Modeling the effects of cutting parameters in MQL-employed finish hard-milling process using D-optimal method

In any finish milling process the minimization of surface roughness is the prime objective, and if the workpiece belongs to the category of ultra-hard steels then the objective of maximizing tool life also gains considerable importance. In this research work, the effects of four parameters, namely, hardened steel's microstructure, workpiece inclination angle, cutting speed, and radial depth of cut were studied upon tool life and surface roughness (in directions of feed and pick-feed). The milling was performed under environment of minimum quantity of lubrication (MQL), using coated carbide ball-nose end mills. The quantification of the aforementioned effects was done using a new response surface methodology known as the D-optimal method. For tool life, workpiece material was found as the most influential parameter followed by the rotational speed of tool. High values of tool's rotational speed proved unfavorable for tool life but favorable for surface finish. In addition, the effects of workpiece inclination angle and radial depth of cut were analyzed upon effective cutting speed and cusp height and, subsequently, upon surface roughness. SEM and EDS analyses of the worn-out tools were carried out in order to study the effects of different levels of parameters selected upon the severity of different tool damage modes. The major tool damage mechanisms detected were notch wear, adhesion, and chipping. The severity of chipping was relatively smaller as compared to that of adhesion and of notch wear because of reduced effective cutting speeds and feed rate employed.