**2.3. Tapping torque test**

Tapping torque tests (ASTM D5619) were carried out on a CNC machine, installed with a tapping torque set up, as shown in **Figure 1**. The tests were conducted using AISI 1215 cylindrical low carbon steels at the machining speed of 400 rpm as shown in **Table 1**. The workpieces

**2.4. Orthogonal cutting**

where *Fc*

Cutting speed, *vc*

Orthogonal turning process was conducted on an NC lathe machine (Alpha Harrison 400) to cut a steel disk of AISI 1045 plain medium carbon steel. The tool used was a square shape insert with a positive rake angle of 5°, clearance angle of 11°, and a model number of SPGN120308. The tool insert was fixed on a modified tool holder of CSDPN 2525 M12 in a way that the chip will flow freely without any hindrance on the rake surface of the cutting insert during the chip formation process. The steel disk has an initial diameter and width of 150 and 2 mm

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The cutting tool was fixed on a dynamometer, Kistler 9275BA in order to measure the cutting forces. The forces measured were amplified by using the Kistler 5070 amplifier and were recorded on a PC for data analysis. The lubricants were supplied via an MQL system directly to the cutting edge at the tool-workpiece interfaces. The air supply pressure was fixed at 4 bar and the flow rate was set at 0.16 l/hr. The MQL nozzle with an outlet diameter of 2.5 mm was located at a distance of 8 mm between the nozzle outlet and the tool-workpiece interfaces and inclined at 45° to the cutting edge plane. The maximum cutting temperature was captured by using an infrared thermal camera (FLIR T640) within a temperature range of 0–1000°C. The camera was located parallel to the axial direction of the lathe machine facing toward the cutting zone. The schematic diagram of the complete setup assembly on the lathe machine is shown in **Figure 2**. Each cutting test was conducted twice using each of the lubricant sample and new cutting edge to reduce measurement error during the results analysis. At the end of each cutting operation, the cutting forces were recorded and the chips were collected for cutting force evaluation and chip thickness measurement analysis respectively. The cutting force was determined in the Z-axis during the chip formation process. At least 10 chips were measured for their thickness by using a digital micrometer. The specific cutting energy, *U*, was calculated using Eq. (2),

*o*

*t*

MQL lubricants SE; MJO + hBN0.05%; MJO + PIL1; MRPO + hBN0.05%; MRPO + PIL1%

is the undeformed chip thickness [43].

*<sup>o</sup>* <sup>⋅</sup> *<sup>w</sup>* (2)

respectively. The complete cutting parameter is shown in **Table 2**.

is the cutting force, *w* is the cutting depth, and *t*

*<sup>U</sup>* <sup>=</sup> *<sup>F</sup>*\_\_\_\_\_*<sup>c</sup>*

(m/min) 350

**Table 2.** Cutting parameter of the orthogonal lathe machining.

**Description Value**

Feed, *f* (mm/rev) 0.12 Cutting depth, *w* (mm) 2 Disk diameter (mm) 150 Disk thickness, *d* (mm) 2

MQL supply pressure (MPa) 0.4 MQL flow rate (l/hr) 0.16 Nozzle inner diameter (mm) 2.5 Nozzle distance (mm) 8

**Figure 1.** Tapping torque set-up [18].

were predrilled with 5 mm diameter drill bit size. Tapping was performed using an uncoated high-speed steel tapping tool with the size of M6x1.0. The workpiece attached on the jig was later mounted on a dynamometer, Kistler 9345A. The dynamometer was amplified via a multichannel amplifier, Kistler 5070. Approximately, 20 ml of the lubricant sample was poured into the container to lubricate the tools during the tapping process. The result of the torque was recorded using the Dynoware software. Each tapping process was repeated five times for each lubricant sample, prior to averaging the torque values. The efficiency of tapping torque was calculated according to Eq. (1). Efficiency (%) <sup>=</sup> Average torque of reference oil \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ Average torque of lubricant sample (1)

$$\text{Efficiency (\%)} = \frac{\text{Average torque of reference oil}}{\text{Average torque of lubricant sample}} \tag{1}$$


**Table 1.** Tapping torque test parameter.

## **2.4. Orthogonal cutting**

were predrilled with 5 mm diameter drill bit size. Tapping was performed using an uncoated high-speed steel tapping tool with the size of M6x1.0. The workpiece attached on the jig was later mounted on a dynamometer, Kistler 9345A. The dynamometer was amplified via a multichannel amplifier, Kistler 5070. Approximately, 20 ml of the lubricant sample was poured into the container to lubricate the tools during the tapping process. The result of the torque was recorded using the Dynoware software. Each tapping process was repeated five times for each lubricant sample, prior to averaging the torque values. The efficiency of tapping torque

Efficiency (%) <sup>=</sup> Average torque of reference oil \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ Average torque of lubricant sample (1)

was calculated according to Eq. (1).

