Abstract: A radially expansible thin-walled expansion sleeve (13) is expansible under hydraulic pressure by pressing a piston element (17) against hydraulic fluid in a chamber, upon operation of a threaded bolt (18) in a threaded bore (19) in the spindle (11) or an attachment element (12) connected thereto. To prevent loss of the grinding disk, and provide a safety interlock in case of leakage of hydraulic fluid and hence loss of frictional engagement due to the expansion sleeve, a safety lock is formed by a sleeve element (22) which has resilient tines (24) extending therefrom, formed with locking projections (25), radially expanded behind a shoulder (30) formed on a disk holding bushing (1) upon tightening of the bolt (18). The tines (24) have internal camming surfaces (26) which engage against conical surfaces (27) located on the spindle (11) or the attachment (12) thereto.