Large Radius Bending, also known as R Bending or Radius Bending, is the process of creating a bend with a large radius to material thickness relationship.

Figure 1
Figure 1

When the inside radius becomes 8 to 10 times the material thickness or greater, it is characterized as a  radius bends. The two significant factors which characterize large radius bending are the multi-breakage and the increased amount of Springback found in large radius bends.

Multi-breakage occurs in large radius operations, more common in softer sheet metal and less commonly in materials with higher tensile strengths.

Multi-breakage is less likely to occur in a thin sheet of 304-stainless steel and more likely in a softer sheet of H-series Aluminum. This occurs as the workpiece begins to bend and the sheet material begins to separate from the punch tip.

Figure 2
Figure 2

The workpiece actually bends ahead of the punch and ultimately attains somewhat of a polygonal shape that worsens with the angle, figure 2.

Besides the polygonal shape, the material springs back after the bend is completed, so does the final radius. The final radius will be slightly smaller than the punch tip.

This Final radius (Fr) can be calculated from the Punch Radius (Rp) and the Material thickness (Mt) using the following formula:

Fr = Rp – (Rp x ((Rp x .15 + .030) – Mt))

Multi-breakage and Radius

Not only does the formula cover all ranges of material thickness and bend radius, it is also a bit more accurate. So follow along as we go through a sample problem.

Remember, always work from the inner most parentheses out.

  • Material thickness = .031
  • Punch radius = .393 ( A standard metric radius.)
  • Actual Ir = .393 – ( .393 * (( .393 * .15 + .03) – .031))
  • Actual Ir = .393 – ( .393 * (( .088 ) – .031))
  • Actual Ir = .393 – ( .393 * ( .0579))
  • Actual Ir = .393 – ( .0227 )
  • Actual Ir = .370

The actual measurable inside radius (Ir) is equal to .370 instead of a .393 radius.

Excessive SpringBack

Chart 1
Chart 1

In Chart 1 you see the relationship between Springback, material thickness, and bend radius. Note that a 60-mm radius in .8-mm material can develop 30° of springback in mild cold rolled steel.

It is difficult for design engineers to predict springback because so many variables can affect it, such as material properties, tool geometry, and sheet thickness. As a rule, however, the larger the punch radius, the greater the springback, and the greater the bend angle, the greater the springback.                              

The Die Shape

Large dies used for these types of bends often need to bend past the V-shape at 90°. Punches and dies, unless customized, are designed at 90°. This standard 90° V-opening die could not help to compensate for Springback.

Bottom bending and coining of profound radius bends is not possible using a standard V-die because the punch cannot penetrate the die space far enough to compensate for the excess springback of the large radius bend but instead, coins at the two points of contact between the punch, material, and the die faces, so Springback is not eliminated, figure 3.

Because of the need to compensate for Springback modern large V-opening dies have a relieved area which allows for a radius punch to push deeper into the die space, figure 4.

Figure 3
Figure 3

It should also be noted that this relieved die could also decrease die angles from 90° to as little as 73°, included.

Changing the die angle to a lesser angle has the effect of compensating for springback, but at the same time increasing the multi-breakage effect. These dies do not eliminate multi-breakage in the workpiece, they actually make the effect worse.

The addition of a urethane pad at the base of a relieved die will eliminate the multi-breakage phenomenon and allow for a correct and consistent bend radius by forcing the material against the punch radius. In the image below you can see how a relieved type die allows for greater over-bending of the workpiece.

Die width Selection

Calculating the proper die opening for a large radius bending is a little different than the formula for finding the optimal die open previously discussed. You will use a simple formula which takes into account the Punch Radius (Rp) and the Material Thickness (Mt).

Figure 4
Figure 4

For a standard v-opening with no relief you will use:

    Standard V-die = (Rp + Mt) x 2.5      

For the relieved die, because it does require as much of an opening as the standard V-die, we use the formula below:

Relieved V-die = (Rp + Mt) x 2.2.


Specialty Dies and Methods

Figure 5
Figure 5

Both of these phenomena can be prevented by using specialized dies which push the workpiece against the punch during the bend, ensuring that the final radius is equal to the punch radius.

One common die type which can correct these issues is the Winged type of die which uses a spring-loaded half circle. These rotating set of pads hold the workpiece static while they move around the sheet material and punch face, figure 5.

Figure 6
Figure 6

As the punch pushes down through the metal, the winged die while providing pressure against the punch tip. This ensures even forming around a radius and allows for easy over-bending to compensate for springback, which can be significant in a large radius bend.

There is a video showing many of the virtues of the Winged-V’s in the custom tool chapter. While not for everything, they are capable of producing many bends impossible in other punch and die systems. Figure 5 is a good example.

Figure 7
Figure 7

A spring loaded bottom push-back set at the center of the die applies upwards pressure throughout the bend, pushing the workpiece against the bottom of the punch, preventing the multi-breakage phenomenon from occurring. A strong spring loaded bar can also create a nice radius at the cost of added tonnage, figure 6.

If a spring loaded die or a urethane pad is unavailable, a simple piece of high-pressure water hose or like material will work.

Note that Rolla-V and urethane backup pads force the material around the radius of the punch. Therefore, the punch radius is the radius used in the bend deduction calculations.

Additional recommended reading:

July 2014: Why profound radius bending is a different animal

After reviewing this material you should now be able to:

  1. Define a large or radius bend.
  2. Discuss why large radius bends have large amounts of springback.
  3. Explain Multi-breakage and its effect on the radius.
  4. Explain why standard V-dies are inappropriate for large radius bends.
  5. Show the advantages of relieved dies.
  6. Use a back up to eliminate Multi-breakage.
  7. Describe the different ways to back up the bend: urethane, spring loading, etc.
  8. Discuss winged or Rolla-V dies, their purposes and advantages.
  9. Estimate the radius without the backup.
  10. Explain why with a relieved die at 73 or 78-degrees can be mated with a 90-degree punch.

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