# Construction of a Cutting-Tool Wear Prediction Model through Ensemble Learning
## 4 Results and Discussion
### Cutting Force  
Figure 13 shows the cutting force comparison both among different ultrasonic-assisted methods and without assistance for two sets of process parameters planned for the first phase of the experiment. It can be observed that in the first phase of the milling experiment, the cutting force with triple-axis ultrasonic assistance is the highest, while the cutting force with rotary ultrasonic assistance is significantly lower than with the other assistances. The cutting force without assistance is lower than that with dual-axis and triple-axis ultrasonic assistances. Therefore, in this phase, it is evident that rotary ultrasonic assistance can effectively reduce cutting forces. It is speculated that since dual-axis and triple-axis ultrasonic assistance methods include radial vibration, the material's high hardness and toughness prevent effective disruption of the material structure, leading to less effective material removal. Additionally, it increases the contact area and cutting load. In the case of triple-axis ultrasonic assistance, the radial vibration, unable to disrupt the workpiece material structure effectively, is combined with \(z\)-axis oscillation, increasing the contact area and cutting load compared to dual-axis ultrasonic assistance, resulting in an increase in cutting forces. On the other hand, rotary ultrasonic oscillation is along the \(z\)-axis, and its high-frequency oscillation can reduce the contact area in the cutting area. Simultaneously, it creates a vacuum region between the cutting tool and the workpiece material, facilitating air to enter the cutting area. This contributes to chip evacuation, reduced cutting forces, and lower cutting temperatures. Therefore, cutting forces with rotary ultrasonic assistance are the lowest among all the assistance methods. Compared to no assistance, rotary ultrasonic assistance can reduce cutting forces by approximately 16%.  
Figure 12: Surface roughness comparison between hybrid-assisted and rotary-assisted methods for process parameter combinations in L9 orthogonal array.  
Figure 13: Cutting force comparison among both different ultrasonic-assisted methods and without assistance for two sets of process parameters planned in the 1 st phase of the experiment.  
In this section, the laser locally preheating temperature is approximately 620 \({}^{\circ}\)C, which elevates the localized temperature of SKD11 to achieve material softening, resulting in mechanical properties that are distinct from those at room temperature, consequently reducing cutting forces. As shown in Figure 14, with the assistance of laser preheating, the hybrid assistance of laser, combined with rotary ultrasonic assistance, exhibits lower overall cutting forces compared to rotary ultrasonic assistance alone. In comparison to rotary ultrasonic assistance, the hybrid assistance of laser, combined with rotary ultrasonic assistance, can reduce cutting forces by up to 16%. Specifically, hybrid assistance demonstrates lower cutting forces at a spindle speed of 10,000 rpm, a radial depth of cut of 0.1 mm, and a feed rate of 1000 mm/min. Under fixed feed-rate conditions, cutting forces increase with the radial depth of cut. The trend of decreasing cutting forces is less apparent at a feed rate of 2000 mm/min, suggesting that an excessively rapid cutting feed rate may result in inadequate laser preheating due to the fixed laser irradiation point on the front edge of the cutting-tool and its movement with the worktable, causing insufficient preheating time and a less-noticeable reduction in cutting forces. Under fixed radial depth of cut conditions, cutting forces increase with the feed rate. It is inferred that a deeper radial depth of cut may lead to insufficient effective preheating depth by the laser, and the feed rate also affects the laser preheating temperature, resulting in inadequate preheating effects and, therefore, ineffective reduction in cutting forces.