An experimental investigation for the improvement of attainable surface roughness during hard turning process

In this article, an experimental investigation was carried out to improve the machined surface roughness attainable during hard turning. An American Iron and steel institute (AISI) 4340 hard steel workpiece (hardened up to 69 Rockwell C Scale Hardness (HRC)) was machined on a Mori-Seiki SL-25Y (4-axis) Computer numerical controlled (CNC) lathe. Prior to machining, defects were generated on the surface of the workpiece in the form of holes. It was recognized that these pre-machined holes provided intermittent relaxation to the cutting tool and resulted in a lower temperature in the cutting zone, lower average cutting forces and a better quality of machined surface over conventional hard turning. Using the new method, termed ‘surface defect machining’, an improved average surface roughness (Ra) of 0.227 µm was obtained compared to an average value of 0.452 µm using conventional hard turning.