Abstract: A tool for boring cavity holes in concrete walls and the like comprises a cutting blade pivotally mounted about a transverse pin at the end of an elongate body and having a camming profiled portion. A ball is placed at the bottom of the hole which has first been bored with a conventional drill and the tool cutting blade is brought into contact with the ball while causing rapid rotation of the tool body and exerting an axial pressure. The ball cooperates with the blade camming profiled portion so as progressively to orient the blade in a transverse direction.