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Performance Comparison of PCD Insert and Uncoated Insert in Novel CNC Green Machining of SS316L Stainless Steel for Maximizing Material Removal Rate

Aim: This research is about comparing the performance of poly-crystalline diamond (PCD) insert and uncoated cemented carbide insert for improving the material removal rate (MRR) in CNC turning. Materials and Methods: The material investigated in this research was SS316L. PCD insert was set as the experimental group and uncoated insert was set as the control group. The cutting parameters employed in this process were cutting speed, feed rate and depth of cut. Totally 27 samples per group were machined by using the selected parameters. Results: The mean MRR value obtained for the PCD tool is 0.22593 g/s, whereas it is 0.07600 g/s for the uncoated tool. The significance value obtained among the experimental group and control group is 0.000 (p<0.05). Conclusion: Within the limits of this study, the outcome of the research shows that MRR is more when the specimen is machined by PCD insert than machined by uncoated insert. Key-words: Green Machining, Material Removal Rate, PCD Insert, Uncoated Insert, SS316L Stainless Steel, Novel Machining.


Introduction
Turning is a basic conventional machining process in which a cutting tool is used to remove the material from the workpiece. In the seventies, lower productivity with less accuracy of components was the major problem to be rectified as far as the mass production is concerned.
Machining performed without the usage of coolants is termed as green machining, which is environmental friendly. CNC machines are used to carry out the machining with more surface finish (Nataraj and Balasubramanian 2017).
There are 5560 similar works of maximizing MRR in CNC turning available in google scholar and 1350 works available in science direct. A comparison study on the machining performance of PCD and PCBN inserts in dry turning titanium alloy were observed in the findings of (Ren et al. 2019). In that work, the researcher concluded that PCD tools have better performance than PCBN tools. Statistical analysis of different machining characteristics of EN 24 alloy steel during dry hard turning with multilayer coated cermet inserts were studied in the findings of (Das et al. 2019). In that work, the researcher analysed the effect of flank wear on surface roughness parameters, dimensional deviations and effect of MRR on different patterns of crater wear. It is found that the influence of coating material and cutting parameters on material removal rate and surface roughness in the turning process using the Taguchi method (Moganapriya et al. 2018). In that study, the author determined a predictive equation for determining MRR and surface roughness with a given set of parameters in CNC turning. A study on machining experiments of AISI 304L austenitic stainless steel with a PVD (Physical vapor deposition) coated cermet tool was carried out (Kaladhar, Subbaiah, and Rao 2011).
In that study, the researchers highlighted that the feed was the predominant parameter on the machined surface followed by nose radius. In addition, the depth of cut was found as the most important parameter influencing the MRR followed by feed. Among all these papers the best cited paper is: Influence of coating material and cutting parameters on surface roughness and material removal rate in turning process using taguchi method (Moganapriya et al. 2018).
Previously our team has a rich experience in working on various research projects across multiple disciplines (Sathish and Karthick 2020;  Based on all these studies it is found that lower productivity is the main criteria to be improved. Hence the aim of the study is to compare and evaluate the performance of PCD insert and uncoated insert for improving the MRR to increase the productivity.

