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Machining efficiency and hence productivity depend on proper selection of the machine, cutting tool, machining parameters and coolant. Not only the selection of coolant, but its parameters play a vital role in machining efficiency. In the direction of improving coolant performance, high pressure cooling, cryogenic cooling, atomized coolant spray are the demanding trends in the industry and research field (Blau et al., 2015). Cryogenic cooling (Govindraju et al., 2014; Jerold & Kumar, 2005) and atomized coolant spray method (Nath et al., 2014; Lopez et al., 2006) are still in research stage and are costly techniques. Conventional cooling methods supply coolant with maximum coolant pressure up to 6 bar (Palaniswamy et al., 2009). Hence machining process in which coolant is supplied at a pressure greater than six bar is known as high pressure coolant machining.
Following are the major difficulties existing in drilling operation.
First difficulty is in effective cooling and lubrication of cutting zone. In drilling, coolant supplied by conventional method can’t reach the cutting zone at sufficient flow rate and pressure, resulting poor cooling and lubrication action at cutting zone (Sanchez et al., 2012). Cooling and lubrication action become less effective as drilling depth increases, resulting thermal distortion of hole.
Second difficulty is in evacuation of chips (Dhar et al., 2006). In drilling chips are formed at the bottom of hole and accumulate there. The conventional coolant flushing system is incapable to evacuate these chips at faster rate, due to insufficient coolant volume flow rate and pressure. This results in clogging of chips in drill flutes and hole surface (Mellinger et al., 2002) and chips weld to the drill flutes, causing rise of tool temperature as well as work piece temperature. These will result in distortion of workpiece and poor dimensional accuracy of hole.
Third difficulty is in chip breakability. Coolant supplied by conventional method can’t penetrate deeply in to the tool-work piece interface as well as tool-chip interface and hence has poor chip breakability. This result in higher coefficient of friction, larger tool-chip contact length (Palaniswamy et al., 2009), higher thrust forces and hence poor dimensional control of hole.
Hence there is need to study the effect of high pressure coolant supplied directly at cutting zone in drilling using through coolant drill. High pressure coolant approach in drilling can supply coolant at high pressure and flow rate at cutting zone. The performance of machining in high pressure coolant environment was studied by various investigators.
Dhar et al. (2006) studied influence of HPC in drilling operation on chip shape, hole roundness error and tool wear. Study was carried out at fixed cutting speed, feed rate and cutting depth at 40 bar pressure of straight run coolant. The length to diameter ratio of four was maintained. The investigation concluded that better chip formations, less roundness deviation at top and bottom of holes were obtained with high pressure coolant drilling. Lopez de Localle et al. (2000) investigated role of pressurized coolant in drilling and turning operations on Inconel 718 and Ti6A14V using through coolant drill. The study was carried at 110 bar coolant pressure and at different cutting speeds. The results concluded that high pressure drilling with internal coolant results improved tool life, even at higher cutting speed compared to conventional external coolant drilling.