PURPOSE: To simplify the manufacturing process and enhance the strength for an insert core, by forming a male screw at the outer circumference of a ferrite core and then engaging the male screw to the female screw formed on a supporting plate.

CONSTITUTION: The driver groove 123 is formed at the upper edge part of the bar- type core 12 and the male screw 19 is formed at the outer circumference of the core 12. Then the screw 19 is engaged to the female screw 51 formed on the supporting plate 5. The outer iameter of the core 12 is constant from the upper edge through the lower edge, thus producing no problem of strength. Furthermore no other screws are used to eliminate the screw inserting job. Thus no shift is caused to the center shaft between the screw and the core to produce no deflection of the screw.