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Recently I have been working on a chess set utilizing OpenSCAD. All of the pieces are constructed by taking the difference of a cube on four sides utilizing a 3D triangle. I have been able to get most of my files to turn into a STL file, but I'm still running into issues with manifold for some of my pieces. Most of these pieces can be fixed by taking the differences on each four sides and adjusting the size of the base but for the bishop and knight piece I'm running into issue and I don't know how to fix it. I uploaded the files for reference.
1) You're likely using an older version of OpenSCAD. Your designs worked for me using the latest development snapshot.
2) The cause of the problems are likely that your cutouts are dimensioned exactly. This may leave infinitely thin walls. It's smart to let cut-outs slightly larger than necessary.