Quality improvement of deteriorated cutting fluid treated by atmospheric-pressure plasma jet and in-liquid plasma

Friction between a tool and chip, and the heat generated, often limit machining in metal cutting operations. Coolants and lubricants are used in great quantities to reduce friction at the cutting area. However, the cost of cutting fluids is increasing because of the increasingly stringent environmental standards for fluid handling and disposal, and these standards are likely to be further tightened in proposed national and international legislation (Weinert and Inasaki, 2004). Although dry and semi-dry machining is an attractive option, it is currently unavailable in many operations because cooling cannot be achieved (Tasdelen et al., 2008; Werda et al., 2016). Therefore, cutting fluids will continue to be required. As a fluid deteriorates, its cutting power decreases and the work environment becomes polluted. Techniques that extend the life of cutting fluids are therefore needed. One of the key causes of deterioration is decomposition due to bacterial growth (Rabenstein et al., 2009; Griffiths, 1978). Bacteria enter the fluid from both the air and the chip. Because preventing bacteria from contaminating the cutting fluid is difficult, chemical additives are widely used for bacterial control. The widespread use of oil additives has led to the emergence of resistant bacteria, and the additives change the characteristics of the cutting fluid. Therefore, we proposed a sterilization technique using plasma treatment under atmospheric pressure (Miyamoto et al., 2018). Moreover, we suggested that the molecular structure of the fluids was not affected by plasma jet treatment (Miyamoto et al., 2018). In our previous study, the number of bacterial colonies in fluid sterilized using atmospheric-pressure plasma was reduced by more than 90% compared with the number in an untreated fluid (Miyamoto et al., 2018). From this report, however, it is unclear whether the remaining bacteria and surface Junji MIYAMOTO*, Ryo TSUBOI*, Masashi YOSHIDA** and Koichiro NAMBU*** * Department of Mechanical Engineering, Daido University 10-3 Takiharu-cho, Minami-ku, Nagoya, Aichi 457-8530, Japan E-mail: j-miya@daido-it.ac.jp ** Department of Mechanical Systems Engineering, Daido University 10-3 Takiharu-cho, Minami-ku, Nagoya, Aichi 457-8530, Japan *** Toyota Technological Institute 2-12-1 Hisakata Tenpaku-ku, Nagoya, Japan