Die width selection
Most designers and engineers usually place very little importance on achieving the correct inside radius of a formed part. Why? Because the functionality of the part is unaffected if the specified inside radius is 0.062 in. and actual measured inside radius is 0.078 in. So why do we care about achieving an exact inside radius? First we need to look at the current state of forming and the method used.
Air Forming and the 20 Percent Rule
The most common forming method used today is air forming. Air forming is a three-point bending in which the final inside bend radius is determined by the width of the V die (see Figure 1).
As the V-die width is narrowed or widened, it will have the effect of increasing or decreasing the inside radius of the part respectively. This principle is known as the 20 percent rule (20 percent is not the only percentage used). Typical manufacturing facilities use many types of sheet metal. The table below shows the correct percentage to compute the inside radius for some common material types. The 20 percent rule percentages for other materials can be estimated by comparing the tensile strengths of the material with any sheet metal supplier's material characteristics sheet.
Rule of Thumb
In the old days a rule of thumb for selecting a die width was that the die width should be six to 12 times that of the material thickness. No other factors were considered. But this is how many problems with air forming begin. Why? Because between six and 12 times the material thickness a wide range of tooling options exist, and each option produces a different inside radius. Note that these rules do not apply to bottom bending or coining.
To put this concept into perspective, let's examine what might happen on the shop floor. Operator A may choose a six times V-die width from experience, paying attention to the punch tip radius only but not to the achievable inside radius. Operator B, concentrating on tonnage, may choose 12 times the material thickness, again with no real concern for the final measurable inside radius.
Because air forming establishes the inside radius at the V-die opening rather than at the punch tip, variations in V-die width will change the geomety of the final product.
If the radius changes, so does the geometry, in particular, the outside setback (OSSB), or (X) factor. The outside setback is defined as the distance between the tangent point of the radius and the apex of the bend. If the radius and OSSB change, so do the bend deduction (BD), or (X) factor, and the bend allowance (see Figure 2).
Finding a part design that places a feature inside the area that is defined by the outside setback (on the radius) is uncommon, unless that feature is placed on or outside the bend line. Any feature placed inside the area defined by the OSSB or bend line will suffer distortion. The closer to the bend line a feature gets, the greater the amount of the distortion it will encounter (see Figure 3).
The Original Question
Back to our original question, why do we care about achieving exact inside radius? A part feature designed to lie outside the area defined by the OSSB will begin to distort as the radius increases. If the radius is too large, the area of the OSSB will be too large, thus, setting the feature into the area of the new and larger OSSB.
How can these problems be avoided? By using a standard formula based on the outside radius of the final product when selecting a V-die opening. The reason for basing the calculations on the outside radius rather than guessing at six, eight, 10 or 12 times the material thickness is that this method (shown below) will result in a consistent tooling selection regardless of the bending method or type.
Optimum V-die opening: Factor x (outside radius x 0.7071)
- Material 0.125 in. thick or less, bent sharp = 4.0
- Material 0.125 in. thick or less, bent with a radius = 4.85
- Material 0.125 in. thick or more, bent sharp = 5.0
- Material 0.125 in. thick or more, bent sharp = 5.85
When you use the 20 percent rule to find the measurable inside radius specified in the design, based on the inside radius produced at the optimum V-die width, distortions will decrease or disappear completely, and you'll achieve true consistency in forming, regardless of the material thickness, bend radius, type of bend, or forming method.