The frame consists of an inner section with a support for the cover (3), and an outer section (11). This is height-adjustable relative to the inner section. A spacer ring (17) for the cover is adjusted to the new height. The ring is fastened to the cover. The outer frame section is shaped like a hollow cylinder with lower bevelled edges. It has internally accessible projections etc, for a drawing tool. The spacer ring has a damper insert. The inner frame section has radial channels for injection of an packing material (16).
