# Design and Verification of a New Fixture for Machining of Porous Blocks for Medical CAD/CAM Systems
## 2 Methodology
Clamping of these porous blocks should be conducted in way that firmly clamps the required shape in place, while not using excessive force, in order to prevent any possible damage to them. With all these requirements and objectives in regard, Figure 2 shows the whole design process of the new fixture. Regarding the workpiece, a porous block of 50 \(\times\) 40 \(\times\) 10 mm and a standard disc of \(\O\)98.5 mm and thickness of 10 mm were used for this study (Figure 2a). From Figure 2b it can be seen that the design process involved a rectangular cut-out in the disc that would facilitate the porous blocks insertion. This inner shape depends on the porous block size, in this case, it is 51 \(\times\) 41 mm for the block used in this study. An edge clearance was incorporated into the design that ensures compatibility with specific block dimensions. A clearance of 0.5 mm on each side was left on the inner rectangular shape of the fixture body where the porous block would be inserted, which results in its dimensions being 51 \(\times\) 41 mm. This clearance was specifically added to address potential tolerances in the porous block's fabrication process, which can occur due to the inherent challenges of manufacturing highly porous materials. Regarding the further design, four 3 mm holes were drilled in disc that would allow fixation of two steel plates (1 mm thickness), which were designed as well with cut-outs in them, and with chamfered (10 \(\times\) 10 mm) inside edges (Figure 2c). These chamfered edges allow placement of silicone tabs (10 mm in diameter and 1.5 mm thickness), which will support the bone block fixation. Also, four M3 \(\times\) 18 mm pan head slotted screws were used to mound and fix the steel plates and the porous block between them. A total of eight silicone tabs were placed (four on each side) for porous block clamping, which will enable a much softer contact preventing any possible damage to the block during its locating and clamping in the fixture. Completely designed fixture is shown in Figure 2c. In the second step, FEM analysis was conducted in order to verify the fixture's design functionality where different stress loads are applied to the porous block. The focus of FEM analysis was on the behavior of porous block during its clamping and machining on CAD/CAM system, in order to verify its stability and fixed location during workpiece machining. FEM analysis was conducted on porous block workpiece for Von Mises stress, displacement, and strain under three different preloads of 10 N, 35 N, and 50 N, respectively. Regarding the predefined conditions for FEM analysis, entire fixture body (from PEEK material) has been defined as a fixed rigid body. Silicone tabs in contact with the porous block have defined pressure on the porous block body. Also, fixed geometry of the porous block has been defined on its surrounding surfaces, limiting its potential movement as well. External load, i.e., force, has been defined on top surface of the porous block (simulating the machining of bone graft in that region). Also, no penetration contact was defined for all components of the fixture (Figure 3a). In order to verify the accuracy of the FEM analysis, a mesh independence study was performed for the porous block. Figure 3b shows the plotted results of the mesh independence study used to select the optimal size of the FE model for the porous block. The conducted mesh convergence study confirms that the FEA model has converged to a solution, justifying mesh independence. Based on this, the mesh for the bone block consists  
Figure 3: Showing (**a**) predefined conditions for FEM analysis, (**b**) mesh convergence study, and (**c**) FE mesh of fixture with porous block. Figure 2: Fixture design process involving (**a**) initial disk, (**b**) cut-out of required shape, design of steel plate and assembly, and (**c**) cross-section of assembled designed fixture.