118 for an axial displacement of polishing tool 88 along a longitudinal axis 120 are provided also at this point it goes without saying that the elements shown are to be understood only schematically and that of course other types of rotary drives or of linear drives may likewise be used fig6 on an enlarged scale and with further details shows the lower end of hand 78 this lower end is entirely surrounded by a housing 128 within housing 128 and below rotary drive 116 there is arranged a first pinion 130 driven thereby connected with a second pinion 132 via a toothed belt 134 the second pinion 132 rotating about axis 120 second pinion 132 drives a rotor 136 being journalled via a plurality of bearings within a seat 137 being stationary with the housing at its lower end rotor 136 is provided with a sleeve 138 in which a polygonal tube 140 is arranged non rotatably but axially displaceable polygonal tube 140 is radially journalled within sleeve 138 via roller bearings 142 as a consequence polygonal tube 140 rotates with sleeve 138 however may axially move within sleeve 138 with almost no friction rotor 136 terminates downwardly in a lower end 142 and configures a radial closure plane there a tapering section 145 of polygonal tube 140 extends through the closure plane and protrudes outwardly at the free end thereof a tumbling disk 144 is arranged further details of which being explained below with regard to fig8 a first bellows 146 is attached with its upper rim to the.