April 15, 2002
Looking to nest parts tightly, but can't win the battle against the material's natural grain? Take heart—bottom bending could be your key to success.
Air forming is a three-point bending process. The metal is in contact with, and bent by, the two top corners of the die and the punch tip.
Hey, it's a brave new world—a world in which the reality of precision sheet metal manufacturing has to include optimizing the number of parts that can be punched or laser-cut from a single sheet of material.
To get the most from a blank, individual parts are placed at odd angles, both in relationship to each other and to the natural grain of the material. While this is of little relevance to coding, punch press, and laser cutting operations, it is a big problem at the forming stage—a real big problem.
It becomes a problem at the press brake for one main reason—air forming. Air forming is the most common method of forming used today in the production of precision sheet metal parts and is listed by all major press brake manufacturers worldwide as the preferred method of sheet metal bending. However, nested parts are a problem for air forming because, as a three-point forming method, both the bend angle and inside radius are somewhat at the mercy of the sheet material's hardness, thickness, and natural grain direction (see Figure 1).
Take, for example, three nested parts. They are identical except for variations in the natural grain direction of the material. This variation alone ensures that the press brake operator will end up fighting a battle of bend angles.
If the operator is fighting bend angles, you can be sure that dimensional variations are going to result. More than likely, if the operator is worth his salt, he will end up hand-sorting the material into separate piles based on natural grain direction, just to ensure the quality of the finished parts.
And if that weren't enough, there is an even bigger problem—surface finish. If those same parts had been grained for finishing or deburring, the natural grain direction would be hidden, compounding the level of angular variation the operators will have to face.
The graining or finish makes it impossible for the operator to determine the natural grain direction of the material, so sorting at this point really isn't an option. What does that mean to you? It means a lot of handwork and lost profit for that project.
The answer to this situation could be bottom bending.
Coining. For the record, bottom bending and coining are two different forming processes. Though you may find some argument on this point, it is nonetheless a fact. The difference between them is that coining requires an excessive amount of tonnage to place the sharp-radius punch tip required for the process into and past the neutral axis of the bend. The amount of tonnage applied totally depends on the operator's skill, as well as on the quality and type of tooling and equipment used.
For reference, a "sharp" bend is one in which the attempted inside bend radius is less than 63 percent of the material thickness. It is a function of the material, not the punch tip. In other words, any bend less than 63 percent of material thickness is technically a sharp bend and is sharper than the material's normal capability. *
Left: Air bending can be used to achieve a 90-degree bend. When the pressure is released, the material will spring back slightly and the bend will be more than 90 degrees.
Middle: Bending past 90 degrees compensates for springback. The amount of overbending required depends on many factors, such as material type and thickness.
Right: Another bending process, bottoming, continues to lower the punch until it contacts the die. This forces the material back to the set angle of the die which, in this case, is 90 degrees.
Coining requires the punch tip to penetrate the neutral axis of the material, which lies at 0.44 of the material thickness. Unfortunately, it works well only on light-gauge mild steel and, again, requires intense tonnage.
Bottom Bending. Bottom bending is similar to coining when done properly, except that tonnage is greatly reduced. The lower tonnage requirements are attributable to the angular clearance between the complementary angle of the punch and the included angle of the V die (see Figure 2). And unlike coining, bottom bending can reproduce an inside bend radius of up to three times that of the material thickness, as well as a sharp inside bend radius.
The amount of clearance between the V-die face and the punch face is (or at least should be) equal to the springback in the material that is being formed. For example, 5052 H32 aluminum has approximately 1 to 1½ degrees of springback and would be bottom-bent using an 88-degree punch angle and a 90-degree bottom V-die angle. This allows the material to be brought up to 92 degrees complementary and then forced back to 90 degrees. Moreover, through the process of bottoming, the springback is removed from the material, and the bend angle set by the V die.
As a note, bottom bending (like coining) works only on lighter gauges of material. However, bottom bending also is viable for stainless steels, aluminum, and cold-rolled steel.
If the part is bottom-bent correctly with low tonnage, the bend angle can be set regardless of the part's natural grain direction or the added grain produced by a finishing process, such as deburring. Low-tonnage bottom bending also can compensate for other variables in the material—small variations in material thickness or hardness, for example.
All of this leads back to our original idea, forming parts from an optimized blank. A process that can form quality parts regardless of natural grain direction naturally can accommodate any configuration of parts on a sheet.
Rather than spend valuable production time sorting, repairing, or remaking parts that were troublesome because of grain direction variations, material thickness, or hardness, you can add low-pressure bottom bending to your manufacturing repertoire and use the time saved to produce more and better-quality products.
* Sixty-three percent minimum inside bend radius is based on the naturally occurring inside radius achieved in mild cold-rolled steel.