# Design for Additive Manufacturing and for Machining in the Automotive Field
## 3 Results
### Time and Cost Comparison between LPBF and CNC  
The geometry of the bracket designed for CNC is illustrated in Table 3, which also sums up the mass and the predicted mechanical response of the three bracket geometries: the original one, the one optimized for LPBF and that optimized for CNC alone. By adopting the same requirements listed in Section 2.1, according to the FEA, the bracket to be produced by CNC resulted in being about 30% heavier than both the original bracket and the one redesigned for LPBF. In addition, the geometry for CNC alone was about 9% less stiff than the original part, whereas the bracket derived from TO for LPBF achieved a 3% gain in stiffness. Most of all, the part optimized for LPBF was below the elastic limit since, apart from local singularities, the Von Mises stresses were generally lower than the yield point. The stress values were instead much more critical on the original bracket, where they risked overcoming the tensile strength of the material. \begin{table}
\begin{tabular}{c c} \hline \hline
**Details of Building Job from Industrialization** \\ \hline Build time (12 parts + 3 specimens) & 26 h \\ Powder waste & \(\approx\)500 g \\ Supports volume & 72,500 mm\({}^{3}\) \\ Part volume & 477,000 mm\({}^{3}\) \\ Supports type & 90\% Block; 7\% cones; 3\% Lines \\ Machine & SLM 280 Twin HL (double laser) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Industrialization data. A time and cost analysis was carried out to define the convenience of CNC and LPBF for the production of the corresponding optimized parts. Figure 4 compares the workflow of the two processes and their respective lead-time for different volume batches. A holistic methodology is proposed for the AM process, including all the main steps to obtain the final part, combined into a recursive cycle in case of unsatisfactory results of the thermo-mechanical analysis. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Original** & **LPBF + CNC** & **CNC Alone** \\ \hline \hline  & 88 & 85 & 114 \\ \hline  & 0.34 & 0.33 & 0.37 \\ \hline  & 464 & 297 & 351 \\ \hline \hline  & & & \\ \end{tabular}
\end{table} 
Table 3: Comparison of mass and mechanical response for the three bracket versions. Figure 3: Output of the optical dimensional control: (**a**) as-built part; (**b**) finished part; (**c**) histogram of the distribution of geometrical deviation on the finished part. Figure 5 groups the results of the cost and time analysis. In particular, the graphs in Figure 5a compare the variation in the cost per part as a function of the production volume for the first order and for the following reorders. For both production methods, the cost per part quickly decreases for increasing production volumes. In particular, if LPBF is considered, nesting is fundamental in order to exploit as much as possible the volume of the build chamber [33]. However, due to space limits, the SLM 280 machine used here cannot contain more than 12 brackets at the same time. For this reason, the cost per part decreases very quickly from 1 to 12 nested parts, but for larger batches, the cost remains substantially unchanged. In general, LPBF is economically more convenient than CNC. In fact, unless a single part must be produced, the total cost by CNC is systematically higher than by LPBF and becomes 85% higher for batch volumes larger than 12, where all the curves become substantially constant. Interestingly, this means that, although the aluminum bracket was redesigned according to a function-driven design strategy that gives priority to the part performance rather than to cost benefits, LPBF remains economically advantageous over CNC provided that more than a single part is produced [34]. For reorders, all costs are markedly inferior as compared to the first order. In this case, CNC is particularly advantageous for small batches, with a cost curve that is almost constant and that shows the break-even point with LPBF at four units.