## 4 Discussion
### Surface Roughness  
Ti-3 Al-2.5V finds widespread application in the biomedical and aerospace fields, where stringent surface finish standards are imperative. Particularly in biomedical engineering,  
Figure 4: Burr width measurement. attaining minimal surface roughness in the micro-milling of titanium grade 9 is crucial. This not only diminishes the necessity for extensive post-milling procedures but also improves biocompatibility. Additionally, it extends the lifespan of devices by reducing friction and wear. Table 6 presents the results of the analysis of variance (ANOVA) for the surface roughness, evaluating the contributions and significance of different machining parameters. The contributions of three key parameters, namely, the feed rate (F) in \(\upmu\)m/tooth, cutting speed (Vc) in \(\upmu\)m, and depth of cut (\(\text{a}_{\text{p}}\)) in \(\upmu\)m, are highlighted. These parameters had a substantial impact on the machining process. The feed rate parameter contributed the most, at 62.96%, followed by Vc at 20.32% and ap at 9.27%. The statistical significance is indicated by the associated \(p\)-values, where lower values suggest higher significance. All three parameters were considered significant, as their \(p\)-values were less than 0.05, signifying their influence on the machining outcomes. Previous research in the field has identified key process variables crucial for achieving the desired surface finish in the micro-milling of titanium, which encompass tool coatings, cutting speed, tool edge radius, depth of cut, and feed rates [40; 41]. Current research, as detailed in Table 6, corroborates the significance of these factors in influencing surface roughness. Among these variables, feed rate exerts the most substantial impact, contributing 62% to the overall outcome, and cutting speed and depth of cut also play significant roles. Notably, the feed rate's influence is twice as pronounced as that of the other two factors. These factors demonstrated significance in the micro-milling of other titanium grades as well. These results emphasize the consistent importance of these parameters in optimizing surface quality in the micro-milling of titanium. Figure 5 presents the main effects plot for surface roughness, highlighting the major influencing factors. The plot clearly illustrates that surface roughness tends to increase with higher feed rates and cutting velocities. This increase in surface roughness linked to higher feed rates can be attributed to increased heat generation due to a higher rate of material removal, resulting in a rougher surface. Conversely, an increase in the depth of cut tends to reduce surface roughness. Additionally, higher feed rates can lead to a larger contact area between the tool and the workpiece, resulting in greater cutting forces and, subsequently, an increase in surface roughness. Surface roughness is also influenced by cutting speed. However, as cutting speed rises, the temperature also increases, potentially promoting the formation of a built-up edge (BUE). BUE formation becomes more likely with materials that have higher ductility due to increased friction and interference with continuous chip formation, ultimately leading to a less favorable surface finish. In the current research, a decreasing trend in surface roughness was observed for titanium grade 9 as the depth of cut increased. A plausible reason for this trend can be found in the literature, which suggests that deeper cuts may yield a better surface finish due to the work hardening of the surface layer. Additionally, cutting forces tend to decrease with an increase in specific cutting energy and cutting forces, while specific cutting forces decrease as the depth of cut increases. These reduced cutting forces result in less elastoplastic deformation and ultimately lead to improved surface roughness. The standard deviation for surface roughness is displayed in Figure 6.