Analysis and Applications of Minimum Quantity Lubrication (MQL)

Doctoral thesis, 2008

One of the greatest obstacles to the implementation of MQL is “lack of knowledge” which also limits the expansion of its usage. The presented work aims to take a step toward a better understanding of the mechanisms that occur in MQL assisted machining. The tests were therefore designed in such a way that both the air and oil components of the aerosol were investigated separately. The results are presented in terms of several parameters such as tool life, temperature, chip morphology, tool/chip contact area, forces etc. Cooling power of the compressed air was modeled since De Chiffre proposed that cooling power may affect the chip and so the cutting zone. Special inserts equipped with thermocouples were used to measure the temperature on the clearance face of the insert. It was observed that the temperatures in the clearance side of the tool are lower for MQL and air assisted turning compared to dry turning. However, the decrease is not as high as emulsion assisted cutting. This decrease in the tool temperature increased the tool life both for compressed air and for MQL assisted turning compared to dry turning. It was observed that oil particles lubricate the contact between tool and chip at only very short engagement time machining. In order to investigate the effects of different degrees of tool engagement, special work pieces were prepared with different numbers of grooves of the same width. It was clearly observed that the contact length (compared to dry cutting) decreases as the material/tool engagement intermittence increases. This confirms that the lubricating effect is improved as the degree of intermittence increases in MQL machining. Tribological analysis was performed according to the force analysis introduced by Albrecht. It was observed that the decrease in friction coefficient is very small with the application of compressed air. This is due to a decrease in contact length and nothing to do with the lubrication of the contact area. The decrease in contact length was connected to the decrease in chip-curling radius. The direct cooling power of the compressed air via chip was modeled since it was proposed by De Chiffre that this cooling power affects the chip up-curling. However, the calculations show that this power is negligible. It is proposed that the cooling of tool material disturbs the thermal balance between tool and chip material which in turn increases the chip up-curling and then decrease the contact length. This opposes the lubrication theories proposed by Childs, Williams, Trent, De Chiffre…