Abstract: An expansible drill is provided for drilling and undercutting holes. The drill comprises a rotatable tubular drill body having a drill head at one end for cutting a bore. At least one undercutting arm is located within a slot in the drill body and attached by a hinge such that at least a part of the arm is pivotal radially outwards. A guide is provided for moving the arm radially outwards, the guide comprising a shaft which is located within the drill body and constrained to rotate therewith. A locking mechanism selectively prevents or allows the shaft to move axially relative to the drill body as force is applied to an end of the shaft.