the gear rim not shown must in this case be constructed appropriately in the embodiments of fig4 and 5 the guide slots 19 and 19a have been illustrated as running parallel to the machine axis it is however possible for the slots to run at an angle to the machine axis even to have different inclinations the pinions and the gear rim must be provided with a conical toothing which is however within the scope of knowledge of the average machine constructor finally in fig6 there is shown an embodiment in which a toothed member 25 which carries the preforming rollers and which corresponds to the member 10 of fig2 is rotatable by means of a toothed rack 26 which is movable up and down for this purpose the toothed rack is provided with a bolt 26a which projects rearwardly through a radial slot in the base plate 27 rotatably mounted at the rear side of the base plate is an annular member 28 which is retained by two rings 29a and 29b the bolt 26a projects through a cam slot 28a in the member 28 and is provided behind the member 28 with a washer and nut 30 although in fig6 only one slot 28a is shown in the member 28 it is to be understood.