Boiling of Coolant Near the Cutting Edge in High Speed Machining of Difficult-to-Cut Materials

This study investigates film boiling of coolant as a cooling inhibitor in a narrow wedge-shaped space between the tool flank face and the machined surface of a workpiece, observed during high-speed turning of stainless steel SUS304 and nickel-based superalloy Inconel 718. The boiling, likely triggered by high surface temperatures at both the face and surface close to the cutting edge, impedes coolant access to the tool tip area and efficient cooling. Therefore, the impact of coolant pressure on the boiling zone size was initially explored across pressures ranging from 0.1 to 20 MPa. A burn mark band due to coolant boiling, distinctly visible on the flank face of an insert with a yellow hard coating, expanded over cutting time. The film boiling area width, or the distance from the flank wear area to the band, decreased with increasing coolant pressure, reflecting the enhanced cooling ability and tool life with high-pressure coolant. Applying Boyle–Charles’ law to film boiling indicated that vapor pressure was directly related to coolant velocity rather than pressure. In contrast, the boiling area width increased with increasing cutting speed.