A drilling system for producing deep holes in a workpiece, the drilling system comprising: a holder body having at least a first mounting screw hole extending along an axis; a drill head attachable to the holder body, with the holder body having a top surface forming a first mating surface, and the drill head having a bottom surface forming a second mating surface that engages the first mating surface of the holder body when attached thereto, wherein the drill head has at least a second mounting screw hole extending along an axis; at least one dovetail protrusion and at least one corresponding dovetail groove, with the at least one protrusion or groove formed on the opposing first and second mating surfaces, wherein the drill head is assembled to the holder body by rotating the drill head relative to the holder body about the axis of the tool until the at least one dovetail protrusion engages with the at least one dovetail groove and locks the drill head into place in association with the holder body, and at least one screw attaching the drill head to the holder body, wherein the at least one screw has a conical head and the drill head has a conical seat for receiving the conical screw head, characterised in that the axis of the at least a first mounting screw hole and the axis of the at least a second mounting screw hole are arranged to line up such that when the at least one screw is received in the first and second mounting screw holes (18, 30) and tightened, the conical seat of the screw head causes the drill head to rotate about is center axis in the direction of tool rotation until the at least one dovetail protrusion engages with the at least one dovetail groove, and the at least one screw carries substantially no load during drilling. a holder body having at least a first mounting screw hole extending along an axis; a drill head attachable to the holder body, with the holder body having a top surface forming a first mating surface, and the drill head having a bottom surface forming a second mating surface that engages the first mating surface of the holder body when attached thereto, wherein the drill head has at least a second mounting screw hole extending along an axis; at least one dovetail protrusion and at least one corresponding dovetail groove, with the at least one protrusion or groove formed on the opposing first and second mating surfaces, wherein the drill head is assembled to the holder body by rotating the drill head relative to the holder body about the axis of the tool until the at least one dovetail protrusion engages with the at least one dovetail groove and locks the drill head into place in association with the holder body, and at least one screw attaching the drill head to the holder body, wherein the at least one screw has a conical head and the drill head has a conical seat for receiving the conical screw head, characterised in that the axis of the at least a first mounting screw hole and the axis of the at least a second mounting screw hole are arranged to line up such that when the at least one screw is received in the first and second mounting screw holes (18, 30) and tightened, the conical seat of the screw head causes the drill head to rotate about is center axis in the direction of tool rotation until the at least one dovetail protrusion engages with the at least one dovetail groove, and the at least one screw carries substantially no load during drilling.