State-of-the-Art and Future Perspectives of Environmentally Friendly Machining Using Biodegradable Cutting Fluids

: The use of cutting ﬂuids has played a vital role in machining operations in lubrication and cooling. Most cutting ﬂuids are mineral oil-based products that are hazardous to the environment and the worker, cause severe diseases and pollute the environment. In addition, petroleum resources are becoming increasingly unsustainable. Due to environmental and health issues, legislations have been established to ensure that the consumption of mineral oil is reduced. Consequently, researchers are making efforts to replace these mineral oil-based products. Vegetable oils are grasping attention due to their better lubricating properties, ease of availability, biodegradability, low prices, and non-toxicity. In this study, a detailed review and critical analysis are conducted of the research works involving vegetable oils as cutting ﬂuids keeping in view the shortcomings and possible solutions to overcome these drawbacks. The purpose of the review is to emphasise the beneﬁts of vegetable oil-based cutting ﬂuids exhibiting comparable performance to that of mineral oil-based products. In addition, an appropriate selection of non-edible vegetable oil-based cutting ﬂuids along with optimum cutting parameters to avoid a scanty supply of edible oils is also discussed. According to this research, vegetable oils are capable of substituting synthetic cutting ﬂuids, and this option might aid in the successful and cost-efﬁcient implementation of green machining. Nanoﬂuids was and The performance of was examined based on force, torque, R a and friction co-efﬁcient. Results showed that at 1.5 wt.% of nanographene, the values of thrust force, torque, R a , and friction co-efﬁcient were reduced by 27.4%, 64.9%, 33.8% and 51.7%. The addition of an adequate proportion of nanographene stabilized the lubrication ﬁlm, effectively boosting the machining performance.


Introduction
The shearing action between tool and workpiece during machining results in chip formation, which generates high heat at the contact surface. This heat increases the cutting zone temperature, which needs to be reduced; otherwise, this could cause rapid tool wear, degrade surface integrity and reduce dimensional accuracy [1,2]. This poor surface finish and tool wear lead to low productivity. The increased cutting force required due to the non-removal of the chip also increases energy consumption. Thus, to reduce this friction as Figure 1. Comparison of the environmental burden of dry, flooded, and minimum quantity lubrication (MQL) [23].
The MQL method is the most used alternative to flooding. Reduction in power consumption and cutting fluid amounts can be made with the help of this technique. In MQL, a mixture of oil with pressurised air is injected into the cutting region. Oil provides lubrication, and cooling is obtained by the air. The compressed air exposure helps in removing away the chips as well. Thus, minimum surface roughness (Ra) and tool wear are obtained. Additionally, the cost of cutting fluid as well the disposal is reduced due to less cutting fluid use [25]. However, the performance of the MQL system is limited at higher cutting speeds as a small portion of the cutting fluid can reach the cutting zone. This factor leads to less lubrication in comparison to the flooding technique [26]. This uncertainty in lubrication ability forces manufacturers to use the flooding method having more quantity of disposal [27].
Petroleum reservoirs are limited to some specific countries of the world [28]. Because of their high demand, the prices of petroleum products are taking a toll. Hence manufacturers have to invest more in purchasing these cutting oils and invest further to dump them carefully and sustainably. These factors have increased the unit cost of products [29,30]. This literature review aims to look into the usage of vegetable oils to replace the mineral oil-based cutting fluids and minimise mineral oil usage in the major manufacturing processes. This approach will help industries to manufacture products sustainably.
Cutting fluid should primarily offer adequate lubrication and cooling. Vegetable oils as neat oils and in the form of emulsion have been tested as cutting fluids to ensure a green and sustainable manufacturing system [18,[31][32][33]. Vegetable oils are mainly composed of triglycerides which consist of glycol molecules that have three long-chain fatty acids [34][35][36][37][38][39]. These fatty acids are attached to the hydroxyl group with the aid of ester linkages [40][41][42]. The triglycerides structure provides desirable lubricating properties in an effective manner [38,43,44]. Wear and friction are reported to be lowered through the generation of the lubricating film between the surfaces in contact [45][46][47][48][49]. This film is generated due to longer and polar chains of fatty acids [14,50]. Additionally, vegetable oils are easily available, cost-effective, less toxic, and eco-friendly [42,[51][52][53].
A large portion of cutting fluids needs to be disposed of, which contribute to environmental pollution by contaminating soils, rivers, and underground water reservoirs The MQL method is the most used alternative to flooding. Reduction in power consumption and cutting fluid amounts can be made with the help of this technique. In MQL, a mixture of oil with pressurised air is injected into the cutting region. Oil provides lubrication, and cooling is obtained by the air. The compressed air exposure helps in removing away the chips as well. Thus, minimum surface roughness (R a ) and tool wear are obtained. Additionally, the cost of cutting fluid as well the disposal is reduced due to less cutting fluid use [25]. However, the performance of the MQL system is limited at higher cutting speeds as a small portion of the cutting fluid can reach the cutting zone. This factor leads to less lubrication in comparison to the flooding technique [26]. This uncertainty in lubrication ability forces manufacturers to use the flooding method having more quantity of disposal [27].
Petroleum reservoirs are limited to some specific countries of the world [28]. Because of their high demand, the prices of petroleum products are taking a toll. Hence manufacturers have to invest more in purchasing these cutting oils and invest further to dump them carefully and sustainably. These factors have increased the unit cost of products [29,30]. This literature review aims to look into the usage of vegetable oils to replace the mineral oilbased cutting fluids and minimise mineral oil usage in the major manufacturing processes. This approach will help industries to manufacture products sustainably.
Cutting fluid should primarily offer adequate lubrication and cooling. Vegetable oils as neat oils and in the form of emulsion have been tested as cutting fluids to ensure a green and sustainable manufacturing system [18,[31][32][33]. Vegetable oils are mainly composed of triglycerides which consist of glycol molecules that have three long-chain fatty acids [34][35][36][37][38][39]. These fatty acids are attached to the hydroxyl group with the aid of ester linkages [40][41][42]. The triglycerides structure provides desirable lubricating properties in an effective manner [38,43,44]. Wear and friction are reported to be lowered through the generation of the lubricating film between the surfaces in contact [45][46][47][48][49]. This film is generated due to longer and polar chains of fatty acids [14,50]. Additionally, vegetable oils are easily available, cost-effective, less toxic, and eco-friendly [42,[51][52][53].
A large portion of cutting fluids needs to be disposed of, which contribute to environmental pollution by contaminating soils, rivers, and underground water reservoirs [54]. Due to these environmental and health hazards, legislations have been established to [54]. Due to these environmental and health hazards, legislations have been established to minimise the usage of these cutting fluids to ensure environmental sustainability [19]. The suitability of a substance concerning environmental protection is examined by the Global Warming Potential (GWP) [55,56]. US Environmental Protection Agency defines GWP as it is a measure of how much energy the emissions of 1 ton of a gas will absorb over a given period of time, relative to the emissions of 1 ton of carbon dioxide (CO2) [52]. Climate changes are noticed with variations in GWP [57]. In Figure 2, the GWP of some oils is shown, which depicts mineral oils have 45 times negative impact on the environment compared to CO2 [52]. For vegetable oils, it ranges between 5-15 times showing them to be more degradable. The above-mentioned characteristics have increased the trend of using vegetable oil-based cutting fluids [58][59][60]. It is necessary to select novel, innovative and sustainable cutting operations to provide an excellent surface finish, which will cause a reduction in cost and increase productivity [61][62][63]. Several reviews have been published on this topic during the last decade. For example, R. et al. [64] recently published a review article that focuses on the accomplishment of sustainable green manufacturing using several vegetable oil-based metal cutting fluids. This article analyses the economic, technical, and environmental liabilities of these green fluids in association with machining operations. Debnath et al. [9] reviewed the then developments of vegetable oil-based metal cutting fluids and their performances in machining. Nagendramma and Kaul [65] attempted to highlight the then-current status of ecofriendly/biodegradable lubricants for the formulation of new generation lubricants. The present article attempts to present the state-of-the-art of understanding of sustainable machining using vegetable-oil based cutting fluids for turning, drilling and milling operations. It also provides a guideline for industries to ensure green manufacturing through sustainable cutting fluid use.

