insertion projection 30 but instead only over a sub region as is evident from fig1 recess 31 2 is open toward the free end of insertion projection 30 i e in the insertion direction recess 31 2 also opens up radially outward with no undercut insertion projection 30 comprises on the back side located opposite abutment surfaces 31 1 a compression screw receptacle 32 that is equipped with a pressure surface 32 1 fig6 and 9 illustrate that recess 31 2 has a concavely inwardly curved geometry between the two abutment surfaces 31 1 and in particular can form a partly cylindrically shaped cross section fig7 to 10 depict in more detail the configuration of insertion projection 30 fig9 clearly shows the concave inward curvature of recess 31 2 that adjoins the convex abutment surfaces 31 1 it is clear from fig1 that insertion projection 30 has in its region adjoining abutment surfaces 31 1 a substantially circular or oval cross sectional conformation fig8 illustrates the region of compression screw receptacle 32 pressure surface 32 1 being incident at an angle u03b4 to longitudinal center axis m of insertion projection 30 this angle of incidence u03b4 is preferably in the range between 20 u00b0 and 60 u00b0 in order to achieve an optimum draw in effect for bit holder 20 fig7 furthermore shows that pressure surface 32 1 is arranged at a distance equal to distance dimension a from the attachment region of insertion projection 30 onto support member 21 abutment surfaces 31 1 are arranged at a distance equal to distance dimension b from the attachment region of insertion projection 30 onto support member 21.