# Design for Additive Manufacturing: Tool Review and a Case Study
## 4 Results and Discussion. Case Study: A Bicycle Stem
This will be the starting point for the redesign of the part. The file was exported in STEP format, which allows its import into the Inspire(tm) tool of Altair(tm). This topology optimization tool is intuitive and useful for novel users and works with the same calculation engine as the HyperWorks tool from the same company, as well as the OptiStruct software solver. The first action for a TO approach consists of assigning the material from the library to the imported pieces and, as we have mentioned, it will be polyamide 6. It is important to define within the assembly which volumes will be used for optimization and which will not. We will refer to the optimizable volumes where the material removal happens \"design spaces\" and those that cannot be altered \"non-design spaces\", as illustrated in Figure 5. The volume of the design spaces will be increased as much as possible to allow the solver to identify the most suitable solution. The first step is to perform a static stress calculation according to the defined loads to verify that the part meets the objectives and, above all, that the parts have room for improvement for subsequent redesign. We make a stress simulation and observe the safety coefficient of each of the parts of the piece, as illustrated in Figure 6.  
Figure 4: Main dimensions of the bicycle stem designed in the software SolidWorks(tm). Figure 5: Definition of loads and constraints, as well as design and non-design spaces in the Inspire(tm) tool for the modified stem geometry. The result clearly expresses that the part is susceptible to optimization since the values of the safety factor greatly exceed a factor of 1.2 that we accept as the admissible limit. Once the need to reduce the mass of this part has been determined, we define a topological optimization that maintains the mentioned safety factor of 1.2, minimizing the mass and maximizing the rigidity of the part. The result will be a poorly defined organic geometry that we will reconstruct using NURBS surfaces that help us to generate a continuous and well-defined surface, as illustrated in Figure 7. This result will also be analyzed by a static calculation to validate the adopted design solution. With this approach to the redesign of the piece, a notable decrease in weight has been achieved, going from 46.36 g to 24.93 g, which mean a reduction of 53.8% in the optimized piece without losing functionality and according to the design specifications defined at the beginning. Again, this solution is validated by means of finite element analysis (FEA) tools. This solution not only will reduce costs and production times but can also mean important savings in energy consumption with the use of the redesigned part. This optimized model is intended to take a further step towards weight reduction. Using the lattice tools that have already been mentioned and, more precisely, Ntop software, we intend to further reduce the weight by replacing the continuous material of the part of the piece that can be optimized by generating a microstructure of trusses without losing mechanical properties and maintaining its resistance. It is important to emphasize that the tools used do not allow the validation of the results by means of calculation by finite elements. The simulation strategy should be based on a multiscale homogenization based on the analysis of a representative volume element. Therefore, the result presented below is merely illustrative of the redesign process for mesostructure gratings. Figure 6: Result of the finite elements analysis expressed as safety factors of the part using the InspireTM tool. Figure 7: Result of the topological optimization on the left, the middle image shows the highest stress concentration areas and, on the right, the smoothed surface using NURBS, carried out with Inspire software. The import/export of files is again done with STEP files. The lattice structure will be applied in the main bodies in a combination of a shell operation and the internal lattices.