Cutting Fluids
Cutting fluids (CFs) are delivered to the chip production zone gravitationally or under suitable pressure to increase cutting performance through cooling and/or lubrication effects [66]. In general, these effects are proportional to the quantity of heat produced. There are four primary types of CFs, according to popular belief [11,67]: 1. Mineral oils that are used undiluted are known as straight or neat oils. They do, however, commonly contain additional lubricants such as fats, vegetable oils, esters, and extreme-pressure compounds based on chlorine, sulphur, and phosphorus to in- Several reviews have been published on this topic during the last decade. For example, R. et al. [64] recently published a review article that focuses on the accomplishment of sustainable green manufacturing using several vegetable oil-based metal cutting fluids. This article analyses the economic, technical, and environmental liabilities of these green fluids in association with machining operations. Debnath et al. [9] reviewed the then developments of vegetable oil-based metal cutting fluids and their performances in machining. Nagendramma and Kaul [65] attempted to highlight the then-current status of eco-friendly/biodegradable lubricants for the formulation of new generation lubricants. The present article attempts to present the state-of-the-art of understanding of sustainable machining using vegetable-oil based cutting fluids for turning, drilling and milling operations. It also provides a guideline for industries to ensure green manufacturing through sustainable cutting fluid use.

Cutting Fluids
Cutting fluids (CFs) are delivered to the chip production zone gravitationally or under suitable pressure to increase cutting performance through cooling and/or lubrication effects [66]. In general, these effects are proportional to the quantity of heat produced. There are four primary types of CFs, according to popular belief [11,67]: Mineral oils that are used undiluted are known as straight or neat oils. They do, however, commonly contain additional lubricants such as fats, vegetable oils, esters, and extreme-pressure compounds based on chlorine, sulphur, and phosphorus to increase lubricity, and other agents to improve wear and corrosion resistance, dura- Mineral-soluble oils (emulsions) are composed of oil and emulsifiers, which allow the oil to disperse in water prior to use. These oils are frequently used in diluted form, with concentrations ranging from 3-15 per cent. The amount of emulsifier employed determines the size of the oil droplets, which typically ranges between 1 and 10 µm [68]. Although the high concentration of water (up to 99 per cent) provides good cooling, wet machined components are prone to corrosion. 3.
Semi-synthetic fluids (or micro-emulsions) are a mixture of synthetic and soluble oil fluids with features of both types of substances. Natural and manufactured emulsifiers, oil droplets, and transparent emulsions comprise them. They offer high corrosion resistance, lubrication, and contaminant tolerance.

4.
Synthetic fluids are mineral oil-free solutions manufactured from alkaline inorganic and organic compounds, which are often laced with corrosion inhibitors. They are diluted with water prior to use, with concentrations ranging from 3% to 10%. These fluids typically provide the best cooling performance, according to industry standards.
Degradation of cutting fluid is a key factor when we must dispose of the used CF. A large portion of cutting oils needs to be disposed of after their usage. After being dumped, the cutting fluids contribute to environmental pollution by contaminating soils, rivers, and underground water reservoirs, ultimately contaminating the crops being harvested in such agricultural areas [54]. Apart from being harmful to the environment, these cutting fluids directly affect the health of workers in machine shops. Asthma, cough, and other infections are dominantly found in the operators working in such environments. Even skin cancer and lung cancer are also reported in workers [69]. Due to these environmental and health hazards, legislations have been established to minimise the usage of these cutting fluids to ensure environmental sustainability [19]. Another significant characteristic of CFs is their lubrication, which helps to lubricate the contact regions between the rake face and chips, the flank face and the machined surface, and reduces adhesion and abrasion at low cutting speeds [13]. As a lubricant, CF decreases friction and, as a result, the heat generated by friction. Lubricant qualities in cutting fluid aid in reducing rake-face friction and increasing shear angle in low-speed sliding friction cutting [70]. As a result, thinner and more tightly curled chips are produced, decreasing the heat generated in the shear zone and cutting power consumption [71].
There are a number of other considerations to weigh when selecting a cutting fluid [72]:

