You need to know the metal you are forming, and you need to find the neutral line , the point at which the metal neither stretches or contracts during the forming operation.
B.E.

You are judged not by what you know, but by what you can do.

Hi Berkshire,
Thanks a lot for your reply. But can you please provide the formula for calculating the same? And how do it is arrived?

Maybe this will help.
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*1degree) 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
B.E.

You are judged not by what you know, but by what you can do.

Hi Berkshire,
Thanks a lot for your insights.But I noticed a formula for the pipe forming to be L = pi*(D - thk).. I could not understand why they mentioned the thickness in this. Could you please explain me on this?

sudhakarn ,
So I get from your next question that you are bending pipe or tubing , not making it.
The same rules apply . you take the diameter of the pipe and you calculate the neutral line , the point at which the tube neither stretches nor shrinks. for most pipe this is pretty close to the center of the pipe. so your formula is L { Length} equals Pi times ( diameter minus thickness). This does not make sense to me , for most pipe for a starting point you should be using L equals the (radius of the desired bend, plus half of the diameter of the pipe times one 360th of the desired bend angle times pi ) then multiply the result of that times your desired bend angle. This is what the formula I gave you does. Now since you are bending pipe not a solid bar the neutral line ,( this is what the 0,0078 times material thickness did for you.) that number for tubing will not be accurate , but for a first attempt will be close. Bend a piece of pipe check the results and adjust the formula

An example for 2" ( 2 3/8" actual) dia pipe on a 10" inside Radius at 90 degrees L = ( 0.017453 X 10 ) + (0.0078 X 2.375) x90 0.17453 +.0185 = 1931 x90 = 17.375.
Now depending on the hardness of the tubing you will find this to be a little off but after you have bent a test sample you can correct. Be leery of bend allowance and K factor calculations in 3D modelling programs, they are only as good as the data inputted. Remember GIGO.
B.E.

You are judged not by what you know, but by what you can do.

Hi Berkshire,
Thanks a ton for your insights. Let me explore these stuff and get back to you.

Hi Berkshire,
I need some clarity on this. Suppose I want to form a pipe of 50mm dia OD and 40mm Id (thk 5mm),what is the rectangular area of the sheet I need to take ( along with thickness).Need your guidance please.

As I told you the formulas are not exact , but you are looking at between 134 and 139mm depending on the hardness of your pipe and of course the pipe length. Make a piece and check the diameter. Have you not done any exercises with the formulas I gave you?
B.E.

You are judged not by what you know, but by what you can do.

sudhakarn
Are you bending pipe/ tube from completed pipe stock, or are you fabricating pipe or tube from flat sheet stock?
Please let me know what you are doing . The formulas are similar in both cases the difference is where the neutral line lies.
B.E.

You are judged not by what you know, but by what you can do.

Hi Berkshire,
Thanks a lot for your support. I am forming a pipe out of a flat stock.

sudhakarn,
In that case use the formula I first gave you. 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.
B.E.

You are judged not by what you know, but by what you can do.

Hi Berkshire,

Thanks again for your support and interest.