Oh, I thought in your initial post that the plane was the surface you were trying to use and the box was the bounding box you were getting.
When I look at either end of your cylinder, I’m seeing a circle with radius 10 centered on 0 and a bounding box with correct points for that:
|Bounding Box Min||-10.000|-10.000|0.000|
|Bounding Box Max||10.000|10.000|0.000|
And one at z=15 for the other end.
Is it really the bounding box you’re asking about or the way Rhino highlights a slightly larger plane when you select that face? Also, if I select the face of the actual extrusion, then I get a circle as expected. It’s only if I convert to a brep that I get the visual effect- but the bounding box you’re reporting still works fine for me.
Upon reviewing your file I noticed that the NURBS surfaces have noot been shrunk. This discrepancy is causing the issue you’re experiencing. The bounding box you’re seeing represents the untrimmed surface. In NURBS geometry you must shrink the surface post-trimming to realign the control point grid accurately with the modified shape.
In brief: Execute the “ShrinkTrimmedSrf” command prior to using the bounding box function.
To elaborate NURBS surfaces are defined by a control point grid. When these surfaces are trimmed, the original, larger NURBS surface often remains beyond the trimmed area, invisible but still present.
The problem isnt with the bounding box command itself as it accurately matches the underlying NURBS structure. The issue arises because this structure hasnt been updated to match your visual modifications leading to a bounding box that appears oversized but is, in fact, accurate to the unaltered NURBS surface.
Hope this clarifies things, It will 100% solve your issue.