A surface normal is basically the indication of the positive direction of the surface. In BobCAD you will only be able to see these if you have rendered the surfaces and have the Normal option turned on. To see these normals render a solid/surface, then Click on Drawing Options, then Surfaces and then finally Surface Normals. This will display red lines that are perpendicular to the actual surface, these are the normals.
To reverse a normal: Start with nothing selected. Click on Solids in the CAD window, then Surface Modifcation and then Reverse surface. Then click on a Surface. When you render that surface with normals again you will see it reversed.
Ther fact that the surfaces are doing this is expected. When you mirror a surface you want to see the inverse of the shape. Well, unfortunately, you also have the normal that gets reversed as well.
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