In recent years, with the increasing demands on the mechanical and other properties of the parts, some difficult-to-machine materials with hardness as high as 50–70 HRC, such as the hardened tool steel AISI D2, have emerged. It is widely used in industrial fields such as bearings and molds due to its mechanical strength, fatigue resistance and wear resistance.
It is accompanied by waste liquid pollution, low processing efficiency, and difficult to guarantee product quality as machining these materials in traditional processing technology.  In the past 30 years, many researchers have been focusing on the dry hard cutting technology in order to avoid its substantial disadvantages. Although dry hard cutting technology has the advantages of high processing accuracy, high production efficiency, and less environmental pollution,  it also has disadvantages such as the high cutting temperature, large cutting force, and short tool life.  Nevertheless, these green machining technologies, such as low temperature cold air (LTCA), minimal quantity lubrication (MQL), and oil on water (OoW) have always been focused by various researchers in recent years  because they have been considered to be the replacement for dry machining.
The LTCA cutting technology is a cooling and lubrication technique that sprays low-temperature compressed air into the cutting zone to reduce the temperature of the cutting zone. In 2010, Sun et al.  have conducted an experimental study on the LTCA in turning of titanium alloys and confirmed that it has a longer tool life and higher machined surface quality than the dry cutting. Furthermore, Su et al.  also have come to the conclusion that the LTCA cutting can significantly prolong the tool life and improve the quality of the machined surface. Next year, Boswell et al.  from Australia have conducted cutting tests by using the LTCA + minimal quantity lubrication (MQL) technology. According to the findings of the research, the LTCA + vegetable oil MQL technology is the most promising cooling and lubrication technology. The machining performance of Ti-6Al-4V titanium alloy under the condition of LTCA has been studied by Liu et al. in 2013.  They have pointed out that, compared with dry cutting, LTCA can significantly reduce the cutting temperature, prolong tool life, as well as obtain the best machined surface. Subsequently, Jozic et al.  have optimized the cutting parameters in milling 42CrMo4 under the conditions of the cutting fluid, LTCA, and dry cutting by using the orthogonal experiment in 2015. It has been discovered by Kopac et al.  that the residual compressive stress conducive to improving the machined surface quality can be obtained after the low temperature machining. Moreover, Arruda et al.  have considered a significantly longer tool life in cutting API 5L X70 steel under LTCA condition.
The MQL cutting technology is also a cooling and lubrication technique that sprays the minimum quantity of lubricant oil mist mixed up with compressed air into the cutting zone to decrease temperature and friction in chip-tool-workpiece interface, reduc tool wear, and improve the quality of the machined surface. Anthony-Xavior et al.  have investigated the influence of the coconut oil, emulsion, and oily cutting fluid on the surface roughness and tool wear in turning AISI 304 steel. It has a great impact on improving the surface roughness and extending the tool life. Besides, Saini et al.  have experimentally investigated the influence of variables such as cutting speed, feed, and depth of cut, etc. on the cutting force and tool tip temperature in turning AISI 4340 steel at different environmental conditions of dry and MQL cutting. The results obtained in this paper showed that the main cutting force is largest among the three-component cutting forces, and that MQL cutting has an excellent lubricating effect when compared with dry cutting. In 2017, Pervaiz et al.  have experimentally evaluated such important indicators as the surface roughness, cutting force, tool life, and other important indicators focused by the machinists in turning titanium alloy Ti-6Al-4V at the condition of low temperature MQL. Masoudi et al.  have considered that the MQL system significantly increases the cutting efficiency after researched the influences of different parameters on the tool wear, cutting force, and surface roughness in machining AISl 1045 steel. Currently, Makhesana et al.  have further investigated the surface roughness, chip-tool interface temperature, and tool life in turning AISI 4140 steel with coated carbide tools under the cooling and lubrication conditions of vegetable oil-based MQL (VMQL) and minimum quantity solid lubricant (MQSL). The results showed that VMQL and MQSL have significant advantages compared to dry cutting.
The OoW droplets cutting technology refers to the processing technology that sprays a small amount of compressed and atomized vegetable oil and water into the cutting zone to reduce the cutting temperature and lubricate the chip-tool-workepiece interfaces. In 2015, Lin et al.  have analyzed and compared the tool wear at two cooling and lubrication conditions of the OoW droplets and LTCA in turning of titanium alloy Ti-6Al-4V. They have considered that it can obtain lower surface roughness values and lower tool wear by using the OoW cooling lubrication technology. Then, Wang et al.  have studied the tool wear mechanism in turning compacted graphite cast iron at different OoW droplets cooling conditions in 2017. The cooling and lubrication technology has been shown to prolong the life of the cutting tool. Moreover, the influence law and internal mechanism of the nozzle position and diameter on surface roughness and tool wear have been investigated by Yao et al.  under the condition of OoW droplets cooling and lubrication.
According to a large number of literatures, cooling and lubricating techniques such as the LTCA, MQL, and OoW can all achieve satisfactory results, and each has its own advantages. Such difficult-to-machine materials as the hardened AISI D2 tool steel necessitates the use of an innovative cooling and lubrication technology to minimize the cutting temperature at the cutting zone. In this paper, an innovative low temperature oil on water (LTOoW) mist cooling and lubrication technology was proposed by combining the three cooling and lubrication technologies of MQL + OoW+(LTCA). And the influence law and mechanism of the cutting speed, feed, and depth of cut on the cutting force, surface roughness, cutting temperature, as well as tool wear, will be experimentally investigated by using the PCBN cutting tool in turning the hardened tool steel AISI D2 (60 ± 1HRC).