Types of Bending
(A) Air Bending
It is a bending process in which the punch touches the work piece and the work piece does not bottom in the lower cavity. As the punch is released, the work piece springs back a little and ends up with less bend than that on the punch (greater included angle). This is called spring-back. The amount of spring back depends on the material, thickness, grain and temper. The spring back will usually range from 5 to 10 degrees. The same angle is usually used in both the punch and the die to minimize set-up time. The inner radius of the bend is the same as the radius on the punch. In air bending, there is no need to change any equipment or dies to obtain different bending angles because the bend angles are determined by the punch stroke. The forces required to form the parts are relatively small, but accurate control of the punch stroke is necessary to obtain the desired bend angle.
(B) Bottoming
Bottoming is a bending process where the punch and the work piece bottom on the die. This makes for a controlled angle with very little spring back. The tonnage required on this type of press is more than in air bending. The inner radius of the work piece should be a minimum of 1 material thickness. In bottom bending, spring-back is reduced by setting the final position of the punch such that the clearance between the punch and die surface is less than the blank thickness. As a result, the material yields slightly and reduces the spring-back. Bottom bending requires considerably more force (about 50%~60% more) than air bending.
(C) Coining
Coining is a bending process in which the punch and the work piece bottom on the die and compressive stress is applied to the bending region to increase the amount of plastic deformation. This reduces the amount of spring-back. The inner radius of the work piece should be up to 0.75 of the material thickness.
(D) V Bending
In V-bending, the clearance between punch and die is constant (equal to the thickness of sheet blank). It is used widely. The thickness of the sheet ranges from approximately 0.5 mm to 25 mm.
(E) U Die Bending
U-die bending is performed when two parallel bending axes are produced in the same operation. A backing pad is used to force the sheet contacting with the punch bottom. It requires about 30% of the bending force for the pad to press the sheet contacting the punch.
(F) Wiping Die Bending
Wiping die bending is also known as flanging. One edge of the sheet is bent to 90 while the other end is restrained by the material itself and by the force of blank-holder and pad. The flange length can be easily changed and the bend angle can be controlled by the stroke position of the punch.
(G) Double Die Bending
Double die bending can be seen as two wiping operations acting on the work piece one after another. Double bending can enhance strain hardening to reduce spring- back.
(H) Rotary Bending
Rotary bending is a bending process using a rocker instead of the punch.
The advantages of rotary bending are:
- Needs no blank-holder
Compensates for spring-back by over-bending
- Requires less force
- More than 90 degree bending angle is available
General bending guidelines are as follows:
- The bend radius should, if possible, be kept the same for all radiuses in the part to minimize set up changes.
- For most materials, the ideal minimum inner radius should be at least 1 material thickness.
The minimum flange width should be at least 4 times the stock thickness plus the bending radius. Violating this rule could cause distortions in the part or damage to tooling or operator due to slippage.
- Slots or holes too close to the bend can cause distortion of these holes. Holes or slots should be located a minimum of 3 stock thickness plus the bend radius. If it is necessary to have holes closer, then the hole or slot should de extended beyond the bend line
- Dimensioning of the part should take into account the stack up of dimensions that can happen and mounting holes that can be made oblong should be.
-
Parts should be inspected in a restrained position, so that the natural
flexure of the parts does not affect measurements. Similarly inside
dimensions in an inside bend should be measured close to the bend.
Some Structures with Steel Curving (Bending)/ Examples of Steel Curving (Bending)
McDonald’s Arches
Chicago Metal Rolled Products have curved large rectangular tubing to form the parabolic arches for the new flagship McDonald’s which opened in downtown Chicago. The tube bending company matched the customer-supplied templates putting multiple radiuses into 50-foot-long tube to minimize costly weld splices and to reduce the time required for fabrication and erection on the fast-paced project. To meet the project’s tight schedule, Chicago Metal completed all the tube bending within three days after the customer supplied the material. The new design has two 60-foot-tall arches that span much of the entire site and help support the roof of the two-story restaurant. Each large arc is comprised of two 20 x 12 tubes covered by plate on all four sides. The arches are 20 inches wide and vary in thickness from 36 inches at the base to 24 inches at the top (Refer to Figure 1, 2 and 3).
University of Phoenix Stadium
For the roof trusses of the University of Phoenix Stadium in Glendale, Arizona, Chicago Metal Rolled Products’ tube bending machines curved 402 tons of 12 x 12 x 5/8 and 12 x 12 x ½ square tubing to a variety of radiuses from 1000 to 1200 feet. Across the width of the field span 256-foot-long lenticular trusses so-called because both the top and bottom chords are curved, creating a profile that resembles a convex lens. Tube bending from Chicago Metal Rolled Products of sixteen such trusses are incorporated in the two retractable roof panels. (See Figure 4 and 5).
Illinois Institute of Technology Train Tube
A new McCormick Student Center at the Illinois Institute of Technology in Chicago, designed by Dutch architect Rem Koolhaus, was to be linked to Chicago’s elevated “El” train system. Koolhaas’ solution to train noise was to create a steel-and-concrete tube to encase trains as they pass over the single-story, building. Beam bending provided by Chicago Metal Rolled Products produced 104,000 pounds of W12 x 58# beams the “hard way” to form a series of half ellipses with radiuses of approximately 12’, 24’ and 34’. (See Figure 6).
Ratner Athletic Center at the University of Chicago
Beam bending to form a reverse curve saved over $24,000 worth of weld splices (See Figure 7).
Fabricators in U.S. claim to put curves into wide-flange beams up to 44 inches tall that weigh 285 pounds per foot and do it “the hard way”—along the longest axis of the cross section. Its latest equipment acquisition is the largest beam roller ever built for anyone (See Figure 8).
