There is a subtle difference between CAD programs and Google Sketchup. CAD operates on exact measurements, (mathematical solids in 3D) whereas Sketchup operates on 3d meshes, that are inexact by definition. For example, in a CAD a sphere is recorded as an ideal sphere (center point + radius), whereas in Sketchup it is a surface made up of lots of little triangles. For some applications that is OK, for others not.
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