Drill bit for drilling an undercut hole

A drill bit for drilling an undercut hole in an object, comprising:

• a body having a shank at one end;
• a drill bit projecting from the other body end;
• a thrust transmitting member adapted for slidable movement on the body and having a contact portion which comes into contact with the surface of the object to be drilled;
• a guide groove extending axially in the outer surface of the drill bit and having a sloped surface formed at the bottom thereof;
• an undercut cutting member movably mounted in the guide groove and including an undercut cutting edge at the bottom thereof corresponding to the sloped surface;
• a pull-up spring coupled to the undercut cutting member;
• a working force transmitting mechanism coupled intermediate the thrust transmitting member and the undercut cutting member, to press down the undercut cutting member against the resistance of the pull-up spring in response to movement of the body and the thrust transmitting member with respect to each other thereby causing the undercut cutting edge to protrude outwardly of the drill bit outer surface;
• the thrust transmitting member includes a cylindrical sleeve including an upper end, the contact portion at the sleeve bottom, and an elongated hole in the sleeve;
• the body includes a lower portion adapted to fit inside the upper end of the cylindrical sleeve;
• a lock pin projecting from the body and engageable with the elongated hole in the cylindrical sleeve;
• the pull-up spring is mounted between the contact portion of the cylindrical sleeve and the undercut cutting member;
• and
• the working force transmitting mechanism includes
• (1) a plurality of balls housed consecutively in an inverse U-shaped hole made in the body to form a train of balls,
• (2) a pair of connecting rods projecting from the body to make contact with respective ends of the train of balls,
• the lower end of one of the connecting rods engageably contacting the contact portion of the cylindrical sleeve, and
• the lower end of the other connecting rod engageably contacting the undercut cutting member.