Indexable center drill structure

An indexable center drill structure, comprising:

• a holder, having an insert holding portion with an insert attachment seat, the insert attachment seat having at least two screw holes arranged in spaced relationship in a longitudinal direction, said insert attachment seat defining a pair of inner edges of two opposing sides and respective arcuately contoured end sections each being spaced each from the other in a direction transverse with respect to the longitudinal direction on opposing sides of the insert attachment seat, each of the pair of arcuately contoured end sections forming a segment of a circle having a center point external to an external surface of said insert holding portion, each of said arcuately contoured end sections being disposed at a distal end of a respective inner edge of a corresponding side of the insert attachment seat and in correspondence with a position intermediate the screw holes;
• an insert, accommodated in the insert attachment seat, the insert having at least two positioning holes corresponding to the screw holes of the insert attachment seat, outer edges of two sides of the insert respectively have a pair of positioning notches arranged along the transverse direction in correspondence with the arcuately contoured end sections of the insert attachment seat;
• and
• at least two screws, coupled to the holder and the insert;
• wherein when the insert is accommodated in the insert attachment seat, the screws are threaded in the screw holes and the positioning holes, and the arcuately contoured end sections are engaged in the positioning notches, so that the insert is fixed and threadedly secured to the insert attachment seat in the longitudinal direction and in the transverse direction, and a distal end of the insert extends out of the insert attachment seat.