an arc of a circular cross section and the other four are linear tracks having a substantially planar cross section i e formed in the vicinity of intersecting planar walls preferably the guideways 14 formed by the rolling tracks are arranged equidistant from each other in relation to the center line of the anchoring base provided by the projections 16 and the tool shank holder 11 in the embodiment illustrated in fig1 5 four guideways 14 are provided on one side of the center line and the other four guideways 14 are provided in the opposite side of the center line therefore as shown in fig1 bearing balls 15 are arranged according to the vertex of two quadrangles i e an external quadrangle 17 and an internal quadrangle 18 formed by the guideways 14 the internal quadrangle 18 is closer to the geometric axis of the tool shank holder 11 and is formed within the external quadrangle 17 in the embodiment illustrated in fig1 5 the external quadrangle 17 is provided with bearing balls 15 which roll in guideways 14 having an arc of circular cross section and which have a larger diameter than the bearing balls 15 of internal quadrangle 18 which roll on guideways 14 having a planar cross section this construction is not restrictive since all bearing balls 15 may have the same diameter or may have a different diameter as illustrated in a similar manner with respect to the cross section of guideways 14 wherein bearing balls 15 roll all of them may be in the shape of a circular arc or rectilinear or any other appropriate cross sectional shape the purpose of the guideways 14 forming a rolling track.