(A) Air Bending
It is a bending process in which the punch touches the work piece and the work piece does not bottom in the lower cavity. As the punch is released, the work piece springs back a little and ends up with less bend than that on the punch (greater included angle). This is called spring-back. The amount of spring back depends on the material, thickness, grain and temper. The spring back will usually range from 5 to 10 degrees. The same angle is usually used in both the punch and the die to minimize set-up time. The inner radius of the bend is the same as the radius on the punch. In air bending, there is no need to change any equipment or dies to obtain different bending angles because the bend angles are determined by the punch stroke. The forces required to form the parts are relatively small, but accurate control of the punch stroke is necessary to obtain the desired bend angle.
(B) Bottoming
Bottoming is a bending process where the punch and the work piece bottom on the die. This makes for a controlled angle with very little spring back. The tonnage required on this type of press is more than in air bending. The inner radius of the work piece should be a minimum of 1 material thickness. In bottom bending, spring-back is reduced by setting the final position of the punch such that the clearance between the punch and die surface is less than the blank thickness. As a result, the material yields slightly and reduces the spring-back. Bottom bending requires considerably more force (about 50%~60% more) than air bending.
(C) Coining
Coining is a bending process in which the punch and the work piece bottom on the die and compressive stress is applied to the bending region to increase the amount of plastic deformation. This reduces the amount of spring-back. The inner radius of the work piece should be up to 0.75 of the material thickness.
(D) V Bending
In V-bending, the clearance between punch and die is constant (equal to the thickness of sheet blank). It is used widely. The thickness of the sheet ranges from approximately 0.5 mm to 25 mm.
(E) U Die Bending
U-die bending is performed when two parallel bending axes are produced in the same operation. A backing pad is used to force the sheet contacting with the punch bottom. It requires about 30% of the bending force for the pad to press the sheet contacting the punch.
(F) Wiping Die Bending
Wiping die bending is also known as flanging. One edge of the sheet is bent to 90 while the other end is restrained by the material itself and by the force of blank-holder and pad. The flange length can be easily changed and the bend angle can be controlled by the stroke position of the punch.
(G) Double Die Bending
Double die bending can be seen as two wiping operations acting on the work piece one after another. Double bending can enhance strain hardening to reduce spring- back.
(H) Rotary Bending
Rotary bending is a bending process using a rocker instead of the punch.
The advantages of rotary bending are:
- Needs no blank-holder
Compensates for spring-back by over-bending
- Requires less force
- More than 90 degree bending angle is available
General bending guidelines are as follows:
- The bend radius should, if possible, be kept the same for all radiuses in the part to minimize set up changes.
- For most materials, the ideal minimum inner radius should be at least 1 material thickness.
The minimum flange width should be at least 4 times the stock thickness plus the bending radius. Violating this rule could cause distortions in the part or damage to tooling or operator due to slippage.
- Slots or holes too close to the bend can cause distortion of these holes. Holes or slots should be located a minimum of 3 stock thickness plus the bend radius. If it is necessary to have holes closer, then the hole or slot should de extended beyond the bend line
- Dimensioning of the part should take into account the stack up of dimensions that can happen and mounting holes that can be made oblong should be.
-
Parts should be inspected in a restrained position, so that the natural
flexure of the parts does not affect measurements. Similarly inside
dimensions in an inside bend should be measured close to the bend.Some Structures with Steel Curving (Bending)/ Examples of Steel Curving (Bending)
McDonald’s Arches
Chicago Metal Rolled Products have curved large rectangular tubing to form the parabolic arches for the new flagship McDonald’s which opened in downtown Chicago. The tube bending company matched the customer-supplied templates putting multiple radiuses into 50-foot-long tube to minimize costly weld splices and to reduce the time required for fabrication and erection on the fast-paced project. To meet the project’s tight schedule, Chicago Metal completed all the tube bending within three days after the customer supplied the material. The new design has two 60-foot-tall arches that span much of the entire site and help support the roof of the two-story restaurant. Each large arc is comprised of two 20 x 12 tubes covered by plate on all four sides. The arches are 20 inches wide and vary in thickness from 36 inches at the base to 24 inches at the top (Refer to Figure 1, 2 and 3).
University of Phoenix Stadium
For the roof trusses of the University of Phoenix Stadium in Glendale, Arizona, Chicago Metal Rolled Products’ tube bending machines curved 402 tons of 12 x 12 x 5/8 and 12 x 12 x ½ square tubing to a variety of radiuses from 1000 to 1200 feet. Across the width of the field span 256-foot-long lenticular trusses so-called because both the top and bottom chords are curved, creating a profile that resembles a convex lens. Tube bending from Chicago Metal Rolled Products of sixteen such trusses are incorporated in the two retractable roof panels. (See Figure 4 and 5).
Illinois Institute of Technology Train Tube
A new McCormick Student Center at the Illinois Institute of Technology in Chicago, designed by Dutch architect Rem Koolhaus, was to be linked to Chicago’s elevated “El” train system. Koolhaas’ solution to train noise was to create a steel-and-concrete tube to encase trains as they pass over the single-story, building. Beam bending provided by Chicago Metal Rolled Products produced 104,000 pounds of W12 x 58# beams the “hard way” to form a series of half ellipses with radiuses of approximately 12’, 24’ and 34’. (See Figure 6).
Ratner Athletic Center at the University of Chicago
Beam bending to form a reverse curve saved over $24,000 worth of weld splices (See Figure 7).
Fabricators in U.S. claim to put curves into wide-flange beams up to 44 inches tall that weigh 285 pounds per foot and do it “the hard way”—along the longest axis of the cross section. Its latest equipment acquisition is the largest beam roller ever built for anyone (See Figure 8).
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