Edible Vegetable Oils as Cutting Fluids for Turning Operation
The turning operation has been conducted by many researchers using edible vegetable oils, and performance was compared with the mineral oils or dry turning. Coconut oil is an abundant vegetable oil worldwide, and it has less viscosity compared to other vegetable oils. Coconut oil was tested as a biodegradable cutting fluid in a study, and a performance comparison was made with a commercial mineral oil [73]. EN8 Steel was used as a workpiece, and machining was executed on a CNC lathe at various FR, DOC, and V C where high V C , medium FR, and medium DOC were recommended as optimum cutting parameters after machining, as shown in Table 1. The minimum R a value obtained using Coconut oil was 0.93 µm compared to 1.37 µm of mineral oil. In addition, the cutting force was also minimised to 180 N using Coconut oil compared to the 200 N value of mineral oil. Less fluidity and the high saturated fatty acid ratio of Coconut oil enabled it to be a good cutting fluid compared to commercial mineral oil.
Properties of vegetable oils as cutting fluids can be enhanced by the addition of nanoparticles of highly conductive materials [74,75]. This increases the thermal conductivity of the cutting fluid to remove the heat of cutting. A research work comprised of adding nano boric acid particles (0.25% by weight) in the Coconut oil, and its performance as cutting fluid was equated with drying cutting and conventional cutting fluid [76]. Machining was conducted on AISI 1040 Steel with the help of a precision lathe machine and at constant FR, DOC, and V C . The conventional cutting fluid and nanofluid reduced cutting forces by 8.5% and 14.5%, respectively, when compared with the dry cutting environment. In the same manner, R a values were reduced to 24.24% and 12.19%. Tool wear value for dry cutting was 0.16 mm, whereas, for conventional cutting fluid and nanofluid, the values were 0.15 mm and 0.14 mm. As far as cutting temperature was concerned, its values came out to be as high as 280 • C, 200 • C and 150 • C for dry, nano-enhanced, and conventional fluid environments. So, the addition of nanoparticles enhances the performance of cutting fluids to a great extent.
Soybean and Cottonseed oils are abundant vegetable oils worldwide. In another research, Soybean and Cottonseed oils were used as cutting fluids [77]. Their performance was compared with Servocut oil while turning AISI D2 Steel using the CNC lathe machine. Flooded lubrication technique was used, and turning was performed by Ti-Al-N coated carbide tool at different FR, DOC, and V C . The lowest FR and moderate values of V C and DOC mentioned in Table 1 proved to be the best for surface quality. A minimum R a of 1.2 µm was obtained using Cottonseed oil, whereas R a value for commercial Servocut oil was 1.62 µm. This indicates that an excellent surface finish is obtained using Cottonseed oil.
Canola oil has been tested as a cutting fluid as well [14]. A study collated the performance of Canola oil and commercial synthetic oil to promote environment-friendly turning operations [78]. A conventional lathe machine was used for turning with a cemented carbide tool for two specimens, one specimen of Mild Steel and the other of Carbon Steel. The specimens were machined at different speeds, but the highest speed proved to be optimum, as indicated in Table 1. R a for Mild Steel was 3.08 µm in the case of synthetic oil compared to 2.99 µm of Canola oil. Likewise, R a value for Carbon Steel specimens was 4.856 µm using commercial oil and 4.697 µm with the help of Canola oil. The same trend was observed for tool wear where tool wear for Mild Steel with the help of Canola oil-based cutting fluid was 5.276 mm, and for synthetic oil, the tool wear came out to be 3.987 mm. The Carbon Steel tool wear value in the case of synthetic oil was 13.6112 mm compared to the Canola oil value of 9.2743 mm. Thus, canola oil-based cutting fluid can help in refining surface finish and reducing the tool wear. As discussed previously, the MQL technique is often used to minimise the use of cutting fluids [79,80]. In one research work, the turning operation was conducted on Titanium alloy using a polycrystalline diamond tool to optimise various parameters resulting in a decrease in R a [81]. Palm oil was used for MQL, whereas for flooding, 5% water emulsion of Vasco 1000 (phenol-free, chlorine-free) was used, and machining was conducted at FR, DOC, and V C showed in Table 1. High speed and low values of FR and DOC produced minimum R a values. Analysis of R a values indicated that using MQL with vegetable oil was the best approach to reduce R a . The combination of Palm oil with MQL provided R a value of 0.89 µm. For flooding using commercial cutting fluid, R a value was 0.99 µm and for dry condition R a value was 1.75 µm. Hence using MQL in combination with Palm oil based cutting fluid can be an effective and environmentally friendly perspective.
Just like many other edible oils, Groundnut oil is easily available and non-toxic vegetable oil. Researchers employed Groundnut oil as a cutting fluid while turning Stainless Steel, and a performance comparison was made with the commercial water-soluble mineral oil [82]. Machining was conducted on a manual lathe using an HSS tool at various V C , FR, and DOC, whose values are indicated in Table 1. R a was chosen as the performance evaluation criteria. The minimum R a value of 3.84 µm was obtained using Groundnut oil compared to 9.21 µm of mineral oil. The soluble oils have excellent cooling capacities but having good lubricating properties. Groundnut oil can be used as an effective cutting fluid.
Another research was conducted using Groundnut oil-based cutting fluid while comparing its performance with Cottonseed oil-based cutting fluid [83]. The turning operation was performed on a lathe machine using EN8 steel specimens. Both the oils were used in two different concentrations, i.e., 10% and 20% concentrations of oils in 90% and 80% water. So, four different cutting environments were tested to find out the best option. Turning was performed at V C, and DOC showed in Table 1. Temperature variations using Groundnut oil and Cottonseed oils are shown in Figure 3 clearly indicating lower temperature values in the case of 80% water and 20% oil.
Energies 2021, 14, x FOR PEER REVIEW in a decrease in Ra [81]. Palm oil was used for MQL, whereas for flooding, 5% water sion of Vasco 1000 (phenol-free, chlorine-free) was used, and machining was cond at FR, DOC, and VC showed in Table 1. High speed and low values of FR and DO duced minimum Ra values. Analysis of Ra values indicated that using MQL with veg oil was the best approach to reduce Ra. The combination of Palm oil with MQL pro Ra value of 0.89 μm. For flooding using commercial cutting fluid, Ra value was 0. and for dry condition Ra value was 1.75 μm. Hence using MQL in combination with oil based cutting fluid can be an effective and environmentally friendly perspective Just like many other edible oils, Groundnut oil is easily available and non-tox etable oil. Researchers employed Groundnut oil as a cutting fluid while turning St Steel, and a performance comparison was made with the commercial water-solubl eral oil [82]. Machining was conducted on a manual lathe using an HSS tool at vario FR, and DOC, whose values are indicated in Table 1. Ra was chosen as the perfor evaluation criteria. The minimum Ra value of 3.84 μm was obtained using Ground compared to 9.21 μm of mineral oil. The soluble oils have excellent cooling capacit having good lubricating properties. Groundnut oil can be used as an effective c fluid.
Another research was conducted using Groundnut oil-based cutting fluid whil paring its performance with Cottonseed oil-based cutting fluid [83]. The turning ope was performed on a lathe machine using EN8 steel specimens. Both the oils were u two different concentrations, i.e., 10% and 20% concentrations of oils in 90% and 80 ter. So, four different cutting environments were tested to find out the best option. ing was performed at VC, and DOC showed in Table 1. Temperature variations Groundnut oil and Cottonseed oils are shown in Figure 3 clearly indicating lower te ature values in the case of 80% water and 20% oil. Further its can be seen that Groundnut oil performed better than Cottonseed o relation between VC and DOC with Ra is shown in Figure 4. Further its can be seen that Groundnut oil performed better than Cottonseed oil. The relation between V C and DOC with R a is shown in Figure 4.  The figure indicates the recommendation of high VC with low DOC to achieve an excellent surface finish. So, to obtain the best results, Groundnut oil with a 20% concentration in water must be used as cutting fluid, and turning must be conducted at high speed and low DOC to generate minimum Ra.
When only lubrication is the major requirement of the cutting process, pure oil can be used as a cutting fluid. However, when both cooling and lubrication are needed, emulsions are used, which assists in cutting down the consumption of a fluid by mixing the oil in water. From an environmental perspective, the consumption of oil is reduced, and less amount of oil needs to be disposed of. Additionally, if emulsions of edible oils are used, less edible oil consumption helps food security. Emulsions can be prepared by mixing a small quantity of oil in water and stabilising the emulsion with additives. Hence both cooling and lubrication effects are obtained with small consumption of cutting oil [84]. Vegetable oil emulsions along with proper additives are an effective choice as cutting fluids so that consumption of oils is minimised. False Walnut oil, Groundnut oil, and conventional mineral oil emulsions were used as cutting fluids in a study [85]. Emulsions of oil in water were prepared using additives (Tween 80 as an emulsifier, Butyl Hydro toluene BHT as an antioxidant, banana sap as a corrosion inhibitor, biocide, and distilled water). Turning was conducted on annealed AISI 1330 alloy Steel using HSS as cutting insert, and various FR, DOC, and VC showed in Table 1 were used. High VC, medium FR, and low DOC proved to be the best cutting parameters. Groundnut oil provided a Ra value of 5.05 μm compared to 5.11 μm of mineral oil. This research indicates the importance of vegetable oils as cutting fluids in the form of an emulsion. Maximum Ra was noted in the case of mineral oil (1.46 μm).
Emulsions of some other vegetable oils have been used as cutting fluids as well. Experiments were conducted using non-toxic Peanut, and Palm oil emulsions and performance were compared with commercial Servo-cut oil [86]. All three emulsions were prepared using 95% water, 4% oil and 1% additives. The turning operation was performed on a high-speed CNC machine at different cutting parameters shown in Table 1 and the above-mentioned cutting conditions. The performance was evaluated based on temperature and Ra. Variation in Ra under the three emulsions is shown in Figure 5. The figure indicates the recommendation of high V C with low DOC to achieve an excellent surface finish. So, to obtain the best results, Groundnut oil with a 20% concentration in water must be used as cutting fluid, and turning must be conducted at high speed and low DOC to generate minimum R a .
When only lubrication is the major requirement of the cutting process, pure oil can be used as a cutting fluid. However, when both cooling and lubrication are needed, emulsions are used, which assists in cutting down the consumption of a fluid by mixing the oil in water. From an environmental perspective, the consumption of oil is reduced, and less amount of oil needs to be disposed of. Additionally, if emulsions of edible oils are used, less edible oil consumption helps food security. Emulsions can be prepared by mixing a small quantity of oil in water and stabilising the emulsion with additives. Hence both cooling and lubrication effects are obtained with small consumption of cutting oil [84]. Vegetable oil emulsions along with proper additives are an effective choice as cutting fluids so that consumption of oils is minimised. False Walnut oil, Groundnut oil, and conventional mineral oil emulsions were used as cutting fluids in a study [85]. Emulsions of oil in water were prepared using additives (Tween 80 as an emulsifier, Butyl Hydro toluene BHT as an antioxidant, banana sap as a corrosion inhibitor, biocide, and distilled water). Turning was conducted on annealed AISI 1330 alloy Steel using HSS as cutting insert, and various FR, DOC, and V C showed in Table 1 were used. High V C , medium FR, and low DOC proved to be the best cutting parameters. Groundnut oil provided a R a value of 5.05 µm compared to 5.11 µm of mineral oil. This research indicates the importance of vegetable oils as cutting fluids in the form of an emulsion. Maximum R a was noted in the case of mineral oil (1.46 µm).
Emulsions of some other vegetable oils have been used as cutting fluids as well. Experiments were conducted using non-toxic Peanut, and Palm oil emulsions and performance were compared with commercial Servo-cut oil [86]. All three emulsions were prepared using 95% water, 4% oil and 1% additives. The turning operation was performed on a high-speed CNC machine at different cutting parameters shown in Table 1 and the abovementioned cutting conditions. The performance was evaluated based on temperature and R a . Variation in R a under the three emulsions is shown in Figure 5. Similarly, temperature variation is shown in Figure 6. It is evident from the Figures that Peanut oil is not a good cutting fluid in comparison to Servo-cut and Palm oil. Servocut performed far better in temperature reduction, and Palm oil proved best for minimizing Ra. The performance of vegetable oils in their virgin and emulsion states may differ from each other. So, research work was conducted to compare the performance of two vegetable oils in their emulsion and virgin states [87]. Palm oil and Coconut oils in their virgin and emulsion states were utilized as cutting fluids. Emulsions of both the vegetable oils were prepared in the same proportions (i.e., 32% oil, 32% surfactant and 36% water). Turning operation for Titanium Alloy (TC4) was conducted on OPTIMUM lathe machine under the mentioned cutting conditions and varying cutting parameters shown in Table 1. Tool wear, temperature, and chip reduction coefficient were considered as output parameters. The tool wear using virgin Palm oil and emulsified Palm oil was reported to be 122.37 μm and 323.33 μm, respectively. Whereas for virgin Coconut oil and emulsified Coconut oil, the tool wear was found out to be 517.7 μm and 1046.3 μm. The optimum Similarly, temperature variation is shown in Figure 6. It is evident from the Figures that Peanut oil is not a good cutting fluid in comparison to Servo-cut and Palm oil. Servo-cut performed far better in temperature reduction, and Palm oil proved best for minimizing R a . Similarly, temperature variation is shown in Figure 6. It is evident from the Figures that Peanut oil is not a good cutting fluid in comparison to Servo-cut and Palm oil. Servocut performed far better in temperature reduction, and Palm oil proved best for minimizing Ra. The performance of vegetable oils in their virgin and emulsion states may differ from each other. So, research work was conducted to compare the performance of two vegetable oils in their emulsion and virgin states [87]. Palm oil and Coconut oils in their virgin and emulsion states were utilized as cutting fluids. Emulsions of both the vegetable oils were prepared in the same proportions (i.e., 32% oil, 32% surfactant and 36% water). Turning operation for Titanium Alloy (TC4) was conducted on OPTIMUM lathe machine under the mentioned cutting conditions and varying cutting parameters shown in Table 1. Tool wear, temperature, and chip reduction coefficient were considered as output parameters. The tool wear using virgin Palm oil and emulsified Palm oil was reported to be 122.37 μm and 323.33 μm, respectively. Whereas for virgin Coconut oil and emulsified Coconut oil, the tool wear was found out to be 517.7 μm and 1046.3 μm. The optimum The performance of vegetable oils in their virgin and emulsion states may differ from each other. So, research work was conducted to compare the performance of two vegetable oils in their emulsion and virgin states [87]. Palm oil and Coconut oils in their virgin and emulsion states were utilized as cutting fluids. Emulsions of both the vegetable oils were prepared in the same proportions (i.e., 32% oil, 32% surfactant and 36% water). Turning operation for Titanium Alloy (TC4) was conducted on OPTIMUM lathe machine under the mentioned cutting conditions and varying cutting parameters shown in Table 1. Tool wear, temperature, and chip reduction coefficient were considered as output parameters. The tool wear using virgin Palm oil and emulsified Palm oil was reported to be 122.37 µm and 323.33 µm, respectively. Whereas for virgin Coconut oil and emulsified Coconut oil, the tool wear was found out to be 517.7 µm and 1046.3 µm. The optimum conditions for minimum cutting temperature (32.2 • C) were V C = 90 rpm, FR = 0.10 mm and DOC = 0.3 mm using virgin Palm oil as cutting fluid. As far as chip reduction co-efficient is concerned, its minimum value of 1.2 was attained at optimum conditions of V C = 215 rpm, 0.25 mm/rev, and 0.9 mm DOC with the help of pure Palm oil as cutting fluid. So, it can be concluded that emulsions of both the oils proved better as cutting fluid and out of the two oils Palm oil has the best performance.
Another research selected emulsions of Melon Seed oil and Beniseed oil as CFs and compared performance to the mineral oil while machining on a conventional lathe machine [88]. The turning operation was conducted on AISI 304 L Steel using a tungsten carbide tool. The V C , FR, and DOC (values are shown in Table 1) were used as the input parameters. To evaluate the performance of the oils as CFs, surface roughness, and temperature values were measured. High values of speed and medium DOC were recommended as the best input parameters for cutting and their values are mentioned in Table 1. Maximum R a was noted in the case of mineral oil (1.46 µm). Using Beniseed oil provided R a value as 1.50 µm and a value of 1.46 µm was obtained for mineral oil. Likewise, for temperature, a minimum temperature of 37.9 • C was in the case of Melon Seed oil. On the other hand, for Beniseed oil and mineral oils, the values of temperatures were 43.4 • C and 46.3 • C, respectively. The fatty acid composition along with properties like flashpoints, density, etc. enables emulsions of vegetable oils to be used as CFs.

