Shadow zone in MQL application and its influence on lubricant deficiency and machinability during micro-milling

Abstract

Minimum quantity lubrication (MQL) in metal cutting helps enhancing machinability while ensuring sustainability. In the absence of bulk heating in micro-milling, the cutting fluid needs to be deposited on the tribologically active chip-tool-workpiece region to perceive enhanced lubrication effect. There are few factors that can restrict the oil droplets from depositing on the tool. One such restriction that inherently appears as “Shadow Zone” during end-milling process is reported in this article for the first time. This shadow zone hinders certain percentage of droplets from reaching to the cutting edges resulting in reduced wetting capability of the MQL supply even when the overall oil flow rate remains unchanged. Lubricant deficiency induced solely by this shadow zone during micro-milling under varying parameters is assessed here. For fixed nozzle orientation, detrimental effect of the shadow zone intensifies with an increase in the axial depth of cut and top-burr height. At 50 µm axial depth, the shadow zone restricts 3.7% droplets from depositing on the active cutting edge and thereby compels the MQL assisted cutting process to rotate in incompletely lubrication condition. For unchanged oil flow rate, the corresponding deficiency increases to 16.4% at 200 µm axial depth. When compared with dry micro-milling of Ti-6Al-4 V, shadow zone reduces effectiveness of the MQL supply in controlling tool wear from 25.4 – 5.5% in the entire tool life when axial depth is increased from 50 – 200 µm. Similar degree of reduction in MQL effectiveness towards controlling surface roughness, burr width, and cutting forces is also observed.