When drawing arcs that will eventually intersect to form a rounded corner, ensure they use the exact same number of segments. You can change this by typing a number followed by "s" (e.g., 12s ) immediately after selecting the Arc tool.
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When the math gets that tiny, SketchUp effectively gives up, leading to missing faces, holes, and the classic crack artifacts. The solution is often counter-intuitive: . sketchup round corner crack
This is the single most effective trick for dealing with crack and precision-related errors. When drawing arcs that will eventually intersect to