@fredrik :

Yes, your tips working. What I am thinking is when I sent the file ( chairDesign02.3dm ) to my friend then open the file in another program ( SW ), he cant give a thickness also ( the error message is "degenerate geometry". Is there another tips so with the same technique ( use old curves, then network ) then we can shell them without a problem. I tried to delete one of the point, but it will delete the surface its self. So, not working.

@keith :

I think the result is not common. The thickness is not constant.

Thanks Guys....