tool holder 1 the adjacent rocking levers 12 are pivotally connected to one another through pivot pins 13 and recesses 15 in fig1 it is shown diagrammatically that the rocking levers 12 are mounted on hinged bearings 14 of the tool holder 1 if now the knives 9 11 on reaching the highest position 22 of the edge of the bore hole are pressed back in the direction 18 in the tool holder then the hinge pins 13 at the connecting recesses 15 of the knives 9 11 with the parallelogram is pressed up in the direction 18 the tilting levers 12 pivot about the bearing bolts 14 in the direction of rotation 23 the knives 8 10 engage the opposite face end of the rocking levers 12 and carry out an advancing movement in the downward direction 19 and are thus moved to the lowest points 21 on the bore hole edge 33 with the diagrammatically shown mechanism it is therefore seen that the force reacting against the downward direction of advance serves for the automatic control of the remaining knives in the downward direction 19 on the contrary however the knives 8 10 control the advance of the other knives 9 11 from the force diagram in fig1 it is clear that the automatic control of the knives 8 9.