Non-Edible Vegetable Oils as Cutting Fluids for Turning Operation
Non-edible vegetable oils are an excellent choice as cutting fluids because their usage does not harm the food chain, which may be a drawback in the case of edible oils. Jatropha and Pongamia are non-edible, easily available, and biodegradable vegetable oils. A study was carried out while consuming Jatropha and Pongamia oils as straight cutting fluids after epoxidizing (to boost oxidative stability) them, and their performance was compared with commercially available mineral oil-based cutting fluids [89]. The turning process was performed on an AA 6061 workpiece using the HSS tool at different FR, DOC, and V C , out of which less DOC, FR, and elevated speeds were optimum, as mentioned in Table 2. The value of roughness using Jatropha was as low as 0.7 µm, and for Pongamia oil, it was up to 0.8 µm. On the contrary, the R a value was more than 1 µm in the case of mineral oils. Not only less R a values were obtained using vegetable-based cutting fluids, but additionally, a reduction in cutting forces was observed. The minimum cutting force for mineral oil was 465 N, and for Jatropha, the value was 235 N. On the other hand, Pongamia oil reduced the cutting force to a minimum, i.e., 100 N. Jatropha is highly viscous oil in its crude state, so it does not flow easily. Researchers utilised the trans-esterification process and added 0.05% by weight of boron nitride in Jatropha oil to increase its performance as a cutting fluid [90]. A CNC lathe machine was used in experimentation to machine AISI 1045 specimens using a commercial cutting fluid, Jatropha oil as cutting fluid, and modified Jatropha oil. MQL technique was adopted to transport the cutting fluid to the hot zone, and turning operation was conducted under constant cutting conditions shown in Table 2. Cutting force was minimised to 375 N by using the modified Jatropha oil (with 0.05% boron nitride) compared to simple Jatropha Oil (425 N) and commercial cutting oil (475 N). In the same manner, a reduction in cutting zone temperature was observed where the maximum values of temperature for commercial oil, Jatropha oil, and boron nitride enhanced Jatropha oil came out to be 235 • C, 225 • C, and 215 • C. This research emphasised that non-edible oils have all the desirable characteristics to be used as cutting fluids after some chemical modifications.
The effectiveness of Jatropha oil-based cutting fluids has been tested in another experimental work [91]. Emulsion of Jatropha oil in water by 1:9 was prepared to be used as cutting fluid using emulsifier and additives shown in Table 2. Properties of Jatropha oil are enhanced by the addition of anti-corrosive and anti-oxidation additives. The turning operation was conducted on a lathe machine using the Tungsten Carbide tool at different V C , DOC and FR values shown in Table 2. To examine the performance, tool wear and R a were considered as output parameters. The results indicated that the minimum R a in the case of Jatropha based cutting fluid was 0.56 µm in comparison to 1.40 µm of mineral oil clearly indicating dominating performance of Jatropha oil. As far as tool wear was concerned, minimum wear in the case of Jatropha was 0.09 µm when compared with 0.07 µm of mineral oil. So, it can be pointed out that mineral oil performed a little better in the case of tool wear. But if we consider surface quality and biodegradability factors as well, Jatropha emulsion performs exceptionally well.
The properties of nanoparticles of a single material can be limited to a certain extent. These properties can be enhanced by adding nanoparticles of different materials to the base fluid. A combination of nanoparticles of different materials in a liquid is called a hybrid nanofluid. A hybrid nanofluid combines the physical and chemical properties of two or more nanofluids [92,93]. Research was carried out in which Carbon Nano Tubes (CNT)/Molybdenum Di Sulfide (MoS 2 ) nanoparticles were added by (0.5, 1, 1.5, 2, 2.5, 3) wt.% with a hybrid ratio of 1:2 in Sesame oil [94]. The formulated vegetable oil was emulsified with the aid of Sodium Dodecyl Sulfate. The performance of these hybrid nanofluids as cutting fluid was compared with the conventional cutting environment. Turning operation on AISI 1040 Steel was conducted on a lathe machine at constant cutting parameters shown in Table 2. Cutting force, tool wear, R a and temperature were determined to find out the best cutting environment. The results revealed the best performance in terms of reduction in cutting force and R a at 2 wt.% of hybrid nano cutting fluid. Cutting force was reduced by 32% and 27.3% in comparison to dry cutting and conventional cutting fluid. At the same time, R a values were reduced to 28.5% and 18.3%, respectively. As far as cutting temperature and tool wear were concerned, 3 wt.% of hybrid nano cutting fluid performed superiorly. Cutting temperature was reduced by 43.4% and 28% in comparison to dry and conventional cutting fluid. Further tool wear reduction was by 81.3% and 75%. Hence other hybrid nano cutting fluids can opt as good alternatives to conventional cutting fluid. Neem oil is non-edible vegetable oil with anti-bacterial properties. So, research work conducted using neem oil as a cutting fluid and performance were compared with commercial cutting oils. The performance comparison was based on temperature reduction criteria [95]. The oils were used as cutting fluids while turning Mild Steel at different V C and FR showed in Table 2. Additionally, the optimum values of V C and FR are mentioned in Table 2, which indicated less cutting zone temperature at low V C and FR. Results indicated that cutting zone temperature was most effectively lowered down (up to 39.53 • C) using Neem oil. For the soluble and straight oils cutting zone temperatures were as low as 40.51 • C and 43.21 • C, respectively. Consequently, Neem oil reducing cutting forces and being non-edible and anti-bacterial is an excellent choice for usage as a biodegradable cutting fluid.
Jojoba oil is a non-edible oil having preference over other vegetable oils due to its more oxidative stability. A study used Jojoba oil based nano cutting fluid to compare its performance with the commercial mineral oil for the turning operation of Ti-6Al-4V alloy [96]. The mineral oil used was LRT 30, and cutting fluids were used in their pure form as well as by adding MoS 2 nanoparticles. Constant FR and V C values shown in Table 2 were adopted. Machining was performed under dry conditions, using the MQL technique (for pure fluids of both LRT 30 and Jojoba oil) and MQL technique with MoS 2 enhanced nanofluids. Nanofluids were prepared for both the oils with 0.1%, 0.9%, and 0.5% by weight concentrations of MoS 2 . The nanofluids were homogenised using 0.1% by weight of lauryl sodium sulphate as an emulsifier and magnetic stirring. A lathe machine was used for turning operation selecting PVD coated carbide tool. The concentration of MoS 2 by 0.1% of weight both in mineral and Jojoba oils produced the best results for all the output parameters. A maximum cutting force of 112 N was used in dry cutting, whereas for Jojoba, a minimum cutting force of 73.62 N was needed comparing to that of LRT30 nanofluid (76.4 N). Similarly, for R a , a minimum value (0.1358 µm) was obtained in the case of Jojoba nanofluid compared to LRT 30 nanofluid (0.1407 µm) and dry cutting (0.2289 µm). A similar tendency was noticed for tool wear, where minimum tool wear (0.1 mm) was obtained in the case of nano Jojoba oil. For nano mineral oil, the tool wear was 0.14 mm, and for dry cutting, the value was 0.19 mm. The high value of viscosity index and the long-chain fatty acid structure of Jojoba oil help it to be more effective and provide better results as cutting fluids. Further, at 0.1% concentration, more particles of MoS 2 provide a high value of thermal conductivity.
Gingelly is a natural and biodegradable oil of sticky nature to provide better lubrication and heat transfer. In previous research, Gingelly oil was selected as a cutting fluid to compare its effectiveness with the mineral cutting oil and dry cutting [97]. The turning operation was performed on a medium-duty lathe using AISI 1014 Steel as a workpiece under constant FR and V C shown in Table 2. Cutting forces and R a values were measured in all three cutting environments. The minimum cutting force of 243 N was required in the case of Gingelly oil. On the other hand, for mineral oil-based cutting fluid and dry cutting, required cutting forces were 258 N and 285 N. Similarly, a minimum R a value of 0.86 µm was obtained in the case of Gingelly oil compared to 1.3 µm and 1.1 µm for mineral oil-based and dry cutting, respectively. Bio-based oil usage as cutting fluid is environment friendly as well as energy-efficient.

