post 20 below the hollow truncated split cone 22 and fixed in place by a tightening up screw 28 a cylindrical nut 26 is mounted around the rack post 20 and threaded onto the hollow screw 27 from the top a spring 29 is mounted around the rack post 20 and stopped between the cylindrical nut 26 and the flat top of the hollow truncated split cone 22 by turning the cylindrical nut 26 on the hollow screw 27 the tension of the spring 29 is relatively adjusted and therefore the tension of the spool of thread which is mounted on the hollow truncated split cone 22 is relatively adjusted referring to figures from 7 to 9 the a hollow truncated split cone 31 is mounted around the rack post 30 and revolvably secured in place by locating rings 311 312 a locating rod 34 is fastened to one elongated slot 195 of the horizontal supporting wall 193 by a screw 35 having a top hole 36 which holds a spring 38 and a steel ball 37 on the spring 38 a hollow cylindrical rack post holder 33 is mounted around the locating rod 34 and fixed thereto at the desired elevation by a tightening up screw 39 to hold a bearing 32 and the bottom end of the rack post 30 in the bearing 32 because the rack post 30 is mounted in the bearing 32 and supported on the steel ball 37 above the spring 38 less tension is employed to the spool of thread which is mounted on the hollow truncated split cone 31 referring to figures from 10 to.