Tool wear induced burr formation and concomitant reduction in MQL wetting capability in micro-milling

Abstract

During Minimum Quantity Lubrication (MQL), ensuring the reachability of the oil droplets to the cutting region is important to achieve better lubrication effect and enhanced machinability. During end-milling, the side walls and top-burrs can obstruct a fraction of oil droplets from reaching the cutting region resulting in the formation of “Shadow Zone”. For a fixed nozzle orientation, shadow zone size is governed by the axial depth and top-burr height. For a constant slot depth, shadow zone expands as the burr height increases. Since a worn-out tool produces taller top-burrs, increasingly larger fractions of the oil droplets fail to reach the cutting zone as the tool undergoes progressive wear. This article, for the first time, investigates the reachability issue of the MQL oil droplets and corresponding loss in wetting potential of a constant MQL oil flow rate owing to the growth of top-burrs with tool wear during Ti-6Al-4V micro-milling at varying aspect ratios. At 50 μm axial depth, burr height increases from 14 to 120 μm within 46.8 s of micro-milling. Corresponding expansion of shadow zone results in an exponential loss in lubricated area from 38 to 407 μm2. At a higher axial depth of 200 μm, the burr height increases from 21 to 190 μm within 125 mm cutting length, and consequently, 1036 to 3776 μm2 wetted area is lost. Inappropriate wetting compels the cutting edge to rotate in partially dry condition, and thus, MQL gradually becomes ineffective in controlling machinability (surface roughness) with machining duration.