Optimization of Surface Roughness and Diameter Error in Thin-Walled AA6063 during Internal Turning under Minimum Quantity Lubrication

Thin-walled aluminum alloy 6063 (AA6063) components are commonly used in landing gear manufacturing through internal turning. However, tool and workpiece deflection cause diameter errors, altering the depth of cut, inducing chatter, and degrading surface finish. While cutting fluids reduce friction and vibration, conventional mineral oils harm the environment. This study employed the Taguchi L9 (3³) design and the Multi-Response Performance Index (MRPI) to optimize internal turning under minimum quantity lubrication (MQL) using virgin coconut oil (VCO). MRPI combines surface roughness and diameter error into a single index for parameter selection. The optimal settings—depth of cut 0.8 mm, feed rate 0.075 mm/rev, and cutting speed 200 m/min—yielded 4.72 µm surface roughness and 0.09 mm diameter error, demonstrating that MQL with VCO enhances machining quality and supports sustainable manufacturing.