I've made a simply (big) knob for potentiometer. After applying boolean modifier (difference) for create a starred hole (starting from a starred cilynder), the object come with "Non-Manifold"... Why? Anyone can help me? Thamks in advice.
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It looks like the cut has its normals inside out. In edit mode select everything with A. Then recalculate the normals using Shift+N. If that doesn't work make sure that your cutting object is manifold before cutting.
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$\begingroup$ Thanks... it work great! I'm only curious to how occour this... $\endgroup$ Jul 7 '20 at 8:54
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$\begingroup$ there are many ways that this can happen, most of the time blender has to make assumptions on how you want your normals positioned sometimes it guesses wrong, sometimes the user accidentally assumes the normal is in one direction or the other. A telltale sign that something is wrong is the coloring notice how the cut in your photo appears darker than the rest of the faces, that is a signal that the shading is being calculated differently than the other faces. it's usually because the normals that are used in the calculation are facing the wrong way. $\endgroup$– MichaelJul 8 '20 at 22:01