Today, the 3D printer and CNC machines read files (three dimensional files) that then create parts. If you have a flaw on the virtual model, it’s duplicated in the CAD file. There’s no machinist to back you up by finding your flaw and giving you a call. That flaw will get produced and, produced very well I might add.
When parts don’t fit, it’s usually too late. The 3D printers or CNC use whatever parameters they’re given, period. So it’s very important to get the model right. Computer aided design has given us the tools to fit parts with exact clearances needed. The computer can’t do all the work though. You have to know what clearances are acceptable or appropriate for the materials that are being used. Also, heat, loads and the type of materials used, have a lot to do with what clearances are necessary.