Generate mesh on a half torus

I am trying to generate a mesh on half of a torus. I generate the whole torus as a rotational object by following rotational symmetric object.
Then I perform the boolean operation to obtain half of the object, as introduced in half sphere.

Then I encounter the problem. The generated geometry only contains the rotation of the first (maybe) piece of splines. What I want is half of the torus. What am I doing wrong below?

Thanks for your help.


from ngsolve import *
import netgen.gui
from netgen.csg import *

spline = SplineCurve2d() # create a 2d spline
R = 1                    # define the major radius
r = 0.2                  # define the minor radius
eps = r*1e-2

pnts = [ (0,R-r), (-r+eps,R-r+eps), (-r,R),
         (-r+eps,R+r-eps), (0,R+r), (r-eps,R+r-eps), (r,R), (r-eps,R-r+eps) ]
# define the splines using the control points
segs = [ (0,1,2), (2,3,4), (4,5,6), (6,7,0) ]

# add the points and segments to the spline
for pnt in pnts:
    spline.AddPoint (*pnt)

for seg in segs:
    spline.AddSegment (*seg)
rev = Revolution ( Pnt(-1,0,0), Pnt(1,0,0), spline)

bot          = Plane(Pnt(0,0,0), Vec(0,1,0))
finitesphere = rev * bot
geo = CSGeometry()
geo.AddSurface(rev, finitesphere.bc("surface"))
geo.NameEdge(rev, bot, "bottom")


We recommend to switch to opencascade geometric modeling, find here an example revolving a 2D cross section: