the minimum bending angle is 15 degrees in all cases.

Free-form bending requires adapting the part to the process. Programming the part as a curve with a continuously changing radius results in a set of bending instructions specific to free-form bending (see Figure 4).

Free-form Bends for a Roof Rail
Bend No.Rm
Bend Angle
Form (YC)
Roll (C)
Roll (U1)

Figure 4

Like the original part, it is a steel tube 65 mm in diameter with a 2-mm wall thickness. It has a single bend with a radius that varies continuously.

Although the part made with free-form bending isn't identical to the part formed by rotary draw bending, the theoretical part is similar to the actual part (see Figure 5). The biggest change from rotary draw bending to free-form bending was the bend time. It dropped from 48 seconds to 25 seconds.

Sabine Neff

Lang USA
12055 Universal Drive
Taylor, MI 48180
Phone: 734-946-0417

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The FABRICATOR is North America's leading magazine for the metal forming and fabricating industry. The magazine delivers the news, technical articles, and case histories that enable fabricators to do their jobs more efficiently. The FABRICATOR has served the industry since 1971.

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