Abstract: A clamp for duplicating machines for lock keys comprising a pair of jaws (1,1',2,2') which are mounted on a threaded pin (4) rigid with a clamp support carriage (3), means for approaching said two jaws (1,1', 2,2') when clamping the key, and elastic means (8,6) interposed between said two jaws (1,1', 2,2') to keep them spaced apart. The clamp comprises, in a position misaligned with the line of action of the applied clamping force and at the opposite side with repect to the gripping faces of the jaws (1,1', 2,2'), a projecting portion causing, during the clamping stage, a moment opposite to the reaction moment caused by the clamped key.