## 4 Measurement Results  
The schedule of test plates printing was designed to simulate the regular use of printer: the first sets were produced in the same conditions (repeatability), and after the machine was used for printing various other parts, additional groups of test plates were printed in different time intervals (reproducibility).  
To verify the accuracy, as well as repeatability and reproducibility, five measurands were selected on two geometrical features: cone and largest cylinder in _XZ_ plane. The largest cylinder hole was chosen because of the pre-test that showed that there are large deviations in cylindricity for small holes. Additionally, since the point of this paper is to find repeatability and reproducibility, it was better to choose as large a diameter as possible. However, because there is a bigger problem in cylindricity when it is not printed in the _XY_ plane, only the top hole in the _XZ_ plane shown in Figure 6 was selected. According to [5], mentioned earlier in the text, this hole was also chosen because all the holes in the _XZ_ plane are made with a supporting structure and according to [5], it gives better dimensional stability. The cone was chosen because of its height and to establish a deviation from its position on the test plate. Only two geometrical features were chosen because of the costs of examining the products with the CT method.  
Measurements were carried out using _Nikon XT H 225_ industrial Computed Tomography (XCT) machine at X-ray energy of 140 kV and X-ray tube current at 150 uA. These parameters were chosen to provide relatively fast scanning times, while still providing sufficiently small focal point size and therefore adequate resolution. The X-ray source used in this experiment links the focal spot size and X-ray power approx. 1 um per 1 W. In this case, 21 W of X-ray power resulted in a focal spot of approx. 20 um in diameter and that sets the fundamental hardware resolution limit, prior to software subsampling. Final CT scan data had a voxel size of 64 um due to the fact that several test plates (each set) were scanned together, in order to ensure similar measurement conditions within each set. Imaging was performed using a 400 mm \(\times\) 300 mm 14 bit flat panel detector with 127 um pixel size,  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Day** & \multicolumn{3}{c}{**Sample Mark**} \\ \hline
1 st day & 1.1 & 1.2 & 1.3 \\
2 nd day (+1 day) & 2.1 & 2.2 & 2.3 \\
3 rd day (+2 days) & 3.1 & 3.2 & 3.3 \\
15 th day (+2 weeks) & 4.1 & 4.2 & 4.3 \\
16 th day (+2 weeks and 1 day) & 5.1 & 5.2 & 5.3 \\
17 th day (+2 weeks and 2 days) & 6.1 & 6.2 & 6.3 \\
29 th day (+4 weeks) & 7.1 & 7.2 & 7.3 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Plan of printing by days.  
Figure 5: Printed test plate.  
exposed at 333 ms. CT data were acquired with 1440 projections with a two-frame averaging. Scanned test plates were reconstructed using _Volume Graphics VGStudio Max 3_. The coordinate measurement system was aligned with the measured object as presented in Figure 6. Finally, voxel size calibration was performed using an externally calibrated ball-bar standard with a nominal sphere-to-sphere distance of 30 mm. This procedure ensured consistency across all the measured sets of test plates. All measurements in _VGStudio Max_ were made after the _Advanced surface determination_ algorithm was used to define surfaces. Unlike the standard ISO 50 edge determination algorithm, this algorithm uses an adaptive threshold to provide an improved surface definition, reducing the influence of noise and varying material thickness.  
In order to check all three axes of the 3D printer, one measurand (distance) per axis was analysed. For the \(x\) and \(y\) axis, the deviation of the cone axis position from the CAD model was chosen, and for the \(z\) axis,