Characterization of Cutting Parameters in the Minimum Quantity Lubricant (MQL) Machining Process of a Gearbox

Introduction

The process of cutting raw material plays a key role in the development of parts, and is often conclusive to achieve the requirements that they must fulfil before being installed on mechanisms and machines.

The process of machining is not always successful due to multiple involved factors that determine the expected outcome in terms of part quality. Not selecting the appropriate level for one of those factors may lead to an increased number of rejected workpieces, or the necessity of a more demanding equipment maintenance, thus rising production costs. For instance, the maximum temperature achieved during machining, or the type of lubrication is closely related to the tool wear, and must then be controlled to achieve the desired results (Hussain, Taraman, Filipovic & Garrn, 2008). The use of cutting fluids are the solution to these phenomena, but these refrigerants and coolants is harmful to the environment and the health of workers. Not using flood cooling is a feasible solution in order to improve industrial jobs’ security and decrease the generation of waste in the machining processes (Hadad & Hadi, 2013).

In this context, a solution to prevent the use of large quantities of cutting fluids is applying a minimum quantity lubrication (MQL) conditions. This system allows a faster chip evacuation compared to conventional cutting fluids (Mao, Zou, Huang & Zhou, 2013), since pressurized air is used along with a small quantity of lubricant. During the cutting process, chips absorb most of the generated heat. With MQL, system cooling and chip evacuation functions are solved. Ueda, Hosokawa & Tanaka (2006) researched the influence of MQL on the temperature reduction in a drilling process, and they found that with MQL the temperature in the cutting area reached 330 °C, sensibly lower than the 430 ºC achieved in a dry process.

MQL application is not limited only to solve the problems describe above. Studies show that it also makes it possible to reduce the final roughness of the machined surface. Dhar & Islam (2007), demonstrated that using an MQL system, better surface roughness results are obtained, compared to those reported by conventional processes with cutting fluids. Moreover, Jiang, Li, Yan, Sun & Zhang (2010) analyzed the effect of cutting parameters on the average surface roughness (R_a) under different cooling and lubrication conditions including MQL cutting, cutting with fluid and dry cutting. These authors concluded that the surface roughness under MQL cutting conditions was the best one, whereas the dry cutting conditions resulted in the worst one. Under the three cooling and lubrication conditions mentioned above, the study showed that the roughness R_a is sensitive to both the feed per tooth and the axial depth of cut. On the contrary, it is not sensitive to the cutting speed, nor to the radial depth of cut. Along the same lines, Dasilva, Bianchi & Fusse (2007) used MQL in a grinding process and they concluded that the technique can be efficiently applied to this process, since the roughness values were substantially reduced.

In more recent studies, Mao, Tang, Zou, Zhou & Yin (2012) conducted experiments on hardened AISI 52100 steel under different lubrication conditions, in order to measure the surface quality using MQL lubrication system in a grinding process. As opposed to dry grinding, they concluded that MQL results in a significantly improved surface finish and grinding quality due to lubrication and cooling with water and/or oil. MQL is considered an alternative to the grinding method with cutting fluid, because it is a more environmentally friendly and economical technique. The study of the oil-water MQL grinding shows that the process temperature and the thickness of the affected layer could be significantly reduced compared to the pure oil MQL grinding. It proves that the first MQL process has a better cooling condition than the latter.