## 3 Test and Analyses Results  
The analysis of the measurement data was based on the visual evaluation and interpretation of the spectrograms. This approach was based on the adopted assumption: if the results are promising, high-quality measuring equipment and specialized software for signal analysis will be purchased for advanced research. The observations presented in this paper show that the threat of infiltration of 3D printers is so high that it deserves further research aimed at its elimination. The tests began from measuring of printer vibrations in standby mode [25]. Spectrograms for X and Y axes are presented in Figure 8. Horizontal lines at frequencies of approx. 85 Hz and 120 Hz are visible in both spectrograms, and correspond to vibrations induced by the stepper motors operating also in standby mode, maintaining the rotor in a balanced position. These frequencies in further analyses were omitted. In addition, the accelerometer indications for the Z axis were omitted because of scarce information content that could be used to reconstruct printed objects. Figure 7: The objects printed during 3D printer tests: (**a**) trihedron, (**b**) tube, (**c**) tetrahedron. Figure 9 presents waveforms and their spectrograms obtained for printed objects based on signals recorded during 1 min measurement sessions. It results in the repeatability of characteristic printing phases. Figure 8: The spectrograms for printer vibration signals in standby mode. Figure 9: _Cont._  
To make it easier to distinguish details on the above-mentioned spectrograms obtained from the 3D printer vibration measurements while printing the objects under investigation, the time scale used for further analyses was increased. It has been observed that the printhead always starts printing a given object from the same starting point. Therefore, the printing phases presented on the spectrograms are easy to identify, and their distinctive features can be easily compared. Figures 10-12 show spectrograms of the vibration signals recorded during the X and Y axis movement of the printhead (upper and lower spectrograms, respectively). From their comparison, it can be concluded in which direction the printhead was moving at a given moment. For example, when printing a trihedron, the movement of the head along the X axis is marked on the upper spectrogram with a clear line at the frequency of 340 Hz, when printing the right side b. A similar relationship can be observed when printing on the side of the square a (movement of the printhead along the Y axis). The double lines marked  
Figure 9: The spectrograms of printer vibrations (1 min measurement sessions) during the printing of: (**a**) trihedron object, (**b**) tetrahedron object, (**c**) tube object. on both spectrograms with yellow rectangles covering the frequencies 320 Hz and 360 Hz represent the printing phase of the hypotenuse c. These lines indicate alternating X and Y axis motion of the printhead. Figure 10 shows the spectrograms based on the vibration signals recorded when the walls of the triangle were printed (trihedron, Figure 7a). A few characteristic printing steps can be distinguished. These steps are marked as I, II, III, IV, V and VI, and P1, P2, P3 and P4. They are defined in the following way: \begin{tabular}{l l} P1, P2, P3 & the printhead performs the positioning movements (without printing), the platform is lowered by one level; \\ I & the printhead prints the side of trihedron (outer path) from point A to B; \\ II & the printhead prints the hypotenuse (outer path) from point B to C; \\ III & the printhead prints the side of trihedron (outer path) from point C to A; \\ P4 & the printhead performs the positioning movements (without printing); \\ IV & the printhead prints the hypotenuse (inner path) from point C to B; \\ V & the printhead prints the side of trihedron (inner path) from point B to A; \\ VI & the printhead prints the side of trihedron (inner path) from point A to C. \\ \end{tabular}