As a former HS math teacher, I often want to give Rhino very bad grades… What am i doing wrong?
Here are two Simple bent piece of material. I want to develop them into a flat surface To be able to have then wirecut… In blue are the parts.

I drew an extrusion that follow the 1/2 thickness profile to find the median plane( in Black.
So mathematically from my 5th grade math class, what I am looking for is the intersection of the blue part with the black extrusion. As result I am expecting a surface that has the contour of the blue part… Well not according to Rhino.
FIrst Part what I selected:

You’ll find that users on this forum will give great advice when you also include your model - please attach it to this thread. Otherwise it’ll be guessing…

The intersection of your blue polysurface and the black surface would be a curve in Rhino. The Rhino Intersect command is different than the Rhino BooleanIntersection command which is what you apparently used.

I believe the Rhino command that you want to use is called 'Trim"

To get what you want use the blue polysurface to trim the black surface or trim the black surface using the curve that the intersect command will create as the cutting object.

Nathan,
You are very correct and Here is the model.HVP-2B NegBar.3dm (104.7 KB)

Jim,
Yes you are correct, Since Rhino does not really deal with solid, MAthematically the intersection would be a set of curves with the profile of the blue part. The intersec function. I did not know that function. Thanks

I did not think of using the Trim function. Thanks for the advice it is a very elegant solution.

Yes I used the Booleanintersection. I still do not understand the difference between intersec and Boolean intersection. Both should give me the same result.

Thanks Pascal,
Yes it does and I see now. But still do not understand the behavior of the Booleanintersection in my case.

On a slightly different subject on the same model, Once I get my surface, I cannot find the center of the holes. When I duplicate the edge I get a curve made of millions of points instead of a circle. Any way to rebuild the edge to have it simplified as a circle?

Hi Jean-marc - try UntrimHoles and then retrim with the clean circle - if it is a solid, you can extrude the circle through, and use BooleanDifference. You could try ReplaceEdge, it might work if it is only a trim in a surface - I’ve never tried it on a hole but no reason why not. Hm- no, it does not work in my test.