in association with the rotary shaft 6 in accordance with the detection signal from the encoder 12 numeral 16 designates a box type casing enclosing the endless chain 4 and the housing 7 of the driving sprocket 5 is attached to the back side of the casing 16 numeral 17 designates a cover attached to the front side of the casing 16 and it is attached to arm supporting bases see fig3 which will be described later then as shown in fig1 and 3 supporting bases 19 having a pair of guide bars 18 parallel to the endless chain 4 are attached to the casing 16 in the space of the parallel extending endless chain 4 the suppoting bases 19 are provided with a pair of arm supporting slide bases 20 adapted to be moved toward and away in the opposite directions along the guide bars 18 numeral 21 designates cylinder units for sliding the slide bases 20 in the just mentioned manner and they are arranged between the slide bases 20 and the supporting bases 19 a supporting plate 22 is extended from each of the slide bases 20 across the back of the endless chain 4 and the base end of a rotary plate 23 is pivotably attached to each supporting plate 22 by a pin 24 then the pair of rotary plates 23 are arranged on the sides of the spindle 1 near thereto each of the rotary plates 23 is adapted to make a 90 degree turn about the pin 24 and for this purpose a cylinder unit 25 is connected between each rotary plate 23 and the associated slide base 20 also as shown in fig3 a slide support 26 is vertically fitted to the forward end of each rotary plate 23.