1) The physical limitations of the metal make it difficult to accomplish a bend radius less than material thickness without Coining or Bottom Bending the sheet metal.
2) When Air forming with a radius that is technically “Sharp” the bend takes on a parabolic shape as opposed to a true radius.
Making a radius measurement extremely difficult to make and by extension the value for the k-Factor hard to predict with any accuracy
3) In the bend radius, the material goes through a few changes. Those changes being that on the outside of the bend radius
longitudinal and transverse the material is being stretched or “expanding”. On the inside surface, the material is being compressed. Because compression cannot exceed expansion the maximum k-Factor cannot be greater than 50% as the compression cannot exceed the expansion.
The minimum k-Factor described by the following formula:
(4 – Pi) / Pi
Then using reverse calculations, we calculate the minimum k-Factor for an Air Bend, and that value is 0.27324
Minimum k-Factor = (4 – Pi) / Pi = 0.2723
Then taking the minimum k-Factor and subtracting that value from our maximum k-Factor we calculate a value of 0.226
We next take the .22568 value and divide that by 3, the number of sections represented in the gold line section.
0.22568 / 3 = 0.7558666
We then multiply the 0.07558666 by the Inside bend radius divided by the material thickness minus 1. then adding the minimum k-Factor of 0.27324 to that value.
The k-Factor then equals: .07558666 * ((Ir / Mt)-1) + 0.27324
For a Material thickness of 0.062 and an inside radius of 0.093, we would have a k-factor of .349
That is the background for the calculations. Now you can let the calculator do the work for you.
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