Abstract: A tool mounting and demounting assembly for a drilling machine includes a cylindrical support mounted on a rotatable drill shaft into which a tool holder can be inserted. The tool holder has a groove which is engagable by a hook of a tongue mounted on the cylindrical support. A collar surrounding the cylindrical support can be biased to a position which forces the hooks inward so as to engage the groove for mounting a tool. A tool support holds the tool holder along an axis coaxial with that of the drill shaft so that the cylindrical support can be lowered onto the tool holder. The surface of the tool support engages and raises the collar for releasing the hooks from the groove so as to permit the demounting of a tool.