Highly accurate hole making technology of Ti6Al4V experimental elucidation of process for hole diameter in the depth direction to be determined

Ti6Al4V, which is one of difficult-to-cut metals, is widely used in an aircraft structure, parts of a gas turbine and medical equipment so that a hole making operation of Ti6Al4V is indispensable to fasten the parts. When a highspeed drilling by a conventional twist drill is applied to hole making of Ti6Al4V, it is very difficult to obtain highly accurate hole diameter in the depth of hole, also roundness and inlet–outlet edge quality due to a rise of cutting temperature caused by its small heat conductivity. Moreover, it is well-known that Ti6Al4V causes transiently phase transformation from α phase (close-packed hexagonal lattice) to β phase (body-centered cubic lattice) as soon as it reaches the phase transformation temperature of about 883 °C (1621 °F). Since cooling effect by coolant upon inner surface of hole being drilled would be considerably different between conventional drilling and orbital drilling, to make clear highly accurate hole making technology in the depth direction of Ti6Al4V a lot of hole making tests of φ15 mm × 258 mm depth were executed supplying coolant by conventional drilling of φ15 mm twist drill and by orbital drilling of φ11 mm endmill having 6 blades. In order to elucidate the process for hole diameter in the depth direction to be determined, both cutting speeds of conventional drilling and orbital drilling were set to nearly equal and they were varied at 12 values in the range from 23 m/min to 85 m/min. Hole diameter and roundness measured simultaneously at six positions in the depth direction of hole were drawn in relation to depth of hole and hole diameter in the depth direction was compared and considered deeply between the two drilling methods. Consequently, it is ascertained that although the hole diameter drilled by conventional drilling becomes smaller from top to bottom in the depth direction of hole, the hole diameter drilled by orbital drilling becomes slightly larger in the depth direction of hole since the temperature at the area neighboring inner wall of hole being drilled would be maintained under phase transformation temperature of Ti6Al4V over a drilling operation except the exit area of hole.