## 3 Results and Discussion  
### Effects of Cutting Parameters on Cutting Forces in Micro-Milling  
Figures 4 and 5 present the graphs of the cutting forces Fx, Fy, and Fz generated during micro milling at cutting depths of 50 \(\upmu\)m and 100 \(\upmu\)m. An analysis of these figures reveals that an increase in cutting depth and feed rate leads to a significant rise in the cutting forces Fx and Fy. The increase in these two parameters leads to greater friction in cutter and the specimens during each rotation and feed movement compared to previous conditions. This increased contact subsequently causes a rise in cutting forces. Essentially, with the grow in feed and depth, the cutter removes more chip within the same time period, thus showing an increase in the press on the cutter. However, an inverse relationship is observed in micro-milling experiments, where an increase in spindle speed leads to a noticeable decrease in cutting forces. Under the same cutting conditions, coated carbide tools consistently generate higher cutting forces compared to other cutting tools [42]. A higher spindle speed implies that the cutter side of the endmill contacts the specimens at a higher velocity. This higher speed results in a rise in temperature in the machining zone, leading to the thermal softening of the specimen [43]. Heat through machining plays an important role in reducing cutting forces. This widely observed effect is particularly significant in micro-milling because of the extremely small size of the produced chips [30]. There exists an inverse relation between spindle speed and cutting forces; every increase in spindle speed causes a decline in cutting forces. However, the contact between the cutter and the specimen simultaneously causes a significant temperature rise at the tool tip [44]. Figure 4: Feed of 0.25–0.5–1–2 and depth of 50 \(\upmu\)m in cutting forces: (**a**) Fx, (**b**) Fy, and (**c**) Fz. Another notable finding in this study is that the Fx cutting forces observed during micro-milling at cutting depths of 50 \(\upmu\)m and 100 \(\upmu\)m are significantly much higher than the Fy cutter forces. The higher magnitude of Fx can be attributed to its alignment with the primary material removal direction. The Fx component is required to exceed the shear strength of the specimen during machining, thereby resulting in a greater force. In contrast, the Fy component, unlike the material removal operation, is primarily related to the contact between the cutter and the specimen, which explains its lower magnitude [45]. In Figure 6, an increase in feed rate and spindle speed typically leads to a reduction in the Fx/Fy ratio. For the experiments conducted at a cutting depth of 50 \(\upmu\)m, the maximum Fx/Fy ratio was 1.60, while the minimum ratio was 0.80 (Figure 6a). In contrast, for the tests performed at a cutting depth of 100 \(\upmu\)m, the maximum Fx/Fy ratio was 1.43, whereas the minimum ratio was 0.43 (Figure 6b). As the feed increased, the chip thickness also increased, with a corresponding rise in Fx and Fy. However, the Fx cutting force exhibited a more rapid increase compared to Fy. This phenomenon can be assigned to the fact that at higher feed rates, a greater portion of the workpiece is engaged per spindle revolution, necessitating a higher feed force. The highest cutting force was recorded at a cutting depth of 100 \(\upmu\)m, a feed rate of 1 \(\upmu\)m/tooth, and a spindle speed of 30,000 rpm. In contrast, the lowest cutting force values were observed at a cutting depth of 50 \(\upmu\)m, a feed rate of 0.25 \(\upmu\)m/tooth, and a spindle speed of 10,000 rpm. These insights offer practical guidance for selecting cutting parameters to minimize excessive forces, which can prolong tool life and improve product quality in industrial micro-milling setups. A different result was found in this paper. When examining the cutting forces at the feed rate parameter of Fz = 0.25 \(\upmu\)m/tooth in Figure 14, it was observed that spindle speed does not play a significant role.