Sheet Metal Oil Tank Help...

[JB_205519]

Hello, I am trying to document an existing oil tank in Creo to manufacture a new on with some modifications.  The existing tank is fabricated from two pieces of aluminum welded together with no compound curves.  Neither piece is stretched or shrunk, they just have equal perimeters and are welded together.

I templated the existing tank surfaces manually (posterboard), scanned them, and traced them in 2-d cad to generate the shapes of the existing tank.  I built one with posterboard, and it works as far as replicating the existing tank.  To make the new tank I want to have Creo models for visualization and fabrication.

What is the easiest way to attack this in Creo?  I have never started with a flat part and worked into a formed part, always the other way around.  I have roughly solid modeled the part and converted to sheet metal (shell, .050" thick), but it does not like it when I try to flatten it.  
All of my sheet metal design has been straight bends, sheet metal boxes etc.  I have no experience in surfacing.  any ideas on best practice for this part?  I have attached a .pdf drawing of the solid version I did in creo, and the 2d patterns I created form the part.
Thanks!

[tbraxton]

The most likely cause of the issue is that your surfaces are not developable (it can be bent without stretching or compression).

Based on your flat pattern drawing I would try to start with the flat pattern and bend/form in sheet-metal mode to get your 3D shape. I am not able to comprehend the 3D shape from your illustrations so it is hard to see what may be needed. If you can not get the shape using bend features then it is going to be much more difficult.

In theory you can model the "formed" 3D shape and then flatten it but the constructs used to get the 3D shape must support the flat pattern. I have designed formed metal shapes that will not flatten in Creo, other SW is used to get the flat pattern and in many cases that flat pattern must be iterated in prototypes to get it ready for production.

6000 series AL parts red & orange (these do not flatten in Creo)

[JB_205519]

It really is a simple part, no stretching or forming.  I would like to model it solid and then convert, so I can develop the shape of the tank and then convert to flats for manufacture.  

Here is a step file to help visualize...

[pausob]

Instead of using sheetmetal mode, consider using the flatten-quilt tools in the surfaces menu in the Part modeling tab.

Your sample data, shelled, and then the outer surface flattened:

[tbraxton]

Your surfaces are not developable. They must be a smooth surface with zero Gaussian curvature. Developable surfaces include the cone , cylinder , elliptic cone , hyperbolic cylinder , and plane .

The upper shape does not have zero Gaussian curvature. You can query your model using the shaded curvature analysis tool. If it reports non zero values then the surface can not be flattened without distortion.

I put a surface of revolution (cone) in your model (yellow surface) that aligns on the symmetry plane of the upper surface. You can see that it has zero Gaussian curvature but is not congruent with your desired topology. If you run the analysis on your model you will see that the top surface does not satisfy the requirement of zero G curvature. The good news is that the bottom surface does and should flatten in sheet metal.

Use the flatten quilt with caution, it makes approximations that may not actually yield a flat pattern blank that can be made into your part. I would test that method on paper before blanking metal with it.