Contradictory effects of machining conditions on wear in the main cutting zones of the twist drill

The present work is a fulfilment study on previous contribution presented by Mokas et al (2018) through investigation on the effect of chisel wear on the machinability of hardened C22 steel under drilling operations using twist high-speed steel (HSS) grade drill bits. Therefore, the evolution of wear in twist drill bits with respect to both flank and chisel edges has been reconsidered in order to sort out the most dominant wear rate in drill edges. Experimental investigation has been conducted on annealed and hardened C22 steel according to the planning experience methodology (L8). The input parameters are cutting regime elements, cutting speed, feed rate, and drill diameter; meanwhile, the output parameter is the chisel wear related to tool life. Results show that eventually, drilling annealed material generates much better tool life. Relatively to flank wear, chisel wear has great effect in some cutting conditions. In the annealing condition, flank wear is still most dominant and depends on the cutting speed. The higher cutting speed the lower the tool life. When drilling hardened material chisel wear becomes competitive with the flank wear as for some cutting parameters it is faster than flank wear. The feed rate is rather more influencing and the controversial effect of the cutting speed is confirmed with the chisel wear. Results are of great importance for industrial application where drilling hardened steel is required.