A self-locking clamping apparatus is provided for coupling a tool unit to a tool supporter. The clamping apparatus includes a pair of locking spheres, a canister member that is matable with the tool unit and which includes a pair of opposing apertures for admitting the locking spheres, and a lock rod reciprocally movable within the canister member along an axis and having opposing ramp portions for radially moving the locking spheres through the canister apertures into locking engagement with the tool unit. The ramp portions are inclined at a small angle of approximately 5.degree. with respect to the axis of movement of the lock rod to render the lock rod self-locking as a result of stiction between the surfaces of the ramp portions and the locking spheres. The small angle of the ramp portions allows the use of a small, lightweight pneumatic actuator which may be powered by shop air. Finally, a biasing mechanism in the form of one or more coil springs prevents the lock rod from being shaken out of a locked position from externally applied vibration.