The screw-on part (1) has a first clamping surface. The cover part (2) has a second clamping surface. The panel (11) is clamped between the clamping surfaces by connections on the screw-on and or cover part. At least one of the two clamping surfaces is positioned on a movable body (7) in the form of a wedge with one wedge surface (9) forming the clamping surface. The other wedge surface (8) is movable on a flat guide surface (5) on the screw-on or cover parts and tilted at an angle to the clamping plane (6).
