Patent ID: 8192114

Claim:
A combination of a center drill and a drill holding tool, wherein: the drill holding tool includes a shaft and a drill-holding portion, the drill-holding portion includes a recess axially corresponding to an axis of the drill holding tool, the recess is defined by two sidewalls and a bottom surface, a threaded hole is defined through the bottom surface and two reception holes are separately defined in the two sidewalls of the recess, two positioning bars are separately received in the reception holes and are independent pieces, the center drill is partially engaged with the recess, the center drill includes a positioning hole defined through a center thereof and two second straight surfaces are defined in two sides of the center drill, two positioning notches are defined in the two second straight surfaces of the center drill, the first and second straight sides are matched to each other, a trough is formed by each of the two positioning notches and one of the reception holes to receive one of the positioning bars, a bolt extends through the positioning, hole and is threadedly connected to the threaded hole, two bits extend from two ends of the center drill, cutting edges are defined between the bits and the second straight sides, and the center drill placed in the recess is coaxial with the drill holding tool.