This paper describes the mechanism of long pitch helix type chip formation of twist drill drillable deep hole under continuous high feed rate. The long pitch helix type chip forms after the segment type chip formation. In drilling S 50 C, when the top of chip catches the flute surface after the contact with the drilled hole, the cutting ratio becomes small. If the frictional force is kept in same level, the long pitch helix type chip formes. However, the crack occurred at the second main cutting edge progresss under insufficient plasticity of chip and then the segment type chip formes. However, the crack did not progress and then the long pitch helix type chip formes when the cutting temperature at the point 1.0 mm rear from the first main cutting edge on the flute becomes apprmoximately 400°C. In order to form the long pitch helix type chip in early stage, the following conditions can be considered : (i) Enlargement of frictional force between chip and the flute (Prevention of crack progress). (ii) Change of cutting ratio under no constraint by the drilled hole and the flute (Reduction of cutting ratio change by them). In order to realize these conditions, the drill with chamfered margin and the drills with zero-degree rake angle were tested. These drills showed the effective results.
