Abstract: A self-centering steady of the closed type includes a housing (H), an actuator (14), a center gripping arm (11) and a pair of side gripping arms (12, 13) intended to grip a workpiece (P) under the action of the actuator (14). The actuator is provided with cam surfaces (31, 32) for engaging the side gripping arms (12, 13) in order to cause the workpiece engagement and/or disengagement movements. Each side gripping arm (12, 13) is connected to the actuator (14) and is slidably received in a rotating element (26) provided in the steady housing (H). The center gripping arm (11) is slidably received in a slide guide (G) provided in the steady housing (H). The rotating elements (26) together with the side gripping arms (12, 13) and with the center gripping arm (11) form a closed structure which prevents dust and scraps from entering the steady housing (H), both with the gripping arms (11, 12, 13) in the retracted and in the extended positions.