[Paul_]

I am going to make something like this; an half cylinder on Ø400 mm, on that i will weld an half cone that ends on Ø 200 , On the sides of this item i will weld an flang so i can bolt it with the other half. I have not decided the thickness yet

Now to my question, i need a decent tolerance on this (since i am going to press this against another surface with a sealant), so i was thinking about milling after roll bending. But i am a bit worried that there are alot of tensions in the sheat after the roll bending so that it will deform when milling, is this a problem (can i maybe add extra thickness to compensate for that?)

[victorofga]

I believe you need stressrelieving..
its actually heat threatening..

I remember for one work, while i worked in machine shop.. that was a 160 mm dia pipe, welded on this pipe two disc and the outer pipe was rolled and welded onto..
outside it was around 450 as much I remember and wall thickness were 12 mm

even it was closed shape and stress relieved, it was very hard to keep 0.15 mm tolerance..

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to milling flat only the flanges might works, but this shape even for the least temperature changing will change its geometry..
clamping this part to the table also will try your patiency..

milling that halfround geometry... looking very hopeless..

[Paul_]

I think it is ok if it change shape with changing temperatures since it will be pressed against another item which is made of steel (or aluminium not decided yet) so the gap between the items will stay intact?

Did you use heat threatening in the case you are describing?
Do you know who much the shape change after heat threatening?

[victorofga]

from practical view aluminum would be better option.. its pending what it will be used..

when I concerning part changes its shape and dimensions by heat, I thinking on when its happening within machining..

this part not simply expanding, also due its geometry it will ""opening up"" making the flat flange on the two side becomes angled relatively to each other..

so part wont be flat..

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let see some practical examples..

if this shape is used for as an ""oilpan"" , like you can find under a car engine then a pressed relatively thin sheetmetal or cast aluminum will work well.

building a large silo type structure, the tolerance will be almost neglectible..

as you concerned about tolerance, that's why I tried to call your attention what you can expect..

when its get ready and bolted together , from that point wont be matter if shape or say, its geometry changing..
but,,, if it made not flat, then theres a chance you can not pull together the two half..