# Spare Parts Made by Additive Manufacturing to Improve Preventive Maintenance
## 3 Results: Case Study
### Characterisation of the Spare Parts  
Two non-destructive tests were carried out on the manufactured parts:  
* Hardness measurement: the hardness test performed on the materials is the shore D hardness method, given that all the parts (both printed and the regular parts) were rigid polymers. According to the norms UNE-EN ISO 868 and 7619, the applied load is \(F=44.45\)\(N\). Two results were noted for each regular spare part:
* The maximum value of the shore D hardness displayed for one piece;
* The stabilised shore D hardness value during 10 s for one piece, which is marked as D/10 in the tables of hardness results. * Roughness measurement: the objective of this test is to determine if the surface of the printed parts is close to the original parts, by evaluating and comparing their roughness parameters values to ensure their work in real situation otherwise operational. The spare parts have been measured in four different areas with a Taylor Hobson profilometer, according to ISO-4287, as shown in Table 4[36]. The same direction and side have always been chosen to measure the profile of surface, and the cutoff equal to 0.4 mm was selected. The radius of the stylus is 5 um. A measure length equal to 2 mm (five cutoff) and a Gaussian filter were used. The parameters used to define the roughness and waviness of the spare parts were Ra, Rq and Rt. In this research, the 2D parameters were used given their wide use in the industry. These measurements were obtained using the profile method, in order to know the initial parameters and their variability on the surface. Equation (2) shows the arithmetic mean profile (Ra) of the heights, along axis Z measured in a length L in axis X. Equation (3) presents the value of the heights square mean of the profile and Equation (4) denotes the maximum height (Rt) that is the sum of peak height (Rp) and valley depth (Rp). Table 5 shows the results of measurements on original spare parts and AM printed parts. The hardness was improved compared to the original parts. A slightly higher \begin{table}
\begin{tabular}{c c c} \hline
**Parameter** & **Description** & **Expression** \\ \hline \(Ra\) (μm) & Arithmetic mean profile height & \(Ra=\frac{1}{L}\int_{i}^{x}|Zi|x\mathrm{d}x\) (2) \\ \(Rq\) (μm) & Profile means square height & \(Rq=\sqrt{\frac{1}{L}\int_{i}^{x}Z^{2}x\mathrm{d}x}\) (3) \\ \(Rt\) (μm) & Maximum height over the measured profile & \(Rt=Rp+Rv\) (4) \\ \hline \end{tabular}
\end{table} 
Table 4: Parameters profile surface according to ISO 4287. Figure 10: Spare Part 3 selected (**a**) Top side and (**b**) Bottom side. hardness can also be observed in the manufacturing process that uses the MultiJet 3D technology, compared to the SLS technology, as is shown in Figure 11.  
The results obtained from measuring the roughness of the spare parts with the methodological conditions described above are shown in Table 6. The values obtained for Ra, Rq and Rt are shown graphically in Figure 12. The roughness of replacement parts can vary according to their functionality. As a rule, high Ra values were used when adhesion is a criterion for the usability of the replacement part and lower values were used to avoid wear. The parameter Rt indicates the maximum value between peak and valley and is complemented by Ra to the extent that it provides us with a clearer idea of the dispersion of the profile, although Rt is usually proportional to Ra, i.e., the higher the Ra the higher the Rt. According to the graph in Figure 12, the Ra increases in regular spare parts. In the case of the regular Sp3, the roughness increases, perhaps due to the type of material of the regular part, which is TPFE (Teflon). However, the parts made in AM have an apparently stable roughness. It seems that the surface quality of the part made using SLS is slightly better than the surface quality of those parts made through the multi-injection process.