# Design and Verification of a New Fixture for Machining of Porous Blocks for Medical CAD/CAM Systems
## 2 Methodology
of a total of 97,459 tetrahedral elements and 139,616 nodes (corresponding to an element size of approximately 1.08 mm), which will be used for the FEM analysis (Figure 3c). Based on a literature review [31; 32; 33], and according to [32], expected force applied to the bone block was 35 N, but considering the possible variations in machining conditions, this FEM analysis was conducted under three different static preloads of 10 N, 35 N and 50 N in order to test porous blocks stability during machining. The properties of synthetic bone block material were based on [31; 32], while other material properties were selected from the SOLIDWORKS database. The reason for choosing steel 316 as a material for steel plates was because it has found its use in medical industry for its good properties and resistance to corrosion. The mechanical properties of materials used for this FEM analysis are shown in Table 1.  
Third step involved manufacturing the fixture, based on its design. For this step, a rectangular 51 \(\times\) 41 mm shape and appropriate holes, according to the design, were machined on GU 600 (INDEX, Esslingen, Germany) CNC machine from a PEEK disc. PEEK disc was used because it is strong and resistant material with small weight and its ability to absorb loads (density: 1.52 g/mL, stress at yield: 110 MPa, tensile modulus 5100 MPa, tensile elongation at break: 5%, flexural strength: 178 MPa and flexural modulus: 4800 MPa). Two steel plates were also cut in required shape from 1 mm stainless steel sheet on a UNIJET PJ5 AX (PTV, Hostivice, Czech Republic) waterjet machine. Fourth step involved testing the fixture with the workpiece on a 5-axis CNC milling machine. A custom-designed 3D model of bone graft was selected as a test object (Figure 4). Bone grafts are used today as a bone substitute in oral surgery applications, and as such have a very complex and irregular shape, due to them being designed for each patient personally. And its machining, due to its complex shape, presents a challenge. Dental CAD/CAM system CC Power (INTERDENT, Celje, Slovenia) was used for machining of porous block equipped with MillBox CIM software. Appropriate machining \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Young Modulus E** & **Poisson Ratio** & **Density Q** & **Source** \\  & **(GPa)** & **m** & **(kg/m\({}^{3}\))** & **Source** \\ \hline Steel plate & & & & \\ (steel 316) & 192.9 & 0.27 & 8000 & * SW database \\ \hline Bone block & 13.7 & 0.3 & 480 & [31; 32] \\ \hline PEEK disc & 3.9 & 0.4 & 1310 & SW database \\ \hline Silicone tabs & 112.4 & 0.28 & 2330 & SW database \\ \hline \hline \end{tabular}
* SW—SOLIDWORKS.  
\end{table} 
Table 1: Properties of materials used for FEM analysis. Figure 4: Three-dimensional model of customized bone graft used for CAD/CAM machining. strategy using Strategy Editor within MillBox CIM software was selected. This involved defining several steps. First, appropriate CNC machine was selected, in this case CC power machine. Next, block holder was selected which will hold the designed fixture in place. Regarding the toolset selection, long tools were selected. For material that will be machined, and since porous block material was not available in the software database, PEEK was selected. Regarding the restoration type, since this is a filter to select among the several restoration types inside MillBox CIM software (crown, bridge, coping, inlay, and outlay), inlay was selected because bone grafts are not available in the software database, and inlay presents the freeform shape. A total of 6 operations were performed during machining. Cutter material, which is made of titanium, was used for all operations. External rough milling (on both sides) was performed using end mill with 2 mm ball tip diameter, external finish milling (both sides) was conducted using end mill with 1 mm ball tip diameter, and profile milling and connectors reduction were completed using end mill with 0.6 mm ball tip diameter.