The use of tool flank wear and average roughness in assessing effectiveness of vegetable oils as cutting fluids during turning—a critical review

Vegetable oils have significant potential to replace conventional cutting fluids for green machining due to their lower environmental impact. The effectiveness of these oils when used as cutting fluids has been assessed using several measures, such as tool wear, cutting forces, surface roughness of workpiece, cutting zone temperature, vibration, and chip formation. Among these, the two most commonly investigated parameters in the past are tool flank wear and average surface roughness (R_a). The use of flank wear and average roughness in assessing the effectiveness of the oils as cutting fluids, however, has resulted in confusing and contradicting findings in many of the published literature. One common anomaly found in many of the published works is the improvement in surface roughness in spite of an increase in tool flank wear. This contradiction is mainly due to the poor correlation between the major flank wear and average roughness R_a. Moreover, since R_a is a measure of the average absolute height of the roughness profile, and thus is insensitive to changes in the overall morphology of the surface profile of the workpiece as a result of tool wear, the use of R_a as the sole roughness measure could potentially lead to erroneous conclusions. In this paper, the major issues and anomalies in previously published research are highlighted and discussed critically. More reliable measures of surface finish quality that could be used to assess the effectiveness of vegetable oils as cutting fluids are proposed and demonstrated using simulated and real surfaces produced by finish turning. The cross-correlation method is shown to provide a more reliable means of assessing the deterioration of the surface morphology due to gradual and excessive tool wear compared with R_a.