Optimisation of Drilling Parameters Using Grey Relational Grade on AA7075 Plates Using Coated and Cryogenically Treated Drills

Drilling is the one of the basic operation of machining to make holes. It is the essentially used in aerospace, automobile and medical industries. In this study, an attempt is made to study the effect of cryogenic treatment and AlTiN coating on drills by varying parameters (speed, feed, thickness of plates) on the diametrical error (De) and surface roughness (Ra) on AA7075 plates. The control factors provide better diametrical error and surface roughness were determined by the grey relational analysis. Response surface methodology (RSM) is used to develop the interactions among control parameters. In addition, analysis of variance was employed to determine the most significant control parameters on diametrical error and surface roughness. Four type of drills (conventional heat treated (H), coated drill (CO), cryogenically treated (CY) and cryogenically treated +coated (CYC), thickness of plate, feed rate and spindle speed are considered as input parameters, and L36 full factorial design with mixed orthogonal array selected for experimentation. As a result, it is found that type of drill bit used and feed rate are most significant parameters with percentage contributions 36.54 % and 31.17 % respectively. From the grey relational analysis, low thickness, high feed rate and low spindle speed with cryogenically treated +coated drills are found as optimal parameters combination. Confirmation test is conducted for optimal parameters combination and an error in grey relational grade found as the 3.04 % when it compared to predicted value.