Drill and method for forming hole

A drill comprising:

• a drill main body having a shank portion at a rear side of the drill main body and a cutting portion at a front side the drill main body;
• a group of lands including a first land to an n-th land formed at the cutting portion;
• a group of cutting portion grooves including a first cutting portion groove to an n-th cutting portion groove, each of the cutting portion grooves being formed between two adjacent lands from the group of lands;
• a pilot blade which is formed at a tip of the cutting portion and forms a pilot cutting hole in a material to be machined;
• a diameter-enlarging spiral thread portion having a group of threads of a diameter-enlarging spiral thread which is formed on each of the first land to the n-th land of the group of lands so as to continuously follow the pilot blade, the diameter-enlarging spiral thread gradually forming a spiral convex part while cutting the pilot cutting hole so as to enlarge a diameter of the pilot cutting hole;
• a finishing spiral thread portion having a group of threads of a finishing spiral thread which is formed on each of the first land to the n-th land of the group of lands so as to continuously follow the diameter-enlarging spiral thread portion, each of the finishing spiral thread having the same height, and cutting and finishing the spiral convex part so as to have a flat wall surface;
• a spiral dent formed between a spiral thread and a spiral thread of the diameter-enlarging spiral thread and the finishing spiral thread in a way that each adjacent cutting portion groove from the group of cutting portion grooves can communicate with each other;
• a diameter-enlarging spiral blade portion which is a crest edge of the diameter-enlarging spiral thread formed by the diameter-enlarging spiral thread and a groove surface of each of the first cutting portion groove to the n-th cutting portion groove of the group of cutting portion grooves and has a group of blades of a diameter-enlarging blade, the diameter-enlarging blade performing a segmented diameter-enlarging cutting or a split diameter-enlarging cutting to the pilot cutting hole formed by the pilot blade;
• and
• a finishing spiral blade portion which is a crest edge formed by the finishing spiral thread and a groove surface of each of the first cutting portion groove to the n-th cutting portion groove of the group of cutting portion grooves and has a group of blades of a finishing blade, the finishing blade performing a segmented diameter-enlarging cutting or a split diameter-enlarging cutting to a diameter-enlarging culling hole formed by the diameter-enlarging spiral blade portion.