How to cut a subd "torus" side without affecting the form

i am in top view, and i would like to delete some part of this round sudb objet. but when i delete parts, the circle is deformed. how to keep a perfect circle whitout the global closed subd objet ?
thanks !!

dear @emile_francois
as fare as i uderstand SubD 's can not represent mathematical precise circles - they can only approximate them.

For arc s its even more a visual approximation:

(see screenshot below)

(left) if you do a simple SubD Surface starting with a curve (_curve, Degree 3, SubD-Friendly = yes,
SubD s will always have zero curvature towards the edge.
therefor the torus with removed segments shows the behaviour you are experienced / shown in your screenshots. (so it s just a technical speciality of subD s)
(middle) _extrudeCrv (subD)), extractControlPolygon (red) will have the structure of the initial curve
because the curve was “subD friendly” - see more info about subD friendly here
(right) if you want to visually approximate what you are looking for, you have to do some more dense EdgeRings towards the opening of the Arc.
a fast start: use
_arc deformable=yes
_line, _divide n=3 als construction for the 1/3 rule
_insertControlPoint 1/3 at start / end
maybe some visual CV-Movements to optimize

Another approach would be to do most of the modelling as long as the torus is closed.
Do the opening at the end of the modellingprocess converting _toNurbs

does this help ? kind regards -tom

a visual approximation might look like this:

blue curves:
_curve Degree 3 SubDFriendly = yes
(or workflow from above post)
_circle deformable= yes, Degree 3, 8 Points


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i draw a torus dashboard like this

i would like to continue to draw differently this shape with keeping the round jig (torus template) because it fits exactly with the exterior shape … head breaking haha
with this “modified torus shape” it is the dashboard, i would like to extend in the same limit, a sofa and other modules…

maybe impossible to make it with the same object … that why i would like to cut this shape in several sequences

When i insert edge loop …

Hi Emile - no - you can get closer by adding subdivisions but free edges of SubDs are alway zero-curvature so they are not suited to representing arcs.