This question comes up regularly as users want to double check the developed length of their sheet metal parts.
The Inventor wiki help shows all kind of nice pictures and formulas for the developed length but if you look more closely you will notice that all the images provided in the wiki are for situations where the bend angle is smaller than or equal to 90 deg.
For bend angles larger than 90 degree, calculating the developed length proves to be a bit more involved.
The general formula of the developed length is:
d = delta value as found in the bend table
b = bend angle as measured in the image below
L1, L2 = the flange lengths measured to the tangency point.
Fig 1: Parameters needed to calculate the developed length of a bend
The most important aspect to understand in Fig 1 is how the L1 and L2 length values are measured.
They are measured "tangent" to the bend (see also the blue lines in Fig4).
Let's take an example. We start out with a 90 degree flange with L1 = 30 mm and L2 =20 mm (= flange height).
Fig 2: Initial 90 degree flange definition
We use a 2 mm bend radius. For that radius we use following bend table in the sheet metal unfold style:
Fig 3: Our bend table definition
We now modify the 90 degree flange to a 95 degree flange.
The most common mistake is to calculate the developed length as
L = L1 + L2 – d = 30 mm + 20 mm – 3.71045 = 46.289 mm
Our L1 and L2 values have changed of course and the L value is incorrect!
Fig 4: Measuring the L2 value
When we measure the L1 and L2 values appropriately, to the tangent line to the bend, filling in the values in the formula is uneventful.
L = L1 + L2 – d = 29.635 + 19.635 – 3.710405 = 45.559595 mm
The calculated result corresponds nicely with what can be measured on the flat pattern
Fig5: Developed length as measured on the flat
Hopefully this gives you an idea on how to reverse engineer Inventor's developed length and assures you a better night sleep, knowing that Inventor does an accurate job in calculating the length J