It looks like you are running up against the tiny face limitation in SketchUp. Instead of trying to draw it at 6mm in diameter, set units to meters and draw it so it is 6m in diameter. Then scale the model down after you are finished.
These are 4mm bolts which I modeled that way.
[image: SeZc_Screenshot-7_11_202012_35_18PM.png]
These are 6mm shoulder screws also modeled with the same method.
[image: 49627196177_7d2ae945ef.jpg]