## 4 Discussion  
According to the obtained results, the null hypothesis was rejected, and a significant difference in surface roughness and milling duration was observed under different milling conditions. It was suggested that the milling duration could be reduced by changing the milling conditions while maintaining the surface properties. Clinically, the ability to fabricate dentures quickly while ensuring accuracy aids in providing superior patient care. It is significant that this fact was made clear this time. In this study, the surface roughness was measured using a surface roughness measuring machine to measure Ra. Other methods, such as atomic force microscopy and microscopes, can also be used to measure surface roughness. However, when considering the clinical tolerance of surface roughness on a complete denture, it was determined that measurement with a surface roughness measuring machine was appropriate. Moreover, although Ra was used in this study to account for the unevenness of the specimen surface, there are other parameters besides Ra, such as RMS, to measure surface roughness. The surface roughness was measured at several points on the specimen; further, we observed a difference in surface roughness at two points on the left and right sides of the specimen. First, on the left slopes, there was a slight trend toward an increase in surface roughness as the feed rate increased with the same spindle speed. At a spindle speed of 20,000 rpm, there was a significant difference in surface roughness between feed rates of 2000 and 3500 mm/min; however, at 40,000 rpm, surface roughness did not significantly differ among the feed rates. This suggests that surface roughness is easily affected by feed rate at low spindle speeds. At feed rates above 2500 mm/min, surface roughness did not significantly differ among different spindle speeds. However, at a feed rate of 2000 mm/min, the surface roughness was significantly lower at a spindle speed of 20,000 rpm than at a spindle speed of 40,000 rpm. This suggests that milling at lower spindle speeds is more likely to affect surface roughness. Ideally, the milling duration should be shortened while ensuring accuracy. However, surface roughness may increase as the milling conditions are changed to shorten the milling duration. Second, on the right slopes, the surface roughness tended to increase with an increase in the feed rate between 2000 mm/min and 3500 mm/min, which was similar to the left slope. When comparing spindle speeds, a significant decrease in surface roughness was observed as the spindle speed increased. One possible reason for the difference in the surface roughness variation according to spindle speed between the left and right slopes could be attributed to the difference in the processing direction. The present study applied scanning line milling, which involves milling from the right to the left end of the specimen. Therefore, on the right slope, the tool cuts down the slope, whereas on the left slope, it cuts up the slope. The main milling methods used are scanning line and contour line milling [24; 25]. In scanning line milling, the tool is moved in a fixed direction and the specimen is machined by moving the tool up and down along the target shape. In contrast, in contour milling, the tool is held at the same height and the target shape is milled by changing the height of the tool each time. Both methods are also used for finishing; furthermore, although scanning line milling is more accurate, contour milling is also useful depending on the milling conditions. The present study only used scanning line milling. Therefore, it is necessary to consider differences in milling methods, including changing the milling method for each processing area. This study was performed using a simple specimen in order to facilitate the evaluation of accuracy. Further studies on the processing methods for specimens with complex curved surfaces, including an actual complete denture, are warranted.