Edible Vegetable Oils as Cutting Fluids for Drilling Operation
Like turning, many researchers conducted drilling operations using edible vegetable oils, and performance comparison was made with dry cutting or commercial cutting fluids. Sunflower oil has a triglyceride structure, and its non-volatile nature can opt as a cutting fluid. So, research was carried out using Sunflower oil as a cutting fluid. AISI 321 SS square blocks were drilled using a drilling machine of 5 KW power, and fresh HSS drills were used for each experiment [98]. FR and V C and drilled hole depth values are shown in Table 3. Cutting fluid was supplied to the cutting area using MQL of pure Sunflower oil and nanoparticles enhanced Sunflower oil. Nanofluids of vegetable oil was prepared by adding 0.5%, 1% and 1.5% by wt. of nanographene. The performance of both MQL conditions was examined based on thrust force, torque, R a and friction co-efficient. Results showed that at 1.5 wt.% of nanographene, the values of thrust force, torque, R a , and friction co-efficient were reduced by 27.4%, 64.9%, 33.8% and 51.7%. The addition of an adequate proportion of nanographene stabilized the lubrication film, effectively boosting the machining performance. Palm oil is an edible vegetable oil having a rich concentration of unsaturated fatty acids of the carbon chain. In a study, drilling of Inconel 718 was conducted using Palm oil as cutting fluid, and its performance was compared with mineral oil-based cutting conditions and air blowing [99]. Palm oil was supplied through MQL, and commercial oil was used in MQL as well as flooding situations. Drilling was performed using Palm oil (MQL), commercial oil (MQL), commercial oil (flooding), and air blowing using different V C and FR values mentioned in Table 3. High values of V C and FR cause more workpiece temperature. The performance of these drilling conditions was investigated based on thrust force and workpiece temperature. Thrust force values for Palm oil, commercial oil MQL, flooding, and air blowing condition were 2638 N, 2886 N, 3443 N, and 3699 N, and correspondingly workpiece temperatures were 125 • C for both MQL conditions, 135 • C for flooding, and 183 • C for air blowing. Workpiece temperature and thrust force reduction make it possible to use Palm oil as an effective cutting fluid through the MQL technique.
Pongamia oil was used by the MQL technique to compare its performance with commercial mineral oil. Researchers conducted drilling of AISI 316 Steel using Pongamia and mineral oil, and an HSS drill was used to drill holes [100]. Machining was performed at various speeds and depths of holes shown in Table 3. At maximum speed, both cutting force and R a came out minimum. The value of cutting torque using Pongamia oil was reduced up to 36%, while thrust force and roughness were additionally 34% less in comparison to the commercial MO. Moreover, continuous chips were formed when vegetable oil was used to ensure that Pongamia is additionally a good alternative to commercial mineral oil.
Emulsions of vegetable oils in water can be prepared to minimise the cost of cutting fluid. A study comprised of drilling experiments using Coconut and Peanut oils with water and drilling parameters was optimised for difficult-to-drill Aluminium-based metal matrix composites using vegetable oil-based cutting fluid [101]. The cutting fluid used was C40P10W50, where C is the weight percent of Coconut oil, P is the weight percent of Peanut oil, and W is the weight percent of water. A4140 was used as an emulsifier to promote the mixing of oils and water. V C and FR showed in Table 3 were the input parameters used at three different levels, whereas cutting force and torque were the output parameters. Less cutting speed and low FR proved to be optimum for machining. The performance of the formulated cutting fluid was compared with the dry drilling process. Vegetable-based cutting fluid reduced cutting force to 94.87 N compared to 120.6 N of dry drilling. Similarly, torque was minimised to 0.72 N-mm, which was 2.81 N-mm in case of dry cutting. Significant improvement in the drilling process can be attained with the help of a combination of vegetable oils as cutting fluids.
Another research for drilling was carried out using Coconut oil-based green cutting fluid, and its performance was compared with the Commercial cutting fluid [102]. Concentrated green cutting fluid was prepared using non-toxic emulsifiers (40%), natural additives (20%) and pure Coconut oil (40%). The composition of emulsifiers and additives is shown in Table 3. Five best cutting fluids available in the market were chosen, and out of them, the best cutting fluid was selected to be compared with the formulated green cutting fluid. The drilling operation was conducted on an HMT FN2 drilling machine using HSS drills. The machining was conducted on EN8 Mild Steel at FR and V C showed in Table 3. In the case of torque, green cutting fluid showed comparable performance to green cutting fluid. Furthermore, higher speeds caused less torque, and at higher FR more torque was noticed. Additionally, green cutting fluid usage required low cutting forces in comparison to commercial cutting fluid. For a sustainable manufacturing operation, cutting fluid toxicity and biodegradability must be focused. As far as current regulations are concerned, they take only machining into consideration, neglecting vital factors of biodegradable ingredient selection and toxicity tests. These vital factors contribute to the overall quality of a product.
Just like turning, Soyabean oil has additionally been used as a drilling cutting fluid. In a research work drilling operation was performed on Al 6063 alloy using a CNC 3axis drilling machine [103]. Drilling was conducted in dry, flooded, pure MQL, and nanoparticles enhanced MQL environments utilising Soyabean oil as biodegradable cutting fluid at different speeds shown in Table 3. For drilling 10 holes at a speed of 30 m/min under dry condition, the R a value was 2 µm, for flooded cutting environment 1.65 µm, for MQL 1.6 µm, and for nanofluid MQL, R a value was 1.3 µm. Nanoparticles cause a reduction in roughness by depositing on the surface.

