Hi Pilou,
re:
> Why the subdivision surface still stay in 6 separate faces and not redone a normal sphere when join or union?
The subdivision surface sphere is not quite exactly spherical.
Since it is not an exact match for a sphere there isn't any mechanism to try and convert it into an exact sphere.
If you want an exact sphere, you can make one with the Draw solid > Sphere command, instead of using SubD conversion.
- Michael
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