used in combination with the covering machine 7 of fig7 when practicing the instant invention using a cone shaped core 6 shaft assembly 24 of fig4 is used shaft assembly 24 itself is composed of a shaft 26 on which there is disposed large and small discs 27 and 28 respectively and mass 25 disc means 27 and 28 are used to center shaft 26 in the center of the cone shaped space enclosed by cone 6 and to carry the axially disposed mass 25 disc 27 is obviously larger than disc 28 so as to conform to the cone shaped space created by cone core 6 mass 25 for optimum results should be located somewhere between the midpoint of shaft 26 and shaft 26 free end nearest the smallest sized disc 28 any free end will do if the core is cylindrical shaped placement of the mass at one end or the other of a core causes the wound yarn on that portion of the core where the mass is located to have a composite density higher than the balance of the core composite yarn density difference is demonstrated most visibly when a cylindrical shaped core is used in combination with a spindle having.