Non-Edible Vegetable Oils as Cutting Fluids for Drilling Operation
Many non-edible oils like Neem, Karanja, Mahua, and Honge have been tested as cutting fluids for drilling operations. Neem and Mahua oils contain more than 40% mono-saturated content that exhibits them to provide lubricity, and they have good lowtemperature properties. Researchers conducted research in which Neem and Mahua oils were used as cutting fluids (epoxidation was conducted to improve oxidative stability), and their performance was compared with that of commercial Servocut-945 [104]. Cutting fluid was supplied through the MQL technique while drilling AISI 304 L Stainless Steel at a constant depth of the hole, and different FR and V C showed in Table 4, where higher speed provided optimum results. Thrust force was minimum when using Mahua oil (351.54 N), and for Neem Oil the thrust force value was 489.45 N compared to the Servo cut cutting fluid environment (674.87 N). In the same way, R a values for Mahua, Neem, and MOs were 2.10 µm, 2.21 µm, and 2.89 µm, respectively. As far as the temperature was concerned its values for mineral, Neem, and Mahua oils were 83.45 • C, 71.46 • C, and 72.65 • C. Vegetable oils, due to their stable thin film formation ability, provide better lubrication, thus ensuring a better surface finish. Blends of oils help in minimising the usage of specific oil to avoid shortage. In another research, experiments were performed in which Mild Steel was drilled on a CNC machine with different cutting fluid environments [105]. Cutting fluids used were petroleum-based SAE 20W40 oil, Karanja oil, Neem oil, and different blends of Neem and Karanja oils. The values of FR, DOC, and V C were kept constant and are exhibited in Table 4. The suitability of these cutting fluids was established based on chip formation, surface finish, and cutting forces. A blend of 50% Karanja and 50% Neem oil proved to be the best of all, providing longer and continuous chips along with less cutting forces and an excellent surface finish. Minimum cutting force (151 N) was required when using a blend of 50% Neem and 50% Karanja whereas maximum cutting force was required for dry cutting (251 N) and for MO-based cutting (301 N) was required. The same was the case for R a values which were 4.1 µm, 3.5 µm, and 1.25 µm for dry cutting, mineral oil-based cutting fluid, and 50% blend of Karanja with 50% Neem Oil, respectively.
Neem oil performs exceptionally well in the form of blends with other oils. Hence another research work was conducted in which drilling was performed on a CNC machine using SAE 20W40 oil, Honge oil, Neem oil, and different blends of Neem and Honge oil [106]. This time performance of cutting fluid was examined based on temperatures of tool and workpiece at constant cutting conditions shown in Table 4. At the same time, the hardness test and R a tests were additionally the performance evaluation criteria. Again, a blend of 50% Honge and 50% Neem oil provided the best results as cutting fluid. Tool and workpiece temperature (40 • C) was minimum using a blend of 50% Honge and 50% Neem oil when equated to dry cutting temperatures (60 • C). This combination outperformed other combinations by providing a minimum R a value of 1.1 µm. Surface hardness test before and after drilling was additionally conducted, which concluded that the difference of surface hardness was minimum when using 50% Honge Oil and 50% Neem oil blend. Consequently, blends of different vegetable oils in proper proportions can prove to be effective as cutting fluids.
Eco-friendly Neem oil was used in another drilling operation by emulsifying it with water [107]. The used additives and surfactants are shown in Table 4. The performance of Neem Oil based cutting fluid for drilling was compared with the conventional cutting fluid in the market. Workpieces of Al grade T6061 were used for drilling using HSS drill at constant FR, and different speeds are shown in Table 4. The cutting force, torque and surface finish were selected to compare the performance of Neem oil-based cutting fluid with commercial fluid. Drilling was conducted under five different cutting environments, i.e., dry, deionized water, Koolkut commercial cutting fluid, Neem oil emulsion and Neem oil emulsion with ionic liquid. After conducting experiments, the results showed that minimum axial force (332.01 N) and torque (1.03 N-m) were observed at a medium drilling speed of 710 rpm, and an 11.7% reduction in R a was observed using an emulsion of Neem oil with ionic liquid. So, a conclusion can be drawn that Neem oil emulsion with ionic liquid as an additive can enhance the performance of vegetable oil-based cutting fluid.
Castor oil extracted from Castor seeds consists of a mixture of triglycerides enabling it to be used as a lubricating cutting fluid. So, researchers conducted Castor oil assisted drilling operation on rectangular blocks of Al 6063 alloy [108]. The drilling operation was conducted on a manual drilling machine at constant FR, and various speeds are shown in Table 4. An uncoated HSS drill was used, and drilling was conducted under dry, flooded and MQL based cutting environments. Considering thrust force, torque and R a as output parameters, vegetable oil-based cutting fluids provided the best results in terms of all the output parameters. The friction coefficient was reduced due to MQL causing convenient sliding of chips over the tool surface. As a result, a reduction in R a is observed.
Non-edible Rapeseed oil with low cost and oil-enriched seeds is one of the largest vegetable oil resources. Consequently, research work was conducted by researchers using Rapeseed oil as a cutting fluid for drilling of AISI 304 SS [109]. Performance of vegetable oil-based cutting fluid and nanoparticles enhanced vegetable oil-based cutting fluid was compared with Boron oil. Nanoparticles of silver were used for the preparation of nano cutting fluid (15% of silver particles were added in 85% of Boron oil or vegetable oil). MQL was adopted in both cases, and drilling was conducted on rectangular prisms at high as well as low speeds and feeds shown in Table 4. R a and cutting forces were selected as performance evaluators. At low speed and low feed, minimum roughness of 1.64 µm was observed using nano cutting fluid. Whereas minimum cutting force of 928 N was consumed with pure Boron oil and the addition of silver nanoparticles in vegetable oil required a maximum cutting force of 1140 N. Further at high V C and feed a minimum R a of 1.36 µm was noted by using silver nanoparticles in vegetable oil compared to all other conditions. As far as cutting force was concerned minimum cutting force of 1146 N was observed in the case of silver enhanced vegetable oil. So the best option for machining is to use high speed and feed using silver nanoparticles enhanced vegetable oil as cutting fluid.