**Figure 1.** Tapping torque set-up [18].

50 Lubrication - Tribology, Lubricants and Additives

**Table 1.** Tapping torque test parameter.

Feed rate, *f r*

**Description Value** Spindle speed (rpm) 400

Lubricant volume (ml) 30

(mm/rev) 1 Hole depth (mm) 12 (through hole) Tapping tool High speed steel, M6

Workpiece material AISI 1215 steel Workpiece dimension (mm) Ø37×12

Orthogonal turning process was conducted on an NC lathe machine (Alpha Harrison 400) to cut a steel disk of AISI 1045 plain medium carbon steel. The tool used was a square shape insert with a positive rake angle of 5°, clearance angle of 11°, and a model number of SPGN120308. The tool insert was fixed on a modified tool holder of CSDPN 2525 M12 in a way that the chip will flow freely without any hindrance on the rake surface of the cutting insert during the chip formation process. The steel disk has an initial diameter and width of 150 and 2 mm respectively. The complete cutting parameter is shown in **Table 2**.

The cutting tool was fixed on a dynamometer, Kistler 9275BA in order to measure the cutting forces. The forces measured were amplified by using the Kistler 5070 amplifier and were recorded on a PC for data analysis. The lubricants were supplied via an MQL system directly to the cutting edge at the tool-workpiece interfaces. The air supply pressure was fixed at 4 bar and the flow rate was set at 0.16 l/hr. The MQL nozzle with an outlet diameter of 2.5 mm was located at a distance of 8 mm between the nozzle outlet and the tool-workpiece interfaces and inclined at 45° to the cutting edge plane. The maximum cutting temperature was captured by using an infrared thermal camera (FLIR T640) within a temperature range of 0–1000°C. The camera was located parallel to the axial direction of the lathe machine facing toward the cutting zone. The schematic diagram of the complete setup assembly on the lathe machine is shown in **Figure 2**.

Each cutting test was conducted twice using each of the lubricant sample and new cutting edge to reduce measurement error during the results analysis. At the end of each cutting operation, the cutting forces were recorded and the chips were collected for cutting force evaluation and chip thickness measurement analysis respectively. The cutting force was determined in the Z-axis during the chip formation process. At least 10 chips were measured for their thickness by using a digital micrometer. The specific cutting energy, *U*, was calculated using Eq. (2), where *Fc* is the cutting force, *w* is the cutting depth, and *t o* is the undeformed chip thickness [43].


$$
\mathbf{U} = \frac{F\_e}{t\_\circ \cdot w} \tag{2}
$$

**Table 2.** Cutting parameter of the orthogonal lathe machining.

**Figure 2.** Orthogonal lathe cutting set-up.

Finally, the sliding region on the tool insert's rake face was analyzed under an optical microscope and the tool-chip contact length was measured for data analysis. Scanning electron microscope (SEM) was used to further analyze the surface morphology of the sliding regions.

**3.2. Tapping torque performance**

**3.3. Orthogonal cutting performance**

**Figure 4** shows the tapping torque and efficiency for all lubricant samples. The reference oil (SE) had the highest tapping torque at 129 Nm. The results reveal that the tapping torque for all modified vegetable oils exceeded the tapping torque of SE. MJO + PIL1% had the lowest tapping torque of 104 Nm correlated with the highest tapping torque efficiency of 124%. The presence of PIL as an additive improves the tapping torque performance. This is because of the addition of PIL in MJO-based oil, which is thermally more stable than SE. The alkyl chain length and hydrogen bonding between the cation and the anion seem to influence the tribofilm formation of PIL [29]. Meanwhile, MJO + hBN0.05% recorded tapping torque of 117 Nm with the tapping torque efficiency of 110%. The presence of hBN particles provided a thin lubrication film that allows the particles to change from sliding friction to the rolling friction [18]. Moreover, the presence of long carbon chain length of MJO-based oil and MRPObased oil which is between 16 and 18 carbon number had enhanced the adsorption ability of the fatty acids on the metal surfaces, thus exhibited better tapping torque performance. MRPO + hBN0.05% and MRPO + PIL1% had tapping torque efficiency of 107 and 106%. It can be seen that the addition of PIL and hBN particles as the additive in MRPO-based oil did not significantly affect the tapping torque performance compared to the MJO-based oils. This scenario is due to the weak tribo-chemical reactions of additives with the MRPO-based oil, thus

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**Figure 3.** The kinematic viscosity values at 40 and 100°C and the calculated viscosity index value.

reduced the adsorption ability of the lubricant molecules on the metal surface [46].

The orthogonal lathe cutting operations were conducted at a constant speed and feed. The cutting force, cutting temperature, chip thickness, specific cutting energy, and tool-chip contact length are the main outputs of this experimental section analysis. The results for each lubricant mixture were compared with the conventional cutting fluid, synthetic ester (SE).
