Die design for double bending
Knowing how steel tends to react when it is bent in a single operation can help you do the job right the first time.
I'm often asked if double bends can be made easily in a single wipe bending operation. The answer is yes; but keep in mind, the results are slightly different than if the bends are made in a multistage bending operation.
When bending in a single operation, you need to pay careful attention to the die design and build techniques. Figures 1 and 2 show typical die design techniques for achieving two different bend angles. Both techniques ultimately produce a double-bent part. However, after the forming has taken place and the upper punch is removed, both parts show evidence of a loss of flatness in a certain area. This curling, or distortion, is caused by the deformation of metal as it passes over, or skids, over the primary forming radius of the punch (see Figure 3).
Remember-reducing the amount of deformation during bending is the key to producing undistorted bends.
The best analogy to use to explain this deformation is a rolling pin being pushed across cookie dough. As the rolling pin passes over the dough, the dough tends to curl up behind the rolling pin. This curling effect is the result of a continuous strain pattern on both sides of the dough. If the diameter of the rolling pin is increased, the amount of deformation, or curling, decreases.
Here's another analogy: Have you ever wrapped a present and used that kind of ribbon that curls when you hold it tight against the blade of the scissors and pull it through? If so, then you know that if you were to pull the ribbon over an object with a much larger radius-such as a large marker-the amount of curling would be reduced. This amount of curling is due to the severity of the deformation in the radial area of the bend. The smaller the bend radius-such as with the scissors (almost zero radius)-the greater the severity of the deformation. Increasing the radius size, such as with a large marker, decreases the amount of deformation.
Figure 4 shows metal being distorted during the bending action. The metal skids over the punch point, causing it to distort and curl. In this case the punch point is the rolling pin, except it does not rotate. In addition, the metal that does not travel over the punch point is bent to a curve shape before the bottom of the stroke. Once the metal is bent, it will be very difficult to flatten it.
Figure 5shows metal being distorted in a die that is attempting to achieve two 90-degree bends. The same basic distortion is happening here.
To help reduce the curling distortion during double bending, it helps to design the dies as shown in Figure 6. Modifications have been made only to the lower forming sections. When a portion of the lower forming section is relieve and removed, the metal is allowed to be pushed closer to its finished position and the amount of bending and unbending of the part's vertical wall of the part is reduced greatly. This small but basic change reduces greatly the amount of deformation that occurs in the straight walls of the product, thus reducing the curling effect.
In addition, increasing the radius on the forming punch also reduces forming severity. Keep in mind that increasing the radius size most likely requires a product change. This proposed change in the product design may or may not be feasible, depending on how it affects the product's fit and function. Changing the bend radius most likely will not affect production time or cost. A good rule of thumb is to use a radius that is four to five times the metal's thickness.
Products that require a critical flatness feature are best-suited to stamping operations that create multiple bends in more than one operation. Commercially available rocker benders are good candidates for creating straight-line bends when more than one operation can be utilized.
For more information on rocker benders, see the previous article published on this site entitled "Die Design for Bending Part II."
Best of luck!