bar support 1 a such that the movable base 8 is normally urged in the upward direction by the coil spring 5 as shown in fig1 and 2 the movable guide 9 has a slot 9 a formed thereat the slot 9 a receives the guide pin 3 so as to vertically move guide pin 3 and the threading bar 2 accordingly as the operating member c is vertically moved the slot 9 a is inclined at an appropriate angle generally 45 u00b0 but not strictly limited to the mentioned angle the slot 9 a is inclined in the direction opposite to the inclined groove region 7 b of the guide groove 7 the inclined slot 9 a is so inclined to give a desired rotation speed of the guide pin 3 as the inclined slot 9 a moves along the inclined groove region 7 b of the guide groove 7 in case the inclination of the inclined slot 9 a is acute the rotation speed of the guide pin 3 is fast and in case the inclination of the inclined slot 9 a is obtuse the rotation speed of the guide pin 3 is slow namely the rotation speed of the threading bar 2 and of the arm 4 a of the threading member 4 is determined by the degree of inclination of the slot 9 a the slot 9 a may be formed as generally extending horizontal in this case the.