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To create void spaces another sketch is needed
Therefore the center point circle tool is chosen from the sketching tools and the the surface of the 3D geometry as the new sketching plane
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Three circels circles are created with a radius of 5 mm
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After closing the tool three new surfaces are created
With the Push/Pull tool the surfaces are selected and pushed completly completely through the 3D geometry
This results in holes inside the geometry which are void spaces
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Another option to create void spaces is to generate new solids and to subtract them with a boolean Boolean operation from the already existing solid
Finally the Design space for a Bookshelf is created and the optimization model setup can start which is discribed described here.