Percussion drill bit, an insert for use therein and a method of drilling a bore

A percussion drill bit for drilling bores, comprises a drill body having a plurality of gauge row inserts embedded in a front end of the drill body. Each gauge row insert comprises a cemented carbide body having a mounting portion embedded in the drill body, and a front end protruding from the drill body. A polycrystalline diamond layer is provided on the front end of the carbide body. The diamond layer has a rear edge disposed in the vicinity of a maximum diameter of the insert. That rear edge is spaced from the wall of the bore during drilling by tilting the gauge row insert at an acute angle of 41 DEG to 51 DEG relative to the rotational axis of the drill bit. The front end of the carbide body is defined by a radius whose origin lies on a center axis of the insert. That origin is disposed axially rearwardly of a plane containing the rear edge of the diamond layer. The length of that radius is 50% to 52% of a diameter of the mounting portion of the insert.