Wanna improve the connection strength between your project's 3D-printed parts, and also have nice clean surfaces? Instead of gluing bits together, or screwing plastic screws directly into your 3D prints, use strong and reusable machine screws and heat-set inserts. Heat set inserts are only a few cents a piece and have a grooved outside, with threads on the inside. Originally they were designed for injection molded parts but they work fabulously for 3D printed plastic as well.