## 3 Results and Discussion
### Assessment of Cutting Noise  
In the recent progress of manufacturing research, environmental sustainability is highly essential toward the welfare of society and machinist. Cutting sound generation is one of the important indexes for attaining environment sustainability. The cutting sound directly affects the machinist's hearing capability as well as their efficiency. Louder sound during machining greatly affects the machinist's health and working capability. Therefore, controlling the intensity of sound in the course of cutting is greatly needed to control its adverse effects on the machinist. It has been observed that an increase in cutting force often leads to a corresponding increase in the noise level produced during the cutting process. The aforementioned factor, nevertheless, influences the energy consumption associated with the cutting operation. In the literature [32], it has been observed that during the cutting action, the presence of fixtures and mechanical parts can lead to the generation of noise. This noise is primarily caused by the mechanical disturbance of these components. Figure 16: Interaction effects of input variables on cutting temperature. \begin{table}
\begin{tabular}{l l l l l l l l} \hline \hline
**Source** & **DF** & **Seq-SS** & **Adj-MS** & **F** & **P** & **\% Impact** & **Significant** \\ \hline ap & 3 & 9389.6 & 3129.9 & 44.03 & 0.006 & 16.73 & Yes \\ s & 3 & 3591.9 & 1197.3 & 16.84 & 0.022 & 6.40 & Yes \\ v & 3 & 32123.1 & 10707.7 & 150.64 & 0.001 & 57.25 & Yes \\ q & 3 & 10788.5 & 3596.2 & 50.59 & 0.005 & 19.22 & Yes \\ Error & 3 & 213.2 & 71.1 & & & & \\ Total & 15 & 56106.3 & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: ANOVA findings for T.  
Moreover, it was noticed that an increase in cutting force often leads to a corresponding increase in the sound level produced during the cutting process. The control of the cutting noise is imperative to prevent potential harm to the auditory system of workers who are present during the machining process. Considering this aspect, the current research measured the cutting sound in every 10 s of one pass of machining and their average values were analyzed for each test. The highest and least cutting noise was obtained as 77.03 and 83.11 dB, respectively. According to the scientific report, the cutting noise should be smaller than 85 dB to prevent human hearing loss [36,37]. Therefore, as the maximum evolved sound (83.11 dB) was lowered to the safe limit of 85 dB, the current study can hence be suitable to achieve sustainable machining. Moreover, to control the noise emission, it is required to analyze the impact of the process variables on cutting noise. Therefore, main effects plot, interaction plots, and ANOVA were developed to analyze the impact of process variables on cutting noise. The cutting noise increased with increasing speed due to the vibration and speed associated in hard turned processes. When the cutting speed, feed rate, and depth of cut increase, the noise intensity increases, as displayed in the main effects plot (Figure 17). The depth of cut was aggressively affecting the cutting noise in comparison to other variables. Moreover, the cutting noise was slowing down when the coolant flow rate was improving. It might have possibly been due to a considerable reduction in friction by applying a higher volume of nano-lubricant in the shearing zone. Figure 18 shows the interaction plots' effects of different couples of input process parameters on cutting noise. According to the interaction graphs, a logical interaction between ap-s, s-q, and v-q was developed, representing a good amount of interaction among all factors. However, other interaction terms such as ap-v and ap-q graphs are mostly parallel, indicating the reduced interactions for cutting noise during nano-based MQL hard turning of bearing steel. Table 6 presents ANOVA analysis results for cutting noise. The cutting parameters with a \(p\)-value less than 0.05 are statistically significant [38].