The adaptor includes a compensating ring (13) which has a first non-through slot so that its circular cylindrical internal faces are divided into two, opposite to which lie the first of the circular cylindrical external faces of the shank section (12) of the tool holder (11) and the second of the circular cylindrical external face of the adaptor ring, thereby forming two ring segments. The ring segment of the second face section has a radial slot, and a screw lying in a radial plane bridges it.