Materials and Methods
The workpiece material considered for turning purpose was SS316L and the cutting tool insert used was PCD and uncoated carbide insert. The workpiece material was difficult to machine. Hence these two inserts were considered in this work. This study was carried out in the CNC turning centre available at Saveetha Industries, Saveetha School of Engineering (SSE), Saveetha Institute of Medical and Technical Sciences (SIMATS), Chennai. The total number of groups involved in this project was 2 (experimental group and control group). In this work, novel machining specimens with PCD insert was set as an experimental group and novel machining with uncoated cemented carbide insert used was set as control group. The sample size per group was 27. Clinical online sample size calculator was used to calculate the sample size with 80% g power. During the calculation, the mean value and standard deviation considered for the experimental work was 0.22 and 0.25, respectively (Ramana, Venkata Ramana, and Kumar 2018). SS 316L is more attractive material because of its properties such as high hardness, toughness, yield strength, excellent ductility, super resistance to corrosion and oxidation. SS316L stainless steel cylindrical rod was cut into the required dimension (20mm diameter and 55mm length) for conducting experiments. The cylindrical rods were obtained from Mehta metals, Chennai. Chemical composition of S316L is given in Table 1. In this work, the turning process was carried out by using a CNC turning centre (Super jobber) shown in Fig. 3 stock of the lathe and the insert was fixed in the tool holder which was fixed in the tool station.
Selected cutting parameters were cutting speed (m/min), feed rate (mm/rev) and the depth of cut (mm) (Frifita et al. 2020). The cutting parameters were set in CNC machines (Senthilkumar, Sudha, and Muthukumar 2015). Weight loss method was used for determining the MRR during machining.
The workpiece having dimensions of 20mm diameter and 55mm length was machined using PCD and uncoated carbide inserts and subsequently MRR was calculated. The MRR per unit time was noted for each experiment trial for both PCD insert and uncoated carbide insert. Cutting speed was set as 100 m/min, feed rate as 0.10mm/min with a depth of cut of 0.6 mm. Before the machining process, weight of the specimen was measured by using a weighing machine. A stopwatch was used to measure the time taken for material removal in seconds during machining and after machining the weight of the specimen was again measured. By using the difference in weight, MRR was calculated.

Statistical Analysis
SPSS v.26 statistical software was used to calculate mean, standard error and standard deviation. Totally, 27 experiments were carried out for analysis for each group so that 95% confidence level can be achieved with P value < 0.05. In this work the independent variables used are cutting speed, feed rate, depth of cut and the dependent variables is material removal rate (MRR).
Independent sample T-test was used to analyze the outputs to identify the significance among PCD and uncoated inserts.

Results
As per the machining setup and constant machining parameters, the calculated MRR values for both PCD and uncoated inserts are tabulated in Table 2.  The outcomes of the independent sample T test is shown in Table 4. It is learnt from the table that among the control group and experimental group a significant difference is obtained P=0.000 (P < 0.05). Bar chart (Fig. 4) shows the comparison of PCD insert and uncoated insert in terms of mean accuracy and standard deviation. PCD insert has better mean accuracy than uncoated insert and it also has slightly better standard deviation than uncoated insert.

Discussion
From the results obtained, the performance of PCD insert appears to perform significantly better than uncoated insert when material removal rate is concerned during machining SS316L. The fischer value obtained for this study is 29.564, with a significance value of 0.000 that is < 0.05 which shows that a significance difference exists between the two tools. Fig. 4 -Comparison of PCD insert and uncoated insert in terms of mean accuracy. The mean accuracy of PCD insert is better than uncoated insert and the standard deviation of PCD insert is slightly better than uncoated insert. X Axis: PCD insert vs uncoated insert. Y axis: Mean accuracy of detection + 1 SD From the bar chart (Fig. 4), it is clearly observed that higher MRR is obtained when the specimen is machined by PCD insert and MRR is lower when the same is machined by uncoated cemented carbide insert (Senthilkumar, Tamizharasan, and Anandakrishnan 2014). This is mainly due to the higher hardness and capability of PCD tool (Mashinini, Soni, and Gupta 2019 Limitations involved in this study is the high cost of the PCD insert and formation of the builtup chip due to high contact temperature. In future work, the same study of comparing coated and uncoated inserts for turning of SS316L can be tried by using low cost inserts which provide more MRR.

Conclusion
The turning was performed on SS316L using PCD and uncoated carbide inserts to compare the material removal rate using CNC machine using a set of cutting parameters. The effect of MRR is dependent on cutting parameters considered. From the results it is concluded that PCD insert provides higher MRR (0.22593 g/sec) than the uncoated carbide (0.07600 g/sec) insert during machining SS316L material.

Conflict of Interest
The authors of this paper declare no conflict of interest.

Authors Contribution
Author KBP was involved in data collection, data analysis and manuscript writing. Author CT was involved in conceptualization, data validation, and critical review of manuscript.