Increasing malicious users have sought practices to leverage 3D printing technology to produce unlawful tools in criminal activities. It is of vital importance to enable 3D printers to identify the objects to be printed and terminate at early stage if illegal objects are identified. Deep learning yields significant rises in performance in the object recognition tasks. However, the lack of large-scale databases in 3D printing domain stalls the advancement of automatic illegal weapon recognition. This paper presents a new 3D printing image database, namely C3PO, which compromises two subsets for the different system working scenarios. We extract images from the numerical control programming code files of 22 3D models, and then categorize the images into 10 distinct labels. These two sets are designed for identifying: (i). printing knowledge source (G-code) at beginning of manufacturing, (ii). printing procedure during manufacturing. Importantly, we demonstrate that the weapons can be recognized in either scenario using deep learning based approaches using our proposed database. The quantitative results are promising, and the future exploration of the database and the crime prevention in 3D printing are demanding tasks.