be determined very easily the stability of the connections 46 is not impaired in any way the head piece 36 per se is preferably made from cast iron it may however be made from any other stable material notice should be taken in this respect however that the lower side of head part 40 should be sufficiently hard so as to avoid unnecessary and rapid wear fig1 shows the preferred embodiment which is based on a combination of the first and second aspects of the invention a head piece or part 36 again includes an insertion part 41 head piece 36 is also provided with a slide section 50 which in the working position of the flat rod is guided along flexible bow 2 and during the return movement along a rail not shown slide section 50 is provided with two projections 52 and the two projections 52 jointly form a receiving opening 54 flat rod 31 is unchanged with respect to the embodiment of fig8 that is why it is only shown partially in fig1 schematically and will not be explained further in connection with fig1 drive belt 9 is arranged as a toothed belt the teeth on the 34 inner surface 34 56 of the belt i e on surface 56 which with respect to the revolving closed path is directed inwardly cooperate with drive wheels not shown on the 34 outer surface 34 58 of the belt which is opposite to flexible bow 2 in the working position of the flat rods recesses 60 are provided in pairs whereby recesses 60.