Evaluation of Surface Roughness Effect on Fatigue Life in Drilling of Hardened Steel

In this study, the effect of surface roughness on fatigue life was investigated in drilling process of AISI4340 steel. Three hole making methods including drilling with and without pre-drill and helical milling were utilized. In order to study the effect of surface roughness and process parameters on fatigue strength, the two main cutting parameters including cutting speed (Vc) and feed rate (fz) were changed at 5 levels using the response surface method. Five main parameter of surface roughness profile including Rz, Rt, Ra, Rq, and Rsm were measured in each experiment. Then, the fatigue life of specimens were obtained using fatigue tests. Regarding the validation experiments, 13 fatigue tests were carried out for each drilling strategy (39 fatigue tests totally). Investigation of surface texture showed the signs of the tool path, scratches by the chip collisions and ploughing on hole surfaces in the conventional drilling. These effects were negligible in other processes. Accordingly, the highest roughness was observed in conventional drilling, drilling with pre-drill and helical milling processes respectively. Also, the fatigue life estimation model based on Rz had the best estimation with an average error of 4.4%. In fact, the fatigue life is more dependent on the difference between adjacent peaks and valleys. It was also observed that a model based on the roughness parameter will predict the fatigue life with lower error compared to a model based on the cutting parameters.