# Design and Verification of a New Fixture for Machining of Porous Blocks for Medical CAD/CAM Systems
## 2 Methodology  
For machining on dental 5-axis CNC machines, today exists a standard size disc with fixed diameter of \(\mathcal{O}\)98.5 mm, and a thickness varying from 10 mm up to 25 mm. The discs are made from various different materials such as titanium alloy, zirconia, chrome cobalt, polymethyl methacrylate (PMMA), polyether ether ketone (PEEK), and wax, which are used for various purposes. Based on these materials, tool cuts out the desired shape while the disc is placed inside a CNC machine holder. However, porous bone blocks do not come in this size, since their machining would not be feasible if they are made as large circular discs. The reason for this is that they would be structurally unstable and break due to their large shape, size, and fragility. Additionally, the size would lead to inefficiencies in material utilization, as large portions of the disc area would remain unused. Today, porous blocks come in rectangular and cubical shapes, depending on their use case. Porous block from SmartBone(r) (Industrie Biomediche Insubri SA, Mezzovico-Vira, Switzerland) was used in this study. It is a bovine bone-derived mineral matrix with resorbable biopolymers and cell nutrients that presents a chemical structure and a morphology resembling human bone. With its 27% open porous microstructure, it mimics the human bone. However, its physical properties have a rigid but not elastic structure, which makes it fragile and thus difficult for machining operations [31]. In that regard, the idea for fixture design was developed in order to use the standard disc shape, as a holder for rectangular porous block placed inside it, where it would be located and clamped for machining on CAD/CAM system. Depending on the size of the porous block from different manufacturers, multiple discs can be prefabricated and equipped with clamping fixtures facilitating this design, in order to expand their versatility. Figure 1 shows the basic workflow of the whole process. It consists of several key steps, and those are fixture design, FEM analysis, fixture manufacturing, fixture testing, and dimensional verification. In the first step, based on the shape of the porous block, a novel fixture was designed from a circular disk, in order to be able to locate and clamp the required shape. For this purpose, SOLIDWORKS v2018 (Dassault Systemes) software was used. Key design  
Figure 1: Workflow for the whole process. features, such as clamping mechanisms and support structures, were modeled to ensure proper alignment and interaction between components. When designing a fixture, several objectives and requirements must be addressed to ensure its functionality, efficiency, and suitability for the intended application. Ensuring that fixture maintains correct positioning and alignment of the workpiece throughout the operation minimizes dimensional and geometric errors. Also, it needs to provide sufficient structural strength to resist deflection, vibrations, and any forces that occur during workpiece machining. It needs to accommodate easy tool access while clamping mechanism does not obstruct machining process. By meeting all these objectives and requirements, a fixture can significantly enhance process efficiency, accuracy, and product quality. However, it should be noted that specific quantitative thresholds for some design objectives, such as allowable deflection or clamping force, could not be established due to the absence of standardized benchmarks for fixtures designed to handle porous materials. Instead, the validation of these objectives was carried out through FEM analysis and qualitative assessment, ensuring the fixture meets practical and functional requirements for machining delicate porous blocks. With all this in regard, when developing the design concept of fixture for clamping and positioning of porous blocks, special attention was paid to its simplicity and ease of placing and removing blocks from the fixture while providing uninterrupted tool access, and without any obstructions during machining process.