The AZ31B alloy is a key material in modern industries where weight reduction is a strategic priority, particularly in the aerospace and automotive sectors. This study investigates the machinability of AZ31B magnesium alloy under three distinct cutting conditions: dry cutting, minimum quantity lubrication (MQL), and nano-MQL. A total of 81 CNC turning experiments were conducted, with 27 trials performed for each machining environment, varying cutting speed ( \(\:{V}_{c}\) ), feed rate ( \(\:f\) ), and depth of cut ( \(\:ap\) ). The environmentally friendly CUTTEX SYN-10 oil and multi-walled carbon nanotubes (MWCNTs) were employed for the MQL and nano-MQL conditions, respectively. Machinability was assessed by measuring the variations in the gains of the arithmetic mean roughness ( \(\:\varDelta\:Ra\) ) and the maximum profile height ( \(\:\varDelta\:Rz\) ), a new surface quality criterion has been introduced to better characterize the machined surface, as well as tool wear and chip morphology. To achieve this, statistical analysis, scanning electron microscopy (SEM), and optical microscopy were performed to study the responses. Overall, the results indicate a clear and progressive improvement in surface roughness gains when transitioning from dry machining to MQL and subsequently to nano-MQL conditions. The average gains increase from 58.24% to 62.07% and reach 71.41% for \(\:\varDelta\:Ra\) , while corresponding values of 65.14%, 66.48%, and 78.60% are obtained for \(\:\varDelta\:Rz\) . This consistent enhancement highlights the effectiveness of nanoparticle-enriched lubrication in improving tribological conditions at the tool–workpiece interface, thereby contributing to superior functional surface quality.
