exact formula to calculate dish end blank

[Kintesh]

i joined this industry before 5 months. i learned many formula to calculate dish end blank diameter. but still i did not found exact formula to calculate diameter.

 

[TomBarsh]

Kintesh- I don't think you'll find an exact formula either.

The issue is that the dished surface is not what is called "developable". Such a surface is one that can't be unrolled, unfolded, etc, to a flat surface. Contrast this with a circular cylinder or a cone, these can be unrolled to a flat surface; meaning they can be "developed". Trying to flatten out a non-developable surface will impose strains in it (conversely, forming a flat plate of metal into a dished head induces strains...and stresses...in the plate).

The result is that there is no simple formula based strictly on geometry to determine the diameter of the blank required for a formed head.

However, as the material properties are known I imagine that it is possible to determine a formula to determine the blank dia. This would require knowledge of the material's properties: modulus of elasticity, yield stress, ultimate stress, Poisson's ratio, and possibly more properties as well. Thus the blank's diameter would vary slightly depending upon the material. I have never seen such a formula, nor heard of anyone discussing the possibility of using the material properties to determine this. It's likely that head manufacturers have worked out rules of thumb that inherently (or explicitly) account for these properties.

 

[berkshire]

Kintesh.
You also have to consider the manufacturing methods used to produce your dish ends. Both press forming and spinning involve some trimming of waste from the finished part. In pressing the part is thinned some what in the manufacturing process, in spinning/flo turning, the part is thickened particularly at the edges. The result is that your blank diameter is at best an educated guess, unless you know exactly how much is trimmed off.
B.E.

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

 

[racookpe1978]

That is why most hemispherical and F&D and elliptical tanks and PV's are bought from a fab shop specializing in rolling and forming dished heads.

They've spent year getting it wrong, and so their "shop practices" and "proprietary knowledge" lets them avoid these assumptions, errors, and rework you will be making. That said, you are also paying for that experience when you buy from them. It will be less expensive to buy from a pro shop.