Drilling of AISI 316L stainless steel: Effect of coolant condition on surface roughness and tool wear

Abstract

This study examines the effect of coolant condition on surface roughness of AISI 316L austenitic stainless steel workpiece and on tool wear during drilling using uncoated carbide drill. The drilling was done under flood cooling, minimum quantity of lubrication (MQL) using palm olein, and dry machining. Drilling was performed on a CNC machining centre using spindle speed of 955 rpm and feed rate of 24 mm/min with drill of 4±0.01 mm diameter, 130° point angle and 30° helix angle. Drilling under flood cooling was performed using commercial water based mineral oil (6% mineral oil) with flow rate of 18.4 l/h. MQL technique applied mist coolant of palm olein with flow rate of 27 ml/h from 5.5 bar air pressure. For dry machining, no coolant was applied. Surface roughness (Ra) was measured with surface roughness tester with setting of 0.8 mm cut-off length and 4 mm sampling length for each measurement. The surface roughness is averaged from twelve measurements at different points on the drilled hole. Tool wear was measured after particular time interval during drilling. It was found that the surface roughness resulted from drilling under flood cooling was significantly lower than that of MQL and dry machining. For surface finish resulted by worn tool, the surface roughness was higher compared to when new tool was used for all coolant conditions. Using the tool life resulted under flood cooling as the benchmark, it was found that dry drilling could only achieve 5% of the tool life while MQL drilling resulted better with 68% of flood cooling's tool life. The cooling conditions showed different tool failure modes as well. For flood cooling, tool failure modes were uniform flank wear and chipping on primary cutting lips. For MQL, excessive flaking on flank face was identified as tool failure mode. For dry machining, the failure modes were margin wear, outer corner wear and catastrophic failure.