Drilling a magnesium alloy using PVD coated twist drills

The present paper describes a research study on machinability in drilling operations of a magnesium alloy and on the opportunity offered by the use of coated tools. Twist drills in HSS steel (10 mm diameter) were used for the experimental work. TiN, CrN, and two different ZrN coatings were produced by means of an arc evaporation PVD process, using different deposition parameters. Each coating was characterised as far as thickness, surface finish and hardness are concerned. Dry machining drilling tests consisting in machining holes 40 and 60 mm deep were performed using both uncoated and coated tools. Three sets of drilling tests were carried out at a fixed peripheral speed (63 m/min) and with feed speeds of 0.27, 0.37, 0.5 and 0.7 mm/rev. In each test condition the tool life was evaluated and the evolution of tool wear and of hole surface finish (roughness) were also monitored. The experimental results showed that an optimal range of feed speeds for each drill surface finish exists. The optimal range corresponds to heavier machining conditions for TiN and for one of the ZrN coating investigated.