# Design and Verification of a New Fixture for Machining of Porous Blocks for Medical CAD/CAM Systems
## 4 Discussion  
This study introduces and validates a novel fixture for machining fragile, biocompatible porous blocks, addressing a crucial challenge in the fabrication of medical and dental implants. The design integrates FEM analysis and experimental testing to ensure stability, accuracy, and material integrity during machining processes.  
The FEM results in this paper demonstrated that stress and displacement within the workpiece remained within safe limits under preload conditions, even at the maximum applied force of 50 N. These results align with earlier studies emphasizing the importance of force distribution to prevent microcracks in brittle porous materials, as highlighted by [35; 36]. Additionally, this study also supports [37], where FEM analysis was used to evaluate porous implants and underscored the importance of balancing structural stability and mechanical performance in biomedical applications.  
Compared to more conventional fixtures that rely on rigid clamping systems [8; 38], the proposed design effectively mitigates localized stress concentrations, which are known to compromise the material structure where clamping can also have an influence on geometric deviations as well [39]. Workpiece stability is of great importance during machining, and it depends on the clamping, especially when the porous materials are involved [40]. Functionally porous implants and their porous morphology, as discussed in [41; 42], demonstrate the critical role of porosity in influencing the mechanical behavior of such materials. Considering that the primary component of porous implants, when they are machined to be porous, is that their mechanical properties are substantially lower than that of bone, this makes them difficult to machine [43]. The designed fixture in this paper addresses these issues by offering a more uniform force application, thus minimizing the risks of workpiece breaking during machining.  
Experimental validation revealed minimal dimensional deviations (deviation range of \(\pm\)0.135 mm, +0.015 mm at peak) thus being compliant with other conducted research on dimensional accuracy in this field [44; 45; 46]. This contributed to improved dimensional accuracy, emphasizing the application of used materials and their properties in achieving precise machining outcomes through the use of PEEK for its mechanical strength, while the flexibility and damping properties of silicone align with findings in [47] where a rubber-based fixture was developed for machining of low-stiffness parts and demonstrates how the viscoelastic and elastoplastic behavior of rubber-like materials effectively mitigates vibrations, enhances dynamic stability, and minimizes machining-induced deformation. This is very important when handling fragile materials.  
Notably, the fixture's hybrid design, balancing rigidity and compliance, sets it apart from traditional clamping systems. Rigid systems, while offering stability, can often induce surface damage or deformation in porous biomaterials [48]. The current fixture design overcomes these limitations by distributing forces more evenly across the surface, a critical requirement when machining biocompatible materials [49]. The focus on biocompatible materials' machinability and the fixture's adaptability reflects advancements in biomedical manufacturing technologies.  
Also, while the clinical effectiveness of a customized 3D printed alloplastic bone material is still being studied, findings suggest that 3D printed customized bone graft materials could be used as an alternative for machining, but for more simple procedures, as it still needs further improvements [50,51].  
Overall, this study advances the machining of biocompatible materials by presenting a novel fixture design that integrates FEM analysis and experimental validation to address challenges in the manufacturing of porous materials. This fixture demonstrates significant potential for enhancing the precision and reliability of CAD/CAM systems in healthcare applications.