Edible Vegetable Oils as Cutting Fluids for Milling Operation
Many researchers utilised edible vegetable oils as cutting fluids during milling processes. Mostly flood cooling technique is used while machining alloys. Researchers performed a side milling process on Carbon Steel with the help of Coconut oil and conventional mineral oil as cutting fluid [110]. Then comparison was made, keeping surface quality in view. Machining was performed using a conventional vertical milling machine at various V C , FR, and cutting depths displayed in Table 5. Moderate values of feed, speed, and DOC provided optimum results. Additionally, the results indicate R a of 2.565 µm in the case of Coconut oil and 2.978 µm for mineral oil using the flood cooling technique. Coconut oil being nontoxic and oxidatively stable is an excellent choice as vegetable-based cutting oil. Just like turning and drilling, Palm oil has been used as cutting fluid by researchers due to its non-toxicity. A research was conducted in which Palm oil was used as a cutting fluid both in flooding conditions and MQL method [111]. Dry milling was additionally conducted to compare the performance with Palm oil. A milling operation was conducted on the ASTM A36 steel plate. Machining was conducted on a vertical milling machine, and values of FR, DOC, and V C are shown in Table 5. Flank wear and cutting forces were selected as output parameters. Maximum flank wear and cutting forces were noticed for dry cutting. Whereas wet milling provided the best results. However, MQL assisted machining provided almost similar results as flooding. Hence MQL may easily opt as a cutting fluid supply method using Palm oil to minimize the usage of cutting fluid and ensure biodegradability.
Another research was conducted with Palm oil-based cutting fluid [112]. In another study, a Palm oil assisted MQL cooling system was compared to the flood cooling of commercial water-soluble oil. The end milling process on difficult-to-drill aerospace grade Titanium alloy was conducted under the above cooling and lubricating conditions. Machining was performed at V C , FR and DOC values shown in Table 5. R a , cutting force and temperature values have been opted as performance evaluation factors. A reduction of 35% in cutting force and roughness was observed for MQL (Palm oil) in comparison to flooding (Commercial oil). Additionally, the cutting zone temperature was additionally reduced by 47% more in the case of MQL when compared with flooding. Hence Palm oil combination with MQL is far more effective than the flooding of commercial oil.
AISI 304 is the most widely used steel and requires massive amounts of cutting fluids for machining. A study was conducted in which AISI 304 Steel was machined under MQL, flooding method, and dry environmental cutting conditions [113]. Here Olive oil was used as a biodegradable cutting fluid while machining in an MQL environment, and for flooding, mineral oil was utilised. End milling was conducted on a CNC machine with FR, experimental cutting environment, and cutting speed shown in Table 5 as input parameters, whereas R a was the response. Higher V C and low FR proved to be optimum parameters for milling. Minimum roughness was measured as 0.37 µm in the case of Olive oil-based cutting fluid, whereas, for mineral oil flooding and dry cutting, R a values came out to be 0.58 µm and 1.58 µm, respectively. Vegetable oil combination with MQL can provide better results compared to the flooding method with mineral oil.
Another research used the MQL technique with Black Mustard oil-based cutting fluid, comparing its performance with the mineral oil-based flood cooling and dry milling [114]. A vertical milling machine was used for end milling of AISI H13 Steel with a coated carbide tool at different V C and feed shown in Table 5. It was noted that high speed and feed cause high temperatures and poor surface quality. The results additionally revealed that using MQL (Mustard oil), reduction of temperature was enhanced to about 52-57% more than dry milling, and 38-45% improvement was noticed compared to wet machining (Commercial oil). In the same manner, the surface finish utilising MQL was improved to 53-64% and 22-40% in comparison to the dry and wet milling process. Additionally, burnt chips were obtained in dry milling, whereas light blue and shining silver chips were obtained using flooded and MQL techniques, respectively. Burnt chips show elevated temperatures due to dry machining; blue chips indicate mediocre temperature because of wet milling, and shining chips show a highly considerable decrease in temperature using vegetable oil.
Sunflower oil has additionally been used as a cutting fluid for milling operations. Research work was conducted in which end milling of AISI 316 SS was performed on a conventional vertical milling machine using a solid Tungsten Carbide tool [115]. Three different cutting conditions were used, i.e., dry MQL (pure Sunflower oil) and nanoparticles enhanced MQL (NMQL). Nanoparticles of Al 2 O 3 were added by 0.2 wt.% in Sunflower oil to prepare nano cutting fluid. End milling operation was conducted at various V C and FR showed in Table 5. Power consumption and surface quality were examined to determine best cutting conditions. A reduction in power consumption by MQL and NMQL was 4.7% and 8.6%, respectively, compared with the dry case. Whereas R a values were reduced by 40% and 44% for MQL and NMQL conditions compared to dry cutting. A smoother surface with an excellent surface finish was obtained using NMQL. Hence higher V C and feeds combination can be used with NMQL to improve productivity without compromising on quality.
As in the case of turning and drilling operations, Soyabean oil has been used as a cutting fluid in milling operations as well. For this purpose, experimental work was conducted for face milling of Inconel-62 and SS-304 using Soyabean oil under two conditions, and dry cutting was additionally conducted [116]. Operator friendly MQL was adopted as a cutting fluid supply method to ensure less consumption of cutting fluid. MQL was opted in two different manners, i.e., using pure Soyabean oil and Multi-Walled Nano Carbon Tubes (MWNCTs) enhanced Soyabean oil. MWNCTs were added in different proportions (0.5, 0.75, 1, 1.25, and 1.5) wt.% to prepare nanofluid. As materials were very hard-to-cut, down milling was opted, which causes more tool vibrations and the role of proper cutting fluid and cutting parameters gain significant importance. PVD coated carbide tools were used for machining, and the experiments were conducted under two stages. In the first stage, tool wear and surface roughness were examined to find out the optimum input parameters of V C , DOC and FR for both the materials. Various values of input parameters were selected for the materials and are shown in Table 5

