When sheet metal is bent,the inside surface of the bend is compressed and the outer surface of the bend is stretched, somewhere within the thickness of the metal lies its Neutral Axis,which is a line in the metal that is neither compressed nor stretched.
What this means in practical terms is that if we want a work plece with a 90 degree bend in which one leg measures A, and the other measures B, then the total iength of the flat piece is NOT A+B as one might first assume. To work out what the length of the flat piece of metal needs to be, we need to calculate the Bend Allowance or Bend Deduction that tells us how much we need to add or subtract to our leg lengths to get exactly what we want.
where:L is the total flat length
A and B are shown in the illustration
BA is the bend allowance value
The location of the neutral line varies depending on the material itself, the radius of the bend , the ambient temperature, direction of material grain and the method by which it is being bent,etc. The location of this line is often referred to as the K factor.
K-factor is a ratio that represents the location of the neutral sheet with respect to the thickness of the sheet metal part.
The only truly effective way of working out the correct bend allowance is to reverse engineer it by taking a measured strip of material, bending it,and then measuring it.