Pipe Forming

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Got some basic doubt here. How do we calculate the forming length of a pipe with thickness. I know the formula is pi*d. But I am a bit confused on the thickness part.
As the unfold gives the same length for both the diameters(which is illogical),how do we consider the calculation for taking the forming length for such pipes. I know it is basic. But I am not an expert in manufacturing and hence need some support.

I am forming a pipe out of a flat stock.


Bend Allowance (BA)
BA = [(0.017453 × Inside radius) + (0.0078 × Material thickness)] × Bend angle, which is always complementary
The length of the neutral axis is calculated as a bend allowance, taken at 50 percent of the material thickness. In Machinery’s Handbook, the K-factor for mild cold-rolled steel with 60,000-PSI tensile strength is 0.446 inch. This K-factor is applied as an average value for most bend allowance calculations. There are other values for stainless and aluminum, but in most cases, 0.446 in. works across most material types.

If you multiply the material thickness by the K-factor (0.446), you get the location of the relocated neutral axis: for example, 0.062 × 0.446 = 0.027 in. This means that the neutral axis moves from the center of the material to a location 0.027 in. from the inside bend radius’s surface. Again, the neutral axis goes through no physical change structurally or dimensionally. It simply moves toward the inside surface, causing the elongation.
Note the two factors shown in the bend allowance formula: 0.017453 (Pi*1 degree) and 0.0078 ( offset for neutral line.). The first factor is used to work your way around a circle or parts of a circle, and the second value applies the K-factor average to the first factor. The 0.017453 is the quotient of π/360. The 0.0078 value comes from (π/360 × 0.446. So this formula will give you the bend allowance per degree.
Now the neutral line moves with the hardness of the metal, however there are published tables you can google to find those offsets for whatever material you are using

And because you are forming out of flat stock, remember the .0078 X T calculates the K factor and this number changes according to the hardness of the material. Depending on your Radius and the thickness of the metal this value can migrate from the center of the material ( 50% ) to closer to the inside edge but as a general rule never less than ( 25% ) This is why you need to make test pieces to determine the exact amount. The other question is how are you making this? Are you using two piece forming dies, or are you bumping the part on a press brake. Rolling tubing on a 3 roll rolling machine always ends up with flat spots at the edge of the sheet.