Non-Edible Vegetable Oils as Cutting Fluids for Milling Operation
To avoid a shortage of edible oils, some non-edible vegetable oils have additionally been tested as cutting fluids to replace them with toxic commercial mineral oils. Researchers performed face milling of Aluminium alloy 7075 using Cottonseed oil-based cutting fluid and compared the performance with commercial soluble oil and dry milling [117]. Cottonseed oil was utilised through the MQL technique once in its pure form and secondly in form of a nanofluid prepared by adding 0.5% by weight of Al 2 O 3 nanoparticles. Table 6 displays the cutting conditions. R a values for dry, mineral oil emulsion, MQL (Cottonseed oil), and MQL (nanofluid) were measured to be 0.1724 µm, 0.1647 µm, 0.1443 µm, and 0.1316 µm. In the same manner for the mentioned conditions, the cutting forces were 234 N for dry milling, 197 N for mineral oil, 189 N for pure MQL, and 187 N for nanoparticles enhanced MQL. Nanofluid of vegetable oil can perform better than flooded lubrication of mineral oil.
Emulsions of various vegetable oils have additionally been tested as cutting fluids for milling. In a research face milling experiments on AISI 1045 Steel were conducted using emulsions of commercial oil, Castor oil, Cottonseed oil, and Neem oil [118]. All the oils were emulsified using water. Milling was conducted on a vertical milling centre, keeping the number of passes, DOC, FR, and V C as input parameters, whose values are shown in Table 6. High speed, moderate FR, and low DOC provided the best results. R a values for all the oils were measured to find out the best option. The least surface roughness (0.786 µm) was obtained using Castor oil, whereas maximum roughness (0.931 µm) was in the case of Cottonseed oil. Mineral oil and Neem oil produced R a values equal to 0.805 µm and 0.790 µm. Consequently, vegetable oils in an emulsion state can additionally out-perform mineral oils. Emulsions of Castor oil and Neem oil are the best options to be utilised as cutting fluids. Castor oil provides good lubricating properties, but its performance is somewhat limited due to viscosity. More viscosity of Castor oil causes flow difficulty. Research was conducted with Castor oil was mixed with Palm oil to enhance the flow of cutting fluid [119]. Castor oil and Palm oil were mixed with each other in six different volume ratios, as shown in Table 7. Cutting fluids were supplied through MQL technique. Vitamin C tablets were added to enhance resistance against corrosion. The performance of these formulated cutting fluids was examined in an end milling operation on hard-to-cut Inconel 690. Experimentation was conducted at constant cutting parameters shown in Table 7. The performance of the cutting fluids was evaluated by tool wear and R a . At various volume ratios, the values of roughness and tool wear are also shown in Table 7. The minimum R a value is obtained at 1:2, and minimum tool wear is noticed at 1:1.5, as shown in Table 6. So medium values of volume ratio are recommended to obtain the best machining results.
Sago starch is extracted from palm stems and is composed of oval-shaped particles of 30 µm diameter, which enable its solution to behave like nanofluids. So it was tested as a cutting fluid [120]. Laser-assisted high-speed milling of AISI 316 SS was conducted on a CNC milling machine using a micro-grain carbide coated endmill. MQL was adopted as a cutting fluid supply method, and the performance of Sago Starch-based cutting fluid was compared to the commercial mineral oil. Water-soluble Sago Starch was used with the additives shown in Table 7. Milling was conducted at various V C and feed values which are additionally shown in Table 7. In the case of commercial mineral oil, R a value of 1.023 µm and tool wear of 5.2960 µm was observed. At the same time, Sago Starch-based cutting fluid provided R a and tool wear values of 1.023 µm and 5.2960 µm, respectively. Consequently, Sago Starch solution can be used as a non-toxic and more effective cutting fluid.
Raw Kapok oil is non-edible oil. So, to encounter a shortage of vegetable oil for food supply, Kapok oil has been used by researchers for milling Mild Steel (AISI 1020) on a CNC vertical milling machine. The machining performance of Kapok oil was compared to that of the commercial mineral oil (SAE 20 W 40) at various FR, DOC, and V C showed in Table 7. Wear, cutting force and R a were determined to evaluate the performance of Kapok oil. It was observed that minimum flank wear and R a is produced using Kapok oil with the set of FR, DOC, and V C equal to 0.09 mm/tooth, 1.20 mm and 647.65 rpm, respectively, with the desirability of 82.1%. As far as cutting force was concerned, commercial cutting fluid provided better results by a minimum cutting force of 659.167 N compared to 659.217 N of Kapok oil. However, this small margin can be ignored, keeping in view the other factors. So raw Kapok oil is a good option to be selected for the milling process.

Discussion
This review helps us to understand that sustainable machining is necessary to achieve excellent products and provides a guideline for industries to ensure green manufacturing. One of the most important factors for manufacturing concerns is the fine surface finish. So, cutting parameters for different materials must be optimized to achieve excellence in machining and obtain a smoother surface finish. Many research works were conducted to  Table 1, Table 2, Table 3, Table 4, Table 5, Table 7 indicate that medium to low feed rates and low depths of cut are the optimum conditions for machining. At higher cutting speeds, more heat is generated at the workpiece surface, causing the softness of the material. The chips will additionally move away easily from the surface at higher speeds. Consequently, excellent surface quality is attained.
The selection of proper cutting fluids is necessary to encounter the heat of cutting as well as a reduction in friction. Mineral oil-based cutting fluids are mostly used, but their toxicity and cost are forcing manufacturers to look for more sustainable manufacturing processes. Sustainable machining is not only limited to achieving high surface quality, but environmental issues, health, and cost of machining are key factors. Keeping in view the environmental and economic aspects, research on the utilisation of vegetable oils as cutting fluids has attained significant importance in recent years. Vegetable oil has begun to replace mineral oil due to environmental concerns worldwide [122][123][124][125]. As mentioned earlier, vegetable oils as neat oils and in the form of emulsion have been tested as cutting fluids to ensure a green and sustainable manufacturing system. Primarily the researchers have selected readily available edible vegetable oils as cutting fluids which, due to their triglyceride composition, exhibit excellent performance as cutting fluids.
However, special care must be adopted for the selection of edible vegetable oils as cutting fluids so that their consumption does not affect the food supply. Usage of edible oils with less demand for food must be focused on. Additionally, the usage of non-edible oils is a good option. Further, the cost of the selected vegetable oil should be considered as important as other factors. Many non-edible trees and crops can be cultivated in barren lands. Special attention must be paid to this aspect as well. It can help us to utilize those lands, and this can be cost-effective as well.
Although mineral oil-based cutting fluids are the most common cutting fluids, using them is harmful to the environment and human health, and more cost is encountered. So, this trend of using mineral oils is shifting towards environment-friendly vegetable oil-based cutting fluids.
Approaches like nearly dry machining, solid lubrication, compressed air cooling, cryogenic cooling and minimum quantity lubrication have been utilised to minimise the consumption of cutting fluids. However, dry machining is never a good option for machining of hard materials. Compressed air helps in the removal of chips, but friction is not reduced. Solid lubricants limit the cooling capabilities, and cryogenic cooling is a very expensive approach [6].
Many research works were conducted using biodegradable cutting fluids, and they proved the efficiency of vegetable oil-based cutting fluids. Analysis of research works on machining of hard materials using biodegradable cutting fluids is conducted in this research. In addition to this, values of desired output parameters like roughness, tool wear, cutting forces etc., have been identified. Values of optimum parameters are provided as well. The efficiency of machining is improved by using vegetable oil-based cutting fluids as less roughness is obtained as well as power consumption, tool wear, cutting forces and cost of disposal are reduced.

Conclusions
The major advantages of the new developed vegetable-based cutting fluids are high biodegradability and environmentally friendliness while providing the same or better performance than mineral-based cutting fluids. This review paper helps us to draw the following conclusions:

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Optimized cutting conditions (speed, feed, depth of cut and cutting environment) help in minimum tool wear, roughness, cutting forces and power consumption. An increase in productivity is noticed at optimum conditions. • Although cutting fluid is a basic requirement when materials are hard to cut, but mostly these cutting fluids are composed of mineral oils causing serious harm to The low thermal conductivity of the vegetable oils is improved by the addition of highly thermally conductive nanoparticles. Nanoparticles of silver, boric acid, graphite, carbon and molybdenum disulphide (MoS 2 ) have been used as additives.

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High concentrations of nanoparticles affect the surface finish by infusion of nanoparticles in the surface and getting trimmed by the upcoming particles. The rolling of nanoparticles formulates a lubrication film that can shear easily and thus providing better surface polish.

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The best way to utilise vegetable oils as cutting fluids is to supply their nanofluids in adequate concentrations to the cutting zone using the MQL technique. Additionally, the focus must be on the selection of non-edible vegetable oils.

Future Work
The recommendations for future research are given as under: • The performance of vegetable oils may limit due to less oxidative stability and low corrosion resistance. So biodegradable and appropriate additives, e.g., anti-corrosive agents, biocides, emulsifiers, antioxidants, must be used to formulate novel vegetable oil-based cutting fluids. • Vegetable oils have poor thermal conductivity. So proper concentrations of nanoparticles like graphene, graphite, copper, MoS 2 , etc., can be used as additives to improve the thermal conductivity of vegetable oil-based cutting fluids.

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The concept of using hybrid nanofluids i.e., combination of different nanoparticles in a fluid, is additionally an emerging idea. • A very large amount of cutting fluids is consumed for machining hard materials. Techniques like minimum quantity lubrication must be adopted to cut down the usage of cutting fluids as the flooding method is not cost-effective. • Non-edible vegetable oils like Karanja, Neem, Honge, Castor, Jatropha, etc. must be utilized as cutting fluids so that the food supply chain is not damaged.