Abstract: A tool holder assembly which includes a support and a chuck for a tool is disclosed in which the tool holder may be responsive to either torque or thrust overload, or a combination of both. The tool holder includes a movable signal member in the form of a longitudinally movable sleeve which may be actuated rearwardly to actuate a proximity switch to stop rotation of the tool holder assembly, for example. Spring means acts between the chuck and the support, with this spring means being preloaded. A hemispherical projection and a conical recess act between the chuck and the support so that if a longitudinal thrust is provided on the tool and chuck, it is transmitted directly through this projection and conical recess to additionally prestress the spring means and to rearwardly move the signal member sleeve.