## 2 Materials and Methods
### Milling the Specimens  
The cutting paths for the specimens were output and the specimens were milled. Milling was performed using three types of tools: a 6 mm diameter square end mill (AE-TS-N 6 \(\times\) 18; OSG CORPORATION, Aichi, Japan), a 3 mm diameter square end mill (AE-TL-N 3 \(\times\) 15; OSG CORPORATION, Aichi, Japan), and a 2 mm diameter ball end mill (WXL-LN-EBD R1 \(\times\) 20 \(\times\) 4; OSG CORPORATION, Aichi, Japan). The milling was performed in the order listed in Table 1 and as follows: roughing using the 6 mm diameter tool, medium roughing using the 3 mm diameter tool, medium roughing using the 2 mm diameter tool, and finishing using the 2 mm diameter tool. These milling conditions were based on the manufacturer's guidelines [22]. To evaluate the surface roughness of specimens used in this study, we adjusted parameters that affect the surface properties during the finishing stage using the 2 mm diameter tool. Specifically, we set three different spindle speeds (20,000,  
Figure 1: Designed specimen and locations of surface roughness measurement on the specimen. 30,000, and 40,000 rpm) and four different feed rates (2000, 2500, 3000, and 3500 mm/min) for the finishing stage; accordingly, 12 combinations were created, and we examined the accuracy of the specimen and milling duration for each combination. Two PMMA disks were used for each combination of spindle speed and feed rate; accordingly, six specimens were milled for each combination (Figure 3). The NC data were imported into the MD-500 operating application (md-gear; Canon Electronics Inc., Saitama, Japan). Three types of tools and a PMMA disk were placed in a predetermined position on the milling machine and the specimens were milled. Each of the three types of tools was replaced for each combination in order to ensure that the wear and tear of the tool, as well as other factors, would not affect the milling conditions. Overall, 12 sets of the three types of tools were used. The milled specimens were separated from the disk to allow distinction between the left and right sides and stored. For the distinction between left and right, the specimens' sides were marked before they were separated from the disk to avoid any possibility of error. The sample size was calculated using an analysis software program (G*Power 3.1.9.7; Kiel University, Kiel, Germany). Based on a previous study [16], the sample size required to investigate the accuracy and milling duration for one combination of spindle speed and feed rate was determined to be six. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Milling Process** & **Tools** & **Spindle Speed** & **Feed Rate** \\  & & **(rpm)** & **(mm/min)** \\ \hline Rough (Front) & \(\Phi\)6 (AE-TS-N 6 \(\times\) 18) & 16,000 & 1300 \\ Rough (Back) & \(\Phi\)6 (AE-TS-N 6 \(\times\) 18) & 16,000 & 1300 \\ Rest rough (Front) & \(\Phi\)3 (AE-TL-N 3 \(\times\) 15) & 18,000 & 1200 \\ Rest rough (Back) & \(\Phi\)3 (AE-TL-N 3 \(\times\) 15) & 18,000 & 1200 \\ Rest rough 2 (Front) & \(\Phi\)2 (WXL-LN-EBD R1 \(\times\) 20 \(\times\) 4) & 26,000 & 2300 \\ Rest rough 2 (Back) & \(\Phi\)2 (WXL-LN-EBD R1 \(\times\) 20 \(\times\) 4) & 26,000 & 2300 \\  & & & 20,000 & 2000 \\ Fine (Front) & \(\Phi\)2 (WXL-LN-EBD R1 \(\times\) 20 \(\times\) 4) & 30,000 & 3000 \\  & & 40,000 & 3500 \\  & & 20,000 & 2000 \\ Fine (Back) & \(\Phi\)2 (WXL-LN-EBD R1 \(\times\) 20 \(\times\) 4) & 20,000 & 2500 \\  & & 40,000 & 3000 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Milling conditions used in this study, including the list of tools, spindle speeds, and feed rates. Figure 2: Locations of the specimens on the CAM software (hyperDENT V9; FOLLOW-MEI Technology Group). The milling process was performed in the order mentioned in Table 1. Tools corresponding to each step are shown in the table. Finishing steps were made at a combination of spindle speeds of 20,000, 30,000, and 40,000 rpm and feed rates of 2000, 2500, 3000, and 3500 mm/min.